Galaxy Structure and Interaction

When Charles Messier first catalogued his 110 deep sky objects so comet hunters would not be confused by objects that are relatively fixed in space, his telescope was not sufficiently large enough to discern the shape or even nature of some of the "nebulae" that he saw. With the advent of better telescope optics and larger mirrors, these nebulae soon began to show up as a complex array of objects of different appearances. V.M. Slipher and Edwin Hubble later demonstrated that these "nebulae" were not part of our local collection of stars, but actually large collections of stars existing in locations far from the Milky Way. Below is a beautiful image of M100, taken by the NOAO telescopes in Chile. This page hopes to help astronomy students better understand the shapes of galaxies as well as how galaxies interact with each other.

Definition and General Comments

A galaxy is a collect of stars, gas and dust bound together by their common gravitational pull. Galaxies range from 10,000 to 200,000 light-years in size and between 109 and 1014 solar luminosities in brightness.

The discovery of `nebula', fuzzy objects in the sky that were not planets, comets or stars, is attributed to Charles Messier in the late 1700's. His collection of 103 objects is the first galaxy catalog. Herschel (1792-1871) used a large reflecting telescope to produce the first General Catalog of galaxies. Below is a photographic collection of the Messier Objects, beginning with M1 (The Crab Nebula) in the upper left corner and proceeding left to right row by row.

Before photographic plates, galaxies were drawn by hand by the astronomer, but now we have gained a great amount of detailed information from telescopic images of near and distant galaxies. Galaxies have certain features in common. Gravity holds the billions of stars together, and the densest region is in the center, called a core or bulge. Some galaxies have spiral or pinwheel arms. All galaxies have a faint outer region or envelope and a mysterious dark matter halo.

The contents of galaxies vary from galaxy type to galaxy type, and with time, since galaxies go through an evolution of change in its lifetime. The most recent information on Quasars indicate that the core of galaxies are initially occupied by massive Black Holes that engulf stars at tremendous rates early in their lives, and slow that consumption as the galaxy ages. We find some galactic types that age in a relatively consistent manner and slowly die with every star winking out of visible existence. Other galactic types regenerate stars from stellar remains and continue to recycle stars.

Almost all galaxy types can be found in groups or clusters. Many clusters of galaxies have a large, supergiant galaxy at its center which has grown by cannibalizing its neighbors. Our solar system is located in outer regions of a spiral galaxy we call the Milky Way. The nearest major neighbor galaxy is Andromeda Galaxy (M31), seen below. There are smaller galaxies in our local neighborhood, such that the Local Group holds about 20 total galaxies.

Above is a 3D plot of most of the Local Group of galaxies, the population of galaxies within 1000 kpc if the Milky Way. Clustering of dwarf satellite galaxies around the great Milky Way and Andromeda spirals can be seen. Remember, LMC stands for Large Magellanic Cloud and SMC stands for Small Magellanic Cloud.

Hubble sequence

Galaxies basically can be organized into three classes: Spirals, Elliptical, and Irregular. The names were given by Edwin Hubble, who arranged the galactic forms in a "tuning fork" diagram. Almost all current systems of galaxy classification are outgrowths of the initial scheme proposed by American astronomer Edwin Hubble in 1926. In Hubble's scheme, which is based on the optical appearance of galaxy images on photographic plates, galaxies are divided into three general classes, seen in the image below. There are two groups of Spirals, and you will learn about their differentiation later.

Elliptical galaxies

Galaxies of this class have smoothly varying brightnesses, steadily decreasing outward from the center. They appear elliptical in shape, with lines of equal brightness made up of concentric and similar ellipses. These galaxies are nearly all of the same color: they are somewhat redder than the Sun. Ellipticals are also devoid of gas or dust and contain just old stars.

NGC 4881 (HST image)

All ellipticals look alike, and NGC 4881 is a good example (NGC stands for New General Catalog). Notice how smooth and red NGC 4881 looks compared to the blue spirals to the right.

M32 (NOAO image)

A few ellipticals, like M32, are close enough to us that we can resolve the individual stars within them. Such a situation exists with M32, a companion to the Andromeda Galaxy.

