When you look up at the night sky, especially during the summer, you'll see a faint band of stars spread across the entire middle of the sky. This band of stars is our galaxy, the Milky Way. The sun is just one of about 200 billion stars in the Milky Way, which is just one of the billions of galaxies in the universe. A galaxy is a large system of stars, gas (mostly hydrogen), dust and dark matter that orbits a common center and is bound together by gravity -- they've been described as "island universes." Galaxies come in many sizes and shapes. We know that they're very old and formed early in the evolution of the universe. Yet how they formed and evolved into their various shapes remains a mystery.
When astronomers look into the deepest reaches of the universe with powerful telescopes, they see myriads of galaxies. The galaxies are far away from one another and constantly moving away from one another as our universe expands. Furthermore, galaxies are organized into large clusters and other structures, which could have important implications for the overall structure, formation and fate of the universe.
Some galaxies, called active galaxies, emit huge amounts of energy in the form of radiation. They may have exotic structures such as supermassive black holes at their centers. Active galaxies represent an important area of astronomical research.
In this article, we'll find out how galaxies were discovered and what types exist, what they're made of, their internal structures, how they form and evolve, how they're distributed across the universe, and how active galaxies might emit so much energy.
Galaxy Types and Parts
Galaxies come in a variety of sizes and shapes. They can have as few as 10 million stars or as many as 10 trillion (the Milky Way has about 200 billion stars). In 1936, Edwin Hubble classified galaxy shapes in the Hubble Sequence.
- Elliptical: These have a faint, rounded shape, but they're devoid of gas and dust, with no visible bright stars or spiral patterns. They also don't have galactic disks, which we'll learn about below. Their classification varies from E0 (circular) to E7 (most elliptical). Elliptical galaxies probably comprise about 60 percent of the galaxies in the universe. They show wide variation in size -- most are small (about 1 percent the diameter of the Milky Way), but some are about five times larger than the diameter of the Milky Way.
- Spiral: The Milky Way is one of the larger spiral galaxies. They're bright and distinctly disk-shaped, with hot gas, dust and bright stars in the spiral arms. Because spiral galaxies are bright, they make up most of the visible galaxies, but they're thought to make up only about 20 percent of the galaxies in the universe. Spiral galaxies are subdivided into these categories: S0: Little gas and dust, with no bright spiral arms and few bright stars Normal Spiral: Obvious disk shape with bright centers and well-defined spiral arms. Sa galaxies have large nuclear bulges and tightly wound spiral arms, while Sc galaxies have small bulges and loosely wound arms. Barred Spiral: Obvious disk shape with elongated, bright centers and well-defined spiral arms. SBa galaxies have large nuclear bulges and tightly wound spiral arms, while SBc galaxies have small bulges and loosely wound arms (recent evidence suggests that the Milky Way is a SBc galaxy).
- Irregular: These are small, faint galaxies with large clouds of gas and dust, but no spiral arms or bright centers. Irregular galaxies contain a mixture of old and new stars and tend to be small, about 1 percent to 25 percent of the Milky Way's diameter.
What are the parts of a galaxy?
Spiral galaxies have the most complex structures. Here's a view of the Milky Way as it would appear from the outside.
- Galactic disk: Most of the Milky Way's more than 200 billion stars are located here. The disk itself is broken up into these parts: Nucleus: The center of the disk Bulge: The area around the nucleus, including the immediate areas above and below the plane of the disk Spiral arms: These extend outward from the center. Our solar system is located in one of the spiral arms of the Milky Way.
- Globular clusters: A few hundred of these are scattered above and below the disk. The stars here are much older than those in the galactic disk.
- Halo: A large, dim, region that surrounds the entire galaxy. It's made of hot gas and possibly dark matter.
All of these components orbit the nucleus and are held together by gravity. Because gravity depends upon mass, you might think that most of a galaxy's mass would lie in the galactic disk or near the center of the disk. However, by studying the rotation curves of the Milky Way and other galaxies, astronomers have concluded that most of the mass lies in the outer portions of the galaxy (like the halo), where there is little light given off from stars or gases.
On the next page, we'll take a walk through the history of galaxies.
History of Galaxies
Let's look at the history of galaxies in astronomy.
