Galaxies | The Schools' Observatory

A galaxy is a large collection of stars, dust and gas. They are surrounded by dark matter, and are held together by gravity. It is thought that there are around 100 billion (100,000,000,000) galaxies in the Universe.

Galaxies come in all shapes and sizes. The smallest galaxies, known as dwarfs, contain just a few million stars. The largest giant galaxies can contain around 1 trillion stars - that's an incredible 1,000,000,000,000 stars. Scientists have estimated that there are at least 100 stars in space for every grain of sand on Earth's beaches!

It is thought that super-massive black holes live at the centre of most galaxies. This includes our galaxy (the Milky Way). Astronomers have found it by looking at the movement of stars close to the centre of the galaxy.

An image of the galaxy M64. There is bright white-yellow sphere in the centre, giving off a glow. Dark brown and black clouds of dust form a pinwheel shape around the centre. Within these clouds are numerous bright white dots of stars, though some appear more blue or red in colour. There are also a few stars visible against the black background. — The spiral galaxy M64

The Milky Way

Our own galaxy is called the Milky Way. It is a large spiral galaxy. Our Solar System sits towards the edge it, about 26,000 light-years from the centre. The closest star to our Solar System is Alpha Centauri, which is just 4.4 light-years away.

As we live inside the Milky Way galaxy, it is hard to see its shape. From Earth, we see a faint, white band stretching across the night sky. The name of our galaxy comes from the Latin words meaning "milky path".

A lake at night with dark silhouettes of trees around the edge. Most of the image is focused on the night sky, where a bright arc sweeps upwards and then down again. Dark brown-red clouds cover sections of the brighter patches in areas. Small, white dots of stars are scattered everywhere in the sky. — Panorama of the Milky Way over Bontecou Lake in Dutchess County, New York

Looking closely at this band, we see that it is made up of billions of stars. A lot of the light coming from those stars is blocked by gas and dust in the galaxy. It is this gas and dust which will ultimately form new stars.

A dark blue sky with numerous white dots of stars, some slightly larger and brighter than others. From the lower right corner, a bright white band arcs across towards the top left. Dark clouds of dust block this light in places, illuminating some parts of the cloud and making the colour more red-brown than dark brown-black. — A closer view of the Milky Way band

Telescopes have accurately measured the positions of stars and found that the Milky Way is a barred spiral galaxy. It has over 200,000,000,000 (200 billion) stars inside it. It is disk shaped with a bulge in the middle. The Milky Way is about 100,000 light-years across and 1000 light-years thick.

The Sun is on one of the spiral arms of the Milky Way, two-thirds of the way out from the centre.

An illustration of the Milky Way galaxy against a black background. In the centre, a bright yellow-white sphere glows. Two patchy, red-brown clouds leave the sphere - one towards the upper right, and one towards the lower left. These then swirl around the centre to create the galaxy's spiral arms. Though the red-brown is visible, it appears the arms are mostly bright, blue-grey clouds. Numerous white dots are scattered along the arms. A yellow arrow points to the 3rd spiral outwards, labelled "Sun".

This work by NASA is licensed under Creative Commons Zero v1.0 Universal

In the same way that the Earth orbits the Sun, the Sun orbits the centre of the Milky Way. It travels at a speed of almost 220 km/s. Even at this speed it takes our Sun around 225 million years to complete one full orbit of the galaxy.

Quasars

A Quasar is an energetic object in space that looks like a star. They are actually galaxies, very far away, with very bright cores in the middle.

They are probably the brightest objects in the whole Universe, many times brighter than normal galaxies. A quasar can be as bright as 1,000 Milky Way galaxies!

Numerous red dots against a black background. In the centre, there is a large, bright, white sphere with small swirls of yellow, orange and red around it. A yellow streak leaves the sphere towards the upper right, but fades into the background towards the top of the image. There is also a white/yellow sphere in the lower right corner. — Quasar PG 0052+251

Astronomers think that quasars are powered by supermassive black holes found at the centre of most galaxies. The power from a quasar comes from material crashing together at enormous speeds. This is because the material is being pulled into the black hole and swirling around.

The material comes from stars which have got too close to the black hole. The gravity from the black hole is so strong that it pulls the stars apart. This makes a string of huge explosions that can be seen across the Universe. The furthest quasar we can see is more than 13 billion light years away!

Galaxy Clusters

Gravity is powerful in space. It allows planets to orbit stars, and stars to orbit around galaxies. It also brings galaxies together. There are many groups, and clusters of galaxies in space that are bound to each other by gravity.

Galaxy groups have less than 50 galaxies and are 3 to 7 million light-years across. The mass can be 10,000,000,000,000 (10 trillion) times that of the Sun. The Milky Way is part of the Local Group of galaxies, which has around 40 members.

Galaxy clusters are even bigger. They can have between 50 and 1,000 galaxies. Reaching 7 to 35 million light-years across, clusters can contain 10 to 100 times more mass than galaxy groups.

If you were to look at a cluster for a very long time, you would see that the galaxies in it were all swarming around a central point.