Nebulae are huge interstellar clouds of gas and dust that range in size from 1 AU to 10 AU, and which are less dense than the deepest vacuum in Earth's laboratories. Just one portion of of a nebula the size of Earth would weigh only a few kilograms! The first nebulae that formed after the Big Bang contained mainly hydrogen and helium, but later nebulae formed with nitrogen, carbon, oxygen and even large molecules. The size, shape and characteristics of nebulae may vary significantly because some are star nurseries and others are the remains of dead stars. Keep reading to learn more about dark nebulae, emission nebulae, reflection nebulae and planetary nebulae.
In fact, dust particles usually blot out light from the stars behind them, though occasionally bright stars behind the dark nebulae will emit enough light so we can see the outlines of a dark nebula.
Surprisingly, scientists discovered that besides the common elements like hydrogen and helium, these clouds contain some simple molecules like water, methanol, carbon dioxide, and ammonia. These molecules can stick together and form grains, which probably play an important role in the formation of the stars.
The Horsehead Nebula, one of the most famous dark nebulae, is located in the constellation Orion.
When stars begin to shine, they heat up the remaining gas around them and that gas starts to shine as well.
One of the most famous emission nebula is M42, which is located in the constellation Orion. With binoculars you can see it as a pale diffuse spot that appears to be shaped like a heart. With powerful telescopes astronomers are able to detect many young stars inside.
An example is Pleiades in the Taurus constellation. Its stars formed about 10 millions years ago, yet it continues to shine with a bluish light.
Planetary nebulae tend to be smaller; they rarely exceed more than few light years across.
With binoculars, you can easily find Planetary Nebula M57 in the constellation Lyra, and it will look like a pale spot. Photographs with larger telescopes clearly show its ring shape.
Once formed, these gas condensations start to grow, attracting more and more gas and dust. Astronomers called them EGGs - Evaporating Gaseous Globules. They look like fingertips emerging from a gas cloud. After some time, they split from the cloud, condense and heat up.
Since they swirl at the same time, the gas around them starts to form a disk around the future star. When the temperature in the star becomes high enough for nuclear fusion to begin, a star is born. The star's wind then expels the gas remnants and the star begins to shine steadily.