Category: astronomy

The Alpha Centauri System

Post author By Ngtriplett
Post date June 11, 2020
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Planets at Alpha Centauri? — Courtesy of Physics.org

If you ask someone what the nearest star is, other than the sun, the likely reply will be Alpha Centauri. As it turns out, Alpha Centauri is actually a system of three stars discovered in 1915. The two main stars are a pair of binary stars Alpha Centauri A and Alpha Centauri B. They are termed binary stars because they orbit a common center of gravity (space.com). The third star in the system is named Proxima Centauri. So why then do people say that Alpha Centauri is the closest star to our sun? As it turns out when you look up at the night sky you see this three star system as a single star.

What Do We Know about the Physical Structure of Alpha Centauri?

Let’s talk about the characteristics of each of the stars in this system. Centauri A has a surface temperature of 5770 K (10,000 F) which is similar to that or our own sun which has an average surface temperature of 5778 K according to evimage.org. According to earthsky.org Centauri A has a diameter 25 percent greater than our sun. The luminosity, or intrinsic brightness of Centauri A is 1.6 times the luminosity of our own sun

Centauri B has a surface temperature of 5300 K (9,000F) and a luminosity which is equal to one half the luminosity of the sun. Centauri is 90 percent the mass of the the sun. If Centauri B were not part of this binary system it would rank as the 21st brightest star in the night sky. (earthsky.org)

Proxima Centauri is a red drawf star which orbits the Centauri A&B pair. Proxima has a mass that is about 12 percent or ouf sun and a radius that is 14 percent the size of our sun (solarsystemquick.com) and is 20 thousand times fainter than our sun. (nineplanets.org)Proxima has a relatively cool surface temperature of 3500 K (5840 F).

Centauri A and Centauri B orbit a common center of mass with an orbital period of 80 years. The average distance between them is 23 AU or astronomical units which is the distance from the Earth to the sun. The distance between Centrauri A&B and Proxima Centauri is about 13,000 AU and it takes almost a half a billion years for Proxima to orbit Centauri A&B. (nineplanets.org)

How Far Away is Alpha Centauri?

Distance Alpha Centauri measured in astronomical units. Courtesy of futurism.com

We have stated that the three star system Alpha Centauri is the closest star to our sun but just how far away is it? Alpha Centauri A and Alpha Centauri B are an average distance of 4.3 light years from earth according to space.com and Proxima Centauri is 4.22 light years from earth. This may not sound like a large difference but it means that Proxima Centauri is 620 billion miles closer than Centauri A and Centauri B. (earthsky.org) Let’s take a moment to discuss the term light year. This is a measure of distance and not of time. A light year is the distance light can travel in a year. This distance is roughly 6 trillion miles or 9.5 trillion kilometers. In order for us to reach Proxima Centauri, the closest of the three stars, we would need to travel at the speed of light for 4.22 years. As a refresher, the speed of light is roughly 670,616,629 miles per hour, stated another nearly 671 million miles per hour. Even if we could somehow travel at that rate it would still take over four years to reach it.

Is Alpha Centauri a Planetary System?

You may be thinking about our star, the sun, and the planets that orbit it. We know this is our solar system and you may be asking yourself if the Alpha Centauri system has any planets orbiting it? The answer is yes, astronomers discovered a planet orbiting Proxima Centauri in 2016. The fact that this star has a planet orbiting it makes this a planetary system. A planetary system is simply a star with a planet or planets orbiting the star. You may ask how is that different than a solar system. The solar system is simply our planetary system which we named the solar system. The planetary system of this planet has three suns, Centauri A, Centauri B, and Proxima Centauri.

The planet orbiting Proxima Centauri is named Proxima b. The planet is 1.3 times as massive as Earth according to space.com and resides in the habitable or Goldilocks zone of the Proxima Centauri. The term habitable zone is a bit misleading as it does not necessarily mean that any planet in this zone can support life. What it does mean is that it is at an appropriate distance that if the planet has water it won’t freeze or boil off the surface. NASA defines the habitable zone as “is the range of distance with the right temperatures for water to remain liquid.”

Proxima Centauri b is is classified as a super Earth planet. Livescience.com defines a super Earth planet as “a planet with a mass between 1 and 10 times that of Earth. The super-Earth classification refers only to the mass of the planet, and does not imply anything about its surface conditions or habitability.” The exoplanet of the Alpha Centauri system has a radius that is 1.08 times the radius of the Earth. (exoplanets.nasa.gov) Proxima Centauri b has an orbital period of 11.2 days which means it takes 11.2 days to orbit Proxima Centauri and is only 4.7 million miles from Proxima Centauri which is about one half of an astronomical unit AU. By way of comparison the Earth is roughly 93 million miles from our own sun which is one AU.

Could Alpha Centauri Host Other Exoplanets?

In 2019 astronomers detected a second potential exoplanet orbiting Proxima Centauri. The object has tentatively been named Proxima c pending verification that the object is indeed a planet. This potential planet appears to reside in the habitable zone which sounds like an exciting prospect. According to space.com “the candidate world, which has a minimum mass about six times that of Earth, orbits 1.5 astronomical units (AU) from Proxima Centauri and is therefore probably very cold. Its equilibrium temperature hovers around minus 390 degrees Fahrenheit (minus 234 degrees Celsius).” Proxima c is 1.5 AU from Proxima Centauri which is a bit of a problem for astronomers. Scientists measure the gravitational effects of an orbiting planet or other object on the star. The effects are minor at on a smaller star at such a great distance.

How to View Alpha Centauri

As mentioned earlier Alpha Centauri appears to be a single star to the naked eye. This star system form the brightest star in the Southern constellation called Centaurus. Only the stars Sirius and Canopus appear brighter making Alpha Centauri the third brightest star in the night sky. Unfortunately for most of us in the northern hemisphere we are unable to see Alpha Centauri. Anyone north of 29 degrees latitude will not be able to view this bright star. Stargazers in portions of the southern hemisphere, including Australia, will note that the star is cirumpolar which means it never sets. If you are lucky enough to be in an area where you can view Alpha Centauri take advantage of it. Some of may never get to gaze directly at the third brightest star in the night sky. Here is a quick video tour of the Alpha Centauri system: Alpha Centauri Virtual Tour

Tags Alpha Centauri, Alpha Centauri A, Alpha Centauri B, exoplanets, nearest star, Proxima b, Proxima c, Proxima Centauri

astronomy physics solar system

Introducing Comet SWAN

On March 25th amateur astronomer Michael Mattiazzo discovered Comet SWANN (C/2020 F8) using the Solar and Heliospheric Observatory’s SWAN (Solar Wind Anisotropies) camera. (skyandtelescope.org) Viewers in the Northern Hemisphere should be able to view the Comet SWAN beginning in Mid May. The discovery of this previously unknown comet got me thinking a bit about these celestial objects. This comet has only been recently discovered so we don’t yet know much about it. I wanted to give a brief description of comets and talk a little about what we do know about these fascinating objects.

According to solarsystem.nasa.gov “Comets are cosmic snowballs of frozen gases, rock and dust that orbit the Sun.” This certainly doesn’t seem like much to get excited about. Let’s look a little closer at these “cosmic snowballs”. Think back to your days in middle school and high school science class when you learned about atomic structure. One of the things that stood out to me is how similar each atom was varying only by number of electrons and/or neutrons. It was easy to draw an atom of any element using the Bohr model because they all had the same basic structure, a positively charged nucleus containing some protons and neutrons, and electrons with their negative charges orbiting the positively charged nucleus. It turns out that active comets which are close to the sun have all have similar structure as well, meaning they all have the same component parts but may vary from comet to comet in size, composition, and place of origin.

