Here is a continuing question from 3-yr-old Samantha. It actually goes to the heart of why we have geology in the first place: black holes and supernovas of earlier suns have led to a cyclic mix of fusion-created heavy element products like oxygen, carbon, iron, and silicon - major constituents of our rocky Blue Marble, water-covered planet. A world like ours could not have existed in the early life of the universe.
Thank you so much for your reply. She (Samantha) still talks about you from time to time. Then out of the blue she asks "Mommy, what are black holes made of?" I don't know! :)
Well, the short answer is a LOT of mass. There are actually at least two different kinds of Black Holes.
A Stellar Black Hole starts with the collapse of a very large star - a star much bigger than our Sun. As the star uses up its hydrogen by fusing it to helium, it starts converting helium to carbon - these stars are a deep red, almost garnet color in a visible light telescope. Rather quickly on a cosmic time-scale, it will start converting carbon and helium into a number of other life-critical elements, all the way up to iron. The fact that the Earth's crust contains elements up into the uranium range suggests other processes, too. All the material we find on our own Earth has come from this thermonuclear process - probably from many ancient stars that reached old age and blew up long ago.
In two words, we are “Star Stuff.”
Somewhere in this winding-down process for this very large, earlier star, there is an initial collapse of the outer blanket of hot gas material down to the star's core, and a "bounce" causing an initial huge blow-out of the outer envelope. This is called a nova, or in some cases a super nova. It produces prodigious, short-lived amounts of radiation from visible light all the to X-Ray energies and beyond. In a distant galaxy, a supernova can look temporarily like a nearby star in our own galaxy.
This outer shell ejection process creates something called a Planetary Nebula - a glowing shell of gas that almost looks like a planet in a cheap telescope. Finally, there is a huge terminal collapse and all the remaining matter, without thermonuclear heat to hold it up, collapses into what becomes a Black Hole. It's called a Black Hole because there is so much mass in such a tiny volume that it bends light. It bends light so strongly - this is an essential part of Einstein's General Theory of Relativity - that light can't get out of a certain volume outside the central concentrated mass. This "edge" where light can't escape from is called the Schwarzschild radius, or the Schwarzschild discontinuity. You can guess who suggested this idea first. If the original star isn't big enough, the mass will collapse back into a White Dwarf - or if there is more mass, it will collapse into a neutron star, a teaspoons of which would weigh tons on Earth (if you could get it here or even weight it).
This is a description of a multi-stellar-mass Black Hole.
There are other, far larger Black Holes. Galactic Core Black Holes are found in the centers of most galaxies including our own – and they form for different reasons and are HUGE. These Black Holes result from too many large stars in the crowded center of the galaxy being in too small a confining space - and they coalesce into each other forming a Black Hole that grows ever larger with time as it gobbles other nearby stars spiraling into it from tidal orbital collapse. In some science fiction books this is called "The Eater" or the Black Monster. We know there is a Galactic Core Black Hole in the Sagittarius constellation - the center of the Milky Way galaxy - because astrophysicists can see huge Doppler shifts in radiated light over a very small angular separation in a tiny area. This zone was originally named "Sagittarius A" - for the first apparent brightest star classified in that constellation by early astronomers. Sensitive satellite detectors indicate that the center of this interesting area radiates light all the way up into the X-Ray range of energies. On one side the Doppler shift indicates that material is rotating TOWARDS us (the absorption bands are blue-shifted), and close by on the other side there is a red shift telling us that it is rotating AWAY from us. The latest indirect calculations suggest this area, called Sagittarius-A* (Sagittarius-A-Star, or "Sgr-A*" for short) is about the diameter of Mercury's orbit around our Sun - but holds a mass equivalent to at least 44 million Suns in that relatively tiny volume. It's hard to see this, as the whole mess is about 26,000 light years away from us, so it's taken some very clever astrometrics by some very smart astrophysicists to get these numbers.
This seems like more than a normal 3-yr-old might be able to absorb. I am struck, however, that this 3-yr-old of yours has such a wide-ranging interest in scientific things. She could not get there without a highly supportive parent who will spend the time at least trying to answer her questions. You must have some rather eclectic conversations with your daughter.