Expert: Philip Stahl - 12/29/2005

Question
Hello. I frequently read that nebulas are made up of "gas and dust."  I'm wondering what kind of gas and dust this is. Is the gas hydrogen, perhaps? And what is the dust made of?

Thanks.
David

Answer
Hello.

Nebulae actually come in a variety of physical types: Diffuse nebulae, which can be emission or reflection, dark nebulae and planetary nebulae.

Let's take the last first. Planetary nebulae are the gaseous, cast off shells of certain stars. The Ring Nebula is an example of this.

The most prominent element in this type of nebula is doubly ionized oxygen (meaning that it has lost two of its outer electrons). Other elements include singly ionized oxygen, neon, nitrogen, as well as permitted lines of hydrogen and helium.

In bright diffuse nebula (such as M 42 in Orion) the gas radiates by virtrue of having absorbed high energy radiation from stars in its vicinity. It then re-radiates this energy (as photons) at lower wavelengths. When one examines the chemical signature of such radiation (using an instrument called the spectroscope) it is found that it is mostly ionized hydrogen. That is, hydrogen less its electron.

The nebula that surrounds the star cluster known as The Pleiades, meanwhile, is what we call a "reflection nebula".When analyzed it shows no bright emission spectrum, hence can't be shining by re-radiation of energy.

Rather, it shows light simply reflected by dust within the nebula itself. Such nebulae tend to have a bluish color, since interstellar dust (mainly in the form of silicates - e.g. particles of silicon and oxygen combined, ices, graphite and metals) tends to scatter the light incident on it  - and always the blue (shorter) wavelengths more than the red (longer) wavelengths.

Note: if NO stars are in the vicinity of a dense cloud of dust and gas (say hydrogen) then the nebulae (like the 'Coal Sack' in Crux) will simply act as an obstruction, blocking everything out in the visible region. There will be neither reflection or emission of light, just a dark glob or 'hole' in the sky with nothing visible behind it. (Though infrared instruments can be used to get around this!)

Many of these 'dark nebulae' also feature dense molecular clouds, in which we believe stellar birth can occur. The bulk of such clouds is believed to come from molecular hydrogen (e.g. pairs of hydrogen atoms bonded to make up molecules) and the bulk of its mass may be due to a combination of the following molecules: formic acid, carbon monoxide, ammonia, acetylene, methane, methyl formate, ethyl alcohol and hydroxyl radicals.

Hopefully, this information will prove useful.