Nebula Facts – A Guide To Nebulae

Nebulae are very large—sometimes even millions of lightyears in diameter. They can often be nests for new stars to be born in, due to all of the matter that makes up the cloud. The materials collect into denser regions, and the density attracts more matter, which will become a star over time. Scientists believe that the other matter inside nebulae can form planets and other things necessary to create new solar systems.

 

 

For a long time, any cloudy object observed was referred to as a nebula. Many astronomers since Ptolemy in 150 CE have been interested in these unusual patches in the night sky. In 964, the Persian astronomer Abd al-Rahman al-Sufi noted “a little cloud” in the area where we later learned the Andromeda Galaxy is.

Things started to pick up fast after the 1600s. Three different astronomers discovered the Orion Nebula independently. In 1715, Edmund Halley (whom the famous Halley’s Comet is named after) published a list of six nebulae, and another well-known name, Charles Messier (many objects in space are called Messier objects), compiled his own list by 1781. Other astronomers published lists, too. The Herschel siblings created three catalogues that contained a total of 2,510 nebulae and star clusters around the end of the 1700s.

Around 1922, astronomers realized that a lot of these objects that had been called nebulae were actually galaxies. That helped astronomers start to narrow down the differences in what they were viewing.

Edwin Hubble, after whom the famous Hubble telescope is named, announced that the light from nebulae all comes from stars.

Since then, other scientists have made many discoveries into the different types of nebulae that exist and their special qualities.

 

 

There is a lot of overlap in types of nebulae, because some are subcategories of other kinds, but these are terms that get used often.

 

Pronounced as “H 2,” these regions are mostly made of hydrogen.  They can come in many shapes and sizes, and can be clumped together or spun out in thin strings, or form strange shapes like the Horsehead Nebula.

They are active star-making regions, and over several million years will create thousands of stars. Over time, as the stars age and die, the gasses that make up the H II region are dispersed. What gets left behind are star clusters, like the Pleiades. Some of our familiar nebulae come from H II regions, like the Orion Nebula. The famous Hubble picture of the Pillars of Creation in the Eagle Nebula also comes from an H II region.

 

A diffuse nebula is one that is spread out and whose edges aren’t easy to define, like very thin clouds in the sky. Diffuse nebulae are usually emission nebulae, reflection nebulae, and/or dark nebulae.

One characteristic they share is that they emit a lot of infrared light from the nebula’s dust, regardless of how visible they are.

 

Emission nebulae are made of ionized gasses that emit light in different colors. They usually have a star close by. Some types of emission nebulae are H II regions and planetary nebulae.

 

When red giant stars get very old, sometimes they start throwing off a shell of ionized gas that keeps expanding due to strong stellar winds blowing it away, a lot like blowing bubbles with chewing gum.

Planetary nebulae don’t really have anything to do with planets, but when William Herschel looked at them through his telescope in the 1780s, he thought they might be matter, forming planets. He called them planetary nebulae, and it became the common name. It was never changed to something more accurate.They last a very short period of time compared to most things in the universe—only a few tens of thousands of years. A star usually lives for several billion years.

Planetary nebulae seem to form from stars that range from having a little less mass than our sun up to about eight times the Sun’s mass. (Stars that have more than eight times the mass of our sun usually explode into spectacular supernovae.)

Some astronomers think planetary nebulae are very important in the evolution of galaxies. When stars are born, they are mostly made up of hydrogen and helium, but over their life cycle, they start creating heavier elements, which get blown away by their stellar winds. Some of these elements are carbon, nitrogen, and oxygen, which are very important for the existence of life in the universe as we know it.

Right now, we know of about 3,000 planetary nebulae in our galaxy, the Milky Way, out of roughly 200 billion stars. Most of them are near the center of our galaxy.

They come in many shapes, but most are spherical, elliptical, or bipolar. The spherical ones are usually produced by older stars, like our sun.