- The Greeks coined the term "galaxies kuklos" for "milky circle" when describing the Milky Way. The Milky Way was a faint band of light, but they had no idea what it was composed of.
- We've known for centuries that our solar system was located within the Milky Way because the Milky Way surrounds us. We can see it throughout the year in all parts of the sky, but it's brighter during the summer, when we're looking at the center of the galaxy. However, to astronomers in the 18th century and earlier, it wasn't clear that the Milky Way was a galaxy and not just a distribution of stars.
- In the late 18th century, astronomers William and Caroline Herschel mapped the distances to stars in many directions. They determined that the Milky Way was a disk-like cloud of stars with the sun near the center.
- In 1781, Charles Messier cataloged various nebulae (faint patches of light) throughout the sky and classified several of them as spiral nebulae.
- In the early 20th century, astronomer Harlow Shapely measured the distributions and locations of globular star clusters. He determined that the center of the Milky Way was 28,000 light years from Earth, near the constellations of Sagittarius and Scorpio, and that the center was a bulge, rather than a flat area.
- Shapely later argued that the spiral nebulae discovered by Messier were "island universes" or galaxies (retaining the Greek wording). However, another astronomer named Heber Curtis argued that spiral nebulae were merely part of the Milky Way. The debate raged on for years because astronomers needed larger, more powerful, telescopes to resolve the details.
- In 1924, Edwin Hubble settled the debate. He used a large telescope (100-inch diameter, larger than ones that were available to Shapely and Curtis) at Mount Wilson in California and resolved that the spiral nebulae had structure and stars called Cepheid variables, like those in the Milky Way. (These stars change their brightness regularly, and the luminosity is directly related to the period of their brightness cycle.) Hubble used the light curves of the Cepheid variables to measure their distances from Earth and found that they were much farther away than the known limits of the Milky Way. Therefore, these spiral nebulae were indeed other galaxies outside our own.
There are still many mysteries surrounding galaxy formation, but on the next page we'll explain some of the best theories about it.
We really don't know how various galaxies formed and took the many shapes that we see today. But we do have some ideas about their origins and evolution.
- Shortly after the big bang about 14 billion years ago, collapsing gas and dust clouds might have lead to the formation of galaxies.
- Interactions between galaxies, specifically collisions between galaxies, play an important role in their evolution.
Let's look at the period of galaxy formation.
Edwin Hubble's observations, and subsequent Hubble Law (which we'll explain later), led to the idea that the universe is expanding. We can estimate the age of the universe based on the rate of expansion. Because some galaxies are billions of light years away from us, we can discern that they formed fairly soon after the big bang (as you look deeper into space, you see further back in time). Most galaxies formed early, but data from NASA's Galaxy Explorer (GALEX) telescope indicate that some new galaxies have formed relatively recently -- within the past few billion years.
Most theories about the early universe make two assumptions:
- It was filled with hydrogen and helium.
- Some areas were slightly denser than others.
From these assumptions, astronomers believe that the denser areas slowed the expansion slightly, allowing gas to accumulate in small protogalactic clouds. In these clouds, gravity caused the gas and dust to collapse and form stars. These stars burned out quickly and became globular clusters, but gravity continued to collapse the clouds. As the clouds collapsed, they formed rotating disks. The rotating disks attracted more gas and dust with gravity and formed galactic disks. Inside the galactic disk, new stars formed. What remained on the outskirts of the original cloud were globular clusters and the halo composed of gas, dust and dark matter.
Two factors from this process might account for the differences between elliptical and spiral galaxies:
- Angular momentum (degree of spin) -- Protogalactic clouds with more angular momentum could spin faster and from spiral disks. Slow-spinning clouds could have formed elliptical galaxies.
- Cooling: High-density protogalactic clouds cooled faster, using up all the gas and dust in forming stars and leaving none for making a galactic disk (this is why elliptical galaxies don't have disks). Low-density protogalactic clouds cool more slowly, leaving gas and dust for disk formation (like in spiral galaxies).
