Posted by Davin Flateau on 14 Feb 2006 at 10:15 am.
Filed under General, Astronomy.
The theories were there; the thousands of pages of equations telling its story could plaster the inside of an aircraft hanger from floor to ceiling. Science had written a tale of black holes that would seem completely absurd if it weren’t backed up by so much physics. Black holes were an inescapable celestial abyss that would crush everything it comes into contact with. Nothing could escape its grasp - not even light itself.
But equations and theories are one thing - astronomers couldn’t be sure that black holes were out there until they actually found one. So how do you find a tiny, invisible object from many light years away hidden in a seemingly endlessly large Universe? A needle in a haystack would be a breeze in comparison to the search for a black hole. They may be invisible, but the effect they have on the environment around them should be easy to spot. Astronomers figured out that matter being crushed into energy near the black hole should light up the sky.
Our Universe is glowing in all kinds of light. Our eyes only see a tiny part of what’s out there, and without special instruments to see the rest, we’d be blind to almost everything happening in the cosmos. The invisible energies tell us a lot, from the low energy infrared radiation of warm dust and gas clouds, all the way up to the high energy x-rays and gamma rays bursting from the most violent events nature can cook up. It’s these high energy light sources that can give away the location of black holes.
Dateline: 1972, Ontario, Canada. If you could put on special glasses that would let you see the sky exclusively in high energy x-rays, one of the brightest stars in the night sky would be one particular star in Cygnus. When University of Toronto astronomer Tom Bolton discovered Cygnus X-1, he knew its massive outpouring of x-rays was the marker for a something cataclysmic happening some 8000 light years distant. Bolton discovered that Cygnus X-1 was actually two objects orbiting around each other. One enormous blue supergiant star about 20 times the mass of our sun paired with a small object that seemed to be sucking the gas from its huge partner. The gas seemed to form a hot, fast-spinning “accretion disk” around the object, emitting tremendous amounts of x-rays.
By studying the orbital dance between the stars and the small object, Bolton figured out that the compact object was also very massive - about 10 solar masses. Finding that mass was a critical piece of the puzzle; the world knew from Chandra 42 years earlier that stars that tiny cannot be that massive without collapsing into a black hole. X-1 marked the spot for Bolton, as he took credit for finding the first black hole in the Universe.
Since Cygnus X-1, astronomers were able to find more black holes from spotting hot accretion disks, and measuring the black hole’s mass through its dance with an orbiting companion. It’s possible that the Universe is littered with these tiny but powerful dark stars.
Getting Big
So far, we’ve been talking about “stellar mass” black holes left over when individual stars collapse; they typically have masses of 4-15 suns. But some astronomers started to think on a larger scale. When studying entire galaxies, they wondered what was happening at the center of these cities of hundreds of millions of stars. Some galaxies showed jets of energy and matter shooting out at tremendous speeds. Could black holes — really big black holes - be the engine driving these galactic energetic jets?
Dateline 1994: Low Earth Orbit. The Hubble Space Telescope with its 2-meter diameter meter high above Earth’s obscuring atmosphere is observing a galaxy 50 million light years away. The galaxy is designated M87, an elliptical galaxy well known by amateur astronomers, and easily visible with a backyard telescope. But Hubble and its astronomers are looking much deeper than any telescope has seen before. They’ve noticed that there is a swirling cloud of gas, dust and stars churning around the center this city of hundreds of millions of stars, and they want to know exactly what’s at the center of the galaxy causing all the mayhem.
By measuring the speed of the huge rotating maelstrom of hot gas and dust, astronomers were able to calculate the mass of the compact object at about 2 *billion* suns. The only explanation was a black hole, in fact the first confirmed sighting of a supermassiveblack hole. It wouldn’t be long until many other galaxies were suspected of harboring one or even more supermassive black holes at their center. Even our own Milky Way galaxy is thought to be home to such a monstrous abyss.
And that’s the long and short of the short history of the science of black holes, although there is even more to the story we couldn’t cover in a single set of articles. Starting from an interesting thought puzzle, it moved through the centuries into the field of theoretical astrophysics, and then finally to the exciting images in the real world of the power these objects have. The story is also a great example of the power of the scientific method, moving from conjecture to hypothesis to scientific theory.
In the past 20 years, the science of black holes has captured the imagination of the public at large, even finding a place in our language (many people describe my office as a black hole for paperwork). We live in exciting times. Newspapers seem to be filled almost daily with amazing new discoveries that changes our view on the Universe we live in; we are living in a golden age of astronomical discovery. Black hole research will continue to show us that we are just beginning to understand our celestial landscape.
