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M51: The Whirlpool Galaxy
Credit: Todd Boroson (NOAO), AURA, NOAO, NSF (via Astronomy Picture of the Day)
M51: The Whirlpool Galaxy. The Whirlpool Galaxy is a classic Sc type spiral galaxy visible with binoculars in the constellation of Canes Venatici. It is 23 million light years distant and is one of the brightest and most picturesque galaxies on the sky. The smaller galaxy appearing below and to the left is behind M51 and is thought to be responsible for the spiral structure of M51.


On a dark night, we can often see a band of light stretching across the sky. This band is the Milky Way galaxy -- a gigantic collection of stars, gas and dust. Far beyond the Milky Way, there are billions of other galaxies -- some similar to our own and some very different -- scattered throughout space to the very limits of the observable universe.

Types of Galaxies

Astronomers classify galaxies into three major categories. Spiral galaxies look like flat disks with bulges in their centers and beautiful spiral arms. Elliptical galaxies are redder, more rounded, and often longer in one direction than in the other, like a football. Galaxies that appear neither disk-like nor rounded are classified as irregular galaxies.

Spiral Galaxies

Spiral galaxies usually consist of three components: a flat disk, an ellipsoidally formed bulge and a halo. The disk contains a lot of interstellar gas and dust, and most of the stars in the galaxy. The gas, dust and stars in the disk rotate in the same direction around the galactic center at hundreds of kilometres per second and are often arranged in striking spiral patterns. The bulge is located at the centre of the disk and consists of an older stellar population with little interstellar matter. The near-spherical halo surrounds the disk, and is thought to contain copious amounts of dark matter: matter that acts gravitationally like "normal" matter but that can't be seen! Astronomers infer the presence of this dark matter by the motions of stars and gas in the disk of the galaxy, as well as older stellar populations in the halo like globular clusters. The young stars in the disk are classified as stellar population I, and the old bulge and halo stars as population II.

Astronomers classify spiral galaxies according to their appearance by using the Hubble scheme. Those with pronounced bar structures in their centers are called "barred spirals" and are classified "SB" (examples are given in brackets). Galaxies with conspicuous bulges and tightly wound spiral arms are called "Sa" (Sombrero galaxy) or "SBa" (NGC 3185). Galaxies with prominent bulges and pronounced spiral arms are classified as "Sb" (M31, M81) or "SBb" (M95, NGC 4725). Other spirals with loose spiral arms and a small bulge are classified as "Sc" (M33, M74, M100) or "SBc" (M83, M109).

There are some galaxies like M84, M85 and NGC 5866 that are disk galaxies without any spiral structure. These galaxies are called "S0" or lenticular galaxies. Though the origin of lenticular galaxies is still debated the most plausible explanation to date is that the gas and stars that would reside in the galaxy disk have been stripped by interactions with the hot gas in clusters and groups of galaxies. From their appearance and their stellar contents, they look more like ellipticals rather than spirals and have often been misclassified due to this fact. For instance, misclassification has occured for both the Messier object examples listed above.

Elliptical Galaxies

Elliptical galaxies are ellipsoidal agglomerations of stars, which usually do not contain much interstellar matter. Photometric studies of elliptical galaxies suggest that they are triaxial (all the three axes of the ellipsoid are of different sizes). Unlike spiral galaxies, ellipticals have little or no global angular momentum, so that different stars orbit the center in different directions and there is no pattern of orderly rotation. Normally, elliptical galaxies contain very little or no interstellar gas and dust and consist of old population II stars only. Elliptical galaxies are classified according to the Hubble scheme into classes "E0" to "E7", in increasing order of ellipticity. Thus E0 galaxies appear round like M89 while E6 galaxies like M110 and NGC 3377 are almost cigar shaped.

The largest galaxies in the universe are giant elliptical galaxies. They contain a trillion stars or more and span as much as two million light years - about 20 times the width of the Milky Way. These giant ellipticals are often found in the hearts of galaxy clusters. For example the giant elliptical galaxy M87 is found in the heart of the Virgo Cluster.

