What Are Nebulae and Why Does Our Universe Need Them?
Nebulae are vast clouds of dust and gases in outer space that act as nurseries for the birth of new stars. Some nebulae are formed as a result of the death of a star. After its lifecycle is complete, some stars explode into supernovas, throwing out vast clouds of debris and gases into space. Other nebulae are formed when interstellar matter such as gases and dust particles come together in space and form clumps due to gravity, creating areas of greater and greater density.
Turbulence deep within these clouds creates areas of high density referred to as "knots". When the knots contain sufficient mass, the gas and dust can collapse from the gravitational attraction. As the knot collapses, gravitational pressure causes the material at the center to heat up, creating a protostar. When the core of the protostar becomes hot enough to ignite fusion, a star is born.
However, not all of the material in the collapsing cloud ends up as part of a star. The remaining dust can also coalesce into planets or smaller objects such as asteroids. According to Hubblesite, three-dimensional computer models of star formation predict that the collapsing gas and dust commonly break up into two or more distinct blobs. Researchers suggest this is why the majority of the stars in the Milky Way are paired or in groups.
One of the brightest nebulae is the Orion nebula, located around 1,345 light-years away. This is also the closest star-forming region to Earth.
However, Orion is not the closest nebula; the Helix Nebula, spotted by Karl Ludwig Harding in the early 1800s, is thought to be the closest nebula to Earth. It is located at a distance of around 655 lightyears but it consists of the ruins of a dead star and can not give birth to new bodies.
How does a nebula is formed?
While space may appear empty, it actually contains a fair amount of gas and dust particles, which is referred to as the Interstellar Medium (ISM). Much of this is composed of gas, with about 75% hydrogen and around 25% helium. The interstellar medium includes both neutral atoms and molecules and charged particles, such as ions and electrons. Normally, these particles are very spread out, with an average density of around just 1 atom per cubic centimeter. (In comparison, at sea level, Earth’s atmosphere contains around 25 x 1018 molecules per cubic centimeter.)
Although this is a very low density, the total amount of matter can add up to quite a bit over the vast interstellar distances. Over time, gravitational attraction can cause this matter to coalesce into larger and larger clumps. In regions of high density, pressure from gravity may cause the gaseous material to heat up enough to ignite fusion. When this happens, new stars may be formed.
However, gravitational attraction is not the only way a nebula may be formed. When a large enough dying star explodes in a supernova (an astronomical event that marks the end of the lifecycle of a star), the material ejected from the explosion, along with other interstellar material swept up in the shockwave, can form a type of nebula called a supernova remnant nebula. These are not always visible, but may emit powerful X-ray and radio waves due to interactions with the surrounding interstellar medium. Remnant nebulae eventually disperse into the ISM, usually after several hundred thousand years or so.
Another kind of nebula that is formed after the death of a low-mass star (having a mass between one and eight solar masses) is called planetary nebulae.
The word planetary is a bit misleading because these nebulae have nothing to do with the planets. Rather, they were given the name by 19th-century astronomer William Herschel, because the new objects resembled a gas giant when viewed in a telescope.
At the end of its lifecycle, the star blasts off its outer layer. The matter from the outer layer becomes distributed around the dying star and causes the formation of a planetary nebula. The radiation released from the star’s expanding core ionizes the debris and gases that resulted from the blast. At this time, when the core is still burning heat, it is called a white dwarf star. As the core of the planetary nebula cools, it becomes a black dwarf star.
Planetary nebulae resulting from the death of dwarf stars (such as the Medusa nebula) and supernova remnant nebula (like the Crab nebula) can not give rise to new stars, but interstellar molecular clouds like the Swan nebula or the Orion nebula act as active star nurseries. Most nebulae are vast, spanning several light-years in size but they have very low densities, for instance, the weight of Earth is 5.972 x1024 kg but a nebula having size the same as of the Earth would weigh only a few pounds.
Types of nebulae
Apart from supernova remnants and planetary nebulae, there are three other types of nebulae. Most fall into the category of Diffuse Nebulae, which means they have no well-defined boundaries. The diffuse nebulae are subdivided into two categories, based on their behavior with visible light — “emission nebulae” and “reflection nebulae”. In addition to these, there are also dark nebulae.
