A nebula is a cloud of gas and dust in outer space. These clouds are often very large, spanning across many light years. There are many different kinds of nebulae in the sky.
Emission Nebulae
The first type of nebulae we are going to explore are called Emission Nebulae, because they emit their own light.
Emission nebulae usually have young stars within their gas clouds. This type of nebula soaks up energy and heat from these stars. Since these stars are so new, they are extremely hot and emit ultraviolet radiation. All this radiation supercharges the hydrogen in the nebula gas cloud. This causes the nebula to glow brightly, like a neon sign, thus creating and emitting its own light.
Emission nebulae are usually red or pink. This is because when the hydrogen is superheated with the ultraviolet radiation it glows red to pink in color. The Orion Nebula is an example of an Emission nebula.
Scientists believe that new stars form inside of nebulae. Sometimes the dust and gas in these clouds begins to contract, or squash together. When things such as clouds contract they get hotter. The denser the cloud gets the hotter it gets. Eventually it gets dense enough and hot enough to ignite its hydrogen fuel, beginning its new life as a star. To learn more about stars Click Here.
Reflection Nebulae
A blue relfection nebula is completely different from an emission nebula. This is because rather than emitting their own light, they are reflecting the light of the stars around them.
Why are they blue?
Reflection nebulae are blue for the same reason our sky is blue. They reflect the light of a star. Our sky is reflecting the light of the sun.
When light passes by a particle of dust, the blue color in that light is scattered , while the rest of the colors in the light are allowed to travel undisturbed. This blue light travels around the cloud bouncing off of dust particle after dust particle until it eventually escapes the cloud and reaches our eyes.
Remember: A blue nebula is called a reflection nebula. It scatters the blue light from nearby stars while the rest of the colors are allowed to pass through the cloud undisturbed. Eventually the blue light escapes the cloud and travels to our eyes.
Planetary Nebulae
Although this third type of nebula is called a planetary nebula, it actually has nothing to do with planets. A planetary nebula is formed when a dying sun-sized star begins to shed its outer layers.
When the Sun begins to die, it will expand and become what is called a giant star. It will grow so large that it may engulf the Earth. After millions of years as a giant star, the Sun will again shrink down to its normal size. As it shrinks, much of its surface layers will be shed, leaving behind a beautiful ring.
This ring is only visible for about 50,000 years. Over time, the nebula mixes into surrounding space eventually becoming too thin to see.
Bok Globule
A Bok Globule is different than the other types of clouds we have been learning about. Bok Globules are very dense. This means that there is a lot of dust and gas close together inside of the cloud. There is so much dust and gas that it blocks out all the light behind it. In the picture to the right, you can see two nebulas. First you can see the blue reflection nebula in the background. In front of the reflection nebula is a dark bok globule blocking the view.
