What is a Nebula?

Carina Nebula
Nebula are huge clouds of gases. Carina Nebula photographed by Hubble Telescope

Sky is full of interesting objects, one of which is nebula. You must have seen the beautiful images of nebulae like Crab Nebula, Orion Nebula, and Ring Nebula etc. These objects are named so differently because of their unique appearances which sometimes look like animals, eyes, rings and other familiar objects. Have you ever wondered what these beautiful structures are? How does such amazing structure form?

Nebulae are huge cloud of gases mostly Hydrogen and extend to very large distances in the interstellar space. The interstellar medium is not empty it consist of small dust and gases. The interstellar medium is 3 to 5 % in the spiral galaxy and it contributes to 20 % in the spiral arms. These dust particles in the interstellar medium consist of silicon, water, oxygen and argon etc. and contribute less than 1 % mass of the interstellar medium. Most of the mass of galaxy is due to stars and dark matter. The density of interstellar medium is not uniform and the gases are found in clumps. These clumps have relatively larger density than the normal density of interstellar medium. These clumps of gases are called nebulae.

The nebulae appear fuzzy with low resolution telescopes that are why they were not easily recognisable in historical observations. All fuzzy objects which do not appear like point stars were initially known as nebulae. You will be surprised to know that initially galaxies were also called the nebulae. The Andromeda galaxy was known as Andromeda Nebula. There were two categories of Nebulae, extra galactic nebulae and galactic nebulae. The extra galactic nebulae are group of huge number of stars which are known as galaxies these days. The galactic nebulae are clouds of gases present in our galaxy. But nowadays only galactic nebulae are called nebulae.

Size of Nebula

The range of size of Nebula is very large, it can be as small as the distance between the Earth and Sun and it can be as large as the entire Milky Way. For example the Tarantula Nebula is extends to the width of 1000 light years and size of Orion Nebula is about 24 light years. Thus we can say that the Nebulae appear in nearly all familiar cosmic scales.

Composition of Nebula

Nearly half of the mass of interstellar medium is present in the form of molecular clouds which are composed of mainly molecular Hydrogen and the temperature of which is very low about 10 K. Carbon Monoxide(CO) is also present in significant amount and the emission from CO help in detection and analysis on Nebula. The density of Hydrogen molecules are found to be nearly 1000 molecules per cubic cm in the region studied by CO emission.

Some regions in nebular clouds are composed of Hydrogen ions which has very less density of around 0.001 ion per cubic cm and the temperature is extremely high up to 10 million K.

The Nebula also contains other elements like Helium, oxygen, carbon and nitrogen etc. The total fraction of these elements is very less about two atoms per thousand.

Types of Nebulae

There are basically two types of nebulae dark nebulae and bright nebulae. The dark nebulae absorb all the light falling on them thus they appear as dark patches in the background radiation. The bright Nebulae has faintly luminous surface, they either emit their own light or reflect the light from the other nearby stars.

There are various subclasses of Nebulae like reflection nebulae, H II regions, diffuse ionized gas, planetary nebulae and reflection nebulae.

Molecular Cloud or Dark Nebula

Molecular Cloud or Dark Nebula is an opaque cloud due to the dust grains present in it. The shape of Dark Nebula is very irregular and they do not have well defined boundaries. The largest dark clouds are visible to naked eyes as dark patches in the background light of Milky Way galaxy. For example, the Coalsack present in the Southern Galaxy. The stars are born in these Molecular clouds.

The gas of cloud mostly contains Hydrogen in the form of H2 molecules. The largest molecular cloud in known as giant molecular cloud and it is extended up to 150 light year having density of 100 to 300 molecules per cubic cm at the temperature of only 7 to 15 K. The stars are also present in the molecular clouds but the opaque nature of cloud does not allow light to penetrate and come out of the nebula. The presence of these stars can be confirmed by the infrared thermal emission from the dust grains and micro wave emission from the molecules.

Horsehead Nebula - Dark Nebula
Horsehead Nebula is a Dark Nebula.
Credit and Copyright: Jean-Charles Cuillandre (CFHT), Hawaiian Starlight, CFHT

The formation of star within these molecular clouds takes place due to gravity. The hydrogen gas in the cloud collapses due to gravity and becomes denser. The high pressure result in very high temperature and once the temperature reaches the threshold temperature required to initiate the nuclear fusion reaction, the star starts glowing. The remaining gas and dust around the star forms planets.

