Black Holes Proven by Hubble Telescope
We've seen in an earlier section that one piece of evidence for a black hole would be material spiraling into a black hole like water draining out of a bathtub, called an accretion disk. On February 27, 1994, scientists using NASA's Hubble Space Telescope discovered just such an accretion disk at the center of the giant elliptical galaxy M87, which lies in the constellation Virgo 50 million light years away from us.
But how did they know that the disk was spinning? They observed an effect well-known to physicists, called the Doppler effect. It is the familiar phenomenon you notice when you listen to the whistle on the train as the train comes by you. As the train is heading towards you at high speed, the frequency of the train whistle is higher and as it travels away from you, its frequency gets lower.
The same thing can happen to light. If the light you are looking at in a telescope is coming from an object that is moving rapidly toward you, its frequency gets higher, shifting more toward the blue end of the spectrum (this is known as a blueshift). Similarly, if the object is moving rapidly away from you, its light is redshifted.
Below is the scientist Benjamin C. Bromley's computer model of how the light would be shifted coming from a rotating black hole. The red and the colors near it on the spectrum (such as orange) represent redshifted light, as the blue and the colors near it on the spectrum represent blueshifted light.
The Hubble detected just such red and blue shifts, proving that the disk was rapidly rotating at the speed of 1.2 million miles per hour on the outside of the disk! Scientists calculated from these measurements how massive the object in the center must be to pull things in that fast. They found it would have to weigh as much as 3 billion suns, and yet still be concentrated into a space no bigger than our solar system. The only explanation for such a massive object would be a black hole.
In the words of Dr. Holland Ford, of the Space Telescope Science Institute and Johns Hopkins University, "If it isn't a black hole, then I don't know what it is."
Click here to view NASA's official press release on this.