Galaxies also contain vast quantities of "dark matter" -- matter that produces no detectable light or other form of energy, but that reveals its presence through its gravitational pull on the visible stars and gas. In the Milky Way, dark matter appears to account for more than 90 percent of the galaxy's total mass. Most of the dark matter resides in a "halo" that surrounds the galaxy's bright disk and extends hundreds of thousands of light-years into space.

Galaxies are sprinkled throughout the universe. Only three galaxies outside the Milky Way are easily visible to the unaided eye -- the great galaxy in Andromeda (the Andromeda Nebula) and the Large and Small Magellanic Clouds. These are some of our nearest galactic neighbors. The farthest galaxies ever observed are more than 10 billion light-years away. These galaxies formed soon after the universe itself was born.

In theory, if the universe lasts long enough, the galaxies will die. Their stars will burn out. Some of the stars will drift away, but some will fall into giant "black holes" that lurk in the hearts of most galaxies. Eventually, all galaxies will disappear from sight.

Galaxies can be classified in several ways. The most common is a system developed by Edwin Hubble, which is based on the shapes of galaxies.
The most beautiful galaxies are called spirals. The Milky Way is a spiral, and so is the Andromeda galaxy (M31).

The most beautiful galaxies are spirals, like M83, with delicate spiral arms wrapping around a bright bulge of stars in the middle.

NGC 1365 is an example of a 'barred' spiral, with a thick bar of stars extending from the core.

The largest galaxies are giant ellipticals, like NGC 1416. Thick lanes of dark dust surround the galaxy's dense core.

NGC 1569 is a dwarf irregular galaxy that gave birth to thousands of new stars about 25 million years ago.

Spirals are flat disks of stars with bright bulges in their centers. Spiral arms wrap around these bulges. Spiral arms probably form as the result of waves that sweep through the galactic disk. Like the waves on the ocean, these "density waves" don't carry material with them. Instead, they influence matter as they pass by. In the case of galaxies, they squeeze clouds of interstellar gas, triggering new star formation. Some newborn stars are massive, hot, and bright, so they make the spiral arms appear bright. These massive stars are blue or white, so the spiral arms look blue-white, too. The gaps between the arms contain older stars, which are not as bright. In some spirals, a wave organizes the stars in the center into a bar. The arms of barred galaxies spiral outward from the ends of the bar. The Milky Way falls into this class of spirals.

A second class of galaxy is the ellipticals. Like spirals, they are named for their shapes: they look like fat, fuzzy footballs. Instead of spreading out into a thin disk, as they do in spirals, the stars in ellipticals wrap completely around the galaxy's heart in all directions. The largest galaxies in the universe are giant ellipticals. They can contain a trillion stars or more, and span as much as one million light-years -- about 10 times the diameter of the Milky Way. Like many large spirals, most of them appear to contain "supermassive" black holes at their hearts -- star-gobbling monsters that are as much as three billion times as massive as the Sun.

The final class of galaxies contains a hodge-podge of shapes -- anything that looks neither spiral nor elliptical. These are the irregulars. These galaxies have no identifiable form. Their stars, gas, and dust spread randomly. These are the smallest galaxies, and may contain as few as one million stars. They may be like the "building blocks" that came together to form the first large galaxies. Many small irregular galaxies orbit the Milky Way.

Astronomers also categorize galaxies by how much energy they produce in their cores. A particular class is called "active" galaxies, because they produce much more energy than "normal" galaxies. The most powerful active galaxies are quasars. They are among the brightest and most distant objects in the universe. A quasar may emit more energy than an entire galaxy of stars from a region no bigger than our own solar system. Astronomers believe these objects contain supermassive black holes at their hearts, which are encircled by disks of gas. A black hole is an object that is squeezed together so tightly that it has extremely strong gravity. Its gravity is so powerful that nothing can escape from it -- not even light. As gas spirals toward the black hole, it is heated to billions of degrees, so it emits enormous amounts of energy, and the quasar shines brightly.

One of the greatest challenges facing astronomers today is understanding how galaxies form. Observations by Hubble Space Telescope and ground-based instruments show that the first galaxies took shape as little as one billion years after the Big Bang, which probably took place about 13 billion to 14 billion years ago. There are two leading theories to explain how the first galaxies formed. The truth may involve a bit of both ideas.

One says that galaxies were born when vast clouds of gas and dust collapsed under their own gravitational pull, allowing stars to form.