We don’t really know why they come in so many shapes, but astronomers hypothesize it may have something to do with the magnetic fields around them.

 

As mentioned above, some planetary nebulae have what is called a bipolar structure. These nebulae look like hourglasses, sometimes with each end of the “hourglass” extending long and thin in either direction from the center. Sometimes they are spread out wide like butterfly wings.

 

Even though they’re classified among them, astronomers aren’t quite sure how, or if, they’re related to planetary nebulae.

 

Reflection nebulae are clouds of interstellar dust that have a star or stars close enough to shine light on them to make them visible to astronomers.

Reflection nebulae and emission nebulae sometimes show up in the same places, because emission nebulae are created when stars are close enough to the dust to start ionizing it and making it bright.Reflection nebulae are usually blue because the dust reflects blue light better than other colors.

Sometimes they’re grouped together as diffuse nebulae.

 

When a massive star reaches the end of its life, sometimes it explodes as a supernova. When this happens, the star expels lots of the material that used to be part of the star.

This process of expelling the old star material creates a shock wave ahead of it that superheats anything it passes through. The shockwave spends hundreds to thousands of years expanding and becomes very large. As the superheated material cools, it forms a shell around the supernova remnant.

Astronomers think supernova remnants are a major source of cosmic rays in the universe. The word “ray” isn’t completely correct: they are particles. One of these particles is carbon.

Scientists use the carbon-14 isotope of carbon to help them figure out roughly when carbon-based lifeforms existed. This is important in the study of fossils, the environment, and ancient people.

Cosmic rays have helped keep the level of carbon-14 in our atmosphere almost constant for around 100,000 years, letting humans use carbon dating with a reasonable amount of accuracy for anything that’s existed within the last 60,000 years.

 

Also known as plerions, pulsar wind nebulae are often found inside the shells of supernova remnants. They have also been found around older pulsars whose supernova remnants have disappeared. They can tell astronomers a lot about how the nebula’s pulsar is interacting with the space around it through careful observation of the pulsar wind nebulae, making them a great find for scientists interested in pulsars.

 

Also known as absorption nebulae, dark nebulae are interstellar clouds so dense that they hide the light behind them. They are like thick storm clouds on Earth.

Astronomers use radio waves and infrared detection to find out what’s behind the dense clouds. A small dark nebulae is called a Bok globule, after astronomer Bart Bok, who observed them for the first time. They are some of the coldest things in the universe and are still mysterious to astronomers, so they’re being heavily researched.

These small areas can usually be found within larger H II regions.

The largest dark nebulae can easily be seen with the naked eye, looking like black or darkened patches in the Milky Way.

 

There are millions and millions of Nebulas in the Universe, some more notable than others. Here’s a small list of the most famous of Nebulae that have been discovered so far.

 

Also called NGC 6543, the Cat’s Eye Nebula is a planetary nebula in the constellation of Draco. It was first discovered by William Herschel in 1786, and it was the first planetary nebula to be observed with a spectroscope in 1864, giving astronomers insight into what makes up planetary nebulae.

The Cat’s Eye Nebula is very complicated in its structure, possessing knots, twists, bubbles, arcs, concentric rings, and a very large (over three light years across) but faint halo extended far away from its center. The reason for its structure isn’t well understood.

Since it is so complex, astronomers think it might have a binary (double) star system inside of it which are causing different forces to act on the materials that make it up.

 

Also known as M1, NGC 1952, and Taurus A, the colorful Crab Nebula was first observed by John Bevis in 1731, and then independently rediscovered by Charles Messier in 1758. In 1920, scientists began to realize it was created by a supernova when Carl Otto Lampland noticed he had seen changes in its structure over time.

It is believed by many to be the result of a supernova (SN 1054) that was recorded by Chinese (most extensively), Arabic, and European astronomers, and possibly Native Americans and Aboriginal Australians.The Crab Nebula has a pulsar as its central star, which makes the surrounding nebula very bright. In good conditions, it can even be spotted with binoculars.