Spiral galaxies

These galaxies are conspicuous for their spiral-shaped arms, which emanate from or near the nucleus and gradually wind outward to the edge. There are usually two opposing arms arranged symmetrically around the center. The nucleus of a spiral galaxy is a sharp-peaked area of smooth texture, which can be quite small or, in some cases, can make up the bulk of the galaxy. The arms are embedded in a thin disk of stars. Both the arms and the disk of a spiral system are blue in color, whereas its central areas are red like an elliptical galaxy.

M100 (NOAO image)

Notice in the above picture of M100, that the center of the spiral is red/yellow and the arms are blue. Hotter, younger stars are blue, older, cooler stars are red. Thus, the center of a spiral is made of old stars, with young stars in the arms formed recently out of gas and dust.

NGC 4639 (HST image)

The bulge of NGC 4639, seen above, is quite distinct from the younger, bluer disk regions.

NGC 1365 (ESO image)

NGC 1365, seen above, is a barred spiral galaxy. Note the distinct dark lanes of obscuring dust in the bar pointing towards the bulge. A close-up of the spiral arms shows blue nebula, sites of current star formation.

NGC 253 core and outer disk (ESO image)

NGC 253, seen above, is a typical Sa type galaxy with very tight spiral arms. As spiral galaxies are seen edge-on the large amount of gas and dust is visible as dark lanes and filaments crossing in front of the bulge regions.

Irregular galaxies

Most representatives of this class consist of grainy, highly irregular assemblages of luminous areas. They have no noticeable symmetry nor obvious central nucleus, and they are generally bluer in color than are the arms and disks of spiral galaxies.

NGC 2363 (HST image)

NGC 2363 is an example of a nearby irregular galaxy. There is no well defined shape to the galaxy, nor are there spiral arms. A close-up of the bright region on the east side shows a cluster of new stars embedded in the red glow of ionized hydrogen gas.

Both the Large and Small Magellanic Clouds (Anglo-Australian Images)

Above is a nice set of images of the close neighbor galaxies of the Milky Way. These irregular galaxies are called the Large and Small Magellanic Clouds, and they are visible from the southern sky. In the image set above, the LMC is in the center, and the SMC to the right. If you want to see a beautiful website that details the appearance of the LMC in various wavelengths, click on Large Magellanic Cloud , or else go to LMC or SMC to see the SEDS site about our neighbors.

Galaxy Colors

The various colors in a galaxy (red bulge, blue disks) is due to the types of stars found in those galaxy regions, called its stellar population. Big, massive stars burn their hydrogen fuel, by thermonuclear fusion, extremely fast. Thus, they are bright and hot = blue. Low mass stars, although more numerous, are cool in surface temperature (= red) and much fainter. All this is displayed in a Hertzsprung-Russell Diagram of the young star cluster.

The hot blue stars use their core fuel much faster than the fainter, cooler red stars. Therefore, a young stellar population has a mean color that is blue (the sum of the light from all the stars in the stellar population) since most of the light is coming from the hot stars. An old stellar population is red, since all the hot stars have died off (turned into red giant stars) leaving the faint cool stars.

The bottom line is that the red regions of a galaxy are old, with no hot stars. The blue portions of a galaxy are young, meaning the stellar population that dominates this region is newly formed.

Star Formation

The one feature that correlates with the shape, appearance and color of a galaxy is the amount of current star formation. Stars form when giant clouds of hydrogen gas and dust collapse under their own gravity. As the cloud collapses it fragments into many smaller pieces, each section continues to collapse until thermonuclear fusion begins.

Initial conditions for a galaxy determines its rate of star formation. For example, elliptical galaxies collapse early and form stars quickly. The gas is used up in its early years and today has the appearance of a smooth, red object with no current star formation.

Spirals, on the other hand, form slower, with lower rates of star formation. The gas that `fuels' star formation is used slower and, thus, there is plenty around today to continue to form stars within the spiral arms.


You have done well so far, and it is now time to look at some really exciting images of galaxies taken by the Hubble Space Telescope. Please go forward to the HST Galaxy Image page. You could go back to the Milky Way, or go to the Syllabus .

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