 Welcome to our segment on nebula. We'll be seeing some amazing scenes from across our galaxy. In recent segments, we've seen two types of nebula. Both have been connected with stars dying. Planetary nebulas such as NGC2371 are all about normal stars at the end of their hydrogen-burning life. And supernova remnants such as SNR0509 are the remains of giant star explosions at the end of their fusion factory life. But the most beautiful nebula come from vast molecular hydrogen clouds where new stars are being born. To understand these areas a bit better, we need to know a little more about nebula. There are three basic kinds, reflection nebula, emission nebula, and dark nebula. Reflection nebula are clouds of interstellar dust grains that are reflecting light from a nearby star. This witch's head nebula is an example. It is reflecting light from the nearby star, Rigel. In this photograph, the blue color of the witch head nebula is caused by the dust grains scattering blue light. The same physical process causes Earth's daytime sky to appear blue. Emission nebula are shining their own light. In order to do that, the gas and dust need to be excited to the point of luminescing. This is accomplished in two primary ways. One, by exploding stars at the end of their lives like the two we just saw, and two, by new stars exciting the clouds they are born in. The Rosetta nebula is a good example of this. This nebula is a vast cloud of dust and gas extending over an area almost 100 light years wide. It would cover our entire solar neighborhood. As parts of a great molecular cloud condense, new stars are created. These hot new stars shine brightly in the ultraviolet. This is exactly the right wavelength for radiation to ionize hydrogen molecules and atoms by stripping away their electrons. This sets off a series of quantum effects that create photons in very large numbers, creating the light that we see with our telescopes. You can see the young, recently formed stars situated within this nebula. They formed from the nebula's material, and they are the stars that make the nebula shine. Areas like the one creating the Rosetta stars are called H2 regions. There are only a few thousand of these in the Milky Way because they only last a few million years. Radiation pressure from the hot young stars drives most of the gas away. The Pleiades are an example of a cluster which has boiled away most of the H2 region from which it formed. Only a trace of blue reflection luminosity remains. Our third kind of nebula, dark nebula, are not shining at all. There are clouds of dust and gas that are positioned in front of bright nebula obstructing its view. The Horsehead Nebula is an excellent example of this. Rising from a sea of dust and gas like a giant horse head, the nebula is one of the most photographed objects in the sky. It is a cold, dark cloud of gas and dust, silhouetted against a bright emission nebula. Here's what it looks like in infrared, where we can see lower wavelength light, the kind of light that can pass through the nebula's dust.