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 emptier 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 denser 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 the Cosmic Cloud Nebula.
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 a 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. Let us discuss some beautiful nebula up in space.
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 (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 Herschel, who observed that they were cloudy like a nebula but round like planets. Some look rounder 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.
Eta Carinae Nebula
When we are discussing The Cosmic Cloud-Nebula, how can we forget Eta Carina? The Eta Carinae Nebula (NGC 3372), also called the Carina Nebula or the Great Nebula in Carina, in the constellation Carina is a spectacular diffuse nebula and one of the largest and brightest in the sky. The Great Nebula in Carina is the showpiece of the southern hemisphere, larger than the Great Nebula in Orion in appearance. Its distance is estimated to be 6500 to 7000 light years and it shows the beautiful and intricate structure in small telescopes.
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.
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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).
I hope you like the above article about “The Cosmic Cloud Nebula”. It’s one of the spectacular phenomena in the sky that mesmerize us all. It always fascinates me and that’s the reason I decided to write about it. The contents are collected from different sources on the internet.