By Bill Pellerin

Houston Astronomical Society

GuideStar Editor

When you look at the sky, what do you see? On a clear night, you’ll see stars, perhaps the moon, perhaps one or more planets. Do you ever see clouds in space? They’re a bit harder to see than many of the bright objects, but they are often the most interesting objects in the sky. These objects are generally called nebulae (the plural of nebula) and the word ‘nebula’ is from a Latin word meaning ‘cloud’. They don’t look like small points of light; they look hazy and, well, cloudy. For a long time, we didn’t know what these objects were, but now we do. If you listen, you’ll often hear observers say that an object is ‘non-stellar’ – meaning that it is not a star – meaning that it is nebulous.

Let’s begin with a bit of history.

One of the kinds of nebulae seen by early observers was a spiral nebula. One of the first observers to see these spiral nebulae was a British fellow named William Parsons, the 3^rd Earl of Rosse. In 1845, he built a very large telescope – called the Leviathian. With this telescope, he saw the spiral structure of M51, the object we now know as the Whirlpool Galaxy. In 1889, Vincent Van Gogh painted the famous ‘Starry Night’ which includes a spiral shape in the sky thought to be inspired by the drawing of Lord Rosse.

By Bill Pellerin

Houston Astronomical Society

GuideStar Editor

When you look at the sky, what do you see? On a clear night, you’ll see stars, perhaps the moon, perhaps one or more planets. Do you ever see clouds in space? They’re a bit harder to see than many of the bright objects, but they are often the most interesting objects in the sky. These objects are generally called nebulae (the plural of nebula) and the word ‘nebula’ is from a Latin word meaning ‘cloud’. They don’t look like small points of light; they look hazy and, well, cloudy. For a long time, we didn’t know what these objects were, but now we do. If you listen, you’ll often hear observers say that an object is ‘non-stellar’ – meaning that it is not a star – meaning that it is nebulous.

Let’s begin with a bit of history.

One of the kinds of nebulae seen by early observers was a spiral nebula. One of the first observers to see these spiral nebulae was a British fellow named William Parsons, the 3^rd Earl of Rosse. In 1845, he built a very large telescope – called the Leviathian. With this telescope, he saw the spiral structure of M51, the object we now know as the Whirlpool Galaxy. In 1889, Vincent Van Gogh painted the famous ‘Starry Night’ which includes a spiral shape in the sky thought to be inspired by the drawing of Lord Rosse.

Lord Rosse didn’t know what he was seeing, though and neither did anyone else. The mystery continued until the early 20^th century – some astronomers holding to the notion that everything that exists is within the Milky Way and some astronomers convinced that there were other galaxies outside the Milky Way. In April of 1920 a debate was held between Harlow Shapley (who had already determined the center of the Milky Way by mapping out the positions of globular clusters), and Heber Curtis. Shapley supported the Milky Way is everything position, and Curtis supported the idea that galaxies existed beyond the Milky Way. It was Edwin Hubble who settled the issue when he determined the distance to the Andromeda Galaxy using Cepheid variables. He announced his conclusion in early 1925, and from then on astronomers knew that these spiral nebulae were, in fact, separate galaxies. They only look nebulous because they are so far away that we cannot easily resolve individual stars.

So, galaxies are not nebulae. What’s left?

Nebulae are usually identified as emission, reflection, or dark. An emission nebula is one which glows (red) because material in the nebulous cloud of interstellar dust is ‘excited’ by radiation from an imbedded star or an imbedded white dwarf. For a planetary nebula, an imbedded white dwarf radiates a lot of UV (ultraviolet) radiation. This radiation strikes the molecules of the gas which absorb this energy and re-radiate the energy as light.

A reflection nebula is one which simply reflects light from a star or a set of stars. This reflected light looks blue to us, for the same reason that the sky looks blue. The blue light is reflected by the cloud and some of that reflected light reaches us. The Trifid Nebula has both an emission component and a reflection component.

A dark nebula looks like a dark spot in the sky because the dark nebula hides any stars or glowing nebulae that may be behind the dark nebula. A common example is the Horsehead Nebula in Orion. Another favorite is Bernard 68 in Ophiuchus.

So, this is how the nebulae look, but knowing this does not answer the question: What are these things?

So, let’s look at nebula in a different light (so to speak). If we think of nebulae as pre-stellar or post-stellar, we end up with a much better understanding of the nature of these objects and their place in the universe.

Pre-stellar nebulae

A pre-stellar nebula is one comprised mostly of the elements that will condense, under the pull of gravity, to form new stars. The visible ones are those that are lit up by the new stars that have already begun to shine. The recipe for star formation is actually rather simple. Get a large cloud of about 75% hydrogen and about 25% helium, often with a little bit of heavier elements, and wait for gravity to do its job. As the material falls together to form a star, it begins to heat up. After some time the heat is high enough to initiate nuclear fusion (hydrogen to helium) and the star begins to shine. It ultimately settles into mid-life on the main sequence and becomes an ordinary star. Its characteristics (color, size, lifetime) depend completely on the mass of the material that formed the star.

There are many examples of star forming nebulae – the best known of these is the Orion Nebula, M42. There are many others, including the Eagle Nebula, M16. The dark nebulae can be star forming regions as well, but the nascent stars cannot be seen visually but they can be detected by infrared telescopes.

Post-stellar nebulae

Stars eventually run out of fuel. Low mass stars fuse hydrogen to helium, then helium to carbon and oxygen. At that point the star is not, and will not become, hot enough to fuse carbon. The result of this failure is a star that simply runs out of energy, but in its last burst of glory it becomes visible to us as a planetary nebula. The outer layers of the star are pushed away from the star late in the star’s life, and then are lit up by the ultra-violet radiation from the white dwarf star in the middle. (Calling what is left a ‘star’ is, in fact, a misnomer. The white dwarf is glowing from the residual heat of the star’s glory days, but nuclear fusion has ended. In the end, it cools off and disappears.)

Well known planetary nebula include the Ring Nebula (M57) in Lyra and the Dumbbell Nebula (M27) in Vulpecula. These planetary nebulae are only visible for 50,000 years or so, so look quickly.

High mass stars (usually defined as being larger than 8 times the mass of the sun), often end their lives as a supernova. A titanic implosion of material from the star produces the most energetic event in the universe, the supernova. After some time, we’re left with a shell of the material that was blown away, and this eventually dissipates.

We have two very good examples of supernova remnants to see. The Crab Nebula (M1) in Taurus is a compact supernova remnant, and the Veil Nebula (NGC6960 is a piece of the Veil) in Cygnus is a very large remnant, about 3 degrees on the sky – so large that you may not be able to see the entire nebula in one telescopic view.

Please go out and look at these, and other, nebulae.  You will need dark skies for most of these. Having an understanding of the nebula’s place in the universe (pre-stellar or post-stellar) makes the viewing all the more enjoyable.