The Structure of Active Comets

The structure of active comets all contain several distinctive features. The nucleus of a comet is the portion which contains mostly ice and gas with trace amounts of dust and other particles and is generally 1 to 10 km in size. It is possible, however for a comet to have a nucleus of up to 100 km. The nucleus of an active comet is mostly in a solid state and fairly stable. The coma is a cloud of water, carbon dioxide and various other gases which are converted directly from solid to gas as they are heated as the comet approaches the sun. This part of the comet may be over a thousand times larger than the nucleus. The nucleus along with the coma form the head of the comet. The hydrogen cloud or envelope is yet another feature of comets, this is an extremely large cloud of hydrogen which envelopes the comet. This cloud may be in the vicinity of millions of kilometers in diameter. Active comets that are nearing the sun have two distinct tails. The dust tail, which is the most obvious and visible feature to the unaided eye, is composed of mainly dust and other gases and may be upwards of 10 million kilometers long. Interestingly, the dust tail always points away from the sun. The dust tail reflects light from the sun making it the most visible portion of the comet. The second tail, the ion tail is composed of plasma and other particles related to the comets interaction with the solar winds associated with the sun. This tail may be much longer than the dust tail reaching lengths of several hundred kilometers.

28+ [ Comet Diagram ] | schematic diagram of a typical comet the ... — Courtesy of http://0osr12g1.adtddns.asia/comet-diagram.html

The Path of a Comet around the Sun

Comets, like planets orbit around the sun. The orbit of a comet follows a much more elliptical path than do planets. The length of time it takes an object to complete one orbit is called a period. The period of the Earth’s rotation around the sun is 365.25 days and the period about its axis is 23 hours and 56 minutes. The period of the moon is 27 days which is the amount of time it takes to orbit the Earth. Comets in general have much longer periods. Any comet with a period of less than 200 years is referred to as short period comets. These comets originate from the Kuiper Belt. In case you were wondering, the Kuiper Belt is “a donut-shaped region of icy bodies beyond the orbit of Neptune. (solarsystem.nasa.gov) If the period of a comet is greater than 200 years it is called a long period comet and originates from the Oort cloud which is “is believed to be a giant spherical shell surrounding the rest of the solar system and is the most distant region of our solar system. Even the nearest objects in the Oort Cloud are thought to be many times farther from the Sun than the outer reaches of the Kuiper Belt.” (solarsystem.nasa.gov) Halley’s comet is perhaps one of the most famous comets of all time and has a period of 76 years making it a short period comet. The Hale Bop comet is a long period comet with a period of over 2500 years.

An article featured on Space.com, author Joe Rao wrote about the period of comet SWAN where he stated “for fun, I fed its orbital elements, which includes the eccentricity of its path around the sun, into an orbital simulator. My simulation suggests Comet SWAN is traveling around the sun in a period of about 25 million years. (space.com) The discovery of this comet is so new that the actual period of orbit remains to be seen but this certainly would be a fascinating development. We would be the very lucky few of humankind to be able to view this very long period comet.

I've heard that the Oort Cloud contains trillions of icy bodies ... — Courtesy of astronomymagahttps://astronomy.com/magazine/ask-astro/2018/04/the-oort-clouds-icy-bodieszine.com

How to View Comet SWAN

The best time to view SWAN will be in the evening hours or just before dawn beginning on May 23rd until June 10th according to skyandtelescope.org. The comet currently is visible using binoculars as well as the unaided eye to those in the Southern Hemisphere. Curious stargazers wishing to view the comet with the unaided eye in the Northern Hemisphere shouldn’t have to wait much longer as it will be visible to you by late May. The comet will reach perihelion, which is its closest distance to the sun around May 27th. At perihelion the SWAN comet will be roughly 64 330 000 km from the sun and the closest it will be to Earth will be on May 12-13th when it will be a mere 83 330 000 km from Earth.

Stay tuned to your favorite astronomy magazine or website to learn more about this newly discovered comet. Michael Mattiazzo, the amateur who discovered this comet has discovered a total of 8 comets dating back to 2004. Keep your feet on the ground, eyes open and head to the heavens and maybe you will discover the next comet or other astronomical object.

Tags Comet SWANN, comets, Hale Bopp comet

astronomy cepheid variable physics solar system supernova

How Do We Measure Distances in Space: Cosmic Distance Ladder Part 2

Post author By Ngtriplett
Post date April 10, 2020
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Last week we discussed several rungs on the cosmic distance ladder including stellar parallax, spectroscopic parallax, and main sequence fitting of stars. We discussed several astronomical terms associated with the cosmic distance such as astronomical unit, light year, and parsec. That discussion got us about halfway up the cosmic distance ladder. Lets ascend the rest of the way.

Variable stars

Have you ever wondered if the brightness of stars ever change periodically? There is a class of stars called Cepheid Variable stars that do just that. According to the NASA website starchild.gsfc.nasa.gov/docs/StarChild/questions/cepheids.html, Cepheid Variables “are stars which brighten and dim periodically. In 1912 a Swiss scientists made a remarkable discovery regarding 25 Cepheid Variables located in the Magellanic cloud. Henrietta Swan Leavitt noticed that these stars seemed to brighten and dim at regular predictable intervals called periods. She postulated that the brighter the Cepheid, the longer the period. As it turns out once you know the period of one of these variables the brightness can be inferred.

Cepheid Variables fall into two general classes: the first are those of a period of pulsation from about 1.5 days to 50 days. These population 1 stars are often found in the spiral arm of galaxies. The second class of Cepheid Variables are Population 2 and are “much older stars less luminous, and less massive than their Population I counterparts.” These are stars “…with periods greater than about 10 days and BL Herculis stars with periods of a few days.” https://www.britannica.com/science/Cepheid-variable

So how do these stars which brighten and dim in predictable patterns help us determine the distance to nearby galaxies? The apparent magnitude of these variables can be plotted at different times to develop a light curve. A light curve is a relationship between brightness and time. Using the information from the light curve and data collected using sensitive photometric equipment, the apparent magnitudes and period of the star can be determined. These values can be plotted on a “period-luminosity” graph to determine its absolute magnitude. Once this value has been obtained you use the distance modulus formula to determine the distance to the Cepheid Variable.

Standard Candles

Cepheid Variables and other astronomical objects, such as supernova, which have a known luminosity across the entire class of objects can be used to determine the distance of nearby galaxies as well as the expansion of the universe. These objects are collectively referred to as standard candles. According to http://planetfacts.org/standard-candle/ “A standard candle is a class of astronomical objects that belong to the same class and have a standard luminosity or brightness. You can actually determine an object’s distance from the earth using standard candles.” This method of distance measurement works like this according to planetfacts.org “…. a technical process which involves comparing the object’s brightness against a known or measured brightness from objects that belong to the same class. For example, you spot a certain object like a star or supernova, and determine that it is a standard candle; you can get its distance by measuring its brightness and comparing it to the known brightness of objects that are similar to it.” In our discussion regarding Cepheid Variables we learned about two different classes of these types of variables. If astronomers were trying to determine the distance of a far away galaxy, for example, and that galaxy had a Population 1 variable in it they could then compare its brightness to a Population 1 variable with a known distance to determine the distance of the galaxy.

The Tully-Fisher Relationship

We saw that by using variables and supernova as standard candles we could determine the distance astronomical objects. Is there a method to use the object which we want to study to determine how far away it is? Well, yes actually. The Tully-Fisher relation is just such a method. According to www.noao.edu/staff/shoko/tf.html the Tully Fisher relation is “a correlation for spiral galaxies between their luminosity and how fast they are rotating.” Scientists know that larger galaxies rotate with greater velocity then smaller galaxies.

Courtesy of spiff.rit.edu/classes/ladder/lectures/distant_gal/distant_gal.html#tf

“The key point of the Tully-Fisher relationship is that the speed of rotation of material in a spiral galaxy is related to the luminosity of that galaxy: high speeds occur in galaxies of high luminosity.” Again we see that by comparing the absolute magnitude against the apparent magnitude the distance of a given galaxy can be determined. The Tully-Fisher Relationship allows us to measure the distance of galaxies up to hundreds of megaparsecs away.

Hubble’s Law

We have now reached the top ring of the cosmic distance ladder. Hubble’s Law can be used to determine distances to the edge of our universe. “What is Hubble’ law” is a question I hope you are asking. Glad you asked, Hubble’s law, according to http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/hubble.html#c1“is a statement of a direct correlation between the distance to a galaxy and its recessional velocity as determined by the red shift. So what exactly is red shift?

