Seagull Nebula
Seagull Nebula | Credit: NASA
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Content co-written by:
Heather Sickels
April 12th, 2017

The word nebula is derived from the Latin nebula, which means 'cloud.' Loosely, a nebula is a visual classification for bright, cloudy objects appearing beyond our solar system in interstellar space. Over time, there have been a variety of objects that have been classified as nebulae. These include certain diffuse comets and galaxies (such as Andromeda, which we now know is a galaxy), which to early astronomers often appeared to be clouds in space. For a long time, galaxies were believed to be Spiral Nebulae until approximately 1922 when astronomers realized that they were a separate phenomena. Galaxies are much larger objects consisting of masses of stars that exist outside the Milky Way. Nebula are now, however, generally understood by scientists to consist of vast, mostly bright clouds of interstellar dust and gases including hydrogen and helium, hundreds of lightyears in diameter.

Facts about Nebulae

  • Nebula are defined as interstellar clouds of dust, hydrogen, helium and ionized gases that are hundreds of light years across.
  • There are multiple types of nebula, including Dark, Emission, Planetary and Reflection varieties.
  • Among the most famous types of these phenomena include the Orion, Horsehead, Crab and Ring nebulae.
  • Some can not be seen in visible light, and require infrared or other types of cameras to be seen.
  • The closest nebula to Earth is called the "Helix Nebula". It is roughly 700 light years from here.

What are Nebulae?

Nebulae form in the deep interstellar medium, the space between the stars, where the gas and dust molecules clump together to form gargantuan nebulous clouds. Inside these, gas and dust aggregate together due to gravity and collision, and when enough critical mass and density is achieved, the stellar fusion ignites into a star. This is why nebulae are often referred to as star nurseries, and they have a characteristic glow from the young stars that are forming and emanating within them. The matter that is left encircling the stars form into planets, moons, asteroids and comets. This is a likely model of how Earth's solar system formed.

How Big are Nebulae?

Most nebula are huge and span light years--even millions of light years across. On average, nebulae occupy trillions and trillions of miles in space. For perspective, the Earth's diameter is just under 8,000 miles and the Sun is about 865,000 miles in diameter. Even a small nebula would easily consume our entire solar system - they are vast regions.

Types of Nebulae

Most of these phenomena can be placed into one of four or five categories: Dark, Emission, Planetary, Reflection and Supernova types.

Dark. Dark Nebula containing carbon, silicon, and a variety of other elements appear dark because they are so dense that they block the light of the stars in the background. These microscopic particles are coated in frozen nitrogen and carbon monoxide, which block visible light, so they cannot be seen with a normal telescope. Instead, they can only be seen as dark shapes in the infrared spectrum. Despite not emitting any light, these nebulae can still become Stellar Nurseries as well as create planets and other large astronomical objects. In the center of the Cygnus constellation of the milky way, there is a dark patch known to be a Dark Nebula. The Coalsack Nebula and the Great Rift are other examples of Dark Nebulae.

Emission. By contrast, Emission Nebulae are huge, fluorescent clouds of gas that have become ionized and glow various colors. The light emitted by the stars embedded within them, or sometimes behind them, charges the gas of the nebula and causes this spectacular glowing effect. Most often, this occurs when the gases of the nebula become so dense that they collapse into a hot, dense point. This ignites to become a star! These types of nebulae are associated with special star formation regions known as Stellar Nurseries. The green-colored Orion Nebula is twenty lightyears across, and it is a great example of this kind of Emission Nebula. This is a Stellar Nursery where stars are being born. Another famous example of an Emission Nebula is the Eagle Nebula, popularized by the incredible images taken by the Hubble Space Telescope. This particular object is also home to the massive gas formations known as the Pillars of Creation. The Eagle Nebula is located in the Serpens constellation and is one of the most spectacular objects in the galaxy seen so far.

Planetary. A planetary nebula is a kind of emission nebula, meaning it consists of ionized gases that emit light, rather than absorb it. Planetary Nebulae appear greenish and round (like a planet) through a telescope and are formed when certain stars evolve past the Red Giant phase of their life cycle. At this point, they shed their outer layers of atmosphere leaving only their cores. These remnants are called White Dwarfs. The White Dwarf continues to emit light, and the leftover dust and gas becomes a Planetary Nebula. Not only do they look like planets but they orbit their stars in the same way planets do. Examples include the Ring Nebula, which resembles a colorful hot spring pool like the ones found in Yellowstone National Park. The glowing shell of ionized gas was ejected from an old red giant star late in its life.

Reflection. Unlike Emission Nebulae, Reflection Nebulae only reflect the light of nearby stars and other nebulae. These nebulae always appear blue, like the sky, and for almost the same reason. The dust in these clouds only scatter light in the wavelengths at the blue end of the spectrum. The Witch Head Nebula, which is 900 lightyears away in the Eridanus constellation, is an example of both a Reflection Nebula and a Supernova Remnant. A Supernova Remnant occurs when a supernova, one of the most powerful phenomena in the universe, sends out matter for hundreds of lightyears in every direction. Another example of a Supernova Remnant is the incredibly beautiful Crab Nebula in the Taurus constellation.

Supernova. A Supernova Remnant occurs when a supernova, one of the most powerful phenomena in the universe, sends out matter for hundreds of lightyears in every direction. Another example of a Supernova Remnant is the incredibly beautiful Crab Nebula in the Taurus constellation.

Future Nebula Research

With the rise of space telescopes, such as Hubble, incredible imagery of these fantastic interstellar formations have come to light. With the introduction of the James Webb Telescope, we can expect to expand our vision of our place in the universe.