Black Holes and Neutron Stars

Black holes and neutron stars won the poll by a hair. So here is my small info dump on them. Most of these concepts are taken from what learned in my textbooks, lectures, and classroom discussions. I am by no means an expert on the subject, I don't go pushing around equations and numbers about them, but I am probably more familiar with some of the subject matter then most. That said, I will try to avoid the math, and just provide some interesting information about these cosmic oddities. I also provide link to wikipedia in some places, because generally it's the easiest thing to link for information on this stuff.

Also, a new poll should be up, so those who wanted dark matter, you get to try again. Same with anyone else interested in the other areas. I replaced Black Holes/Neutron Stars with a good suggestion from the previous thread, cosmic dust. My undergraduate advisor was an expert on the stuff, so I have some insight on it.

Ok, so, black holes and neutron stars:
Black holes and neutron stars are a bit hard to get into right off the bat. You see, they are usually tackled at the end of the stellar life cycle, which is another topic for another time here. But that's OK, we can just say that black holes and neutron stars are possible outcomes from the death of a star. Generally these stars are massive stars.

What happens in these stars is that their core's get so large they cannot support their own mass. The force of gravity in the core is greater then the electron forces that hold electrons and protons apart. As electrons and protons collide, neutrons and neutrinos are formed. A shockwave is formed from this sudden collapse, exploding violently off the star in a supernova. Some supernovae in other galaxies have been known to outshine the rest of the galaxy. In our own Milky Way, supernovae have been seen as bright 'new stars' in the night. It is an awesome, brilliant, and violent death for the star.

It is just the birth of either a neutron star or black hole. The mass of the star really determines whether it becomes a black hole or neutron star. And for our Sun, we don't have to worry about either, as it is not massive enough. It will become a red giant, then cast off its outer shell as a planetary nebula and what remains will be a white dwarf that slowly fades into a black dwarf. For black holes and neutron stars, because of their core collapse, they have matter degeneration.

For neutron stars, what was once a massive core made of protons and electrons, there is now just a small ball of neutrons. Something maybe 1.5 to 2 times the mass of our Sun can be crammed into a 12 km sphere. The only thing really preventing a neutron star from becoming a black hole is that the mass is not great enough to overcome repulsive interactions between nucleons. So it is something in a real delicate balance, if the star wasn't big enough, it would have just become a white dwarf, if to large, a black hole. There is an even finer balance, only theoretical with a few candidates as far as I know right now, for a quark star that is somewhere between a neutron star and a black hole.

Neutron stars also tend to emit energy in the radio and x-ray wavelengths, which make for some interesting observations. They also spin really fast for their size, neutron stars will retain their angular momentum, meaning its still spinning at the rate of the larger star, but since its tiny now, it seems to go a lot faster. There are also some interesting subtypes:
Pulsars are neutron stars that are emitting radiation from one portion of the star, and as it rotates that radiation will point directly at us, appearing to pulsate.
Magnetars have extremely strong magnetic fields, hundreds of times stronger then regular neutron stars, and take longer to rotate (on this scale on a few seconds, like 5-10).
X-ray bursters are binary stars, where one star is a neutron star (occasionally a black hole)and the other is a companion 'donor' star. The donor star will have its matter ripped off through interactions (there is quite a bit  of astrophysics to this portion) and as it falls onto the neutron star, it bursts into intense x-rays, thus the name.

Black holes we all know are the rogue evil stars gone bad out there, so brutal that not even light can escape their wrath, and that are going to devour our entire solar system, galaxy, and universe. Or not quite. It is true that light doesn't escape, and they do warp spacetime with their compact mass. This warping of spacetime creates an undetectable radius around the mass that appears black. This radius is known as the event horizon, or Schwarzchild radius (really interesting read), and light that hits this horizon hits the point of no return.

Black holes are formed much the same way as neutron stars, but instead of the degeneracy pressure holding up in the neutron star case, the black hole keeps on collapsing. It creates an exotic dense state of matter that really isn't well understood because there is no easy way to study it. This matter is pushed into a tiny point called a singularity, where spacetime curvature becomes infinite. The event horizon also marks the point were you cannot escape falling into the singularity.

But lo, there is hope! A brilliant man named Stephen Hawking showed that black holes evaporate. This evaporation is known as Hawking radiation. Essentially, it goes that black holes are not entirely black, but emit small amounts of thermal radiation (heat). As they emit heat, black holes lose mass, this comes from the E=mc^2 concept (which means mass can be converted to energy and vice versa). It is a strange concept though, because the larger a black hole is, the less heat it emits, so the slower it evaporates. Stellar mass black holes could take millions of years, were as one about the weight of a car would take a nanosecond.

On the flip side, black holes also grow from that matter falling into them. They will absorb anything from cosmic background radiation to interstellar dust. There really isn't any known size limit to a black hole. So, we also happen to see some really really large black holes. And these black holes actually might be good for us on a whole. These supermassive black holes are found in the center of most galaxies and can be millions of times more massive then our Sun. There is a strong indication that the Milky Way is home to a black hole 4 million times more massive then our Sun.

These black holes are thought to aid in galaxy formation, as matter is attracted to these black holes, more matter is attracted to that matter. You end up with large swirling clouds of matter where dusts collects and collides and forms new stars. Galaxies grown and live thanks to their supermassive black holes. Without that process, we might not even be here.

OK, that has been more then enough writing for one day. I feel like I am writing a college paper all over again, but without the math. I hope enjoyed the read, there is so much more left to be said, but hopefully this peaked your interest and you might go out and find more if you desire it.

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