When you look up on a dark, clear, moonless night, you might just see a faint fuzzy streak cutting across the sky. That's the Milky Way, called that because it looks like milk has spilled across the sky.
But when you look at it through a telescope, it resolves itself into millions upon millions of stars. Faint and tightly packed, they merge together, forming a fuzzy streak because we lack the resolution in our eyes to separate out the stars.
The same is true when you look at the sky in X-rays. A pervasive X-ray glow permeates the same area of the sky as the Milky Way, and for many years astronomers haven't known if this is due to a vast ribbon of million-degree hot gas and is truly a fuzzy glow, or if it's caused by lots and lots of point sources packed together.
NASA's orbiting Chandra X-ray observatory has finally resolved (haha! I kill me) the issue. Behold:
The image shown is a Spitzer infrared image of a region very near the center of the Milky Way. Chandra stared at the indicated spot for roughly 250 hours (!) -- one of the deepest exposures ever taken using the observatory -- and instead of a faint, diffuse glow it found hundreds of faint point sources of X-rays. This strongly indicates that the glow in X-rays seen by earlier telescopes is in fact the combined glow of millions of such discrete sources!
This is good news; it hardly seemed possible that there was a source of energy that could keep so much gas heated to millions of degrees. But now we have another question: what the heck are the point sources? Most likely they're white dwarfs, stars like the Sun whose lives are over, and all that's left is the hot, dense core. If a white dwarf is in a tight orbit around a normal star, it can draw material off that star. This matter piles up on the dwarf and gets very hot, emitting X-rays. It can also build up to a critical point and undergo massive spontaneous fusion, again blasting out X-rays.
There are most likely other sources of X-rays in there as well, including highly magnetic binary stars, neutron stars, and black holes.
I love this observation. It's everything science should be: an interesting question, a straightforward observation to determine the solution, a willingness on the part of the observatory directors to take a lot of precious telescope time to make the observation, and a result that is so clean it's pretty easy to interpret.
If only all things in science were like this. But of course, sometimes the fun is at the edges, where observations are tricky, the physics uncertain, and the results surprising and leading to new vistas to explore.
Simple and clean, or difficult and fuzzy: science has room for both.
Credit: X-ray (NASA/CXC/TUM/M.Revnivtsev et al.); IR (NASA/JPL-Caltech/GLIMPSE Team)