What is a nebula? Nebula is a Latin word which means ‘Cloud’. But when we’re discussing about the universe, we are not talking about our average clouds that we see during rainy seasons. Nebulae (plural of nebula) in context of universe are interstellar clouds that are made NOT of water vapors BUT of plasma, helium, hydrogen and dust. Now that we know that a nebula is not an average cloud, it is time that we learn about 20 interesting nebula facts and try to get a better understanding of these interstellar clouds.
1. Nebulae are usually formed by gravitational collapse of gasses in the interstellar medium. The particles that collapse have their own gravitational attraction and they clump together to form these clouds.
2. Nebulae are ginormous. They may stretch over hundreds of light years across. A light year is essentially the distance that light can travel in one year. Here is a quick calculation:
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One year = 31,536,000 seconds. Light covers a distance of 300,000 kilometers in a second. Therefore, in one year light is capable of traveling (31,536,000 x 300,000) kilometers = 9,460,800,000,000 kilometers. Let us trivially assume that we have a nebula in form of a perfect circle (which usually never happens). So, the maximum distance between two extreme points of the nebula will be its diameter. If our assumed circular nebula stretches only one light year across then, the diameter of the nebula will be 9,460,800,000,000 kilometers. So imagine how big a nebula can be that stretches hundreds of light years across!
3. Nebulae are often referred to as star nurseries of the universe because stars often form inside the nebulae.
4. We know that nebulae are ginormous but how do they look like? Nebulae have myriads of shapes. Some look like a horse head, some look like a crab, some like a butterfly and some look like cat’s eye!
5. We know that stars can form inside nebulae but how? According to scientists, gas and dust inside these nebulae squash together under their own gravitational pull. As a result, the clouds start contracting and get denser. The denser they become, the hotter they get. Eventually they become so hot that hydrogen present in them gets ignited and new stars comes to life.
6. When a star forms inside a nebula, ultraviolet rays are emitted and the entire nebula is lit up, resulting in what is known as emission nebula. Emission nebulae are usually pink or red in color because of the sizzling hot particle rays but they can have other colors.
7. The Orion constellation that we see has an emission nebula known as Orion nebula.
8. Apart from emission nebulae we have other types of nebulae known as the reflection nebulae. They are so named because they do not emit their own light. They only reflect the light from the nearby stars. Reflection nebulae appear blue in color.
9. An interesting phenomenon is that presence of reflection nebula means the presence of an emission nebula somewhere close.
10. The Orion constellation also has a reflection nebula which is known as the Witch Head reflection nebula because it looks like the head of a witch from the side.
11. We have another type of nebula known as planetary nebula. Don’t be deceived by the name. Planetary nebulae have nothing to do with planets. A planetary nebula forms when a star similar to the size of our Sun runs out of hydrogen fuel in the core and the hydrogen in the outer shell starts fusing. As a result, the star starts to expand and becomes a red giant. Sometime later, the core of the red giant is heated so much that it starts fusing helium atoms that were formed because of hydrogen fusion in the core. Eventually when the helium atom is also used up, the star becomes very unstable. This is when the outer layer of the star is ejected leaving behind the core which is then known as white dwarf. This white dwarf still emits radiations. These radiations ionize the gas atoms present in the surrounding; leading to spectacularly colorful displays known as planetary nebula.
12. Then…we have the supernova remnant. Enormously big stars eventually explode shooting out matter in every possible direction. These ejected materials eventually form a nebula. A classic example of a supernova remnant is the Crab nebula.
13. The coolest place to naturally exist in our known universe is the Boomerang nebula. It is also known as a Bow Tie nebula. It was first observed in 1980 by two astronomers using Anglo-Australian telescope from Siding Spring Observatory. They failed to get a proper view and observed a slight asymmetry in the lobes of the nebula and hence, named it Boomerang nebula. Later in 1998, the Hubble Space Telescope observed the proper symmetrical shape of the nebula which looks very much like an hourglass. The temperature of the nebula is -272 degrees Celsius which is only 1 degree above absolute zero!
14. The famous Eagle nebula has structures known as Pillars of Creation because those pillar-like structures give birth to new stars. The Eagle nebula is also known as Star Queen Nebula and is located in Serpens constellation.
