History of Nebulae Observation
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.
Types of Nebulae
H II Regions
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.
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.
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.
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.
Pulsar Wind Nebulae
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.
Cat’s Eye Nebula
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.
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.
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.
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.
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.