Thanks to instruments such as the Hubble Space Telescope, astronomers know more about the variety of objects in the universe than previous generations could even dream of understanding. Even so, most people don't realize just how diverse the universe is. That's especially true about galaxies. For a long time, astronomers sorted them by their shapes but didn't really have a good idea about why those shapes existed. Now, with modern telescopes and instruments, astronomers have been able to understand why galaxies are the way they are. In fact, classifying galaxies by their appearance, combined with data about their stars and motions, give astronomers insight into galactic origins and evolution. Galaxy stories stretch back almost to the beginning of the universe.This Hubble Space Telescope view reveals thousands of galaxies stretching back into time across billions of light-years of space. The image covers a portion of a large galaxy census called the Great Observatories Origins Deep Survey (GOODS). NASA, ESA, the GOODS Team, and M. Giavialisco (University of Massachusetts, Amherst)
Spiral galaxies are the most famous of all galaxy types. Typically, they have a flat disk shape and spiral arms winding out away from the core. They also contain a central bulge, within which a supermassive black hole resides.
Some spiral galaxies also have a bar that runs through the center, which is a transfer conduit for gas, dust, and stars. These barred spiral galaxies actually account for most of the spiral galaxies in our universe and astronomers now know that the Milky Way is, itself, a barred spiral type. Spiral type galaxies are dominated by dark matter, making up nearly 80 percent of their matter by mass.An artist's concept of what our galaxy looks like from outside. Note the bar across the center and the two main arms, plus smaller ones. NASA/JPL-Caltech/ESO/R. Hurt
Less than one in seven galaxies in our universe are elliptical galaxies. As the name suggests, these galaxies are either range from having a spherical to egg-like shape. In some regards they look similar to large star clusters, however, the presence of large amounts of dark matter help distinguish them from their smaller counterparts.A giant elliptical galaxy has a small neighbor with a HUGE black hole at its heart. NASA/ESA/STScI
These galaxies contain only small amounts of gas and dust, suggesting that their period of star formation has come to an end, after billions of years of rapid star-birth activity.
This actually gives a clue to their formation as they are believed to arise out of the collision of two or more spiral galaxies. When galaxies collide, the action spurs great bursts of star birth as the commingled gases of the participants are compressed and shocked. This leads to star formation on a grand scale.
Perhaps a quarter of galaxies are irregular galaxies. As one might guess, they seem to lack a distinct shape, unlike spiral or elliptical galaxies. Sometimes astronomers have referred to them as "peculiar" galaxies, due to their odd shapes.
No matter what they're called, astronomers want to understand why they often look like oddballs when compared to other galaxy types. One possibility is that these galaxies were distorted by a nearby or passing massive galaxy. We see evidence for this in some of the nearby dwarf galaxies that are being stretched by the gravity of our Milky Way as they are cannibalized by our galaxy.The Large Magellanic Cloud (middle left) and Small Magellanic Cloud (upper center) over Paranal Observatory in Chile. European Southern Observatory
In some cases though, it seems that irregular galaxies have been created by mergers of galaxies. Evidence for this lies in the rich fields of hot young stars that were likely created during the interactions.
The lenticular galaxies are, to some extent, misfits. They contain properties of both spiral and elliptical galaxies. For this reason, the story of how they formed is still a work in progress, and many astronomers are actively researching their origins.Galaxy NGC 5010 -- a lenticular galaxy which has features of both spirals and ellipticals. NASA/ESA/STScI
Special Types of Galaxies
There are also some galaxies that contain special properties that help astronomers classify them even further within their more general classifications.
- Dwarf Galaxies: These are essentially smaller versions of those galaxies listed above. Dwarf galaxies are difficult to define because there is no well-accepted cut-off for what makes a galaxy "regular" or "dwarf". Some have a flattened shape and are often referred to as "dwarf spheroidals". The Milky Way is currently cannibalizing a number of these smaller stellar collections. Astronomers can track the motions of their stars as they swirl into our galaxy, and study their chemical makeup (also known as "metallicity").
- Starburst Galaxies: Some galaxies are in a period of very active star formation. These starburst galaxies are actually normal galaxies that have in some way been disturbed to ignite very rapid star formation. As mentioned above, galaxy collisions and interactions are the likely cause of the starburst "knots" seen in these objects.
- Active Galaxies: It is believed that virtually all normal galaxies contain a supermassive black hole at their cores. In some cases, however, this central engine can become active and drive massive amounts of energy away from the galaxy in the form of powerful jets. These Active Galactic Nuclei (or AGN for short) are widely studied, but it is still unclear what causes the black hole to suddenly become active. In some cases, passing clouds of gas and dust may fall into the gravitational well of the black hole. The material gets superheated as it swirls around in the black hole's disk, and a jet may form. The activity also gives off x-rays and radio emissions, which can be detected with telescopes here on Earth.
The study of galaxy types continues, with astronomers looking back to the earliest epochs of time using Hubble and other telescopes. So far, they've seen some of the very first galaxies and their stars. These little "shreds" of light are the beginnings of the galaxies we see today. The data from those observations will help the understanding of galactic formation back at a time when the universe was very, very young.This simple diagram of galaxy types is often called Hubble's "tuning fork". public domain
- Galaxies exist in a variety of shapes and sizes (called their "morphology").
- Spiral galaxies are very common, as are ellipticals and irregulars. The first galaxies were likely irregulars.
- Galaxies grow and evolve through collisions and mergers.
- “Galaxy | COSMOS.” Centre for Astrophysics and Supercomputing, astronomy.swin.edu.au/cosmos/g/galaxy.
- HubbleSite - The Telescope - Hubble Essentials - About Edwin Hubble, hubblesite.org/reference_desk/faq/all.php.cat=galaxies.
- NASA, NASA, science.nasa.gov/astrophysics/focus-areas/what-are-galaxies.
Edited by Carolyn Collins Petersen.