You may have the heard the term “Doppler” when watching the weather on your local news. The term Doppler shift, with respect to light waves is an indication that the source emitting the waves is either moving toward or away from an observer. If the source is moving toward the observer the frequency of the waves increases as the waves bunch up toward each other. A result of this increased frequency is that the light is blue shifted, meaning the color is shifted toward the blue end of the spectrum. Bluish colors have higher frequencies than do reddish colors. If the source of light is moving away from an observer the light is shifted toward the red end of the spectrum as the frequencies are reduced. Here is a good explanation of Doppler shift both for sound and light waves: https://www.youtube.com/watch?v=h4OnBYrbCjY&feature=youtu.be

Fantastic, now we know about the Doppler shift but what does this have to do with Hubble’s law? In 1929 Edwin Hubble made a shocking discovery. He was able to determine that every other galaxy was flying away from us. Perhaps even more astonishing was the fact that the farther away a galaxy was the faster it was moving away from us. The law which bears his name states that the velocity (v) of a galaxy is equal to the product of the Hubble constant (H) * the distance to a distant galaxy. This relationship is generally used to find the distance of galaxies over a billion light years away.

The velocity of the galaxy is a measure of the recession rate of the galaxy, in other words how fast the galaxy is flying away from us. The Hubble constant has an estimated value of 70 kilometers per second per megaparsec. Not only can this equation be used to determine how far away and how fast galaxies are moving from us but this equation was used to prove that the universe was in fact expanding. So how was Edwin Hubble able to determine that these galaxies were moving away from us, well he was able to determine a red shift of each of the galaxies he observed. Hubble’s law has also contributed to many of astronomy’s deepest and most fascinating issues including providing the “first concrete support for Einstein’s theory of Relativity… It also helps validate theories of Dark matter and Dark energy. A recent discovery in 1998 revealed that the expansion of the universe is accelerating.” this according to planetfacts.org/hubbles-law/

Well I hope these two posts hope clear up how astronomers determine the distance of objects from our own solar system out to the edge of our universe. As technology continues to evolve and develop I would expect that new and improved ways of measuring cosmological distances will soon be devised. Maybe someday technological advances will allow us to go and explore the universe up close and in person.

Tags The Tully-Fisher Relationship

astronomy cepheid variable physics solar system

How Do We Measure Distances in Space: The Cosmic Distance Ladder Part 1

Post author By Ngtriplett
Post date March 28, 2020
1 Comment on How Do We Measure Distances in Space: The Cosmic Distance Ladder Part 1

Part 1

This will be a 2 part discussion about the Cosmic Distance Ladder. Part 1 will cover the Parallax method, Spectroscopic parallax, and Main sequence fitting as methods to determine the distance of stellar objects from Earth. I will begin by defining some astronomical units of measure and then discuss the techniques used to measure the distance of objects throughout our Solar System, the Milky Way galaxy, and the Universe.

Part 2 will focus on the use of variable stars, such as Cepheids and RR Lyraes, as well as nearby galaxies. We will also explore The Tully-Fisher Relationship, and Hubble’s Law to determine the distance of objects deep in our universe. I hope that after reading part 1 & 2 you will have a better understanding of how the distance of objects from our Solar System to the ends of the Universe are determined.

The moon is roughly 250,000 miles from Earth, the sun is 93 million miles from the Earth or 1 astronomical unit away (AU). The next nearest star to the Earth, Proxima Centauri is 4.3 light years (ly)away. The Milky Way Galaxy is 30,000 parsecs (pc) or 30 kiloparsecs (kpc) measured from one side to the other. What do some of these units of measure such as light year, astronomical unit, and parsec mean? How do we actually know how far these objects are from Earth or how far apart they are from each other?

Astronomical Units of Measure

1 AU of astronomical unit is equal to the average distance between the Earth and Sun which is 149,598,000 km (92,955,887 miles.) This unit is used to describe distances within our solar system. Neptune, for example, is 30.1 astronomical units from Earth. This equates to about 2,700,000,000 miles. It is much easier to to calculate using 30.1 au then it is to use 2.7 billion miles.

1 ly or light year is the distance light travels in a year. It may sound as if it is a measure of time but it is actually a measure of distance. 1 light year is equal to 5,878,630,000,000 miles. It is much easier to think and calculate using light years when dealing with the enormous distances associated with the universe.

The Hertzsprung-Russell diagram or HR diagram is “… a graph that is used for making stellar calculations. It gives the relationship between the luminosity of stars against their type and temperature” according to https://universavvy.com/astrophysics-hertzsprung-russell-diagram-explanation. This diagram can be used to classify, trace the life cycle, as well as teach about the how stars work.

An HR diagram plotting luminosity v temperature. Courtesy of mtholyoke.edu

Stellar Parallax and the Parsec

In order for our discussion of parsec to make sense I am going to discuss the phenomena of parallax first and then tie it into our description of a parsec. On the cosmic distance ladder, parallax is the only direct method we have to measure distances to stars. To give you an easy and quick example of parallax hold one arm straight out in front of you and point your thumb to the sky. Now alternate opening and closing your right and then your left eye. The apparent movement of your thumb is an example of parallax.

Astronomers can use this idea to measure the distance of a star. In the image below you can see the position of the Earth six months apart. The angle of parallax can be measured by comparing how the star has shifted as compared to the stars behind it. You can see that a triangle has been formed between the star and the two positions of the Earth six months apart. By using some trigonometry you can determine the distance to the star. This method of measuring distance is limited because the parallax angle for more distant stars is to small for us to measure.

1 pc or parsec is equal to 3.26 ly or 206,280 AU. I know what you are thinking, Han Solo said about the Millennium Falcon “it’s the ship that made the Kessel run in less than 12 parsecs.” https://youtu.be/fjYuw6zWk_Y Maybe “in a galaxy far, far away” a parsec is a measure of time, but in our galaxy it is a measure of distance. The definition of 1 parsec is the distance to an object which has a Parallax angle of 1 arscencond. You can see the term parsec is a combination of parallax and arcsecond. To measure the distance, in parsecs, using parallax angle you use the equation d = 1 AU/p” where p is the parallax angle. Your answer will be the distance in parsecs. This method of calculating distance only works for stars that are within roughly 300 parsecs or 978 light years from Earth, beyond that distance the parallax angles are to small to measure.

Spectroscopic Parallax

According to https://www.atnf.csiro.au/outreach/education/senior/astrophysics/photometry_specparallax.html “The term spectroscopic parallax is a misnomer as it actually has nothing to do with parallax. It is, however, a way to find the distance to stars. Most stars are too far away to have their distance measured directly using trigonometric parallax but by utilising spectroscopy and photometry an approximate distance to them can be determined.”

So then what is spectroscopic parallax and how does it help us determine the distance of stars? Astronomers determine the spectral and luminosity class of a given star. Using photometric equipment the apparent magnitude and color index can be determined. The star is placed on to the HR diagram and its apparent magnitude is deduced. At this point an equation called the distance modulus equation is used to determine the distance in parsecs of the star. According to Australia National Telescope Facility “In practice this technique is not very precise in determining the distance to an individual star…..Nonetheless it is still an important methods for estimating distance to stars beyond direct trigonometric parallax measurement.”

Main Sequence Fitting

Main sequence fitting as a method of determining cosmic distance is done by comparing the relative distance of open clusters to the distance of the Hyades galaxy which has a known distance. An open cluster according to https://www.sciencedaily.com/terms/open_cluster.htm is “a group of up to a few thousand stars that were formed from the same giant molecular cloud, and are still loosely gravitationally bound to each other.” The stars in these clusters are believed to have been formed at the same time and located the same distance away from Hyades. The distance of Hyades was found via direct measurement using stellar parallax.

Scientists will measure the spectra of the cluster to determine “spectral type, luminosity, and temperature and define a main sequence” according to Teach Astronomy https://www.youtube.com/watch?v=hq29cjeR2o4. Scientists compare the shift between the open cluster and the Hyades galaxy on the HR diagram and use the inverse square law to determine the relative distance between the open cluster and the Hyades galaxy. The inverse square law states that the the light will get dimmer by a factor of the square of the distance away from the source of light.