Hydrogen Cloud

The hydrogen clouds or H I clouds are mainly composed of neutral hydrogen rather than ionized of molecular hydrogen. The hydrogen clouds are very easy to detect because they emit characteristic radiation at the wavelength of 21 cm. The neutral hydrogen is found in abundance in those clouds which have enough starlight to dissociate hydrogen molecules into atoms of hydrogen but do not have photons which can ionize the hydrogen.

If the neutral hydrogen were left alone at typical pressure until they reach equilibrium, they could exist either at cold temperature (80 K) or at warm temperature (8000 K), both determined by heating and cooling rates.

Reflection Nebula

The Reflection Nebula is that nebula which appears dark in the absence of light but it is able to reflect the light of nearby stars. The light from the stars are not energetic enough to ionize the hydrogen. The temperature of stars is less than 25000 K cooler than the O type star which would ionize the hydrogen and produce H II region. Example is Pleiades star cluster. Optical observations suggest that the star reflects nearly 60-70 % of light.

Reflection Nebula in Pleiades star cluster
Reflection Nebula in Pleiades star cluster. Image Credit : Mount Wilson Observatory

Emission Nebula or H II Region

The Nebula which contains ionized hydrogen is called H II region or emission nebula or diffused Nebula. The hydrogen gets ionized due to the light coming from the stars having temperature more than 25000 K. The density of clouds ranges from 10 to 100000 particles per cubic cm and the temperature is about 8000 K.

The largest H II regions are 500 light years across and have ionized gas equivalent to at least 100000 solar masses. These enormous H II regions are powered by cluster of massive hot stars rather than any single stellar body.

The only H II region visible to naked eyes is beautiful Orion Nebula.

Orion Nebula - Emission Nebula
Orion Nebula is an Emission Nebula. Credit & Copyright: Russell Croman

Planetary Nebula

Planetary Nebula contains expanding shells of luminous gas expelled by dying stars. They appear round in shape rather than irregular patchy shape. Planetary Nebula has nothing to do with planets; they are named so because of their appearance like planetary disk.

The size of planetary nebula is about 1 light year and contains gas equivalent to 0.3 solar mass. The planetary nebula is considerably denser than H II region and has a surface brightness 1000 times larger.

The Helix Nebula - Planetary Nebula
The Helix Nebula is a Planetary Nebula. Credit: NASA, ESA, and C.R. O’Dell (Vanderbilt University)

Planetary nebula is formed when a star having core of mass less than 1.4 times of solar mass ends its life. During the end stages of life, the star swells up and converts into red giant star. When the fuel of star exhausts, it starts collapsing. The decrease in gravitational potential energy comes out of star and pushes the outer layer into the space. This expanding outer layers form the planetary nebula. The elements present in the planetary nebula are mostly carbon, oxygen, nitrogen and helium etc. The planetary nebula expands form the central star at the rate of 24-56 Km.

The density of planetary nebula keeps on decreasing with the expansion and the expansion stops when the density of planetary nebula becomes equivalent to density of interstellar medium. It takes about 30000 year to complete the expansion.

Supernova Remnant

Supernova Remnant is the cloud left behind the dying star which has undergone supernova explosion. The star which has core mass more than 1.4 times the solar mass undergoes supernova explosion during the end stage of their life. When the fuel of star gets exhausted they collapse due to gravity. The gravity potential energy released appears as shock wave and blows the outer layer of star. The energy of supernova is so huge that the hydrogen present in the outer layer undergoes whole nuclear chain reaction and results in heavy elements like uranium, lead, copper, gold and silver etc.

The material of outer layer is ejected into the interstellar space at the tremendous speed of 12000 km per second. The expanding gas creates a shock wave which helps in the formation of stars in other nebulae. The expanding gas is so hot that its glows in X ray which can be observed from the Earth. The Crab Nebula is second brightest X ray source in the sky after Scorpius X-1. After 1000 year, the Crab Nebula is still losing 100000 times the energy released by sun per second.

Crab Nebula - Supernova Remnant
Crab Nebula is a supernova remnant. Credit: NASA

The example of supernova remnant is Carb Nebula which is the remnant of supernova explosion occurred in 1054. The Crab Nebula is about 10 light years across. The gas is expanding at the rate of 1100 Km per second.


The nebulae are huge clouds of gas mainly hydrogen. The nebulae are called the nursery of star because new stars are born in these clouds. The nebulae are classified in various categories depending upon their composition and formation. The size of nebula varies from solar system to the size of entire galaxy.

I hope after reading this article you have got the answer to question, “What is a nebula?” If you enjoyed this article please like and share. If you want to know more about astronomy, you may follow us on facebook, twitter, google plus or subscribe us for email notification.

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