This image is a 20 minute exposure of a galaxy named M84 (well, OK, so astronomers aren't great at names...), taken by the Hubble Space Telescope's new imaging spectrograph. A spectrograph is an instrument which breaks light up into its colors, much like a (very expensive) prism.
M84 is a galaxy 50 million light-years from Earth. The image above represents a tiny vertical slice near the center of the galaxy, measured on opposite sides of the galactic center at distances ranging from 1500 light-years (top and bottom of the image) to 26 light-years (center line of image). The spectrum represents the light from that slice, broken up into its constituent colors. If the center of the galaxy were standing still, i.e. not spinning, the image would look like a long rectangular bar containing all the colors of the rainbow, from dark blue/violet on the left to red on the right.
However, the zig-zag pattern shows that the center of the galaxy is spinning: green and blue indicate material moving towards the Earth; yellow and red indicate material moving away.
The colors and zig-zag pattern can also tell us the speeds of the orbiting gas and stars, Close to the galaxy's center, the speed of orbiting matter shoots up to 400 kilometers per second, which push the spectroscopic emissions to the far red and far blue (i.e. the material is moving very fast towards or away from us!). The sharp increase in speed can only be explained by the presence of a central black hole, at least as massive as 300 million suns, tugging on the gas and stars.
Reference: Science News 151, May 17, 1997.
Click on the image picture above to jump to the STIS Press Release.
This page is http://solar-center.stanford.edu/explainbh.html
Last revised by DKS on June 2, 1997