Next week we look at part 2 of the Cosmic Distance Ladder which should give you a fuller picture of how astronomers determine the distance of objects throughout our universe.

Tags cosmic distance ladder, cosmology, HR diagram, parallax

astronomy Black holes physics

Black Holes

Most of us have some idea about black holes. How much you know about these objects likely varies on how interested you are in astronomy as well as your scientific background. Science fiction has long portrayed black holes as evil forces of nature bent on destroying mankind. Movies and books often portray these celestial bodies as monsters traveling across the universe eating up everything in its path. Based on these characterizations you might predict the end of our planet will most likely be due to an encounter with a rogue black hole. So before we start preparing for our demise via black holes lets all take a deep breath and discuss the facts about these things.

Escape Velocity

The first thing we should discuss is something that my students often recite whenever the topic of black holes is mentioned. They will automatically say “it is something so dense not even light can escape it” while this is correct what does it actually mean? In order for something, be it light or a rocket, to leave a planet or anything with a gravitational field the object must over come what is called escape velocity. So what does it actually mean? Escape velocity is the speed needed for an object to overcome the gravitational attraction of a planet, or other body. The gravitational attraction can be calculated by the following equation:

Image result for law of universal gravitation — Courtesy of sciencephoto.com

Where F is the gravitational attraction, G is the universal gravitational constant and is equal to 6.67 x 10^-11 m^3/kg * s^2, m1 and m2 represent the masses of the planet and the object being attracted to it and r^2 is the distance between the two objects. You can use this formula to calculate the gravitational attractive force between any two objects in the universe. In order to break free from the gravitational attraction an object must be traveling fast enough to overcome this force. Escape velocity can be calculated by the following formula:

Image result for escape velocity formula — Courtesy of getcalc.com

With this equation, which is derived from the kinetic energy formula and the Universal Law of Gravitation, you can calculate how fast something must travel to leave the planet. An object must be traveling at a rate of 11 km/s in order to leave the Earth. If, for example, one found them self on the surface of a neutron star the escape velocity would be 165,000 km/s.

So we have this equation why can’t we use it to calculate the escape velocity needed for light to leave a black hole? As it turns out this question was answered by Reverend Jon Mitchell in 1783. He was able to determine that, as a bizarre consequence of Newton’s law of motion, an object with the same density as the sun but 500 times its radius would have an escape velocity greater than the speed of light. So for any object, such as a black hole, that has an escape velocity faster than the speed of light it means nothing can escape that object. The more massive the object is and the smaller the object has been compressed the greater the escape velocity.

Features of Black Holes

Another feature of black holes is the event horizon. This is the boundary between the black hole itself and the space around it where objects may safely reside. Once an object has crossed over the event horizon the escape velocity exceeds the speed of light and therefore, no object can escape from it.

According to astronomy.swin.edu “The Schwarzschild radius is the radius of the event horizon surrounding a non-rotating black hole.” The equation for the Schwarzschild Radius is

Image result for schwarzschild radius — Courtesy of perthobservatory.com.au

Any object with a physical radius smaller than its Schwarzschild radius will become a black hole. This quantity was first derived by Karl Schwarzschild in 1916: The Schwarzschild Radius is the size an object must become in order for its escape velocity to exceed the speed of light. In order for the Earth to become a black hole it would need to be compressed down to the size of 0.9 cm or 1/3 of an inch and the sun would need to be compressed to down to 3 km or just under 2 miles. So as we will see later most stars in the sky do not become black holes at the end of their lives.

Image result for event horizon of a black hole — Courtesy of quora.com

There is a point inside the the black hole that general relativity tells us has infinite density and curvature this point is termed the singularity. According to Kip Thorne the singularity is “the point where all laws of physics break down”. Some scientists have suggested that an, as of yet, unknown combination of classical physics and quantum physics may be needed to more accurately describe the behavior and features inside black holes.

Often times black holes will be depicted with material swirling around it prior to being sucked into the black hole. This material is a combination of gas, dust, and other material making up what is called an accretion disk. Physicsoftheuniverse.com defines an accretion disk as “material, such as gas, dust and other stellar debris that has come close to a black hole but not quite fallen into it, forms a flattened band of spinning matter around the event horizon…” We are able to see the accretion disk because the particles which make it up are spinning at a high rate of speed releasing heat, gamma rays and x-rays as the particles collide with each as a result of the black holes large gravitational force.

Image result for accretion disk of a black hole — Courtesy of svs.gsfc..nasa.gov This image depicts a black hole with an accretion disk spinning around it.

So How are Black Holes Formed?

Stellar black holes are formed when a star greater than 25 solar mass, that is 25 times more massive than the sun collapse in on themselves and goes supernova. When the internal pressure can no longer withstand the gravity, stars of this size collapse until all the matter is within the Schwarzschild radius. The resulting supernova ends up as a stellar black hole. These stellar black holes are roughly 3-100 solar masses or 3 to 100 times the mass of the sun.

A second way stellar black holes are formed is when neutron stars have exhausted their magnetic fields and devolve into small (2-5 solar mass black holes) black holes. These black holes are much smaller than those created by stars that are greater than 25 solar mass. As it turns out it is a relative rarity that stars become black holes upon their death. Most stars, including our own sun, are too small to wind up as stellar black holes.

Supermassive black holes are thought to be the at the center of galaxies including our very own Milky Way Galaxy. These black holes are 100,000 times the mass of the sun. Scientists believe that the enormous mass of these black holes is directly related to their location at the center of galaxies. These galaxies provide billions of stars and other material to feed the black hole so that it can grow quickly over time. Galaxies tend to collide with other galaxies which also allows for the supermassive black holes to grow even larger.

Image result for how are stellar black holes formed — Courtesy of steemit.com

Intermediate black holes which may range anywhere from 100 to 100,000 solar mass are created by the merger of other smaller black holes to create a single larger black hole or when a black hole “eats” large amounts of matter around it to increase its mass. Chain reactions caused by the collision of stars is another way that intermediate black holes are thought to be created.

Here is a video that combines many of the topics we’ve already discussed black hole features and how black holes are formed: https://www.youtube.com/watch?v=brmjWYQi2UM

How Do We Know Black Holes Exist?

Since we are unable to see a black hole, how do we know they exist? A fair and interesting question to be sure. Well, we can detect their effects on nearby objects such as gas and stars. Stars that orbit a black hole have a characteristic “wobble” when we observe them. The wobble is caused by a massive object exerting a gravitational pull on the object. The object must be very massive to exert a noticeable effect on its companion star. Scientist can estimate the mass of the black hole by the effect it has on its visible companion. We are also able to detect X-rays from the discs of gas falling into a black hole. This may happen when material from a companion star fall into a black hole for example. Supermassive black holes are thought to eject materials in the form of high speed jets and radio emissions.

Gravitational lensing was used to determine that a black hole had traveled between Earth and a star called MACHO-96-BL5.

These images show the brightening of MACHO-96-BL5 from ground-based telescopes (left) and the Hubble Space Telescope (right). — Gravitational lensing as a black hole traveled between Earth and star MACHO-96-BLG5. Courtesy of spacetelescope.org

According to spacetelescope.org the image above “Hubble Space Telescope looked at the object, it saw two images of the object close together, which indicated a gravitational lens effect. The intervening object was unseen. Therefore, it was concluded that a black hole had passed between Earth and the object.”

So Why Do We Study Black Holes?

Scientist study black holes to learn more about the early universe as some of these black holes were likely created very soon after the big bang. The idea that the laws of physics breakdown inside a black hole pose a challenge for scientists. If our known laws of physics don’t operate at the singularity then what is really going on inside a black hole? New theories such as loop quantum gravity are being investigated to see if they apply at singularities.

Shep Doeleman, senior research fellow at Harvard University and director of Event Horizon Telescope (EHT) stated in the Out There issue of Popular Science: “There’s no environment in the universe like a black hole. Being able to see such an object gives us a ‘natural laboratory.’ We can test long-standing theories about how objects move through space—like Einstein’s general relativity—by watching gravity-driven warps in spacetime impact how light travels. We can also study how black holes help shape the universe by sucking up matter.” And that’s why we study black holes!

