The ultimate fate of a star once it is has died is dependent upon the mass of the star during its lifetime. A star in general falls into one of the following mass categories: low mass, medium mass, or high mass. The determination of a star’s mass occurs at birth as the star is forming. Stars do not acquire mass throughout their lives and do not grow from low mass star to high mass star for example.
You may be familiar with the what happens to a star once its fuel has run out and it is nuclear fusion is no longer occurring in its core. High mass stars generally have the most spectacular death as they transition to a red supergiant before going supernova and eventually reaching their final fate.
A high mass star’s eventual fate will either be as a neutron star if it has a mass of 1.4 to 3 times the mass of the sun or as a blackhole if the star has a mass larger than 3 times the sun. An average star will end its life as a red giant shedding its layers to become a planetary nebula. The red giant core will settle in as a white dwarf. This may not seem like a new revelation but there is a theory that takes the white dwarf to one final stage. We have discussed the life cycle of stars and the death of stars in other posts. Today we want to talk about a very specific and theoretical star known as a black dwarf. Have you ever heard of one? No? Well don’t worry most people haven’t heard of them and absolutely nobody has seen one and no one in our lifetime ever will.
What is a Black Dwarf?
A black dwarf is a theoretical result of a white dwarf cooling down to a point where it no longer emits light or heat. The object still retains its mass in this phase but does not emit any radiation. You may be thinking to yourself that if it emits no light or heat that is why no one has ever seen one. This might appear to be similar to a black hole which had never been observed until the Hubble Space Telescope (HST) produced the first ever image in 2019. It is true that we would not be able to directly observe a black dwarf but the lack of light or heat is not why we haven’t observed one of these objects yet.
It turns out it takes a long time for a white dwarf to cool down enough to enter the black dwarf phase. A really long time. Scientists believe it takes trillions of years for a white dwarf to become a black dwarf. According to Scientific American, it takes “few billion years” for a white dwarf to cool down to the surface temperature as our sun. What that means is that not one black dwarf exists at this moment in time. There simply hasn’t been enough time for one to form. If you take the age of the universe to be around 15 billion years old you can understand why no black dwarfs exist yet. Here is a short video that describes what will eventually happen to our sun as it becomes a black dwarf: https://youtu.be/1mXueDqxvFs
One theory suggests that black dwarf stars eventually “break down and disperse into space” according to Universe Guide. Another idea is that very massive black dwarfs, those between 1.2 to 1.4 solar masses may result in a supernova. According to the article “Fusion can still occur at very cold temperatures – it just takes an incredibly long time and requires some help from quantum mechanics. Eventually, those fusion products should build up enough to choke the black dwarf into a supernova, in a similar way to more massive stars. This explosive fate awaits as many as one percent of all stars shining today,” So if you’d like to wait around and see when the last black dwarf is projected to go supernova you’ll be waiting for a while. How long? Try 10^32,000 in the future according to the article.
Neville’s Phantastic Physics Phun 