Stars and Galaxies

Almost all of the twinkling lights in the night sky are either stars or distant galaxies. The light may have travelled for thousands, if not millions, of miles to reach Earth.

What is a star?

A star is an incredibly hot ball of gas, held together by gravity. They turn hydrogen into helium, giving out lots of energy and once they have used up all of their hydrogen, they begin to die.

What is a galaxy?

A galaxy is a collection of hundreds of billions of stars, held together by gravity as they orbit a central point (often a black hole). There are over a hundred billion galaxies in the universe.

A field of galaxies, seen through the Hubble Space Telescope

Lives of stars

A star starts its life in enormous clouds of dust and gas known as nebulae. It snowballs in size as its increasing gravity pulls in more dust and gas. Once the star has enough mass, the star's gravity is strong enough to fuse hydrogen atoms together to form helium. This gives out lots of heat and light. When the star has used up all of its hydrogen it begins to fuse helium into beryllium and lithium, these fuse to become heavier elements. Each stage gives out less and less energy so eventually the inward pressure from the fusion will become weaker than star's gravitational pull. In small stars the star collapses and cools to form a white dwarf. In big stars the star explodes in a supernova and then collapses to either form a very dense neutron star or a black hole.

Different types of galaxies

Select a galaxy

James Gooding, 2017

All image credits go to NASA (and respective centers/universities)