This is the supernova remnant Cassiopeia A, it can be found about 10,000 light-years from Earth in the constellation of Cassiopeia. Astronomers estimate the star exploded roughly 340 years before it reached the stage we see it in now, therefore making this one of the youngest supernova remnants discovered to date.
Cassiopeia A is immediately an intriguing object because astronomers estimate the supernova should have been visible from Earth after the telescope was invented. Why is this intriguing? Well, we have no historical record of anything like this ever taking place, considering we have historical evidence for astronomical phenomena dating back thousands of years, this is peculiar.
This image is presented in false color to make different elements stand out. In this case, bright green represents oxygen, red and purple depict sulfur, while blue reveals hydrogen and nitrogen atoms. These elements are scattered throughout the nebula, so pick a spot and zoom in to see what’s there.
In some cases, you get smattering of various elements, each part moving faster than the rest of the nebula. One such example is located about a quarter of the way down from the top left corner of the image. You’ll see some filaments of material traveling away at a higher velocity than the rest. It kind of gives the area the same look an apple has when you shoot it with a bullet.
There is a noticeably brighter section of nebula crowning Cassiopeia A. This is simply a region where the gas is hotter and glows.
See a larger image here.