Nebula is Latin for “cloud”

Nebulae are made up of gases, dust, atoms, ions, molecules which formed into clouds.

As mentioned earlier, nebulae were first formed when the universe came into existence. These nebulae are known to have been made up of the original atoms, gas, and dust known as “primordial matter”

Later, as stars exploded into novae and supernovae, the ejected material spread out into clouds also known as nebula(e).

Therefore nebulae can be made up of both primordial as well as star-ejected matter.

Types of Nebulae

Diffuse nebulae

Most nebulae fall under this category. Their characteristics are they have no well-defined boundaries and are extensive. There are two sub categories of diffuse nebulae – the emission nebulae and reflection nebulae

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Emission nebulae

Emission nebulae are very colorful. There colors are caused by the different gases and there are also lit from the inside by young stars in their stellar nursery.

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Reflection nebulae

Reflection nebulae reflect light from the stars from either inside or surrounding the dust clouds. A reflection nebula shines only because the light from an embedded source illuminates its dust; the nebula does not emit any visible light of its own.

Picture courtesy: nasa

Dark Nebulae

Though all nebulae are dark because they do not produce any light on their own, the term “Dark Nebulae” is used to identify nebulae that block the light of something behind them.

Example: horse head nebula

picture courtesy: nasa

There are a number of lanes of dark nebulae in our Milky Way galaxy that prevent us from seeing far into the galaxy in visible light.

Planetary nebulae

Planetary nebulae are distinct in appearance by their circular shape.
These nebulae are created at the red giant stage when the outer shell of the star is thrown off in all directions.

Planetary nebulae were named thus because when they were detected through telescopes during the 19th century, they resembled the newly discovered Uranus and Neptune. This is another name that stuck, even after knowing about the stars and other galaxies.

picture courtesy: nasa

Supernova remnant

These nebulae are the creations of ancient supernovae – the violent explosions of massive stars at the end of their lives. The most famous example is the Crab Nebula, created by a well-documented supernova on July 4, 1054.

picture courtesy: nasa


Neutron Stars

neutron-starspicture courtesy: nasa

When a star begins to collapse its electrons and protons begin to combine resulting in neutrons producing a neutron star.

Neutron Stars are very small – about 20 kilometers in diameter!

But there are unimaginably dense and heavy because they pack about three suns in this very small space.The gravity of a neutron star is about a billion times stronger than Earth’s.

If you took a teaspoon of neutron star it would weigh one billion tons!

But we really can’t even attempt to take a spoonful because if we went close to a neutron star we would be immediately pulled by its strong gravity and have vanished within microseconds!

If a neutron star forms in a multi-star system it strips material off its nearby stars.

Comparative size of a neutron star

comparative-size-of-a-neutron-starpicture courtesy: gsfc.nasa


Neutron stars also have very powerful magnetic fields. These magnetic fields can make the particles around its poles spins so fast that they produce powerful bemaes of radiation. These beams sweep around the poles like huge searchlights.

Some of these beams when oriented towards the Earth , we can see regular pulses sweep by flashing light like from a lighthouse.

These pulsing stars are called pulsars.


A magnetar (a contraction of magnetic star) is a neutron star with an ultra-strong magnetic field. The magnetic field is a thousand trillion times stronger than the Earth’s, and between 100 and 1,000 times stronger than that of a radio pulsar, making them the most magnetic objects known.

It is still not fully known how and under what conditions a magnetron forms. However, some theories suggest that the neutron star must rotate at a dizzying speed – between 100-1000 times/second.

Ultra-powerful magnetic neutron stars are known to erupt without warning, some for hours and others for months, before dimming and disappearing again.

In the constellation Cassiopeia, approximately 18,000 light years from Earth, a magnetar named 1E 2259 is being studied. It suddenly began bursting in June 2002, with over 80 bursts recorded within a 4-hour window. Since then, Magnetar 1E 2259 hasn’t disturbed the depths of space.

Black Holes

We hear many dialogues about getting lost in a “black hole”. Black holes are not empty space. On the contrary. Black holes are so tightly packed with such a great amount of matter in a small space that eve light cannot escape.

We all see objects because light reflects off those objects. But light does not reflect off a black hole. The gravity in black hole is that powerful that it keeps the light trapped.

Imagine packing ten suns in a space as big as New York or Mumbai. We have to squeeze and squeeze the particles so much. That’s is a black hole.

The term was coined in 1967 by Princeton physicist John Wheeler

Black holes were predicted by Einstein’s theory of general relativity, which showed that when a massive star dies, it leaves behind a small, dense remnant core. If the core’s mass is more than about three times the mass of the Sun, the equations showed, the force of gravity overwhelms all other forces and produces a black hole.

Black holes cannot be observed by the traditional telescopes or other instruments because whatever signal we send will never be returned.

So how are black holes detected?

By observing the behavior of its surrounding matter.
Scientists can see the effects of a black hole’s strong gravity on the stars and gases around it.

When a black hole and a star are orbiting close together, high-energy light is produced. Scientific instruments can see this high-energy light.

All matter close to the black holes will be pulled towards the black hole due to its immense gravity and form a disk called the “accretion disk”. As the matter of the accretion disk spirals around the black hole it becomes hotter and the temperature rises releasing X-rays in all directions. Telescopes measure these X-rays to determine and study more on the properties of the black hole.

Most black holes form from the remnants of a large star that dies in a supernova explosion and bigger black holes can result from stellar collision.


Study of black holes have concluded that there are two different sizes of black holes.

On one end there are black holes which are remnants of massive stars known as “stellar” black holes. They are 10-24 times massive than our Sun. These are found throughout the galaxies – up to a billion in our own Milky Way.

On the other end is the “supermassive” black holes which are billions of times massive than our Sun.

Astronomers believe that supermassive black holes lie at the center of virtually all large galaxies, even our own Milky Way. The supermassive black hole at the center of the Milky Way galaxy is called Sagittarius A. It has a mass equal to about 4 million suns and would fit inside a very large ball that could hold a few million Earths. Astronomers can detect them by watching for their effects on nearby stars and gas.

Image of Sagittarius A Black Hole in the center of the Milky Way galaxy taken by the Chandra X-ray Observatory.

Picture credit: NASA

Recent studies have confirmed that mid-size black holes do exist. However they appear to sink to the center of the galaxy after formation, merging with other intermediate black holes to form supermassive black hole.


Quasars are the brightest objects in the universe that even eclipse their own galaxies. They are powered by black holes which are billion times massive than our sun. Quasar is the acronym for “Quasi Stellar Radio Sources”. Quasars are made up of particles that come very c lose to the speed of light referred to as “light speed jets”

Light-speed jets

Quasars exist only in galaxies that have super-massive black holes.

Though light cannot escape from the black hole itself, some signals can break free around its edges. Similarly while some stars and other space matter fall into the black hole some particles accelerate away from it at the speed of light. These particles stream away from above and below a black hole.

Quasars emit energies of millions, billions, or even trillions of electron volts. This energy exceeds the total of the light of all the stars within a galaxy. Quasars are 10 to 100,000 times brighter than the Milky Way and give off more energy than 100 normal galaxies combined.

A quasar

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