Nebulae, Gas, and Dust

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What is a reflection nebula?

A reflection nebula is type of nebula that shines by reflected light. Light from nearby bright stars is reflected by the large amounts of dust in these nebulae. The size of the dust grains causes blue light to be reflected more efficiently than red light, so these reflection nebulae frequently appear blue in color.
I heard that stars form in giant gas clouds in space. Is this true?
Yes this is true! Stars form from collapsing clouds of gas and dust. As the cloud collapses, its density and temperature increase. The temperature and density are highest at the center of the cloud, where a new star will eventually form.
What is the reddening law?
A "reddening law" is really just a description of how interstellar dust absorbs light that passes through it. When starlight passes through a dust cloud, not only does it get dimmer (because some of the light is being absorbed), it also gets "redder" because the shorter wavelength light is affected more than the longer wavelength light. Interstellar clouds are more efficient at scattering and absorbing blue light than red light, so much less blue light gets through them. That makes stars appear redder when seen through a dust cloud. The "reddening law" allows astronomers to correct this effect, and infer what the star would look like without the dust getting in the way. Different astronomers use different reddening laws, depending on who they think has the better model of exactly how much light is absorbed at different wavelengths. Sometimes this creates arguments and controversy, as no one knows what the right law really is. And getting the right version of the law is important, as it allows astronomers to estimate the actual distances and temperatures of the stars in question.
What is a nebula and which one is closest to the Earth?
Generally, a nebula is a cloud of gas and dust in space. Some nebulae are regions where new stars are being formed, while others are the remnants of a dying star. The closest nebulae to the Earth are:
 
Helix Nebula (450 light years away) - closest planetary nebula (a nebula formed when a star blows off its outer layers)
Orion Nebula (1500 light years away) - one of the closest star forming nebulas
What is a giant molecular cloud?
Large, dense molecular clouds are very special environments in space. Composed mainly of molecular hydrogen and helium, with small amounts of heavier gases, they are the birth place of new stars and planets. Molecular clouds that exceed the mass of 100,000 suns are called giant molecular clouds. Giant molecular clouds are the largest inhabitants of galaxies, reaching up to 300 light-years in diameter. They contain enough dense gas and dust to form hundreds of thousands of Sun-like stars. These stars are formed in the densest parts of the clouds. Molecular clouds are very cold, having temperatures ranging from about -440 to -370 degrees Fahrenheit (-263 to -223 degrees Celsius or 10 to 50 degrees Kelvin). They usually do not radiate their own visible light and appear dark when viewed with an optical telescope. In these cold, dense environments, many atoms can combine into molecules. Giant molecular clouds can last for 10 to 100 million years before they dissipate, due to the heat and stellar winds from newly formed stars within them. An average spiral galaxy, like our own Milky Way, contains about 1,000 to 2,000 Giant Molecular Clouds in addition to numerous smaller clouds.
What is the size of interstellar dust? Is it the same size as household dust?
Interstellar dust grains come in many sizes -- from smaller than a tenth of a micron to many microns wide (a micron is one-millionth of a meter). It is not the dust one finds around the house, which is typically fine bits of fabric, dirt, or dead skin cells. Rather, interstellar dust grains are much smaller clumps, on the order of a fraction of a micron across, irregularly shaped, and composed of carbon and/or silicates.
Why does the PPNe (protoplanetary nebula) model have the outflow poles?
This is pretty much unknown and is one of the hottest topics of astronomy research. Why a disk? Because it's a stable structure under gravity and also blocks the stellar winds that may disrupt any other structure formation. The winds are probably the force that powers the outflows. Why the infall? Gravity. This is the weak but ultimate long distance force that governs the properties of all structures in the Universe.

An interesting note is that while the SS has this predominate spin, the 10 trillion stars and their SS's (including ours) in the galaxy have each their own random spins and orientations relative to each other, make up a galaxy that is spinning in a very coherent and unified manner in a plane. The plane of our galaxy is composed of SS's like ours and is flatter than a pancake. It is the same situation in most disk galaxies (they make up 1/3 of the galaxies - the elliptical ones are more like swarms of stars and make up the other 2/3). There are 100 billion such flat galaxies - each with a multitude of randomly spinning SS's.

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