Tags Black holes, escape velocity, event horizon, Event Horizon Telescope, law of universal gravitation, Schwarzschild radius

astronomy Betelgeuse cepheid variable physics solar system supernova

Supernovas and the Death of Red Supergiant Stars

Post author By Ngtriplett
Post date March 2, 2020
3 Comments on Supernovas and the Death of Red Supergiant Stars

The red supergiant, Betelgeuse has been in the news recently because of its unexpected behavior. Astronomers have noted that since October 2019 the red supergiant had been dimming and the star is now less than 40% of its normal brightness. Before we discuss what this means for the star let me introduce you to Betelgeuse.

Betelgeuse is between 640-724 light years from Earth and is the alpha or brightest star in the constellation Orion. Betelgeuse and Bellatrix make up the shoulders of Orion, the Hunter. Betelgeuse used to hold the distinction of being one of the top 10 brightest stars in the sky but has fallen to below 20th since it began dimming in October 2019. Betelgeuse is believed to be between 9 and 10 billion years old with a solar mass that is 12 times more massive than our sun.

find betelgeuse,betelgeuse in orion constellation — Image: Akira Fujii

Red Supergiants

So what is a red supergiant star? Red supergiants are stars of a specific size that are nearing the end of their lives. These stars spend only about 10% of their lives as red supergiants while the prior 90% is spent as a massive main sequence star. These stars have a mass greater than 10 solar masses meaning these stars have more than ten times the mass of our sun. In these stars most of the hydrogen fuel has been exhausted and the core stops producing energy and gravity causes the core to contract. The layer of the star surrounding the core contracts and heats up to a high enough temperature to start fusing hydrogen to helium. The outer parts of the star expand as a result of the star burning hydrogen. The star is producing more energy than necessary to offset the collapse due to gravity. The outer layer expands to several hundred solar radii and the surface temperature cools as a result of the increased surface area. This temperature decrease gives the star its reddish color.

The Dimming of Betelgeuse

Betelgeuse belongs to a class of stars called Cepheid variables or variables. These types of stars, according to universetoday.com “are essentially stars that experience fluctuations in their brightness (aka. absolute luminosity)”. So this means that some dimming from Betelgeuse is to be expected. Betelgeuse is considered to be a semi-regular variable star or slow irregular variable star which means its brightness or luminosity fluctuates in fairly predictable cycles. One cycle lasts for approximately 420 days, a second longer cycle lasts for close to six years, and a third cycle lasts somewhere between 100 and 180 days. The current reduction in brightness is larger than expected which has led to questions about what this means for the red supergiant. Some scientists think that this dimming is simply an extended dimming period lasting longer than the 420 day cycle while others speculate that Betelgeuse may be heading towards its ultimate demise, a supernova explosion. The European Southern Observatory (ESO) posted a video comparing the luminosity of Betelegeuse from December 2019 to January 2020, you can view the video here: https://www.youtube.com/watch?reload=9&v=o1ls7Gr9LTE According to ESO “this video shows the star Betelgeuse before and after its unprecedented dimming. The observations, taken with the SPHERE instrument on ESO’s Very Large Telescope in January and December 2019, show how much the star has faded and how its apparent shape has changed.”

What is a Supernova and When Might it Occur for Betelgeuse?

According to nasa.gov, a supernova is “the explosion of a star. It is the largest explosion that takes place in space.” There is some speculation that Betelgeuse is nearing the end of its life and may go supernova in the near future. Let’s be clear about the meaning of “near future”. In our everyday life the “near future” may be a few days, a few weeks, maybe even a few months. Is astronomical terms, the “near future” may mean anywhere from a few thousand years to over a hundred thousand years. It is all based on your reference. Astronomers are comparing a few thousand years to the age of the universe which is estimated to be nearly 14 billion years old.

When the iron core reaches its Chandrasekhar mass which is about 1.4 times the mass of our sun or 1.4 Solar masses, the pressure of the core can no longer hold up against gravity and the iron core begins to collapse. During this collapse the electrons and iron nuclei get mashed together and electrons combine with protons in the nuclei to form more neutrons. This combing of electrons and protons results in a decrease in pressure which speeds up the collapse. The collapse of the core takes a mere few thousandths of a second.

As the core of the star reaches a size of around 31 miles the core contains a gas consisting of 70% neutrons and 30% protons with a temperature of 100,000,000,000 K. This object is now a proto-neutron star. The outer layers are continuing to fall into the core and a stream of neutrinos are flowing out of the proto-neutron star. This flow of neutrinos results in an enormous release of energy causing the outer layers of the star to be blown away. This flow of neutrinos is responsible for 99% or this enormous energy and light is the remaining 1% of the energy.

The brightness or luminosity of a supernova is 10 billion times greater than that of our sun. A supernova may outshine its own galaxy for several days. So why aren’t we seeing supernovas all the time? Shouldn’t we be able to detect them by the enormous luminosity? Scientists have found that core collapse supernova only occur at a rate of 1 supernova per century per galaxy.

Types of Supernova

There are several different types of supernova. They are classified by the type of light that is emitted by star. This can be thought of as the chemical signature of the star. These signatures help astronomers determine what elements are present in or created by the star.

The first is called type 1a supernova. Scientists believe that this type of supernova occurs when white dwarf stars, those stars who had masses less than 1.5 times the mass of our sun, acquire more mass than its internal pressure can withstand so it heats up and goes supernova. A star that became a white dwarf would not normally be massive enough to go supernova. The white dwarf is thought to have gained the mass from colliding with another white dwarf of from a companion red giant.

The next type of supernova is the type 1b supernova. In this case the star had a mass at least 25 times the mass of our own sun. It certainly is massive enough to go supernova. This type of star is thought to have shed material from its outer envelope later in life which is why there is little hydrogen in its spectrum. This type of supernova does show helium in its spectrum.

Type 1c supernova contain very little hydrogen and helium and are formed the same way the type 1b supernova are formed. The difference between type 1b and 1c is the lack of helium found in the 1c supernova.

Type II supernova or core collapse supernova contain large amounts of hydrogen and helium in its spectrum. It is believed that large stars with masses are larger than 8 times the mass of our sun undergo this type of supernova. The massive explosion results in the creation of a blackhole or a neutron star. This is the type of supernova a red supergiant, such as Betelgeuse, will undergo at the end of its life.

Types of supernovas. Courtesy of astronomy.swin.edu.au

Why Do We Care about Supernovas?

It turns out stars only produce elements up to iron through the fusion reactions in their core. So that begs the question, where do elements heavier than iron come from? Well, the extremely large amounts of energy and the enormous temperatures associated with supernova explosions can cause fusion of the heavier elements. These heavier elements are shot out through the universe during the supernova explosion. Many of the elements we have found here on earth were created in the core of a star during a supernova. The elements that travel through the universe eventually are used to create planets, new stars and anything and everything else in the universe. According to nasa.gov/audience/forstudents “one kind of supernova has shown scientists that we live in an expanding universe, one that is growing at an ever increasing rate.” So that’s why you should care about supernovas!

Elements heavier than iron that are formed in a supernova explosion.
Courtesy of herschel.jpl.nasa.gov

Tags astronomy, Betlegeuse, cepheid variable, physics, redsupergiant, supernova

astronomy physics solar system

The Mysterious Moon of Saturn: Enceladus

Post author By Ngtriplett
Post date February 24, 2020
9 Comments on The Mysterious Moon of Saturn: Enceladus

Image result for Enceladus — Saturn’s sixth largest moon Enceldaus. Courtesy of solarsystem.nasa.gov

The Cassini Mission

The Cassini orbiter was launched from Cape Canaveral, Florida on October 15th,1997. This was a joint effort between NASA, ESA (European Space Agency), and ASI (The Italian Space Agency). The mission was to study and explore Saturn and its system of rings and moons. The trip to Saturn would take seven years and the totality of the mission from the 1997 launch to 2017 plunge into Saturn would be 21 years. A majority, 13 years, of those years being spent studying the ringed planet and its moons. Saturn is approximately 75,000 miles in diameter which is nearly 10 times as large as Earth meaning 750 Earth sized planets could fit into Saturn. Saturn has a mass that is 95 times more massive than Earth. Saturn is roughly 1 billion miles from Earth, when the two are at their closest with respect to each other, which is 7 astronomical units (AU). One AU is the equal to the distance from the center of the Earth to the center of the Sun.

