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Galaxies are systems of billions of stars, held together by their own gravity, usually mingled with dust and gas. There are several types, and it seems that galaxies are transformed by collisions within galaxy clusters.

our place in the universe

The Milky Way is home to our Sun and all the other stars that we can see in the sky. It is a vast spiral system, and our celestial neighborhood sits roughly two-thirds of the way from the center, on the outer edge of a spiral arm. Because the galaxy is essentially disk-shaped, there are more stars in our line of sight as we look along the plane solar system globular cluster in spherical halo solar system globular cluster in spherical halo

dark halo galactic disk dark halo galactic disk our home galaxy

Seen edge-on, the Milky Way galaxy is a thin disk, with the hub forming a huge bulge of old red and yellow stars at the center. The spiral arms and disk contain younger, bluer stars and gas clouds.

the milky way

This long-exposure photograph of the Milky Way reveals the dense star clouds toward its center. The dark areas are created by intervening dust clouds that obscure the brilliant stars behind.

of the galaxy than there are when we look above or below the plane—the dense star clouds of the plane form the band of the Milky Way that wraps around the sky.

The Sun orbits the center of the galaxy roughly once every 200 million years, but the Milky Way does not behave like a solid body, so the inner regions orbit the center faster than the outer ones. The central region is dominated by a dense hub of old red and yellow stars, while the outer disk has a mix of stars. The brightest blue stars are concentrated in the spiral arms, usually in recently formed open star clusters. Stars in the disk and spiral arms of the Milky Way are known as Population I stars— they are typically relatively young. Older stars found in the hub and globular clusters form Population II.

galactic limits

The Milky Way is a disk some 100,000 light-years across and a few thousand light-years deep. The hub is an elongated ball of stars roughly 12,000 light-years in diameter, longest along the axis that points toward the Sun—our galaxy may in fact be a barred spiral (see p.76).


The Large Magellanic Cloud looks like a detached region of the Milky Way. It is rich in gas, dust, young stars, and pinkish star-forming nebulae.

globular cluster

Some of the oldest stars in the Milky Way are found in its globular clusters, such as NGC 5139, also known as Omega (ra) Centauri (left).

globular cluster

Some of the oldest stars in the Milky Way are found in its globular clusters, such as NGC 5139, also known as Omega (ra) Centauri (left).

our celestial neighbors

The Milky Way is not alone in its region of space. For their size, galaxies are relatively crowded together, and our galaxy is a key member of a small cluster called the Local Group. There are at least two dozen other small galaxies in the group, and two other major members—the Andromeda and Triangulum spirals, around 2.5 million light-years away. The Andromeda galaxy is the biggest galaxy in the Local Group, twice the size of the Milky Way. The gravitational attraction between the two is so strong that the galaxies are moving together, doomed to collide and merge billions of years from now.

Around 160,000 light-years away orbit two shapeless "irregular" galaxies of moderate size, called the Magellanic Clouds. Even closer, a tiny, sparse "dwarf elliptical" galaxy is actually colliding with our own, on the far side of the galactic hub from us.

the milky way's black hole

While the galaxy is littered with stellar-mass black holes left over from supernova remnants, a much bigger secret lurks at its heart. The very center of the Milky Way is home to a black hole with the mass of 3 million Suns. This supermassive hole has long since swept the region around it clear of stars and gas, but its gravity still affects the rotation of stars in the hub. Astronomers now think that most large galaxies have supermassive black holes at their centers.

heart of the galaxy

Although the central black hole is dormant, the region around the Milky Way's center contains many violent objects. This X-ray image reveals superhot stars, stellar remnants, and glowing material left by explosions around the black hole.

Types of galaxies

The four major types of galaxies in the universe—spirals, barred spirals, ellipticals, and irregulars—are distinguished by more than just their shape. Each type possesses a unique balance of stars and other material within it, and they display a number of significant features that offer clues to how they may have evolved, and how they may be related.

spiral galaxies

The hub, which is dominated by old red and yellow stars, is surrounded by gas- and dust-rich spiral arms. The space between the arms is not empty, but contains a scattered mix of stars— the arms are prominent only because they contain most of the brightest, short-lived stars. This is a clue that the arms are not permanent, but are regions of increased density that sweep THE andromeda galaxy (M31) around the disk, triggering starbirth as they go.

barred spiral galaxies

In a barred spiral, the hub of a spiral galaxy is crossed by an elongated "bar" of stars, from which the spiral arms emerge. There is some evidence that the Large Magellanic Cloud, for example, is in fact a stunted spiral with a bar and a single arm. It now appears that our own Milky Way is also a barred spiral—the bar just happens to be aligned directly with our point of view.

galaxy ngc 1300
galaxy M87

elliptical galaxies

Ellipticals come in a huge range of sizes, encompassing the very smallest and the very largest galaxies. They are huge balls of mostly old red and yellow stars, each of which follows its own elliptical orbit around the center. They contain very little gas and dust, so little or no star formation is occurring within them. The largest are found only in the center of galaxy clusters—a clue to their origins (see p.78).

irregular galaxies

Usually rich in gas and dust, irregular galaxies such as the Large Magellanic Cloud are more or less shapeless collections of stars. Some appear to have central black holes, bars, and the beginnings of spiral arms. Irregulars are frequently sites of intense starbirth activity, with large glowing emission nebulae. The most active of these "starburst" galaxies are forming stars at a much faster rate than normal spirals. the large Magellanic cloud active galaxies

Most galaxies are the sum of their stars—the radiation they emit comes from their individual stars. But for a substantial number, this is not the case. These "active galaxies" divide into four main types. Quasars and blazars are extremely distant galaxies in which most of the radiation comes from a small, rapidly varying region around the hub. Seyfert galaxies resemble normal spirals but have much brighter cores than expected, and radio galaxies are often insignificant galaxies surrounded by huge lobes of gas emitting radio waves.

Astronomers think all these different types of activity are caused by the black holes at the centers of the galaxies. While in most galaxies the central black hole is dormant, starved of material to feed on, in active galaxies material is still falling inward. In Seyfert and radio galaxies, the effects are relatively restrained, but quasars and blazars, at greater distances from us, are remnants from an earlier, more violent phase.

jet of particles shooting from black hole's magnetic pole multi-wave image of centaurus a

One of the closest active galaxies, this elliptical has become active as it merges with a spiral. The image combines X-ray, radio, and optical pictures.

star-forming galaxies

This box lists some of the brightest galaxies of various types. The majority are in our Local Group of galaxies, although the Whirlpool and M87 are more distant but spectacular examples of their type.

name magnitude type constellation


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