All About Nebulas

A nebula (the Latin word for cloud) is a cloud of gas and dust in space and many can be found in our galaxy as well as in galaxies across the universe. Because nebulas are involved in the birth and death of stars, these regions of space are important to astronomers seeking to understand how stars form and expire.

Not only are nebulas a crucial part of astronomy for astronomers, but they make interesting targets for backyard observers. They're not as bright as stars or planets, but they are incredibly beautiful and are a favorite subject of astrophotographers. Some of the most intricate and detailed images of these regions come from orbiting observatories like the Hubble Space Telescope.

Types of Nebulas

Astronomers divide nebulas into several major groups. One of these is the H II regions, also known as large diffuse nebulas. H II refers to their most common element, hydrogen, the main component of stars. The term "diffuse" is used to describe the large and irregular shapes associated with such nebulas.

Nebulas and the Births of Stars

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H II regions are star-forming regions, places where stars are being born. It is very common to see such a nebula with flocks of hot, young stars within it. Those nebulas may be referred to as reflection nebulas since their clouds of gas and dust are illuminated by—or reflect—the light given off by these bright stars. These clouds of gas and dust may also absorb radiation from stars and emit it as heat. When that happens, they can be referred to as absorption nebulas and emission nebulas. 

There are also cold, dark nebulas that may or may not have starbirth occurring inside them. These clouds of gas and dust contain hydrogen and dust. So-called dark nebulas are sometimes referred to as Bok globules, after the astronomer Bart Bok who first observed them in the early 1940s. They are so dense that astronomers need specialized instruments to detect any heat coming from them that may indicate the births of stars. 

Nebulas and the Death of Stars

Depending on the size of the star, two classes of nebulas are created as stars die. The first includes supernova remnants, the most famous of which is the Crab Nebula remnant in the direction of the constellation Taurus. Thousands of years ago, a giant, high-mass star exploded in a catastrophic event called a supernova. It died when it began to fuse iron in its core, which stopped the star's nuclear furnace from working. In a short time, the core collapsed, as did all the layers above it. When the outer layers reached the core they "rebounded" (that is, bounced") back and that blew the star apart. The outer layers rushed out to space, creating a crab-shaped nebula that is still speeding outward. What's left behind is a rapidly spinning neutron star, created from the remains of the core. 

Stars smaller than the Crab Nebula's progenitor star (that is, the star that blew up), don't die quite the same way. They do, however, send masses of material into space in the millennia before their final death throes. That material forms a shell of gas and dust around the star. After it gently blows its outer layers to space, what's left shrinks down to become a hot, white dwarf. The light and heat from that white dwarf illuminate the cloud of gas and dust, causing it to glow. Such a nebula is called a planetary nebula, so named because early observers like William Herschel thought they resembled planets. 

How Are Nebulas Detected?

Nebulas of all kinds are best detected using telescopes. The best-known exception to this is the Orion Nebula, which is barely visible to the naked eye. It's much easier to observe a nebula using magnification, which also helps the observer see more of the light coming from the object. Planetary nebulas are among the dimmest ones, and they're also the shortest-lived. Astronomers suspect they only last perhaps only ten thousand years or so after they form. H II regions last as long as there's enough material to continue forming stars. They're easier to see because of the bright starlight that causes them to glow. 

Best-known Nebulas

As well as the Orion Nebula and the Crab Nebula, skygazers keep to observe these clouds of gas and dust should get to know the Carina Nebula (in the Southern Hemisphere Sky), Horsehead Nebula, and the Ring Nebula in Lyra (which is a planetary nebula). The Messier list of objects also contains many nebulas for stargazers to search out. 

Sources

• NASA, NASA, spaceplace.nasa.gov/nebula/en/.
• “Nebulae - The Dust of Stars.” Windows to the Universe, www.windows2universe.org/the_universe/Nebula.html.
• “Planetary Nebulae.” The Hubble Constant, 3 Dec. 2013, www.cfa.harvard.edu/research/oir/planetary-nebulae.
• http://skyserver.sdss.org/dr1/en/astro/stars/stars.asp