I’m the host of today’s 365 Days of Astronomy podcast, so please go check it out. (Not just today! They consistently have great people doing shows, with better astronomy credentials than mine.) My topic is galaxy clusters, the largest objects in the universe held together by their own mutual gravity. As with my previous podcast, here are some supplemental images that I hope will help you understand what I’m talking about a little better.

This podcast is dedicated to my parents, Richard and Kathy, who are celebrating their wedding anniversary today!

Types of Galaxies

When most people think of galaxies, they think of spiral galaxies, though there are many other types. Spiral galaxies are indeed very common, and are distinctive because of their arm structures. The image on the right is of M51, also known as the Whirlpool Galaxy. (This galaxy made the news last week because of a supernova near its outskirts.) Spiral galaxies have a lot of relatively young stars (seen as bright patches) and a significant amount of gas and dust, accounting for the milky appearance and dark streaks.

Elliptical galaxies, as the name suggests, are more egg-like in appearance. The giant elliptical galaxy M87, which I mention in the podcast, is shown on the right. There are no clear structures like in a spiral galaxy, though you can see a higher concentration of stars that marks the galactic nucleus at the center. Not a lot of gas and dust, and the stars have a higher average age than in spirals. Elliptical galaxies come in a range of sizes, including the largest galaxies in the universe.

Galaxy Clusters

The Virgo cluster is the closest galaxy cluster to us, and contains perhaps as many as 2000 galaxies. On the sky, the cluster covers an area larger than the full Moon, so it’s hard to get a picture that fully captures both the size and beauty of the cluster, so I picked one that emphasizes beauty. The Virgo cluster is known as an irregular cluster, because it doesn’t have a well-defined center or shape. Even in this close-up view, you can tell there is a high proportion of spiral galaxies, along with a type known as lenticular. Lenticular galaxies (meaning lentil- or lens-shaped) are like spiral galaxies without the arms — they have a lot more structure than elliptical galaxies. Note the two galaxies in the bottom left corner, which are pulling each other into weird shapes, possibly as a prelude to merging into a larger galaxy.

The Coma cluster is a good example of a regular cluster — one that has a well-defined center and a roughly spherical shape. (As you can tell, this image is actually a mosaic of smaller pictures; as with the Virgo cluster, the Coma cluster is fairly large on the sky.) Note that there are a lot of elliptical galaxies — the yellowish galaxies are also elliptical.

Hot Gas and Dark Matter

The images above were all taken using visible light, the stuff we generally think of as light. However, using X-ray light, galaxy clusters look very different. The image to the right is of the Virgo cluster again, only this time using X-rays. The galaxies themselves are still there, but the X-ray image shows a lot of very hot gas, with temperatures on the order of 10 million Kelvins. (The “colors” in this image don’t correspond to color in the visible light regime; instead, white represents the hottest part of the gas, followed by red, scaling down to black for minimal X-ray emission.) As you can clearly see, there is a lot more hot gas in the Virgo cluster than there are galaxies.

I’ve talked about the Bullet Cluster in two previous posts, and probably will mention it again — it’s an important object for understanding the nature of dark matter. Three types of observations were used to create this image: visible light, which shows the positions of galaxies; X-ray light, which is the red and pink regions, showing the location of the hot gas; and an analysis of the bending of light by the mass in the cluster (gravitational lensing) shown in blue. You can see in this picture that the lensing, which shows the location of dark matter, is concentrated near the galaxies, while the hot gas — including the famous “bullet” shock wave — is separated out in the center. Scientists believe this cluster underwent a collision and may be in the process of merging into a single cluster.