Galaxies do not act alone. The distances between galaxies do seem large, but the diameters of galaxies are also large. Compared to stars, galaxies are relatively close to one another. They can interact and, more importantly, collide. When galaxies collide, they actually pass through one another -- the stars inside don't run into one another because of the enormous interstellar distances. But collisions do tend to distort a galaxy's shape. Computer models show that collisions between spiral galaxies tend to make elliptical ones (so, spiral galaxies probably haven't been involved in any collisions). Scientists estimate that as many as half of all galaxies have been involved in some sort of collision.
Gravitational interactions between colliding galaxies could cause several things:
- New waves of star formation
- Stellar collapses that form the black holes or supermassive black holes in active galaxies
So, do galaxies just float around in space or does some unseen force regulate their movement? And what happens when they run into each other? Find out on the next page.
Galaxies aren't randomly distributed throughout the universe -- they tend to exist in galactic clusters. The galaxies in these clusters are bound together gravitationally and influence one another.
- Rich clusters contain 1,000 or more galaxies. The Virgo supercluster, for example, includes more than 2,500 galaxies and is located about 55 million light years from Earth.
- Poor clusters contain less than 1,000 galaxies. The Milky Way and the Andromeda galaxy (M31) are the major members of the Local Group, which contains 50 galaxies.
When astronomers Margaret Geller and Emilio E. Falco plotted the positions of galaxies and galactic clusters in the universe, it became clear that galactic clusters and superclusters are not randomly distributed. They're actually clumped together in walls (long filaments) interspersed with voids, which gives the universe a cobweb-like structure.
The intergalactic medium -- the space between galaxies and clusters of galaxies -- is not entirely empty. We don't know the exact nature of the intergalactic medium, but it probably contains a relatively small density of gas. Most of the intergalactic medium is cold (about 2 degrees Kelvin), but recent X-ray observations suggest that some areas of it are hot (millions of degrees Kelvin) and rich in metals. One of the active areas of astronomical research today is directed at determining the nature of the intergalactic medium -- it may help us figure out exactly how the universe began and how galaxies form and evolve.
Let's look at one final property concerning galaxies and their distributions. For his measurements of galactic distances, Edwin Hubble studied the spectra of light that galaxies emit. In all cases, he noted that the spectra were Doppler-shifted to the red end of the spectrum. This indicates that the object is moving away from us. Hubble noticed that, no matter where he looked, galaxies were moving away from us. And the farther the galaxy, the faster it was moving away. In 1929, Hubble published a graph of this relationship, which has become known as Hubble's Law.
Mathematically, Hubble's Law states that the velocity of recession (V) is directly proportional to the galactic distance (d). The equation is V = Hd, where H is the Hubble constant, or constant of proportionality. The most current estimate of H is 70 kilometers per second per megaparsec. Hubble's Law is a major piece of evidence that the universe is expanding -- his work formed the basis of the big bang theory of the origin of the universe.
Some galaxies spew gases, emit intense light and have supermassive black holes at their centers. We'll learn about active galaxies next.
When you look at a normal galaxy, most of the light comes from the stars in visible wavelengths and is evenly distributed throughout the galaxy. However, if you observe some galaxies, you'll see intense light coming from their nuclei. And if you look at these same galaxies in the X-ray, ultraviolet, infrared and radio wavelengths, they appear to be giving off enormous amounts of energy, apparently from the nucleus. These are active galaxies, which represent a very small percentage of all galaxies. There are four classifications of active galaxy, but the type we observe may depend more upon our viewing angle than structural differences.
- Seyfert galaxies
- Radio galaxies
To explain active galaxies, scientists must be able to explain how they emit such large amounts of energy from such small areas of the galactic nuclei. The most accepted hypothesis is that at the center of each of these galaxies is a massive or supermassive black hole. Around the black hole is an accretion disk of rapidly spinning gas that's surrounded by a torus (a donut-shaped disk of gas and dust). As the material from the accretion disk falls into the area around the black hole (the event horizon), it heats to millions of degrees Kelvin and is accelerated outward in the jets.
Discovered by Carl Seyfert in 1943, these galaxies (2 percent of all spiral galaxies) have broad spectra indicating cores of hot, low-density ionized gas. The nuclei of these galaxies change brightness every few weeks, so we know that the objects in the center must be relatively small (about the size of a solar system). Using Doppler shifts, astronomers have noticed that velocities at the center of Seyfert galaxies are about 30 times greater than those of normal galaxies.
