7. What's making these gamma rays?
Hot doings at the old galaxy M84 tonight! Left, the view from Hubble's Wide Field and Planetary Camera 2, shows dust and gas obscuring something to the rear. Space Telescope Imaging Spectrograph image (right), shows the motion of gas in the center of the left-hand pic.
Is the object at the center a black hole? Perhaps. First of all, it makes no light -- and black holes by definition, are so dense that not even light can escape. But the spectrograph carries other evidence.
A spectrograph measures speed by recording changes in frequency caused by the Doppler ("train-whistle") effect. Spectrum features on the left are "blue-shifted" as the gas that emits them moves toward us because the light waves are compressed. Gas producing the spectrum features on the right is moving away, so its light is "red-shifted."
Why is this convincing evidence of a black hole? Here's the argument: If things do not spin around a central object, they will be sucked in by gravity. The speed of stuff orbiting the object depends on the object's mass. Great speeds require great mass.
The spectrograph tells us that gas 26 light years from the object at the center of this galaxy is moving at 400 kilometers per second. Elementary orbital mechanics, known since Isaac Newton and an absolute non-compute to nincompoopish Why Filers, says that if you know orbital radius and speed, you can calculate the mass at the center. This black hole must have the mass of 300 million suns.
Yikes! Hate to have that land on a sore toe... Good thing M84's 50 million light years away.
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