The Interstellar Medium and Starbirth

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Although the space between stars appears to be empty, it is actually filled with matter known as the interstellar medium, approximately ninety-nine percent of the interstellar medium consists of gas and the remaining one percent is dust. Clouds of interstellar medium are called nebula, derived from the Latin word for cloud (What is the Interstellar Medium?, n.d.). Nebulas are formed when gravitational attraction clumps together parts of the interstellar medium (Villanueva, 2010). There are several types of nebula, which are differentiated by how they affect light from stars (What is the Interstellar Medium?, n.d.). All nebulas are actually dark because they don’t produce light on their own, but when astronomers refer to dark nebulas they are talking about nebulas that contain dust thick enough to completely block light from stars behind it. Dark nebulas appear as black silhouettes in the sky. Emission nebulas are the most colorful and are lit from within by stars forming in a stellar nursery. Reflection nebulas reflect starlight from either outside or inside the nebula. Main sequence stars growing into red giants and casting off outer layers create planetary nebulas. They are called planetary nebulas because nineteenth century astronomers, using primitive telescopes, thought they looked somewhat like the recently discovered Uranus and Neptune. Exploding massive stars at the end of their life span create ancient supernovas, which are also nebulas (Types of Nebulae, n.d.).
All the stars in the universe were born in emission nebulas, extended clouds of hot, glowing interstellar gas. Astronomers believe that shock waves passing through interstellar matter initiates star formation, which happens when gravity starts...

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