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Nebulae - Stellar Nurseries

Nebulae are vast interstellar clouds of gas and dust. They contain mostly hydrogen and helium, but other substances are also present, like relatively huge quantities of alcohol. The word nebula is from Latin, which means "cloud", named after their hazy appearance on the night sky.
Astronomers have categorized the nebulae (plural of nebula) into three groups, depending on their character. These groups are: Emission nebulae, Reflection nebulae and Planetary nebulae. Usually nebulae are a mix of these groups. The first group, emission nebulae, are emit light themselves. A very hot and bright, nearby star ionizes (it strips off the electrons) the hydrogen in the nebula. When the free electrons recombine with protons, light is emitted, usually red light. A subgroup of emission nebulae are HII-regions, where massive stars forming are causing the ionization. Another subgroup of emission nebulae are planetary nebulae, which are made when a dying star is throwing off its' outer shells. Planetary nebulae are not related to planets, other than that their shape may sometimes look like a planet.
A reflection nebula reflects the light that is cast upon its' dust, from nearby stars. They are often blue in colour. A good example of a reflection nebula is the nebula that formed the Pleiades cluster.

Stellar Formation

Stars form from insterstellar clouds of gas and dust, composed mainly by hydrogen and helium, though traces of heavier elements are present. In space, there are two important forces that a cloud/nebula is faced with: The kinetic energy of the gas, which makes the nebula expand, and the potential energy (gravity), which makes the cloud contract. As long as these forces are in balance, the cloud is said to be in a state of hydrostatic equilibrium. To form a star, or multiple stars, this equilibrium must be shattered by the gravity of the cloud. The specific mass a cloud must have in order to collapse is called the Jean's mass, and depends on the cloud's density and temperature, and is normally 10-1000+ MSun.

In case of large clouds (thousands of solar masses), the cloud may collapse on its own and create a cluster of stars. For smaller clouds, there must be a third force which will have to trigger the stellar formation. This third force will make sure the density of the cloud is increased, which will make the cloud collapse. The force can, for example, be a passing by star, galactic collision or a nearby supernova which sends out intense radiation and shockwaves at very high speeds. If the cloud is close to the galactic center, the radiation from an active supermassive black hole may also trigger the stellar formation.
As the cloud collapses the center will get more dense, and therefore warmer. In addition, an accretion disc of gas and dust will be created (especially in the case of massive stars), which feeds the growing protostar. The heat at the center may be so intense that the infalling matter will be repelled back. For a star to continue growing this heat must be channeled away and is normally done so by bipolar outflow from the star, which is a stream of gas that is ejected from both poles of the star. At a certain point, the star will ignite and the intense heat from the star will definitely repel incoming gas and dust. The ignition of a star means that the internal gravity (potential energy of the gas located at the surface, and deeper) of the star is balanced by internal pressure (kinetic energy of the gas), which is a result of nuclear fusion of hydrogen into helium.
The matter in the star's vicinity may create planets. Hence, a new solarsystem is born.

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