In our second year we have to carry out a project about a subject in astronomy. This year we chose for black holes. Now we had to think of a main question, which has become:
Do black holes exist in our Universe?
To answer this question we divided the question into subquestions. My subquestion is:
How can stars evolve into black holes?
This question is very important to answer the main question, because the current theories say that IF black holes are created there are three different sizes:
- very low mass black holes: about 10-4 to 10-6 Msun
- low mass black holes: about 100 to 102 Msun
- heavy mass black holes: about 106 to 108 Msun
If there should be low mass black holes you should think that they have a progenitor of the massrange of a star. Of which kind of an object do you think of when you have an object in the massrange of a star? On the ground of the mass of a black hole you can assume that stars can evolve into low mass black holes.
The other two categories will be explained by colleagues of mine. They think that very low mass black holes originate from primordial black holes. About high mass black holes, which will be in the center of galaxies, they have different theories.
The subquestion is subsequently devided into four subquestions.
As I said to defend my subquestion: when you think that low-mass black holes can exist you will probably think about a stellar origin.
- What are the possible ways for a star to evolve into a black hole?
When you look at the evolution of stars you can see that stars will end up as a white dwarf, neutron star, or black hole. There must be a very narrow border between those kind of remnants. E.g. the border between a white dwarf and a neutron star is equal to MChandrasekhar = 1.4 Msun. If the mass of a star when it dies is larger than MChandrasekhar then it will become a neutron star. Probably there will be a similar kind of border between neutron stars and black holes.- What is the maximum mass of a neutron star?
During the process of supernova forces are exerted. At the moment that a star's life ends there is a critical decision for the core to make: will it collapse directly into a black hole, or will the star go supernova? If the star goes supernova and enough matter falls back onto the arosen neutron star, will the neutron star collapse due to its own gravity into a black hole?
- What is the critical mass that a star will go supernova or that the star will directly collapse?
Two people, Brown & Bethe, have estimated in 1994 that, when stars will evolve normally, there should be about 5x108 low-mass black holes in our Galaxy. But because we haven't noticed any of these black holes (except for maybe SN 1987A and 4U 1700-37), there is something wrong in the assumption that main sequence stars all will evolve equally.
- Which assumption in the evolution-theory of stars is wrong? If something is found: what has to be changed in that evolution-theory to explain that we don't have that many black holes as we have found now (assuming that black holes exist)?
The answers to these questions will be posed here.
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Conclusions
References
Questions for October 19 1999
Acknowledgments
Pages of fellow students
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