You’ll need patience with the following, as it really constitutes a short course in galactic Black Hole physics. ( It's not hard to understand - just loooong. I need Louise to edit these things...)
From Abraham, Chapter 3:
1 And I, Abraham, had the Urim and Thummim, which the Lord my God had given unto me, in Ur of the Chaldees;
2 And I saw the stars, that they were very great, and that one of them was nearest unto the throne of God; and there were many great ones which were near unto it;
3 And the Lord said unto me: These are the governing ones; and the name of the great one is Kolob, because it is near unto me, for I am the Lord thy God: I have set this one to govern all those which belong to the same order as that upon which thou standest.
For decades, astrophysicists have believed that most if not all galaxies must have Black Holes in their centers. There is just too much “stuff” floating around ‘way too close to other “stuff” for it not to all merge due to gravity and orbit-decay. They already knew about white dwarfs and neutron stars - that bigger and bigger original stars gave way to more and more dense “final states”. You can actually “see” one neutron star by its rapidly oscillating magnetic field. It’s like a radar beam sweeping over you as the neutron star spins 1000 times a second. The signal is coherent, which means that the neutron star must be smaller than the distance light can cross in that amount of time. Calculations show that a teaspoon of neutron star "stuff" would weight tons on Earth - that is, if you could transport and then weigh it.
Hmmm. What happens if you throw in a lot more “stuff” - what would you get? Must be something denser (see the Newton paragraph below) - and it will be a real glutton for all the smaller stars and gas and dust whizzing around it. Because of tidal and magnetic drag on the highly conductive material, orbits will decay. Annnnnd... I.... Gotcha!
For almost as much time, astronomers have diligently sought proof of a Black Hole at the center of OUR galaxy. They chose it because it’s closer than other galaxies, so should be easier to image. However, on the face of it this would seem to be a daunting task, as a Black Hole, by definition, radiates nothing - no mass, no light, no signal can escape its Event Horizon. Remember from a previous blog that Black Holes are really dark gray and fuzzy. However, there ARE some indirect ways that we might “see” one.
One way to “see” a Black Hole indirectly is to map stars close to the galactic core. “Our” Black Hole actually has a name these days: Sagittarius A*, pronounced “Sagittarius A-Star” or just abbreviated Sgr A*. It lies in a corner of a bright region in the center of the Sagittarius Constellation, in the center of our Milky Way. This bright spot was designated “Sagittarius A” as the first bright apparent star classified in that constellation centuries ago when astronomers first looked at it. To them, Sagittarius A looked like any other star, but they were using telescopes crummier than the ones you give your kids these days. (That worthless Tasco ‘scope? Galileo would have drooled over it.) As bigger and better telescopes became available, it turned out Sagittarius A was a whole lot more than a single star.
Some basic orbital physics: Thanks to Newton, we know that the gravitational force between two masses goes as a constant (the “G” mentioned in an earlier blog) times one mass times the other mass divided by the square of the distance between the geometric centers of the two masses. Whew, that’s a mouthful. Perhaps you can understand why physicists really prefer to say things in “equation” instead of in English. A quick translation (I didn’t use translate.google.com to do this) gives: F12 = G * M1 * M2/r * r. In shorthand this becomes F=GMm/r^2. This is important, because a star named “S2" close to the center of Sagittarius A has been tracked since 1992 (despite what Wikipedia says). In the vernacular, that sucker is bookin’: it orbits in an ellipse about 5 by 10 light-days across in about 15 years. Days and years here make it seem trivial until you remember the speed of light is 300,000 kilometers per second. This star is moving so fast that it makes the huge nearby stars look like icebergs with a dolphin zipping around nearby. But think of a dolphin moving at the speed of sound. S2 orbits around something that can’t be directly seen - but because of that equation the unseen mass can be measured, and it’s huge: about four million Suns’ worth of “stuff”.
Some basic electromagnetic physics: If matter is being drawn into the monster, it will be accelerating because of that 1/r-squared part of the equation: the shorter the distance, the stronger the pull on it, and the faster it goes. It’s probably a seething plasma as it falls in, because the calculated forces are truly humongous (try dividing anything by a distance squared that approaches zero - it’s like magma expanding and accelerating up a volcano’s throat). Such a seething cauldron of matter will radiate: electrons accelerating in a magnetic field give off electromagnetic energy at wavelengths proportional to the radius of curvature of their ever-tighter spiral motion inward. The Event Horizon of a Black Hole in a busy galactic center, in fact, should be shrieking at all wavelengths. The closer to the Event Horizon, the stronger the pull and the higher the energy - and the higher the frequencies, all the way up into hard gamma radiation. You need a number followed by lots of zeros to describe the energies involved. It’s hard to see the screaming-edge source because of all the stars, gas, dust, and junk in between Sgr-A* and Earth - and it’s also a long ways away to “look” (about 26,000 light years) to see anything.
But astronomers are a persistent lot, and eventually they figured out that certain longer wavelengths can get past all that crap and be picked up by Earth-based radio-telescopes. (They settled on a rather atypical radio wavelength of 1.3 millimeters - not that far from what your cell-phone uses. They chose this wavelength for several reasons, including because it's not a cell-phone frequency.) If you can get a rich billionaire (think the “other” Microsoft billionaire) to pay for it, you can get a big enough array of radio-telescope dishes, spaced far enough apart on the Earth, to get a pretty darn good radial resolution. The shrieking edges of Sgr A* can more or less be made out this way. It’s diameter is no greater than 44 million kilometers - probably less. This is about one-half the size of Mercury’s orbit around our Sun. Now, fit four million Suns into that... and then step back, or scream as you are gobbled up.
In 2004, astronomers were astounded to find evidence of a much smaller (1,300 Solar masses) invisible object orbiting the 4-million-Sun-mass Sgr-A* - a sort of mini-Black Hole. It resides in the center of a cluster of seven massive stars, which orbit it. Astronomers have also identified a number of additional giant stars that circle around in the near vicinity of Sgr-A* (the “lumbering icebergs”).
Now read verses 2 and 3 again. Do you notice what I noticed?