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1. Nebula (plural: nebulae, or in the USA usually nebulas) is Latin for cloud.
If you look through a small telescope, there are objects that look like little cloudy patches in the sky. They were called nebulae, the Latin word for clouds.
2. William and Caroline Herschel made the first big survey of nebulae in the late eighteenth century.
The Herschels were the first to make a serious study of nebulae. William (1738-1822) observed around 2500 nebulae in the northern hemisphere with the help of his sister Caroline (1750-1848). The catalog was completed by William's son John Herschel (1792-1871) who observed the southern hemisphere skies.
3. Modern telescopes can distinguish between true nebulae and star clusters or galaxies.
True nebulae are giant clouds of gas and dust in the spaces between the stars. A good telescope shows that some of the cloudy patches are actually clusters of stars, and others are galaxies millions of light years away.
4. Nebulae are hard to study because they don't shine like stars.
Gas and dust don't give out light, but sometimes they are visible because of the light of nearby stars.
5. The gas in a nebula glows if a bright star energizes it, and dust can reflect a star's light.
Nebulae are made mostly of dust and hydrogen gas. If a nearby bright star shines on the hydrogen gas, it makes the gas glow red. A nebula where this happens is called an "emission nebula". But dust absorbs red light and reflects blue light. If a nebula is lit in this way, it is called a "reflection nebula". Reflection nebulae always look blue. (The Trifid Nebula contains both kinds of nebula - see fact 7.)
6. Some nebulae get no light, but they show up against a light background.
A nebula with no nearby stars is called a dark nebula. These nebulae may look like holes in the sky against a background of stars. They can also be seen if there are bright nebulae in the background, as you can see in the Trifid Nebula.
7. The Trifid Nebula contains emission, reflection and dark nebulas.
Nebulae with names usually get them because of their shapes. The Trifid Nebula looks a bit like a trifid flower. You can see all three types of nebula in this photograph taken by R. Jay Gabany of the Trifid Nebula. The nebula is about 40 light years across, over twenty times the diameter of the Solar System.
8. When stars like the Sun run out of hydrogen fuel their outer layers puff off into what is called a "planetary nebula".
A planetary nebula is often shaped like a ring. In 18th-century telescopes it looked like the round shape of a planet, which is how it got its name. Click here to see planetary nebula NGC 2438 in a photograph taken by Daniel Lopez.
9. Some nebulae are created from the death of giant stars.
When a massive star uses up all its fuel, it explodes as a supernova. The core of the star collapses into either a neutron star or a black hole. The material thrown off in the explosion makes a nebula called a "supernova remnant". Here is the Crab Nebula in a Hubble Space Telescope image. The nebula was the result of a supernova explosion seen in 1054.
10. Some nebulae are star nurseries.
In the right conditions parts of a giant nebula starts to collapse until they are dense enough for stars to form. There are often a number of stars forming in the same nebula. The Orion Nebula is a good example of a stellar nursery.
You can see the nebulae in this article and many others on my Pinterest board Nebulae.