A nebula is cloud of dust and gas in space. The term 'nebula' comes from the Latin for fog or cloud.
Historically, the term nebula was used to describe any object that appeared cloudlike outside of the Milky Way, including other galaxies, but a true nebula is a cloud of dust and gas. The gas normally consists of hydrogen, helium, and ionized gases. They can range in size from millions of kilometers in size, to many light years across. While nebulae can be very large in size, they aren’t very dense. Most nebulae are described as diffuse nebulae, meaning they have no clear edge or boundary.
Stellar nebulae are clouds of dust and gas that will ultimately form a star. The dust of gas collapses due to gravity. As the cloud decreases in size, the heat increases as the size of the cloud decreases. The temperature increases because some of the gravitational potential energy is converted to thermal energy. Once it reaches a critical temperature and a critical density, nuclear fusion can occur.
Another type of nebula is called a planetary nebula. This term was first used by the British astronomer William Herschel in the 16th Century. Planetary nebulae happen around stars with a similar mass to our Sun, towards the end of a star’s life. As the star’s core collapses, a shell of gas is ejected. Stars that are very large undergo a much more violent process towards the end of their life cycle. Large stars become unstable after they have used up their nuclear fuel, resulting in a large explosion known as a supernova. These explosions are huge and can often outshine whole galaxies. The remnants of these explosions will form nebulae.
The Hubble Space Telescope has been used to take images of different nebulae. These images are some of the most impressive that have been taken using the space telescope.