Nebulae are huge clouds of dust and gas where stars are formed. They are either dark or bright. Dark nebulae are made mostly of dust, so the light of the stars behind them gets blotted out, which makes them look like patches in the sky. Bright nebulae glow because they are very very hot. Their colours depend on the concentration of gases in them.
Gases and dust in the nebulae swirl around to form huge clumps which grow larger over time. Then something causes them to collapse (shock waves from stars or something like that). As they collapse, the temperature in them rises. After tens of thousands of years, a core is formed which gets hotter and hotter, starting reactions inside. These reactions cause the cloud of gas, now a star, to shine.
How do they form?
Astronomical nebulae are a collection of gases that have not yet condensed into a star or solar system. These clouds are largely composed of the basic ingredients found inside active stars, most notably hydrogen.
When a main-sequence star reaches the end of its life, it dies in one of two ways:
- The star becomes too weak to keep its mass contained and the star becomes cooler and larger, eventually dying completely as the majority of its component gases extend into interstellar space. Whatever is left near the middle will re-collapse into a super hot, but super tiny white dwarf that eventually burns itself out and becomes a sphere of diamond in space.
- The star encounters mass instability and collapses in on itself, creating a titanic explosion called a Nova or Supernova, depending on the magnitude. The component gases and elements are violently ejected into interstellar space, some at a significant fraction of the speed of light.
In either of these cases, the component gases and elements that make up a star become a loose cloud of “star stuff”. This is what is referred to as a “nebula”.
In astronomy, they are also known as “Star nurseries” because over time, the loose collections of gases and elements will attract to each other (because of gravity, all mass is attracted to other mass), condense and, in the course of their condensation, will grow hot and dense enough to begin nuclear fission. This is how new stars are formed.
Types of Nebulae
There are basically three types of nebulae; emission, reflective and dark.
The Eagle Nebula
Emission nebulae are clouds of gas excited by the radiation, UV and X-rays, from a nearby source. Some a super nova remnants (SNR) like the Ring Nebula, M57, in the constellation Lyra. Others are regions of star formation like the Orion nebula, M43. They are noted for their intense colors and brightness.
Reflection nebulae are clouds of dust that reflect light from a nearby star. The spectrum of these nebulae matches closely to the spectrum of the source star. An example would be the Witch Head nebula illuminated by star Rigel.
Dark nebulae are regions of dust so dense that they obscure the light from stars behind them. Examples are the Coalsack and the Great Rift in our galaxy, both visible to the unaided eye. Another example is the Horse Head nebula, which is back lite by an emission nebula.
There are further refinements in nebula classification that help qualify their nature.
The Beautiful Cat's Eye Nebula
Planetary nebulae are emission nebula and usually surround late stage stars that have ejected their outer mantle of gases. They have a spherical symmetry -mostly though can take on other morphologies.
In the past globular clusters, open clusters and galaxies where once called nebula but we now know better.
In the past globular clusters, open clusters and galaxies where once called nebula but we now know better.
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