It's often said the the Andromeda Galaxy is like our own Milky Way. Sure, there are similarities. The two are destined to collide and probably merge, for one thing. And they share a similar shape, both being spiral galaxies with central bulges loaded with stars.

But there is one big difference: Andromeda's presumed central black hole packs the mass of 30 million suns. The Milky Way's black hole is estimated to be a mere 2.6 million solar masses.

You can find the supermassive black hole in this image of Andromeda from NASA's Chandra X-ray Observatory. See the blue dot? That's actually a relatively cool X-ray source, "only" about a million degrees Celsius. Nobody knows what it is. When Chandra first imaged the region in 2000, researchers thought the blue dot corresponded to the exact center of the galaxy -- the position of the suspected central black hole. But it should have been hotter, so the scientists were puzzled.

Just above the blue dot is a bright yellow dot, several tens of times hotter. That's radiation from around the black hole, astronomers now believe, based on refined Chandra imaging in 2001 (and some help from the Hubble Space Telescope) that showed the cooler, blue X-ray source is actually about 10 light-years south of the galactic center.

The X-rays are given off by gas that swirls toward the gravity sink and is superheated as it approaches the speed of light.

So what about all those other yellow dots? They are most likely what astronomers call X-ray binary systems. In one of these, a normal star is orbited by a dense stellar corpse, either a stellar-mass black hole or a neutron star. The dense object siphons matter from the normal star, setting up a miniature version of the same scenario that generates X-rays in the supermassive black hole at the galactic center.

At just 2 million light-years away, Andromeda is our nearest large galactic neighbor. [Learn more about the impending collision]

-- Robert Roy Britt