Radio galaxies are elliptical (0.01 percent of all galaxies are radio galaxies). Their nuclei emit jets of high-velocity gas (near the speed of light) above and below the galaxy -- the jets interact with magnetic fields and emit radio signals.
Quasars (quasi-stellar objects)
Quasars were discovered in the early 1960s. About 13,000 have been discovered, but there could be as many as 100,000 out there [source: A Review of the Universe]. They're billions of light years away from the Milky Way and are the most energetic objects in the universe. The extreme brightness of quasars can fluctuate over daylong periods, which indicates that the energy is coming from a very small area. Thousands of quasars have been found, and they're believed to be emanating from the cores of distant galaxies.
Blazars are a type of active galaxy -- about 1,000 have been cataloged [source: A Review of the Universe]. From our viewpoint, we are looking "head-on" at the jet emanating from the galaxy. Like quasars, their brightness can fluctuate rapidly -- sometimes in less than one day.
Take a look at the links on the next page for more information about galaxies.
- Is there a hole in the universe?
- How Stars Work
- How the Sun Works
- How the Earth Works
- How Dark Matter Works
- How Black Holes Work
- How Light Works
- How Telescopes Work
- How the Hubble Space Telescope Works
- How Lunar Liquid Mirror Telescopes Work
- If you were to move all the matter in the universe into one corner, how much space would it take up?
More Great Links
- A Map of the Milky Way. http://www.atlasoftheuniverse.com/milkyway.html
- A Review of the Universe - Structures, Evolutions, Observations, and Theories. http://universe-review.ca/F05-galaxy.htm
- A Teacher's Guide to the Universe. http://www.astro.princeton.edu/~clark/teachersguide.html.
- Bennett, J et al. "The Cosmic Perspective (third edition)." Pearson, 2004.
- Chandra X-ray Observatory - X-ray Astronomy Field Guide, Starburst Galaxies. http://chandra.harvard.edu/xray_sources/starburst.html
- Galaxy Classification and Evolution Lab. http://cosmos.phy.tufts.edu/~zirbel/laboratories/Galaxies.pdf
- Henry, J. Patrick et al. "The Evolution of Galaxy Clusters." Scientific American, December 1998. http://atropos.as.arizona.edu/aiz/teaching/a204/darkmat/SciAm98b.pdf
- NASA Imagine the Universe, "The Hidden Lives of Galaxies" book. http://imagine.gsfc.nasa.gov/docs/teachers/galaxies/imagine/titlepage.html
- NASA Imagine the Universe, Active Galaxies and Quasars. http://imagine.gsfc.nasa.gov/docs/science/know_l1/active_galaxies.html
- NASA Imagine the Universe, The Hidden Lives of Galaxies poster. http://imagine.gsfc.nasa.gov/docs/teachers/galaxies/imagine/poster.jpg
- NASA/JPL Galaxy Evolution Explorer (GALEX). http://www.galex.caltech.edu/
- NASA/JPL GALEX. Galaxies and UV. http://www.galex.caltech.edu/SCIENCE/science.html
- Science @NASA. What are Galaxies? How Do They Form and Evolve? http://science.hq.nasa.gov/universe/science/galaxies.html
- SEDS.org, Galaxies. http://www.seds.org/messier/galaxy.html
- Seeds, MA. "Stars & Galaxies (second edition)." Brooks/Cole, 2001.
- Stephens, S. "Galaxy Sorting Handout." http://www-tc.pbs.org/seeinginthedark/pdfs/galaxy_sorting_handout.pdf
- University of Washington Astronomy Department. Lecture" "Galaxies: Classification, Formation, and Evolution." http://www.astro.washington.edu/larson/Astro101/LecturesBennett/Galaxies/galaxies.html
- Windows to the Universe, Galaxies.http://www.windows.ucar.edu/cgi-bin/tour.cgi-link=/the_universe/Galaxy.html&sw=false&sn=1&d=/the_universe&edu=high&br=graphic&back=/pluto/pluto.html&cd=false&fr=f&tour=
- WMAP Cosmology 101: What is the Universe Made of? http://map.gsfc.nasa.gov/m_uni/uni_101matter.html