Discoveries of the Mission

The data, images, and samples collected by Cassini provided scientists with a wealth of information and knowledge about the ringed planet, the ring systems and its many moons. The mission revealed a large hexagon shaped jet stream at one pole of the ringed planet. “The hexagon is just a current of air, and weather features out there that share similarities to this are notoriously turbulent and unstable. A hurricane on Earth typically lasts a week, but this has been here for decades — and who knows — maybe centuries.” said Andrew Ingersoll of the Cassini Imaging Team. The Huygens probe made the first ever landing onto a moon in the outer solar system when it landed on Titan On January 14th, 2005. Titan was found to have rivers, lakes, and oceans. Titan has a thick nitrogenous atmosphere that may be similar to early Earth’s atmosphere. Six new moons were discovered to be orbiting Saturn which brings the total number of known moons orbiting Saturn to 82. With the discovery of these new moons Saturn now boast of having the most moons of any planet in our solar system.

Saturn’s Ring System

Image result for saturn ring structure — Courtesy of explanet.info

The breathtaking rings of Saturn were found to be made up of mostly frozen water mixed with space dust as well as rocky meteoroids. The particles making up the rings vary from the size of a grain of sand to the size of mountains. Vertical structures, which rise as high as 2 miles were seen on the B-ring of Saturn ring system. The rings reach an impressive 175,000 miles from the planet yet the average vertical height is only 30 feet in the main rings.

Image result for saturn's rings vertical structures — Image depicting the vertical structures of Saturn’s rings. Courtesy of universetoday.com

It turns out that the ringed planet did not always have these beautiful rings. This means the rings are younger than the planet itself. The rings are thought to have been created when asteroids, comets, or other bodies smashed into the planet sometime in the last 100 million years making the rings much younger than Saturn itself. The host planet for these rings was formed approximately 4.5 billion years ago. The Cassini mission also provided evidence that Saturn is losing its rings as the particles making up the rings are being pulled into the planet under the influence of Saturn’s strong magnetic field. Scientists predict that the ring structure will be gone in 100 million to 300 million years.

A Little about Enceladus

Saturn’s moon Enceladus, the sixth largest moon of Saturn, was discovered in 1789 by astronomer William Herschel. Enceladus is roughly 790 million miles from Earth and orbits its host planet, Saturn at an average distance of 148,000 miles. By way of comparison our moon orbits the Earth at an average distance of 238,000 miles. Enceladus is one-seventh the diameter of our own moon and much less massive than our moon. Here is a link comparing Enceladus to our own moon: https://solarsystem.nasa.gov/moons/saturn-moons/enceladus/by-the-numbers/

Enceladus boasts five distinct physical features including craters, cracks in the surface, ridges and fissures. Scientists speculate that the presence of cracks and ridges is evidence that the core of Enceladus is still a liquid composition. The smooth plains on the surface of the moon seem to indicate that there was water flowing from deep in the core to the surface. So how does the water make it from deep inside the moon to the surface? As it turns out there are a system of hydrothermal vents that connect to an ocean of saltwater below the surface. Scientists settled on this idea based on the presence of silica nanograins found in the E-rings. These nanograins can only be generated at temperatures above 90 degrees Celsius in areas where liquid water and rock interact. In 2005 scientists discovered that there were plumes of water shooting out from its south pole. These plumes are ejecting water which is continuously shot out into space at 800 miles per hour traveling hundreds of miles to the planets E-rings.

What’s so Special about Enceladus?

So what makes this moon special as compared to the other 81 moons orbiting Saturn? The major finding of Enceladus is not simply that it has an abundance of water, the moon Titan is believed to also have water, but it is what is contained in the plume that was surprising. Cassini was able to directly take samples of the plume from Enceladus as well as from the E-ring of Saturn. The results were stunning. The water contained a mixture of volatile gasses, water vapor, carbon dioxide, carbon monoxide, and organic materials. Similar results were obtained from both the samples taken from the E-ring of Saturn and the icy moon Enceladus.

“This is the first-ever detection of complex organics coming from an extraterrestrial water world,” said planetary scientist Frank Postberg from the University of Heidelberg in Germany. While no life forms have been detected yet scientists have not ruled out the possibility of finding life deep with in the moon or in the rings of Saturn. Scientists noted that most of the larger organic molecules were found in the E-ring. Speculation is that sunlight may have triggered chemical reactions in space which resulted in the complex organic molecules.

So What’s Next?

According to Potsberg the next logical step would be to go back to Enceladus “and see if there is extraterrestrial life.” Plans are already underway for a 2022 mission to the moons of Jupiter to search for habitable conditions on the icy moons containing subsurface oceans. With respect to returning to Saturn’s moon to search for life Potsberg notes “Nowhere else can a potentially habitable extraterrestrial ocean habitat be so easily probed by a space mission as in the case of Enceladus.” When the Cassini mission was close to termination a deliberate decision was made to send Cassini spiraling into the planet to avoid any accidental cross contamination with Enceladus. This suggests that a return trip to the moon was already being discussed and that the researchers wanted to leave no doubt that, if in fact, life was found on Enceladus it was not delivered by Cassini. Will the first extraterrestrial life be found on Enceladus? No one knows for sure but it is an extremely exciting possibility.

Image result for cassini final orbit — Courtesy of cosmosmagazine.com

Tags Cassini, Enceladus, Saturn, Voyager

astronomy Dark Matter physics

What is Dark Matter all About?

Post author By Ngtriplett
Post date February 17, 2020
2 Comments on What is Dark Matter all About?

You may have heard of the terms dark matter and if you read my post from last week then I know you have heard of dark energy. Let’s talk about dark matter, what it is and why it is called “dark”. We know a black hole is called black because it is not visible to us and even light can not escape its gravitational pull. So is dark matter called dark because we can’t physically see it? Well, yes actually. We are unable to detect it directly and are only able to infer its existence by the effects it has on other objects. So let’s take a closer look at what dark matter is and what it does.

What is Dark Matter?

Dark matter comprises about 27% of the universe and dark energy makes up about 68% of the universe which means that only 5% of the universe is comprised of matter as we know it. This “normal” matter is called baryonic matter and is the matter we traditionally think of. Protons, electrons, atoms, anything that makes up everything from people to stars are made from baryonic matter. Dark matter, like dark energy is called dark because we are unable to detect it directly. Dark matter does not interact at all with the electromagnetic spectrum which means it does not reflect light, absorb light, or emit anything we can detect. As of yet no one has been able to directly observe dark matter. Dark matter is thought to be a previously undiscovered subatomic particle that does not respond to the strong or weak nuclear forces.

The key point here is that when scientists account for all the visible or detectable mass in the universe it doesn’t add up to be enough to account for the gravitational effects we observe. As stated above only about 5 % of the universe is normal or baryonic matter so that means the most of the gravitational interactions in the universe are a result not of baryonic matter but of something else. We call this something else dark matter.

Image result for dark matter dark energy pie chart — Courtesy of mcdonaldobservatory.org

There are several ideas as to what actually comprises dark matter. Speculation includes dim brown dwarf stars, white dwarfs, neutron stars and even black holes. Some scientists have dismissed theses objects as dark matter candidates because the gravitational effect needed to make up the “missing mass” doesn’t match the gravitational effect observed by these objects. Others have stated that the “missing mass” may simply be normal baryonic matter that is simply more difficult to detect.