Since many scientists believe they can date the supernova that created the Crab Nebula, it has been heavily studied, especially in the field of pulsar study, which only began in the late 1960s. Knowing the age of the supernova helps astronomers gather information about how supernova remnants form, as well as pulsar wind nebulae, and how they evolve.

The Crab Nebula is roughly oval-shaped and looks like lots of intricately woven filaments—a little like a tangled spider web. The filaments are the remains of the star’s atmosphere and are made of many different gasses and elements.

 

The Eagle Nebula is also known by a lot of names, including Messier 16/M16, NGC 6611, the Star Queen Nebula, and The Spire. The names “Eagle” and “Star Queen” come from what astronomers thought a dark area in the center of the nebula looked like.

It is part of a diffuse emission nebula, or H II region, called IC 4703. It’s located in the constellation Serpens Cauda, and the Eagle Nebula is in the Sagittarius Arm of the Milky Way.This nebula has been especially famous in pictures, due to a photo of a region called the Pillars of Creation that was taken by the Hubble Telescope. The Pillars are active star-forming regions. Dark areas in pictures of the pillars are believed to be Bok globules.

 

First recorded in 1888 by Willamina Fleming, the Horsehead Nebula—which is also known as Barnard 33—is a dark nebula in the constellation Orion, just south of the easternmost star in Orion’s Belt. It’s part of the Orion Molecular Cloud Complex, which is a big group of nebulae and stars in the constellation.

 

The Horsehead Nebula also gets featured in photos because, being a dark nebula, it shows up very well against the surrounding portions of the Orion Complex. It gets its name from its shape, like many nebulae. It especially looks like a horse’s head when it’s viewed with less powerful telescopes from Earth.

 

 

One of the easiest nebulae to see for the amateur observer, the Orion Nebula is known by a few other designations: Messier 42, M42, and NGC 1976. It is a diffuse nebula south of Orion’s Belt in the Orion constellation, and is among the brightest nebulae. It’s even visible to the naked eye with reasonable sky conditions (some light pollution is tolerable), and shows up nicely with binoculars and small telescopes.

A very beautiful nebula and easy to observe, it’s been one of the most photographed and intensely studied objects in the night sky. Astronomers have learned a lot about how both stars and planetary systems form, among many other important things this nebula has taught people on Earth.Some people speculate that the Mayan civilization of Central America may have described the nebula in one of their creation myths. Three stars in Orion form a rough equilateral triangle with Orion’s Sword and the Orion Nebula in the center. The nebula may correspond to smoke or fiery embers of creation in the myth.

Strangely, since it is easily visible in the sky, many early astronomers did not make note of it when they were writing about areas of nebulosity, and even Galileo didn’t mention it despite making observations with a telescope of areas around it. Some people speculate that some of the stars illuminating the nebula have brightened since then. Its discovery is credited to Nicolas-Claude Fabri de Peiresc in 1610, though the first person to publish about it was Johann Baptist Cystat in 1619.

About 700 stars in different stages of their lives make their home in the nebula and over 150 protoplanetary disks have been found, as well. They’re considered to be very early solar systems, just starting to form.

 

Observing Nebulae

One of the easiest targets for observation is the Orion Nebula, since it is visible in places that have some light pollution and doesn’t require a perfectly dark sky to see with the naked eye.

Most people are familiar with the straight row of three stars that make up Orion’s Belt. Once they’ve been located in the sky, approximately halfway between the middle star of Orion’s Belt and an imaginary line drawn between the two stars of Orion’s “feet” is the Orion Nebula. Depending on the amount of light pollution, a fuzzy patch will be seen in the sky.

Using an ordinary pair of binoculars or a small telescope is very helpful for getting a better look at it and other objects in the night sky that are a little hard to see with the naked eye. These can also aid observation in areas with more light pollution, such as larger cities, and help viewers everywhere appreciate these magnificent stellar objects.