Galaxies are the cosmic metropolises in which stars are born, live, and ultimately die. There are as  many as a trillion galaxies in the observable universe, yet each is as unique as a snowflake.

Stars are seldom found alone in the universe. They are formed and live out their lives in massive associations known as galaxies. Typically they contain anywhere from tens of millions of stars on the small side to trillions of stars in the largest ones. 

Stars and dust structures in galaxies are especially easy to study in infrared light

Galaxies come in many shapes and sizes. Just like snowflakes, each is unique, but there are recurring patterns we see again and again. While these patterns were first noted in visible light, infrared observations have greatly advanced our understanding of the various possible structures.

One key advantage of studying galaxies in infrared light is that the brightness of stars is a little less biased. In visible light, more massive stars can give off so much light that star forming areas can look disproportionally more significant than they really are. In the infrared this imbalance is less extreme, so we get a more representative view of how stars are actually distributed.

Many galaxies are also filled with dust clouds that obscure our view of the overall structure. Since dust becomes increasingly transparant to infrared wavelengths of light, this also improves our view of galaxy structure.

At longer wavelengths of infrared light the dust itself begins to glow. This important component of the galaxy's material becomes easy to identify and study, helping us to better understand how gas and dust moves and gathers. This is fundamental to learning how new generations of stars are constantly being formed in galaxies. 

Spiral Galaxies

Messier 81 is a classic spiral galaxy

Spiral galaxies resemble pinwheels, due to the long arms of gas, dust, and stars that appear to be spiraling around the bright bulge at the galaxy's center. Most of the galaxy is spread into a very thin, wide disk that can be hundreds of thousands of light years across but only around a thousand light years thick.

The incredible spiral patterns that form in these disks are kind of like gravitational traffic jams, known as density waves. Stars and gas that are orbiting around the galaxy's center can hit these instabilities, lingering long enough to build up material that forms the spiral arms and the denser dust lanes found there. It is in these regions where stars tend to form in an ongoing process that constantly adds new stars to the disk. Infrared observations make it easy to pick out the warm dust associated with the largest areas of star formation.

A thicker bulge of stars is generally found at the center of a spiral galaxy, which is typically made up of older stars. Astronomers have concluded that all galaxies likely have an incredibly massive black hole at their very centers. A black hole is a point of infinite gravity, from which not even light can escape. Black holes at the centers of galaxies can be millions of times more massive than our Sun. While these are too small to observe directly, often their presence can heat up surrounding clouds of dust which will shine brightly in infrared light.

Our own Galaxy—the Milky Way—is a spiral galaxy. It’s part of a system of at least 30 galaxies known as the Local Group. The largest galaxies in the Local Group are all spiral galaxies.

Barred Galaxies

NGC 253 has a central bar of stars

Many spiral galaxies have an unusual feature running through their very centers—a straight bar-like distribution of stars. While these bars can be very obvious in visible light, galaxies are filled with things that can obscure our view of these bars like dust clouds and star-forming regions. Bars are easiest to see when viewing only the stars in near infrared light, as in this image of NGC 253.

The advent of infrared astronomy revealed that bars were far more common than astronomers had expected. By letting us peer through the dust to the base population of stars, stellar bars can be found in all sizes. Infrared studies of our Milky Way Galaxy reveal that even it appears to have a central bar.

Even though galactic bars appear quite straight, the stars within them follow complex ellipsoid orbits. It appears that bars can be the result of gravitational interactions between galaxies, often forming in the wake of a close encounter.

Elliptical Galaxies
Elliptical galaxies in the Fornax Cluster

Not all galaxies form flat, dusty disks. Another common type is known as an elliptical galaxy. As the name suggests, these objects are either circular or oval in shape. They often show little structure at all, beyond a greatly increasing density of stars at their very center. The smallest elliptical galaxies are less than one-tenth the size of the Milky Way while the largest can be hundreds of times bigger. 

Elliptical galaxies range in shape from circular to oval. The concentration of stars increases exponentially towards their cores, and drops off dramatically in their outer reaches. The stars in them are mostly old. Their orbits do not follow an orderly rotation like we see in the disks of spiral galaxies; instead they swarm around one another like bees around a beehive.

Infrared views of elliptical galaxies quickly reveal another key feature: they have very little dust or gas. The vivid dust structures seen so clearly in mid infrared observations of spiral galaxies are missing. This is why they have no young stars—there just isn't enough gas and dust around to make new stars.

It appears that elliptical galaxies are actually galactic cannibals. The largest ones seem to form from the mergers of smaller galaxies. These massive objects tend to pull in smaller galaxies over time, stripping them of gas and assimilating their stars.

Interacting Galaxies

Dust glows brightly in the infrared where stars are forming rapidly

The space between galaxies is vast, but they still run into one another on occasion. Interactions between galaxies are incredibly slow by human standards, playing out over hundreds of millions of years. When we look into the universe all we see are freeze-frame snapshots of different stages of these enormous events.

In some cases, galaxies may simply pass near one another. This can distort their shapes somewhat, and create features like bars and unusual spiral arm patterns that can persist long after the encounter is over.

When galaxies interact more directly, they can distort one another much more dramatically. During this process, the gas clouds within them collide and compress, and often are driven down into the very cores of the galaxies. These extensive, compressed clouds of gas become huge galactic maternity wards called starbursts. Stars can form at incredible rates within these starbursts, sometimes at rates thousands of times higher than normal galaxies. This starburst activity tends to occur within obscuring coccoons of dust that hide the activity in visible light. However, these huge dust complexes will be heated by the newly-born stars and will glow brightly in the infrared.

Gas and dust driven down into the cores of interacting galaxies can also supply fuel for the supermassive black holes at their centers. As material spirals down into the black hole it heats up and can emit light ranging from x-rays to infrared to radio. Astronomers call these active galactic nuclei, or AGNs. 

Merging galaxies can end up forming elliptical galaxies. In fact, the largest galaxies in clusters are usually giant ellipticals near their centers that, over time, continue to grow ever larger as they cannibalize smaller galaxies.

Published: 09 August, 2013

Fun Fact

Stars are so far apart from each other, that if we were to put two grains of sand into a cathedral, that would be about the correct scale to represent the sizes and distances of stars in our part of the Universe.