The other, which has gained strength in recent years, says the young universe contained many small "lumps" of matter, which clumped together to form galaxies. Hubble Space Telescope has photographed many such lumps, which may be the precursors to modern galaxies. According to this theory, most of the early large galaxies were spirals. But over time, many spirals merged to form ellipticals.

The galaxy-formation process has not stopped. Our universe continues to evolve. Small galaxies are frequently gobbled up by larger ones. The Milky Way may contain the remains of several smaller galaxies that it has swallowed during its long lifetime. The Milky Way is digesting at least two small galaxies even now, and may pull in others over the next few billion years. Galaxy mergers happen fairly often. A large portion of the bright galaxies that we see today may have formed from the mergers of two or more smaller galaxies. Mergers are common because the universe is crowded on the galactic distance scale. The disk of the Milky Way, for example, spans about 100,000 light-years; the nearest major galaxy, the great spiral in Andromeda, which is a little bigger than the Milky Way, is about 2.5 million light-years away. That means the distance between these two galaxies is only about 25 times greater than the sizes of the galaxies themselves. That doesn't leave a lot of "elbow room" for galaxies. Galaxies are very massive, too, so their gravity is strong. When you crowd them together, the attraction can be so strong that two galaxies latch on to each other and don't let go. Eventually they merge, forming a single giant city of stars. The largest galaxies are giant ellipticals. They look like eggs or footballs. They can be 10 times the Milky Way's size and contain more than a trillion stars. Such galaxies probably formed when two or more spirals, like the Milky Way, merged to form a single galaxy. One bit of evidence supporting the merger theory is the large number of ellipticals in dense clusters of galaxies, where mergers must be common. Two giant ellipticals dominate the center of the densely packed Coma Cluster, for example. And the heart of the Virgo cluster contains three giant ellipticals that each span almost one million light-years. Mergers can take anywhere from a few hundred million to a few billion years to complete. They can trigger intense bursts of new star formation, and even create gigantic black holes.

On the human time scale, the Milky Way galaxy is eternal. The starry Milky Way shined down on the first humans and will continue to shine on Earth for billions of years more. Eventually, though, the stars of the Milky Way will burn out. The galaxy will remain, but it will no longer look like a bright pinwheel. In fact, it won't look like much of anything at all -- and neither will the other galaxies sprinkled through the universe. Astronomers aren't sure exactly how the universe will end. But current observations suggest that "dark energy" is causing the universe to expand faster, driving galaxies farther apart. If that's correct, then galaxies will gradually fade from sight as their stars burn out.

The nuclear fires of stars like the Sun will be extinguished in a few billion years. The Sun itself will live for another five billion years or more. The smallest stars consume their fuel much more slowly than bigger stars like the Sun, so they will live for a few trillion years -- thousands of times longer than the current age of the universe. After the stars wink out, all that will remain of them will be crushed corpses. Gravity itself may weaken, so these stellar remnants may lose their planets.

Once every million billion years or so, a small new star may ignite as the result of a collision between two "brown dwarfs" -- objects that were not heavy enough to become stars on their own. Later still, the Milky Way and all the other galaxies will lose their grip on their dead stars. Some of the stars will sail off into space, while others will fall into the giant black holes that will dominate the hearts of all the galaxies. Eventually, the dead stars will disintegrate, and black holes as heavy as galaxies may evaporate in flashes of energy. The universe will contain only a thin mixture of radiation and subatomic particles swimming through the darkness.

Yet even then, long after the galactic cities of stars have disbanded and disappeared, the universe will go on.

The Local Group

The Milky Way galaxy is part of a larger cosmic neighborhood -- a collection of more than 35 galaxies known as the Local Group. These galaxies move through space as a single unit, bound together by their mutual gravitational pull.
The number of galaxies in the Local Group is uncertain because astronomers keep finding new residents of this galactic neighborhood. They found a new one in 1997, about three million light-years from Earth. This new galaxy is a pipsqueak: It contains only about one million stars, compared to hundreds of billions in the Milky Way.

The largest member of the Local Group is the Andromeda galaxy, M31. It lies more than two million light-years away. In other words, when we see M31, we're actually seeing it as it appeared more than two million years ago.

Andromeda is a wonderful laboratory for astronomers who study everything from gas clouds to black holes. It looms so large in our sky that big telescopes reveal many of its individual stars. Yet it's far enough that we can see the entire galaxy, allowing astronomers to study its overall structure and composition.