WIMPs and MACHOs

Weakly interacting massive particles or WIMPS are theoretical particles of non baryonic matter which have somewhere between 10 and 100 times the mass of a proton yet interact very weakly with normal or bayronic matter so they are difficult to observe. If WIMPS are what make up dark matter then there should be 5 times as many WIMPS as normal matter. We should be able to detect them as they do interact with normal matter and the sheer abundance of them should allow us to detect them through collisions with each other. So far no WIMPS have been discovered.

Massive astrophysical compact halo object or MACHO is another candidate for the composition of dark matter. These are objects composed of bayonic matter but are difficult to detect because they emit very little to no light. These include the neutron stars, supermassive black holes, and brown and white dwarfs as mentioned earlier. Because they emit so little light one way to detect them is through gravitational lensing which we will discuss a bit later. It appears that there are not enough of these objects throughout the universe to make up the “missing mass”.

Image result for WIMPS and MACHOS dark matter — Courtsey of slideplayer.com

Other Dark Matter Candidates

Neutrinos are particles that aren’t associated with and don’t interact with baryonic matter. Neutrinos stream from the sun and pass through all regular matter, including us all the time. They are difficult to detect as they do not interact with matter. A new type of neutrino is thought to make up dark matter by some in the scientific community. Sterile neutrinos are a theoretical type of neutrino that have been proposed, they only interact with baryonic matter via gravity.

The Kaluza-Klein particle is a theoretical particle that would interact with the electromagnetic spectrum as well as gravity which should make them easy to detect. These particles are thought to exist in the fifth dimension making them difficult but not impossible to detect. The Kaluza-Klein particle is predicted to decay into particles we can readily observe, such as neutrinos and photons. As of yet though, none of these exotic particles have been observed.

How Do We Know Dark Matter Exists if We Can’t See It?

There are three pieces of evidence used to prove the existence of dark matter even though we can not detect it directly. The first is the speed of stars rotating on the outside edge of spinning galaxies. The stars on the outer edge should move at a much slower rate than those close to the center where most of the baryonic mass of the galaxy is contained. Direct observation has shown that these stars, at the outer edge of the galaxy, are moving at a rate very close to the rate of stars closer to the center. This has led scientists to the conclusion that there must be some mass distributed throughout the galaxy exerting a gravitational effect on the stars farther out from the center.

Stars, as it turns out, orbit their parent galaxy. By using Newton’s equations of force and the Universal Law of Gravitation we know that the force that causes the star to orbit in a circular orbit are equal to the force due to gravity on the star. If these forces were not equal then the star would careen into the center of the galaxy or fly off into space. Close to the center of a galaxy these forces are approximately equal as expected. Stars farther out from the center don’t appear to have these forces equal. So there has to be something going on to keep these stars in orbit. Dark matter is one such explanation.

Image result for dark matter graph — The graph depicts the discrepancy between the expected velocity (A) and actual velocity (B) of a star as the distance from the center of the galaxy, where most of the baryonic matter is located, increases. Courtesy of popscicoll.org

Gravitational lensing is a well documented phenomena in which massive objects distort the fabric of space time. Light must travel along this fabric and if there are massive objects distorting this fabric then the the source of light may appear shifted from the actual position as a result of distortions in space.

Image result for gravitational lensing — A visualization of gravitational lensing. Courtesy of agitatorgallery.com

Gravitational lensing as a result of dark matter. Courtesy of nasa.gov

The third piece of evidence supporting dark matter is the the Bullet Cluster Galaxy merger. Two galaxies collided and due to only about 2% of a galaxy being made up of stars and roughly 5-15% being made up of gas and plasma there is a low probability of any baryonic matter colliding with one another. After the merger of the two galaxies gravitational lensing of background objects allowed scientists to determine where the accumulations of mass were located. It turns out that the dark matter was separated from baryonic matter in large enough quantities to cause gravitational lensing.

Image result for bullet cluster gravitational lensing — Courtesy of nanoqed.org

What if the Missing Mass is the Result of Something Else?

Not everyone is ready to accept that dark matter is responsible for the “missing mass” problem in the universe. Some scientists believe the problem isn’t that mass is missing but rather our theories and equations are incorrect. One particular idea is that Newton’s laws require a modification so that they match the observed behavior of galaxies. Those that support this idea have developed MOND or Modified Newtonian Dynamics as an alternative to dark matter. This theory suggest in situations when the acceleration rates are low Newton’s laws do not accurately describe the motion of galaxies. This alternative to dark matter is as still being developed and refined to better match observation.

Image result for dark matter theory — Courtesy of quantamagazine.org

Scientists know that the amount of baryonic matter which they are able to detect is not enough to hold galaxies together or keep stars from flying off into space rather than orbiting their parent galaxy. What is unknown is the precise mechanism causing the behavior observed by galaxies and stars. Dark matter is certainly the most widely accepted theory about our “missing mass” problem but it is by no means the only theory.

Tags astronomy, cosmology, Dark Energy, Dark Matter, galaxies, gravitational lensing, MACHOs, Modified Newtonian Dynamics, physics, WIMPs

astronomy Dark Energy physics

Shining Some Light on Dark Energy

Post author By Ngtriplett
Post date February 11, 2020
No Comments on Shining Some Light on Dark Energy

You may have heard of the term dark energy but what is it, what does it do and why is it called dark? We know a black hole is called black because it is not visible to us and even light can not escape its gravitational pull. So is dark energy called dark because we can’t physically see it? Well, yes actually it is. We are unable to detect it directly and are only able to infer its existence by the effects it has on other objects. The effects are very real and until recently were not well understood. Let’s take a closer look at dark energy.

The Expansion of the Universe

The universe has been expanding since the Big Bang some 14 billion years ago. So what does this expansion look like? Are the galaxies flying away from each other never to be seen again? Well, sort of. Edwin Hubble noted in the 1920s that the farther away we look the faster galaxies seem to be moving away from us. All galaxies are receding from one another so no matter what direction we look in we will see galaxies receding from our own. Hubble’s Law tells us that the speed at which galaxies recede from each other are proportional to their distance so the farther away a galaxy is the faster it is moving away from us.

Image result for hubble's law — Hubble’s Law shows that the rate at which galaxies are receding are proportional to their distance. Courtesy of space.fm

This idea that galaxies are moving away from each other doesn’t tell the entire picture. It turns out the galaxies themselves aren’t moving but rather the space between them is. The galaxies are not receding away from one another through space they are moving in space meaning that the space in between each galaxy is also moving. One common analogy is to think of the universe as a loaf of bread and the galaxies are represented by raisins. In this analogy a baker making a loaf of raisin bread will sprinkle raisins throughout the batter. Each raisin is some distance apart from every other raisin. When the dough is put into the oven what happens? The dough rises and expands. As the bread begins to cook the dough expands so the raisins become farther apart from each other. The raisins aren’t actually moving it is that the dough in between them is expanding.

Image result for raisin bread analogy of the expansion of the universe — The raisin bread analogy of the expansion of the universe. Courtesy of openstax.org

A second useful analogy is to think of the early universe as deflated balloon. If you were to mark the balloon with several galaxies you would see them recede from one another as the universe, the balloon in this case, expanded due to inflation. Once again it is not the galaxies themselves that are moving but it is the space in between them that is expanding.

Image result for balloon analogy of the expansion of the universe — The balloon analogy of the expansion of the universe. Courtesy of Forbes.com

What Does Dark Energy Actually Do?

As noted earlier the universe has been expanding ever since the Big Bang occurred. For many years scientists believed that gravitational forces would either slow the expansion down or even cause the universe to contract at some point in the future. If you stretch a rubber band and then release it what happens? The rubber band will contract back to its original size. Some scientists thought a similar fate awaited the universe. The Law of Universal Gravitation states that all matter in the universe is attracted to all other matter.

Image result for universal law of gravitation formula — Universal Law of Gravitation. Fg represents the gravitational attraction between two objects. G represents the universal gravitational constant (6.67E-11 Newtons kg-2 m^2. m1 and m2 represent the mass of two objects respectively and r^2 represents the distance between the two objects. Courtesy of Kahnacademy.org

Since the universe is filled with matter it seemed reasonable to think that the attractive forces between matter would cause the universe to slow down or possibly contract. The Hubble Telescope provided evidence, however, that the universe was actually expanding more slowly in the past then it is today. We now know that the rate of expansion of the universe today is actually accelerating!