Emission nebulae
Emission nebulae consist of clouds of ionized gas that emit light at optical wavelengths. They have highly varied densities, and their mass usually ranges from 100 to 10,000 solar masses.
They can occur when an interstellar gas cloud containing a high proportion of neutral hydrogen atoms becomes ionized by hot O-type stars (bluish-white stars with surface temperatures usually around 25,000–50,000 K). These give off large amounts of high-energy photons. These photons break the neutral hydrogen atoms into hydrogen nuclei and electrons which recombine to form neutral hydrogen in an excited state. As the neutral hydrogen atoms return to a lower energy state, they emit photons at wavelengths in the red end of the spectrum, giving emission nebulae a distinctive red color.
Emission nebulae are often called HII regions because they are largely composed of ionized hydrogen (astronomers use the term HII to refer to ionized hydrogen, and HI for neutral hydrogen).
Reflection nebulae
These are created when light from a star is scattered or reflected off a neighboring dust cloud. The brightest reflection nebulae are illuminated by B-type stars. These are very luminous but generally have temperatures lower than about 25,000 Kelvin, cooler than the O-type stars that produce emission nebulae.
The scattered light is slightly polarised. Because the size of dust particles in the cloud is similar to the wavelength of blue light, blue light is scattered the most. The result is that these nebulae often appear blue in color.
Dark nebulae
These contain very high concentrations of dust particles so that they scatter and absorb incident optical light, making them appear as a dark patch in space. They are most noticeable when they are located in front of a brighter area, such as an emission nebula or in a region with a large number of stars. Well-known examples include the Coal Sack Nebula, visible in the southern hemisphere, and the Horsehead Nebula.
Dark nebulae are also very cool, with an average temperature of around 10 to 100 Kelvin. These low temperatures encourage hydrogen formation and this is why dark nebulae generally act as rich star formation regions.
Large dark nebulae containing more than a million solar masses of material and extending over 650 light-yeas or more are known as giant molecular clouds. The smallest ones, called Bok globules, maybe less than 3 light-years across and contain less than 2,000 solar masses of material.
Interesting facts about nebulae
Here are some surprising facts about outer space dust clouds:
- In 1786, Astronomer Frederick William Herschel discovered a complex nebula named the Cat’s Eye (NGC 6543). This dust cloud includes a halo three light-years around and numerous structures that look like knots, bubbles, concentric rings, loops, etc. It was the first planetary nebula spotted by scientists, and until now, astronomers have not been able to completely understand its structure.
- Before the 20th century, the idea of “galaxy” as a separate concept was not known. Astronomers considered nebulae and galaxies to be the same thing, and this is why the first detected galaxy, Andromeda, was previously referred to as the Andromeda Nebula. However, now we know that a galaxy and a nebula are different things. The key difference between the two is that a nebula arises from the interstellar medium, it primarily consists of helium and hydrogen gas, and generally, nebulae have sizes ranging between a few and a hundred light-years. Whereas a galaxy is much larger, spaning between hundred and thousands of light-years, and is made up of dust, stars, nebulae, and solar systems.
- The largest nebula, called “Tarantula,” is 1,800 light-years in size at its widest span and it is located at a distance of around 170,000 light-years from our planet. It is an emission nebula situated in the Large Magellanic Cloud. Tarantula is believed to be harboring as many as 800,000 stars and at present, it is considered the most active star nursery known.
- The hottest stars in our galaxy could have temperatures around 250,000 °C, and they are found in the Butterfly nebula (NGC 6302), a dust cloud that is spread in the form of butterfly wings and is located around 4,000 light-years from Earth in the constellation of the Scorpion.
- In 1995, the Hubble Space Telescope captured an image referred to as the “Pillars of Creation”. The image shows towering tendrils of cosmic dust and gas in the heart of the Eagle Nebula (NGC 6611). The "pillars" are clouds of dust and gas in an active star-forming region of the nebula, with newborn stars hidden in their wispy columns. A later NASA study claimed that a supernova inside the nebula had destroyed the pillars, however, new evidence suggests that the pillars still exist and will likely remain for the next few hundred thousand years, before slowly evaporating away.
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