Copyright © 2007 Dorling Kindersley
Stars are not scattered evenly throughout the Universe. Instead, they are grouped together in great star islands, called galaxies. All the stars we see in the sky belong to our home galaxy, the MILKY WAY. Some galaxies are tiny and contain only a few million stars, but many contain hundreds of billions of stars. Galaxies are classed into three broad groups, according to their shape: elliptical (oval), spiral (if they have spiral arms), and irregular.
A spiral galaxy is roughly disc-shaped and has a bulge in the middle. The disc is formed by arms that curve out from the central bulge. The stars in the central bulge are relatively old. Most star formation takes place on the spiral arms, which are full of gas and dust. In this sideways view of a slightly warped spiral galaxy, dark dust lanes are visible in the disc.
The Large Magellanic Cloud is one of our nearest galactic neighbours in space. It is an example of an irregular galaxy, which means it has little definite structure. It is some 160,000 light years away, and is less than a third as wide as our own Galaxy.
M87, found in the Virgo cluster of galaxies, is an example of an elliptical galaxy. These galaxies lack the curved arms of spirals and can be round or oval in shape. Some of the largest galaxies are elliptical. M87 may be as big as 500,000 light years across.
While working at Mount Wilson Observatory in California, astronomer Edwin Hubble was the first to discover, in 1923, that there are other galaxies beyond our own. Today we still use Hubble’s original method of classifying galaxies into spirals, ellipticals, and irregulars.
The galaxy that is our home is called the Milky Way Galaxy, or just the Galaxy. It measures about 100,000 light years across. Our local star, the Sun, is one of at least 200 billion stars in the Galaxy, and lies in one of the Galaxy’s spiral arms. We also call the faint band of light that arches across the night sky the Milky Way. This band is a just a section of our Galaxy.
On a clear dark night, the faint band of the Milky Way can be seen in the sky. The Galaxy appears as a band because it is a flat disc and, from our position in a spiral arm, we look through the disc side-on. With binoculars or a telescope, we can see the Milky Way’s mass of stars, seemingly packed close together. Dark lanes among the stars show where dust clouds are blocking the light from other distant stars.
Our Galaxy is part of a small cluster of galaxies we call the Local Group. We know of around 30 galaxies in the Local Group, and the largest is the Andromeda Galaxy. It is a huge spiral galaxy, half as wide again as the Milky Way, and contains around 400 billion stars. Although it lies 2.5 million light years away, it is still visible to the naked eye. Andromeda has two satellite galaxies, both small elliptical galaxies, that orbit it as it travels through space.