Galaxy Classification

Galaxies come in many different shapes and sizes, but can generally be classified into one of five categories: Elliptical (E), Lenticular (S0/SB0), Spiral (S), Barred Spiral (SB), and Irregular (Irr).

Galaxies can also have additional subtypes. In Hubble's scheme, Ellipticals have subtypes 0 - 7, which refer to how elongated it is. Spirals and Barred Spirals have subtypes a, b and c, which relate to the tightness of the spiral arms, the color of the galaxy, and the bulge-to-disk ratio.

Check out the links to the left to learn more about the subtypes. There is also an image of Hubble's Tuning Fork in the next section, which might be helpful for determining how the color and the bulge-to-disk ratio are related to the spiral subtypes.

Irregular galaxies consist of small galaxies like the Magellanic Clouds, as well as much larger ones like those shown below. How do you think these larger irregular galaxies formed?

Try classifying these galaxies using Hubble's Classification Scheme.

Our Milky Way galaxy contains over 100 billion stars. To get an idea of how many stars this is, look at the GAIA 3D Star Map. Each dot represents a star in our galaxy... and only 2 million are being shown!

While this might seem like a lot of stars, it turns out our galaxy isn't all that big. The largest known galaxy in the universe (IC 1101) has an estimated 100 trillion stars!

How do we measure the number of stars in a galaxy when we can't resolve them? This is done by measuring the luminosity of the galaxy and dividing by the average luminosity of a star. Another similar method involves using a mass-to-light ratio. For stars on the Main Sequence, the luminosity is related to the mass via \(L \propto M^{3}\). Thus by measuring the luminosity of the galaxy we can get a mass estimate and then divide by the average stellar mass.

There is another more sure-proof method for measuring galaxy masses. It's the same way astronomers measured the mass of the Sun! Newton's derivation of Kepler's third law relates the orbital period \(T\), the semi-major axis \(a\), and the enclosed mass \(M\) via $$ T^2 = \frac{4\pi}{GM} a^3 $$ We can determine the period and semi-major axis by measuring how far away the galaxy is and its rotation speed via the doppler shift. When comparing the two mass estimates, however, they are vastly different.

We can use our very own Milky Way as an example. Astronomers have determined that nearly all of the luminous material is within 15 kpc (about 50,000 light years) from the galactic center. The image below represents the observed rotation curve. What would we expect the curve to look like based on the first observation? Well, try giving it some thought, then click the image to find out if you were right.

As illustrated, distant stars and gas are orbiting much faster than expected. In actuality, however, this discrepancy begins at much shorter distances, and similar results are found in other galaxies. So where is all this missing mass coming from? Astronomers aren't sure, so they call it dark matter.

Just like the HR diagram for stars, a similar diagram exists for galaxies called the Galaxy Color-Magnitude Diagram. If we plot color against luminosity, most galaxies fall into one of two regions: a red sequence and a blue cloud. A smaller proportion of galaxies can be found between these two regions in what is called the green valley.

Can you guess which galaxies populate the Red Sequence and Blue Cloud? Click the image to see if you were right.

Galaxies are not uniformly distributed across the universe, but are concentrated along interlocking filaments. This filamentary pattern is called the Cosmic Web.

On smaller scales, collections of galaxies are placed into several different categories, depending on the number of galaxies that are present.

Group -- smallest collection of galaxies, with up to a few tens of members.

Cluster -- contains anywhere from a hundred to several thousand galaxies that are gravitationally bound. Most of the baryonic mass is actually found in the Intracluster Medium rather than the galaxies.

Supercluster -- collection of galaxy clusters. These structures are not gravitationally bound.

Wall -- collection of superclusters along a filament. The Sloan Great Wall can be seen in the middle of the top portion of the Cosmic Web image to the left.

The Milky Way is part of a group of galaxies known as the Local Group.

The two largest members are our own Milky Way and the Andromeda Galaxy. The Triangulum Galaxy (M33) is the last of the major galaxies; the remaining ones are much smaller dwarfs.

While most galaxies display redshift (meaning that they are moving away from us), the Andromeda galaxy has a blueshifted spectrum, meaning that we are actually on a collision course.

When the Milky Way and Andromeda meet 4 billion years from now, what impact do you think it will have on our galaxy? Record your predictions, then watch the video and see how well it matches the simulation.

While galaxies are generally moving away from each other in what is called the Hubble flow, astronomers have found that our motion is offset from this cosmological expansion. Something, it appears, is trying to pull us in. This gravitational anomaly has been called The Great Attractor, and represents the center of the Laniakea Supercluster.