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Star Formation

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Western University
Astronomy 1021
Chris Racknor

Jan 24 12 Star FormationStars form from the gravitational collapse of an interstellar cloud Stars start shining when temperature is high enough for nuclear fusion Many pictures we have of stars are photo shopped the youngest star clusters are all surrounded by large amounts of dust and gas the interstellar medium ISM It seems that large interstellar clouds somehow become stellar nurseries The Interstellar Medium The space between the stars seems largely empty at visible wavelengthsThere is a fair amount of material between the stars and this is called the interstellar medium Most of the interstellar medium is in the form of low density gas mostly hydrogen and helium in clouds called nebulaeThese clouds do contain a small about 1 amount of heavier elements mostly in small solid grains referred to as dist The interstellar medium has and overall composition very similar to the suns interstellar dust is very small at about one micrometer in size more similar to particles in smoke than the dist we are familiar withNebulae Although interstellar matter is everywhere we look the interstellar medium is most prominently visible in the nebulae Different Types of Nebulae Often a combination of all three can be seen in one image These are not the only type of nebulaeEmission nebulae are made of hot gas and emit light Reflection nebulae are made of dust and reflect light Dark nebulae are made of dust that absorbs light2519The density of an average nebulae is 1010 particles per cubic centimetre air is about 10 particles per cubic centimetre Nebulae is very large upwards of 300 light years acrossEmission NebulaeHot stars that illuminate gas clouds and heat up the gas A low density cloud of gas emits photons at specific colours only This means emission nebulae are low density clouds of gas emitting at their specific coloursAn example is the Orion Nebula which has lots of hydrogen alpha emissionReflection Nebulae Star illuminates the gas and the dust cloud and star light is reflected by nearby dust Appear blue because blue light is scattered more easily than red light by the dist cloud same phenomenon that makes the sky appear blue Dark Nebulae Patches of sky that look completely black not a black hole Dense clouds of gas and dust absorb all the light from the stars behind Absorption is caused by microscopic dust particlesSince dist is less effective at absorbing in the infrared less light is absorbedYou can see some stars around the edge of the nebulae because the dist is thinner but those will appear reddened because dust blocks blue light more effectively than red light The clouds are opaque at visible wavelengths but transparent in the infrared Gas becomes a star through gravitational contraction Since the interstellar medium is everywhere we have plenty of clouds and nebulae
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