The Orion Nebula
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Nebulae - The Dust of Stars
Why would we call nebulae stardust? Because the gas of which they are
composed both creates and is created by stars. Stars are composed of
very dense, high temperature gas. Nebulae are also composed of gas,
but they are cooler and less dense. Water is at least
1,000,000,000,000,000 times as dense as the gas found in nebulae.
Nebulae come in a variety of shapes. But their appearance is usually
linked to the energy source which is lighting them up. For every
region of gas which we can detect in the optical region of the
spectrum, much more gas exists which is not easily visible. Something
must happen for the nebulae to reveal themselves.
So, what happens to make nebulae glow? It depends on the type of
nebulae. Some are involved in the formation of new stars. These are
stellar nurseries called HII
regions. The young stars found in HII regions are extremely hot
and provide a lot of energy for lighting up nebulae. Some nebulae are
the products of stellar death: supernova
remnants and the planetary nebulae surrounding white dwarfs.
Some types of gas clouds are not visible in the optical regions of the
electromagnetic spectrum. We need to explore some other wavelengths
to observe these type of gas clouds. For example, neutral
hydrogenemits energy in the radio region of the electromagnetic
spectrum. Dust absorbs the optical and ultraviolet light of stars and
re-emits it as infrared radiation.
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