Clouds in space
Published on May 24, 2007 at 2:17 am.
4 Comments.
Filed under nebula.
It has been cloudy here the last few days. But, that got me to thinking about clouds in space. Interstellar space is not empty. There are lots of things out there. Granted, these things are so far apart, and space has such low density that it is more empty than the best vacuum pumped by the best vacuum pump on Earth, but it isn’t totally empty. And, sometimes, some of that stuff out there is a bit more dense than others, and it betrays itself to observers on Earth. Often this looks like a cloud-like feature. So, it is no wonder that the term that evolved to describe these cloud-like features was nebula, which means “cloud” in Latin.
So, all of these cloud-like features are called nebulae. Studies have shown that the most common element in a nebula is hydrogen, with helium coming next. In fact, nebulae have about the same composition as stars. And there is good reason for that! Nebulae are generally associated with stars. Stars form from nebulae, and stars create nebulae (different types of nebulae). So, it is no wonder that stars and nebulae have about the same composition.
Nebulae can be categorized in several ways. They can be classified by their appearance, by their physical state, or by their origin. One of the simplest classifications is by appearance: diffuse nebula, planetary nebula, dark nebula, etc. A diffuse nebula is one that appears cloud-like, such as the nebula IC 2118 at the top of the page (sometimes called the “Witch Head Nebula” due to its shape). NGC 7293, the Helix Nebula, seen just above, is an example of a planetary nebula. A planetary nebula has absolutely nothing to do with planets. The name came from William Herschell, who observed that they were cloudy like a nebula, but round like planets. Some look more round and planet-like, and others don’t look round at all. We now use the term planetary nebula to denote a particular type of nebula created when the outer layers of a star like the Sun dissipate into space as the star is dying. But, the term “planetary nebula” refers to a specific type of nebula given off by a star at the end of its life. For example, the star Eta Carina, seen below, is a mass-loss star, and it is shedding material into space in explosive bursts. This creates a nebula, but it is not considered a planetary nebula.
So, origin makes a difference in how you classify a nebula. But, so does how you see it. For example, consider the following image. This is M 20, the Trifid Nebula. You can see that part of it is reddish pink, and part is bluish. But, you also see dark clouds in the way of the pretty colored parts behind. These three features are called emission nebula, reflection nebula, and dark (or absorption) nebula.
The names tell all. The reflection nebula is scattering light from stars. The blue light scatters more (and the stars themselves are bluish), so the nebula appears blue. The emission nebula is shining through emission of light from hydrogen atoms. Hot, bright stars in the vicinity of the nebula excite the hydrogen gas (cause electrons in the hydrogen to move to higher energy levels). Then the electrons move back down to lower energy levels, they give off light. This light is what you see when you look at an emission nebula. The dark nebula is simply material that is in the way. It is not scattering or reflecting light, and it is not emitting light. It simply gets in the way of the other light, so it is also sometimes called an absorption nebula. As a rule, the dark nebulae also are much cooler, and atoms stick together to form molecules and sometimes even small grains of material called interstellar dust. So, these dark nebulae, especially if they are somewhat linear in appearance such as these, are sometimes called dust lanes.
We can also talk about a nebula that is collapsing form new stars (a protostellar nebula), or a nebula created when a supernova explodes (a supernova remnant, such as the Crab Nebula or the Veil Nebula). The Orion nebula, seen above, is an example of a nebula where stars are forming. The newest, hottest stars then excite the gases of the nebula, and they emit light. In fact, these stars have the energy to even ionize the hydrogen. In that case, we call this an H II region (H I is used for neutral hydrogen, and H II means ionized hydrogen).
But, all of these are nebulae because they appear cloudy in the telescope.
-Astroprof
Images courtesy of NASA, HST, AAO, CFHT
Astrolink [Global Edition] » Astrosphere for May 25, 2007 | Latest astronomy news in 11 languages on May 25, 2007 at 1:31 pm: 1
[…] The Astroprof gives you a detailed explanation of nebulae. […]
Debbie Soltis on May 27, 2007 at 12:00 pm: 2
Loved your article and descriptions of pictures. Very clear and understandable. I teach astronomy at the high school level and appreciate your readable and interesting comments!
Lab Lemming on May 29, 2007 at 3:52 am: 3
Istn’t eta carina a high mass star?
Astroprof on May 29, 2007 at 8:32 am: 4
Yes, Eta Carinae is an extremely high mass stars. Such stars tend to be quite unstable and shed mass into space, becoming mass-loss stars. This material lost from Eta Carinae created the Eta Carinae Nebula, but the star itself still has an enormous mass left — well over 100 times the Sun’s mass.