In the 1990s scientists were surprised to learn that the expansion of the universe was expanding rather than slowing down. Many scientists now believe that this expansion is being driven by a force that acts opposite of the attractive force of gravity. They believe there is a repulsive force driving this accelerated expansion. This expansion appears to be occurring faster as the universe continues to expands. The term they gave this force is dark energy.

Image result for expansion of the universe — This diagram reveals changes in the rate of expansion since the universe’s birth 15 billion years ago. The more shallow the curve, the faster the rate of expansion. The curve changes noticeably about 7.5 billion years ago, when objects in the universe began flying apart at a faster rate. Astronomers theorize that the faster expansion rate is due to a mysterious, dark force that is pulling galaxies apart. Courtesy of NASA/STSci/Ann Feild

How do we know that there is something driving this expansion? We know this because we can measure how dark energy impacts the expansion of the universe. What we don’t know is what dark energy is. Dark energy is distributed evenly throughout the universe. As a result of this distribution, dark energy does not appear to have any local gravitational effect rather dark energy effects the expansion of the entire universe as a whole. Scientists have been able to accurately measure the expansion rate of the universe and this in part has helped confirm the presence of dark energy. Most estimates have the universe comprised of 68% dark energy.

What is Dark Energy?

This is the million dollar question. Nobody knows for sure what dark energy is. They can measure how it impacts the expansion of the universe but nobody has been able to directly detect it or determine its composition. Albert Einstein determined that is was possible for more space to spontaneously be created. He developed a cosmological constant in an early draft of his gravitational theory. He would later call this his “greatest blunder.” This cosmological constant was needed by Einstein to show that the universe was static and not expanding. Some scientists today are reevaluating Einstein’s cosmological constant in hopes that it can be used to explain the very expansion it was created to refute. Others have hypothesized that dark energy is a fifth fundamental force they call “quintessence”. The original four fundamental forces are: gravity, electromagnetism, weak nuclear force and strong nuclear force. This potential fifth fundamental force is described as a fluid-like substance which is a repulsive force that may be driving the expansion of the universe. If you are looking to pick up your own Nobel Prize maybe detecting and describing dark energy will get you an invite to the award ceremony in Stockholm, Sweden to collect your prize.

Tags Dark Energy, expansion of the universe, law of universal gravitation, quintessence

astronomy gravitational waves physics special relativity

Gravitational Waves

A brief history of gravitational waves

Albert Einstein predicted the existence of Gravitational waves in his famous 1916 paper describing general relativity. A century after Einstein’s prediction of these mysterious gravitational waves was made, proof of their existence was detected in September 2015. Einstein, as it turns out was not the first scientist to describe or predict gravitational waves but he was the first to accurately describe the phenomena. Einstein wrestled with the idea of gravitational waves for many years after publishing his paper on general relativity which indicated that these waves could, in fact, be a consequence of his theory of general relativity.

A British physicist Oliver Heaviside first proposed gravitational waves in 1893. In 1905 Henri Poincare predicted the existence of gravitational waves in his paper On Electron Dynamics where he states that a consequence of space-time geometry gravitation must produce waves that travel at the speed of light in a fashion close to electromagnetism. While there is some argument as to who first described the concept of gravitational waves it seems clear that Einstein was the first to correctly describe gravitational waves through his theory of general relativity.

What is a gravitational wave and what causes it?

A gravitational, wave according to NASA’s space place website, is “an invisible (yet incredibly fast) ripple in space. These waves travel at the speed of light through space-time which is “incredibly fast” indeed. These ripples in space physically alter the fabric of space-time as they travel. These waves stretch space in one direction and squeezes space in a direction perpendicular to the direction of stretch. These waves travel at the speed of light, in all directions, through space-time away from the source of the gravitational wave.

Gravitational waves are caused by massive objects which are accelerating around each other and may cause this ripple in the fabric of space-time when they eventually collide or merge with each other. Neutron stars or black holes are examples of objects that are massive enough to cause gravitational waves. Events which may be described as cataclysmic, such as the merger of two neutron stars or black holes or a neutron star going supernova likely produce the strongest of these waves.

How are gravitational waves detected?

The first gravitational waves that were verified were detected by LIGO (Laser Inferferometer Gravitational-Wave Observatory) located in Livingston, Louisiana and its twin inferferomter in Hanford, Washington on September 14th, 2015. The event that caused these waves is believed to be the merger of two black holes that occurred 1.3 billion years ago. The black holes reportedly collide at nearly .5c or 1/2 the speed of light to form a single massive black hole. The result is the release of an enormous amount of energy, in this case the amount of energy that was converted was equal to 3 times the mass of the sun. This process occurs in accordance with Einstein’s equation E=mc^2 which states that mass can be converted to energy. The mass that is converted to energy is discharged in the form of gravitational waves. It is these gravitational waves that were detected by the twin LIGO detectors in September of 2015.

Courtesy of Physics.Org: diagram of LIGO Interferometer and gravitational waves

The LIGO equipment consists of two 4 kilometer detector arms in an “L” configuration which can detect the distortion of space by as little as 1/10,000th the diameter of a proton. These distortions are the result of extremely violent events such as the merger of black holes, neutron stars, or a neutron stat going supernova. According to the LIGO Caltech website the 4 kilometer arms were “long enough that the curvature of the Earth was a factor in their construction.”

Both_aerial — An aerial view of LIGO Hanford and LIGO Livingston. Courtesy of LIGO Caltech

The Virgo interferometer is located in Italy which has arms that are 3 kilometers in length and there are plans for two more detectors, one will be located in India and will be a joint operation between LIGO and three research facilities in India. Another detector will be an underground detector called KAGRA located in Japan. Here is a quick link describing how LIGO detects gravitational waves https://www.sciencemag.org/news/2016/02/gravitational-waves-einstein-s-ripples-spacetime-spotted-first-time

Earth based interferometer can detect waves with a frequency of 30-400 hertz (Hz). These ground based detectors have the ability to detect waves that are longer than the 3-4 kilometer arms of the detectors. Space based interferometers which are slated for deployment in the 2030s are projected to be able to detect waves with a frequency of .1-100 milliHz. LISA, or Laser Interferometer Space Antenna consists of three probes that have the ability to detect waves to much lower frequencies than their ground based counterparts. Scientists are attempting to develop methods of detecting subtle variations from pulsars located within the Milky Way using “pulsar arrays” which are located in Europe, Australia, North America, and one being developed in China. These variations may be caused by the propagation of gravitational waves through our home galaxy. The pulsar arrays can detect frequencies from 1-320 nanoHz

What can we learn from gravitational waves?

So we know Albert Einstein predicted gravitational waves in his paper on general relativity and we know that the technology used to detect these waves is amazing but why should we study these waves? Why should we care about waves that may have been generated millions or billions of years ago?

Gravitational waves are unrelated to electromagnetic radiation and this allows us a fuller picture of events in the universe. Black holes for example are invisible to electromagnetic radiation but can be studied by the gravitational waves they create as they merge or collide with one another. Scientists are hoping to answer some fundamental questions regarding black holes and how they end up pairing and circling one another prior to colliding.

The study of gravitational waves led scientists to the origin of heavy elements in the universe. In 2017 scientists were able to witness two neutron stars merging by detecting the gravitational waves associated with the merger. Scientists were able to detect the heavy element strontium in the aftermath of the neutron star merger and the resulting explosion and burst of gamma rays known as a kilonova. LIGO Caltech defines a kilonova as “a phenomenon by which the material that is left over from the neutron star collision, which glows with light, is blown out of the immediate region and far out into space.” It is from this event that the scientists were able to prove that the heavy element strontium was created in the explosion of a neutron star. The study of gravitational waves may reveal information about the rate of expansion of the universe, the origin of black holes, and the composition of neutron stars.

Tags Albert Einstein, Black holes, general relativity, gravitational waves, interferometer, KAGRA, LIGO, LISA, neutron stars, pulsar, pulsar timing arrays, VIRGO