Aardvark Communications:

A Practical Guide to Astronomy


Galaxies


Huge Groups of Stars

About a billion years after the Big Bang, when the Universe was still very young, stars began to group together. These groups are made of billions of stars and are called Galaxies. Galaxies travel through space together to form clusters. We don't know exactly how many galaxies there are in all, but there are probably billions and billions.

There are four main shapes of galaxies:

Spirals turn like a pin-wheel, or like cream swirling in a cup of coffee. Our Galaxy (the Milky Way) is a spiral.

Barred Spirals look like two tadpoles joined at the head. They have a large centre with two long swirling arms. Looks a little like the letter 'S'.

Elliptical or oval-shapes galaxies look like squashed balls. It is thought that they contain much older stars.

Irregulars contain a lot of dust and form no definative shape.

Our Own Galaxy: The Milky Way

The Milky Way contains about 250 billion stars, any of which may have orbitting planets like our solar system. Some solar systems in our galaxy are binary (two stars revolving around each other). Others consist of dying stars.

Our galaxy is the spiral-shaped type with several arms. Our Sun is about two-thirds of the way out from the centre hub and is located on the arm called Orion Spur. The Sagittarius arm is located between us and the galactic center, the Perseus Arm wraps itself behind us.

Some scientist believe there is a black hole at the centre of our Galaxy that may be slowly sucking in the surrounding light and gas. If we wanted to travel the diameter of our galaxy it would take about 100,000 years- travelling at the speed of light.

Just as the moon rotates around the Earth, and the Earth rotates around the Sun, our Sun rotates around the Galaxy's center. It takes 225 million years to rotate around the galaxy. This is called a galactic year.

Galactic Neighbours

The Milky Way is part of a cluster of about 20 other galaxies. This is a small cluster, since some clusters contain thousands of galaxies. Our group of 20 is part of a larger group known as the Virgo Supercluster. The largest galaxy in our group is called the Andromeda Spiral. It is about 2.3 million light years from Earth and contains about 400 billion stars. Andromeda can be seen from Earth with the naked eye.

The closest known neighbouring galaxy is the Magellanic Clouds, named after the famous explorer who first spotted them in 1521. In the 'clouds' are two irregular galaxies, one larger, containing about 10 billion stars, the other smaller and farther away.

Other galaxies that are close enough to study from the Earth are: M81 (a spiral), M51 -the Whirlpool, another spiral, about 14 million light years away, reaches out to a smaller galaxy. Then there is M77 -another spiral, M87- which sends out strong radio waves and M104-a spiral nicknamed the Sombrero because of its shape.
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