[dreadgeek]

Since black holes were invoked in this thread, I thought it might be interesting to delve into these a tiny bit more because these objects are SO cool!

So to understand black holes one must understand that in the interior of all stars there is a battle raging. The mass of the star, which is what creates gravity, wants to cause the star to collapse on itself (remember that gravity is ALWAYS attractive) but the fusion process (caused by that same gravitation) generates heat that prevents the star from collapsing. For 'main sequence' stars of .4 to 1.4 solar masses (using our sun as the baseline) near the end of their lifetime, as the fuel is exhausted, heat briefly triumphs over gravity and the star expands to a red giant. This will be the end-game cycle for the Sun. It will eventually expand out to 1 AU (Earth's orbit), completely obliterating our planet and then collapse into a cooler, white dwarf star about the size of the Earth.

Larger stars > 1.4 solar masses but < 4 solar masses will expand out to red supergiants and then collapse into neutron stars.

Truly massive stars > 4 solar masses go through one of the most interesting and violent transitions in the Universe. In these stars gravity fully triumphs over heat and the core of the star becomes iron. This increases the mass of the star while not generating more heat. At some point the star collapses in a supernovae and then collapses further into itself creating a black hole. When this happens very high energy particles may be ejected in the x-ray part of the spectrum which is one way we can detect black holes.

So why are black holes 'black'? They are black because the gravitational force is so extreme that not even light can escape it. If the black hole is part of a binary system it will tend to capture its partner and begin sucking stellar mass from it, feeding itself. This is another way we can detect black holes. As mass approaches the event horizon (boundary) of a black hole, it will be accelerated to a substantial fraction of the speed of light (C) which can cause atoms to be ripped apart with photons being ejected in the x-ray band.

We cannot detect black holes directly (because no signal can escape or be reflected from it) but we can infer them by the presence of mass being accelerated to a substantial fraction of C because of the gravitational tidal forces.

One more interesting bit about black holes. As you pass the event horizon the tidal forces grow more extreme. Imagine that you pass through a black hole feet first. Since your feet will be closer to the center of mass than your head, your feet will start to accelerate faster than your head causing your body to be stretched out. As you fall deeper, your molecules will be strung out in a process that Neil Degrasse-Tyson calls "spaghettification" and then your very atoms will be ripped apart. What's really interesting about this is that, counter-intuitively, if you were to fall into a supermassive black hole this *wouldn't* happen because the tidal forces are less intense but if you were to fall into a 'normal' black hole (a stellar-mass black hole) then you would undergo that process. So, ironically, you might actually *survive* falling into a supermassive black hole long enough to appreciate what was beyond the event horizon although you would still NEVER be able to get a signal out so you wouldn't be able to report what you found.

Cheers
Aj