15. We have something known as Dark nebulae. They are so called because the interstellar clouds are so dense that they block any light from background stars and/or reflection nebulae. Dark nebulae can be observed against the backdrop of light. Dark nebulae are capable of blocking light because the dust particles which are of the size of sub-micrometer have coats of frozen nitrogen and carbon monoxide that can block light at visible wavelengths. Just because Dark nebulae are invisible without the backdrop of light such as Milky Way, it never means that they are not important. They can give birth to planets, stars, moons etc. Great Rift and Coalsack Nebula are two examples of dark nebulae that are visible to naked eyes as they appear as darker patches against the bright background of our Milky Way.
16. About 5 billion years from now, our Sun will become a nebula.
17. The nebula closest to Earth is the Orion nebula which is at a distance of 1,300 light years from us. The nebula stretches to a distance of 25 light years.
18. Until 1920s, scientists used to classify distant galaxies as nebulae.
19. Some nebulae may have parts of other nebulae inside them. A typical example is the Gum nebula which contains the Vela nebula inside it.
20. Nebulae outside the Milky Way are referred to as extra-galactic nebulae.
Some notable nebulae names:
- Ant Nebula
- Barnard’s Loop
- Boomerang Nebula
- Cat’s Eye Nebula
- Crab Nebula
- Eagle Nebula
- Eskimo Nebula
- Eta Carinae Nebula
- Flame Nebula
- Fox Fur Nebula
- Helix Nebula
- Hourglass Nebula
- Horsehead Nebula
- Lagoon Nebula
- Orion Nebula
- Pelican Nebula
- Red Square Nebula
- Ring Nebula
- Rosette Nebula
- Tarantula Nebula
That concludes our list of 20 interesting nebula facts. If you know and want to share a few more facts, feel free to do so using our comments section.
Additional Nebula Facts:
Date: 12th January, 2016. This section on nebula facts is being added newly on request from one of our student readers.
(i) Planetary nebulae have average temperature ranging between 8,000 Kelvins and 23,000 Kelvins (7726.85 degrees Celsius and 22726.85 degrees Celsius). However, closer to the surface of star, the temperature can be as high as 100,000 Kelvins or 99726.85 degrees Celsius.
(ii) It is estimated that there are at least 10,000 planetary nebulae in our Milky Way Galaxy. However, so far only 2,000 have been detected.
(iii) A planetary nebula has a life span of tens of thousands of years, which though very high is actually very insignificant compared to astronomical standards.
(iv) Planetary nebulae are primarily composed of carbon, neon, nitrogen, oxygen and other elements that have more or less the same weight as the predominant ones. Small amounts of metals and helium are also present in planetary nebulae.
(v) Planetary nebulae expand at a rate of 20 km per second per second. This expansion rate can be as high at 30 km per second per second (km/s2).
(vi) The gas cloud of a planetary nebula has a density ranging between 103 to 106 particles per centimeter. This density decreases as the age of the nebula increases.
(vii) Nearly 90% to 95% of the light emitted by a planetary nebula is green, i.e. the light has a wavelength of 500.7 nm. This green color comes from the oxygen ions present in a particular state inside the nebula. The state of the oxygen ions is known as ‘forbidden lines’.
(viii) Nebula M27 was the first planetary nebula that was discovered on July 12, 1764. It is a Dumbbell nebula and is located at a distance of 1,200 light years from our earth.
(ix) The Pillars of Creation of the Eagle Nebula no longer exist. They were destroyed some 6000 years ago by a nearby supernova explosion. The reason we still see the non-existent Pillars of Creation is that they were actually located at a distance of 7,000 light years from us. Since light takes time to travel, what we see is actually an image from the past. The light that had already traveled for (7000 – 6000 = 1000 light years) before the supernova explosion took place is still reaching us. Thus, we will continue to see the Pillars of Creation till 2995 CE (remember that NASA first detected the Pillars of Creation back in 1995 using Hubble Space Telescope). After 2995 CE, we (or rather our future generations) will actually see the supernova explosion that tore away the Pillars of Creations. Isn’t that amazing? So basically our Universe is a gigantic time machine and everything we see in our universe is thing from the past because light from an event can come to use only after the event has actually taken place.
(x) Nebulae that are outside the confines of our Milky Way are known as extra-galactic nebulae.