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.
SPIRAL GALAXY ESO 510-G13
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.
BIOGRAPHY: EDWIN HUBBLE American, 1889–1953
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.
