The image here is of the very center of the Andromeda galaxy. If you look in the middle of this image, you’ll see a smudge of blue. As first demonstrated in an article in Astrophysical Journal, the central blue region is a collection of about 400 young blue stars. They are concentrated in a region about a light year across, and are moving at about 1,000 km/s.
Hoag’s Object
Hoag’s object is perhaps the most famous example of a rare type of galaxy known as a ring galaxy. It is comprised of a core of older stars surrounded by a ring of young hot blue stars. When it was first discovered by Arthur Hoag in 1950, it was thought it might be an example of the gravitational lensing of a more distant galaxy by a closer one. Later observations showed that the ring and core have the same redshift, and so are of equal distance. More recent images such as the Hubble view seen here clearly indicate that the usual shape is not due to lensing. The galaxy really is shaped the way it looks. Just how it got that way is a bit of a mystery.
Survey Says
The image below is the first complete x-ray survey of the Andromeda galaxy. The Andromeda galaxy (also known as M31) is about 2.5 million light years away. It is a spiral galaxy very similar to our own Milky way, so surveys of Andromeda are very useful in understanding galaxies like ours.
Massive Speed
We now know that most galaxies have a supermassive black hole in their center. One of the ways to determine the mass is called reverberation mapping. By looking at variations in the brightness of active galactic nuclei, you can determine the size of the black hole. But we can only do this for about 40 galaxies, so it would be nice to have another way to determine black hole mass. It turns out there is, using a relation known as the M-sigma relation.
Through a Glass Darkly
Dark energy and dark matter make up about 96% of the universe. Baryonic matter, the stuff of stars, planets, you and me, makes up only 4% of the universe. And most of that isn’t actually in stars and planets and us, but between the vast expanse of galaxies. It makes up the intergalactic medium (IGM) and it is notoriously difficult to observe.
Hubble’s Tuning Fork
Edwin Hubble is perhaps most famous for discovering a relationship between the distance of a galaxy and the speed at which a galaxy moves away from us. This relation is now known as Hubble’s Law, and is evidence for the expansion of the universe. But Hubble’s primary interest was in galactic nebulae (what we now just call galaxies).
Dusty Starburst
A starburst galaxy is a galaxy that’s producing stars at a very high rate. The rate of star production is so high that it would use up the available gas and dust in a time period much shorter than the typical age of a galaxy. We often see starburst galaxies in the midst of a merger or close encounter with other galaxies, which would indicate that starburst periods can be triggered by galactic collisions.
Radio Galaxy
A radio galaxy is a galaxy that emits large amounts of radio waves. They are powered by the galaxy’s supermassive black hole.
Push Me, Pull You
If the universe is expanding, how is it possible that galaxies can collide with each other? It turns out that not only are colliding galaxies possible in an expanding universe, the fact that they occur is due in part to dark matter and dark energy.