Describe the appearance of stars, double stars, asterisms, constellations, open clusters, nebulae and globular clusters in the night sky


Stars & Double Stars

Stars appear as points of light in the sky. They are like our Sun and can vary in size and heat. Although they are bright they are millions of miles away, the nearest is 4 light years away. They may twinkle towards the horizon. Many stars are binary stars and consist of two or more stars that share a gravitational bond. These are called double stars although this name also applies to stars that look binary because of the angle we see them but are not connected.

Constellations & Asterisms

A constellation is a group of stars that appear to make a pattern in the sky. When we look at them they look close together, but in reality they are usually very far apart and may not be part of the same group of stars they appear next to. At different times of the year we can see different constellations due to the rotation of the Earth and the tilt of our axis, in the same way we have seasons. As our planet rotates we appear to stay still while the sky rotates.

An asterism is a pattern of stars that may or may not be linked to a constellation. Examples are the Plough in Ursa Major, the pattern made by the saucepan shaped brighter stars, or the Summer Triangle, a shape formed between the bright stars Vega (constellation of Lyre), Deneb in Cygnus and Altair in Aquila.


Clusters: Open & Globular

There are clusters of stars around the galaxy. These have sometimes been mistaken for galaxies but we know they are not as there are usually only several thousand stars quite close to our galaxy, if not in it.

If they actually were galaxies there would be millions of stars there and they would be much further away. In fact we can see clusters of stars in other galaxies.

There are two types you should know about: 
- Open clusters
- Globular clusters

Open clusters

Open clusters are groups of stars close to each other in space. They form no specific symmetry and are usually very bright, indicating that they are young stars.

There can be anywhere from a dozen to thousands of stars making up no particular shape, and these are found around the galactic plane.

A good example of an open cluster is the Pleiades, or Seven Sisters, above the constellation Taurus. You can compare your eyesight to a friend by seeing how many you can spot with your eyes. From a city you may see seven, the record is nineteen!

Globular clusters are spherical shaped with more stars nearer the nucleus. They resemble a fuzzy ball.

These clusters are located around the galactic nucleus.

The stars are usually very old red giants and white dwarfs packed tightly together. There are thought to be between 100,000 to over a million stars in a typical globular cluster.

An example of a globular cluster is M13 in Hercules.


Nebulae (the plural of Nebula) are blurry patches of light in space that are not other galaxies but are in fact mostly in our own galaxy.

They are places where stars are born, are dying or have died.


Double Star
Asterism of Ura Major
  • What is an asterism?
  • What is the difference between an asterism and a constellation?
  • Describe the appearance of open clusters, globular clusters and nebulae.
More Asterisms...

Orion's Belt, Square of Pegasus, Teapot in Sagittarius, Sickle in Leo. How many can you find?.


Find out about Mizar and Alcor in Ursa Major

Constellation art in Stellarium Software
Open Cluster
Globular Cluster
See also

...On these pages

  • Double Stars
  • Clusters
  • Nebula - Star Birth
  • Nebula - Star Death