The Milky Way is the Local Group's second-largest galaxy. It has a large collection of satellite galaxies -- perhaps a dozen or more. The largest and best known are the Large and Small Magellanic Clouds, which are visible from the southern hemisphere. Both are less than 200,000 light-years away.

The Milky Way's closest satellite is a small galaxy in the constellation Sagittarius. It lies "behind" the center of the Milky Way as seen from Earth, so it is hard to see. It was only discovered in 1994. The Milky Way is devouring this galaxy, so someday it will lose its identity as its stars join the larger Milky Way.

Models of how galaxies behave suggest that the Milky Way has probably swallowed many smaller galaxies during its lifetime. And several billion years from now, the Milky Way and Andromeda may merge to form a galactic giant.

Number of Galaxies
35+

Largest Galaxies

Milky Way
Type: Spiral (perhaps barred)
Diameter: 100,000 light-years
Stars: 200-400 billion
Satellite Galaxies: 12-15

Andromeda (M31)
Type: Spiral
Diameter: 125,000 light-years
Stars: 300 billion
Satellite Galaxies: 12-16
Distance: 2.5 million light-years

M31

The M31 Andromeda galaxy, is the closest large galaxy in our celestial neighborhood, at a distance of just 2.2 million light-years. The galaxy's spiral arms are clearly visible in this telescopic view, as are red star clusters around the galaxy's edge. M31 is barely visible to the unaided eye on autumn evenings.

The Milky Way
In summer, in a clear place with dark night skies, an irregular glowing band arcs high overhead. The ancients likened it to a stripe of milk spilled across the sky.
Today, we know that this band of light is the combined glow of countless millions of stars in the flat disk of the Milky Way galaxy, our galactic home. The Milky Way is a spiral galaxy, so seen from above it would look like a pinwheel. The Milky Way consists of a bulge of stars in the core, probably a thick bar of stars flanking the core, and bright spiral arms wrapping around the core. This pinwheel is about 100,000 light-years in diameter, but only about 2,000 light-years thick, so it forms a thin disk. Our solar system is about 27,000 light-years from its center.

The vast Milky Way contains a myriad of features, including star clusters, stellar nurseries, and a jumbled region of stars, gas, and magnetic fields in the core.

In the 18th century, William Herschel suggested that the Sun resides in a rotating disk of stars. Since the band of the Milky Way is roughly equally bright all around the sky, Herschel suggested that the Sun is in the middle of it.

What Herschel didn't know is that our galaxy is full of dust. Elements like silicon, carbon, and iron are forged in the cores of stars and released into space late in the stars' lives. The dust obscures our view. The problem is like being dropped into a forest on a foggy day. You can see many trees in all directions, but you can't see very far in any direction.

Fortunately, the dust is concentrated in the Milky Way's disk. The disk is surrounded by a roughly spherical halo that is relatively free of dust. But the halo contains about 200 globular star clusters, which are ball-shaped groups of hundreds of thousands of stars.

In 1917, Harlow Shapley noted that most of the globular clusters appeared on one side of the sky. Based on this, he proposed that the Sun is near the edge of the galaxy's disk. He reasoned that the spherical halo of globulars is centered on the core of the Milky Way's disk. This meant that, from our off-centered vantage point, we see more globular clusters on one side of the sky.

In spite of this breakthrough, the dimensions of the Milky Way and any detailed idea of its structure were poorly known. Most modern information came after the beginnings of radio and infrared astronomy. The reason is simple. Like visible light, radio and infrared are forms of energy, but with longer wavelengths. These wavelengths pass through the dust in the Milky Way, so they can reach radio and infrared telescopes on Earth. Recall that on a foggy day, your car headlights have a tough time penetrating the fog, but your car's radio works just fine.

Radio astronomy provided the first new key to studying the disk of the Milky Way. Everything in the galaxy orbits the center of the disk. Objects nearer the center orbit faster than objects orbiting farther out. So by measuring the motions of many clouds of gas and dust, radio astronomers gave us our first murky insight into the structure of the galaxy's disk.

Today, we know that the disk contains between 200 billion and 400 billion stars. A black hole perhaps four million times as massive as the Sun sits in the middle of the Milky Way. It's surrounded by giant stars, clouds of dust, and magnetic fields that make the core a dynamic place.

The galaxy's main constituent is invisible "dark matter" that permeates the halo, extending several hundred thousand light-years in all directions. This material reveals its presence only through its gravitational pull on the Milky Way's visible stars and gas clouds. Dark matter may account for 90 percent of the Milky Way's total mass.