What Do Galaxies Look Like?

This document contains links to several sites with nice pictures of galaxies so it may take a while to bring up, depending on network activity and the speed of your connection. Later I'll make smaller versions of these pictures which you can click on to bring up the full-size picture when I figure out how to reduce them.

The Milky Way Galaxy is a spiral galaxy. If we could view it from the outside, it would probably look like the two galaxies below (NGC 891 and NGC 5033). NGC 891 is an edge-on spiral galaxy (inclined by 84 degrees) in the constellation Andromeda that has many similarities to our Galaxy and so is often considered a ``twin''. You can see the dust lanes in the disk. The V-band CCD image was taken with the Lowell Observatory 1.1-meter Hall telescope. The high dynamic range of the image was compressed for display using a nonlinear intensity mapping. A focal reducer gave a field of view 9 arcminutes on a side for this image.

NGC 5033 is another spiral galaxy sometimes considered a twin of the Milky Way. It is inclined by only 64 degrees so we can see more of the spiral arm structure. This picture is from the Digital Sky Survey of STSCI and is 15 arcminutes on a side.

The Andromeda Galaxy (M 31) is the closest large spiral to us (at only 2 million light years) and is visible to the naked eye in the constellation Andromeda. It is 1.5-2 times more massive than the Milky Way.

GIF picture

Below is a picture of its nucleus (from the Hubble Space Telescope) that probably has a massive black hole of mass up to 70 million solar masses.

JPEG version. HST text explanation. I have a brief summary of massive black holes in the Local Group that is a bit technical.

Another spiral galaxy close to us is M 33 in the constellation Triangulum. It is a small galaxy that lies close to M 31.

Messier 100 is a beautiful face-on galaxy about 56 million light years away in the Virgo cluster and was the first target of the HST project to determine the Hubble constant that sets the scale (and age!) for the universe. The first picture below was taken with the Anglo-Australian Telescope (AAT).

The next picture was taken with the Hubble Space Telescope (HST). Since HST was able to resolve individual stars in M 100 (even at its great distance), we were able to measure distances to individual Cepheid variables in the galaxy and get a much more accurate value for the distance to M 100 than we ever could before. There is some question as to where exactly M 100 lies in the Virgo cluster, so Cepheids in other Virgo cluster galaxies will be observed to get a convincing distance to the Virgo cluster.

GIF picture.
Enough of spirals! Now for some ellipticals. Messier 87 is the huge elliptical at the center of the Virgo cluster. It probably got so big by gobbling up other cluster members that wandered too close. It is nearly circular so is of type EO. The following picture was taken with the AAT.

The galaxy holds the best evidence for a supermassive black hole in its nucleus. The Hubble Space Telescope observed a spiral disk at the core of M 87 and derived a mass of 3 billion solar masses from the Keplerian velocity structure of the disk material. The region contains only a small fraction of the number of stars that would be needed to create the observed gravity field (so a black hole is the only thing left that it could be). The gas disk is in the inset in the picture below. The picture also shows the central part of the galaxy with the jet of material shooting out perpendicular to the nucleus disk.

GIF picture and text explanation. Here's a pointer to a plot of the data of large velocities in the disk and a text explanation.

Most galaxies are small. Messier 32 is a dwarf elliptical companion of M 31 that is bright round blob just above M 31's disk in the picture above. It is a little closer to us than M 31 and probably has a 3 million solar mass black hole in its nucleus. It is slightly flattened so is of type E2. NGC 205 is another dwarf elliptical companion of M 31 and is below M 31's disk in the picture above. Leo I is a dwarf spheroidal in the constellation Leo. As you can see in the picture below, it is not particularly gorgeous, but then again most galaxies are not spirals!

Other galaxies are pretty ratty looking (look like a mess) and are called irregulars (astronomers are usually not that imaginative with their names). NGC 6822 (also called ``Barnard's Galaxy'') is a dwarf irregular.

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last updated 17 Nov 95