Galaxy Superclusters

We live in the Milky Way Galaxy, a beautiful spiral armed galaxy filled with hot gas and young stars.

Did you know that many galaxies, including the Milky Way, actually formed in clusters? Galaxy clusters are groups of 30 or more galaxies that are all gravitationally bound to each other. Galaxy clusters nearby one another can form a supercluster of galaxies, though they may not all be gravitationally bound, just spatially coincident with each other. The Milky Way is part of the Local Group, which contains roughly 40 galaxies. This group is a sub-portion of the Virgo supercluster, which contains over 2500 galaxies within 100 million light years of us! These clusters contain spiral galaxies, like the Milky Way, but also elliptical galaxies which are disk shaped collections of older stars. Some popular clusters you may have heard of are the Fornax cluster, which also lies inside the Virgo supercluster, and the Coma cluster, which is a separate cluster of over 1000 galaxies located over 300 million light years from us. The image above shows some of the superclusters of galaxies in the Universe, with the Virgo cluster at the center. Each white dot is an entire galaxy, so those white regions throughout the image are collections of hundreds of galaxies! 

Image Credit: R. Powell

Hubble Classification of Galaxies

The Hubble tuning fork diagram was developed by Edwin Hubble as a way of classifying galaxies based on their shape. Basically, astronomers look through a telescope at a galaxy, and classify it as an elliptical, spiral, or barred spiral. These classifications were discussed in the last ADYK. If the galaxy looks like a fuzzy bright blob, it's classified as an elliptical (E), and a number is assigned based on how round it looks (1=circle, 9=cigar shaped). If a galaxy has deifned circular rings, but not exactly spiral arms, then it's an "S0".  If the galaxy appears to have spiral arms, it's either a regular spiral galaxy (S), or a spiral with a bar like structure through the center (SB). They are also assigned a letter based on how tightly wound the spiral is. "Sa" means tightly wound arms, while "Sc" is very loosely wound arms. These classifications are handy for astronomers, as they can relate the properties of galaxies to their classification. Hubble originally thought that this diagram represented a time sequence, i.e. all galaxies started as round ellipticals (E0), and evolved over time into spirals (SBc or Sc). Astronomers are now able to measure the ages of many galaxies, and we've found some young ellipticals and old spirals, so the tuning fork diagram does not represent a galaxy evolution sequence. One thing to remember is that this classification is based on actual galaxy shape, not observed. For example, from Earth's perspective, you might view a spiral galaxy edge on, but think it's an E7 elliptical because it appears that way. Because of this, it's not easy to classify these galaxies accurately. You can actually help astronomers do this! Check out the next ADYK!

Spiral and Elliptical Galaxies

 

Spiral Galaxy M81

 

Elliptical Galaxy M87

Galaxies come in two different types: Spirals and Ellipticals. When most people think of a galaxy, they think of the classic spiral galaxy with a twisted arm shape (like M81 pictured above). These bright spiral arms contain most of the stars in the galaxies, which is why they are so bright. The number of spiral arms, how compact they are, and the direction they face varies between galaxies. Most spiral galaxies have a central black hole where all the spiral arms meet. Elliptical galaxies (like M87 pictured above) don't really look like a galaxy at all. They are essentially a huge collection of stars, often in an oval shape, and look like a bright fuzzy cloud, or out of focus star. These galaxies have little structure, other than the density of stars increases towards the center, where a black hole often lives. The next ADYK will discuss Edwin Hubble's galaxy classification scheme, a way of categorizing these types of galaxies based on there shape and structure.

Active Galactic Nuclei

Active Galactic Nuclei…. You may not know what they are, but you have to admit that they sound pretty crazy!  Many galaxies have bright centers due to stars bunched up around a super massive black hole. Active galactic nuclei (AGN) are the super bright cores of distant galaxies. AGN have supermassive black holes that are actively accreting a lot of matter. This in fall of matter creates heat, turns the region around the black hole into plasma, and emits copious amounts of light. The image above is what the active nucleus of NGC 5141 looks like in X-rays! The bright yellow center means lots of x-rays are being emitted, and therefore this region must be very hot and active. AGN are so bright that we can see ones (with good telescopes) that are as far as 12 billion light years away! When we look at AGN, we are most likely seeing light from very large and compact galaxies that existed in the beginning of the universe. There are three main types of AGN: Syefert, Blazars and Quasars. The differences lie in the type of radiation emitted.  Astronomers think that there may only be one "type" of AGN, and they look different because we are viewing them at different angles. Seyferts, Blazars and Quasars will be discussed more in future ADYK.

Image credit: Radomski, J. et al.  2003, AJ 587, 117

Free Astronomy Apps

If you have an iphone or other smartphone, you've probably discovered that there are quite a few astronomy apps out there. Most of them cost a few bucks,  but there are some pretty cool free ones out there. Here's what I have downloaded to my iphone that you should check out for yourself:

Exoplanet Database: This is an awesome app if you're into exoplanets! It's a database with information about all the different exoplanets that have been discovered, and includes size, period, distance, and other useful information. You can read news about exoplanets, view where any exoplanet is located in the galaxy, make handy graphs of exoplanet properties, and even get links to scientific papers about the exoplanets. The coolest part about this app… it sends a text message to your phone every time a new exoplanet is discovered!

APOD Viewer Lite: An application that lets you view the astronomy picture of the day. It shows you the image, explanation, and lets you look back at previous images. Very nice app to quickly view today's APOD.

NASA: Basically this is NASA.gov in app form. You can find news,  launch schedules, pictures, videos, telescope info, and anything astronomy you can imagine. Excellent educational app.

ArXiv: This app allows you to search the astrophysics arXiv for scientific papers, and download the pdf to your iphone (using a pdf reader app). This is more for the serious astrophysicists, but anyone can peruse through the papers and maybe you'll find something interesting.

Galaxy Collider Lite: A cute app that shows you what would happen if two galaxies collided. I'm not sure how scientifically accurate the simulations are, but it's cool to watch the galaxies interact regardless. You can set the number of stars, mass of the galaxies, speed of collision, etc. and observe how that changes things

If you've discovered any other cool free apps, post them in the comments section below! Also feel free to leave your opinion about different non-free apps as well!