Elliptical galaxies also constitute some of the smallest galaxies in the universe. These galaxies are called dwarf elliptical galaxies and dwarf spheroids. Relative to normal ellipticals they are very faint, and are often found in galaxy clusters or near large spiral galaxies. For instance, there are 9 dwarf spheroids like Leo I which are satellites of our Milky Way galaxy.

Irregular Galaxies

A small percentage of the large galaxies we see nearby fall into neither of the two major categories. This irregular class of galaxies is a miscellaneous class, comprising small galaxies with no identifiable form like the Magellanic clouds (the Large Magellanic Cloud and Small Magellanic Cloud are two satellite galaxies of the Milky Way) and "peculiar" galaxies that appear to be in disarray like NGC 1313. There is no discernable disk in these systems, although they often have copious amounts of gas as well as high rates of star formation. Irregular galaxies are often found to be gravitationally interacting with galaxies nearby, which often accounts for their ragged appearance.

Galaxy Evolution, Interactions and Mergers

Galaxies were once thought of as "island universes" evolving slowly in complete isolation. Today we think that just the opposite is true: gravitational interactions of galaxies with each other, and even the coalescence of two galaxies into one, or mergers, are commonplace in the Universe! We see striking examples of merging galaxies in the local Universe, such as NGC 2207 and its companion IC 2163 and the Mice. These interacting systems often sport long tidal tails of gas and stars, a result of the mutual gravitational pull of each system. During the merger, the gas in each galaxy disk flow to the galaxy centers, becomes very dense, and forms stars at an alarming rate. This inflowing material also feeds the supermassive black holes at the galaxy centres, which heat up the infalling material to millions of degrees and eject some of it along powerful jets. All of these mechanisms make merging galaxies very bright, such as Arp 220, are among the most luminous objects in the local Universe.

What do mergers leave behind? Both observations of actual systems and simulations of merging galaxies on a computer suggest that merging spirals create elliptical galaxies. The gas in the progenitor spiral galaxies is used up in making stars which subsequently eject heavier elements and dust from the system, and the collision is forceful enough to randomize the orbits of the stars in the incoming disks into a spheroidal shape. Different types of galaxies are therefore intimately linked by galaxy evolution and mergers: spiral galaxies evolve into elliptical galaxies, and irregular galaxies are galaxies in the process of becoming one or the other!

Galaxy coalescence doesn't only happen between two large galaxies: in fact, most large galaxies are constantly swallowing up the smaller, dwarf galaxies that surround them. Our Milky Way is no exception to this rule: it is currently ripping appart our nearest neighbor, the Sagittarius Dwarf!

The Ask an Astronomer team's favorite links about Galaxies:

  • NED: Nasa Extragalactic Database. Information on 4.6 million Extragalactic objects, this site is often referred to by professional astronomers.
  • The Messier Catalogue: The Messier Catalogue was a catalogue of objects which looked a bit like comets (ie they were fuzzy) to Messier. Turns out that lots of them are galaxies (eg. M31, M51). This is a site maintained by SEDS (Students for the Exploration and Development of Space.
  • The Interactive NGC Catalogue Online: The NGC (New General Catalogue) is another catalogue of galaxies. This site is also maintained by SEDS.
  • Chris Mihos's Java Applets: a dynamical Astronomy Javalab; you can play with simulations of various dynamical problems in Astronomy, including merging galaxies, cannibalism of galaxies, a spiral galaxy rotation curve and how galaxies behave in clusters.

Previously asked questions about Galaxies:

General questions:

Formation and evolution:


Classifying galaxies:

Groups and Clusters:

Observing galaxies:

How to ask a question:

If you have a question about Galaxies which isn't answered above, submit it here. If you have a question about another area of astronomy, find the topic you're interested in from the archive on our site menu, or go here for help.

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