biddie67 wrote:Ann - if you aren't already a teacher professionally, you definitely have the natural skills to be one!! Thanks for gathering all the pictures and info on a level that I could understand it.
After reading it, and at the mention of some analogies to the behavior of stars, I couldn't help wonder if black holes might have some extraodinary ends to their overall cycle that might be compared to the stars and their red giant or supernova phases ??
Please don't jump on me folks for creating theories - I'm probably guilty of having a lot more "what if" imagination than solid technical knowledge .....
Thanks for the praise, Biddie! Actually I am a teacher.
This will have to be a quickie, though, since I'm pressed for time.
Astronomers believe that black holes are created from massive stars. They aren't sure how supermassive black holes like the ones we find at the centers of galaxies got started, but bystander posted a theory about that in the Breaking News folder recently. Go search for it there.
Anyway, astronomers believe that "normal" black holes are created from massive stars at the end of their lives. During its lifetime, a star will shed mass. The Sun is doing that right now in a so called stellar wind. All stars do it, but the more massive a star is, the more violent is the stellar wind and the more mass it will lose during its lifetime.
However, the mass that stars lose is mass in their outer layers. The cores of stars will generally get more massive as long as the stars live.
The Sun now weighs one solar mass (what a surprise).
The Sun now shines by converting hydrogen in its core into helium, but about a billion years from now the Sun will have exhausted the hydrogen in its core. The core will now be all made of helium. When that happens the Sun will become extremely unsettled. Its core will shrink, releasing heat, and this heat will make the Sun's outer parts expand prodigiously. The Sun will grow huge, definitely huge enough to swallow Mercury and Venus. The shrinking core will keep heating until it gets hot enough to start converting the helium in its core into carbon and oxygen. When that happens the outer layers of the Sun will shrink a bit, but the Sun will still be huge compared with its size today. As long as the helium supply in its core lasts, the Sun will shine moderately steadily as a "red" (really yellow) giant.
But the Sun will exhaust its central helium, too, just as it must exhaust its central hydrogen. When that happens the Sun will not again be able to shine steadily. In fact, it will not be able to generate heat in its core and protect its core from shrinking even further. The Sun will get the hiccups, so to speak, and it will get more and more unbalanced. It will expand and shrink, shine brighter and fainter, and eventually it will just shed all its outer envelope and turn its core into a white dwarf, a tiny, hot but cooling stellar ember. Eventually it will have radiated all its heat into space and become dark and cold.
The end of the Sun will be relatively undramatic because the Sun is not very massive (even if it is, believe it or not, more massive than more than 90% of the stars in our galaxy). But if a star starts out with a lot more mass than the Sun, it may build up a core that weighs more than 1.4 times as much as the Sun weighs today. Such a massive star will be able to fuse the carbon and oxygen that it builds up in its core into even heavier elements. I don't have the strength or the time to look up the sequence of elements that will be produced, but if a star is sufficiently massive it will build silicon in its core, and after silicon, iron. The iron is the end of the line for the star. The reason why the star builds up heavier and heavier elements in its core is that the fusion of elements releases energy and protects the core from collapsing under its own weight. But once the star has reached the "iron stage" the fusion must stop, because iron will not release energy if you try to fuse it into heavier elements. In other words, the core has run out of energy supplies once it has turned itself into iron. Therefore, the iron core must shrink catastrophically, releasing a shock wave of heat. The star will explode as a supernova.
When a supernova explodes, it is the outer layers that are blasted into space. The core will not explode, because it has already shrunk and become incredibly dense. The core is left behind as the rest of the star explodes, and most very massive stars will turn themselves into neutron stars as an end product. A neutron star is incredibly small and dense, much, much smaller and denser than a white dwarf. A typical white dwarf is the size of the Earth, but a typical neutron star may be the size of Manhattan (okay, that was probably an exaggeration). But the neutron star still weighs more than a white dwarf. A neutron star must weigh more than 1.4 solar masses.
If the core that the star has built up is still more massive - and astronomers are not sure exactly how massive - the core can't even settle down as a neutron star. The general hypothesis is that a neutron star can't weigh more than six to eight solar masses. If it is still more massive, it must collapse into a black hole.
A black hole can be regarded as "mass point" that has all the mass of the core that collapsed, but it has zero volume. (It does have something called the event horizon which indeed extends some distance from the black hole and which marks the boundary beyond which nothing can't escape the gravity of the black hole, but we don't need to discuss that here.)
After a black hole has formed it is very stable. However, Stephen Hawking has shown that black holes must eventually evaporate. However, the lifetime of a massive black hole, the kind that you find at the centers of galaxies, is believed to be trillions of years, much, much longer than the current age of the universe. Smaller black holes evaporate faster, but they still live so long that you aren't likely to see a mighty explosion in the universe caused by the final, catastrophic evaporation of a black hole.
Ann
(Have we done that before?)
