Modern Physics


Nebulae are large clouds found in interstellar space formed mostly of cosmic dust and gases such as helium and hydrogen . Some nebulae arise from the explosion of massive stars that are in the final cycle of their lives. This explosion, known as a supernova, throws the star’s matter in all directions, giving rise to one or more nebulae, which can have different shapes and extensions, ranging from the size of the Sun to a few hundred light-years.

How are nebulae formed?

Nebulae can be formed during the final event of the life cycle of stars  — supernovas, however, a nebula can also arise from the mere agglutination of atoms by the action of gravity .

Interstellar space is not as empty as it seems, in fact, its density is about 1 atom per cubic centimeter (on Earth , the density of atoms is on the order of 10 19 atoms per cm³). Despite being extremely “diluted” in space, these hydrogen and helium atoms can eventually attract each other by the action of the gravitational force.

At a certain point, a nucleus formed by these particles begins to have temperatures and pressures sufficient to carry out fusions between atoms , which, in turn, lead to the emission of electromagnetic waves capable of exciting the mass of gas around it, which starts to emit electromagnetic radiation at visible wavelengths.

What are the types of nebulae?

There are different types of nebulae , with different sizes and characteristics. The main types of nebulae are:

  • emission nebulae,
  • reflection nebulae,
  • dark nebulae,
  • planetary nebulae.

Next, we will see a little more about the characteristics of each of these types of nebulae.

  • emission nebulae

Emission nebulae are large masses of gases excited due to high temperatures . They usually have hot cores that emit ultraviolet radiation , responsible for exciting the cooler portions of gas that, when cooled, emit visible light. Emission nebulae are basically composed of hydrogen, so they have a reddish color, characteristic of this gas. They are the most visible nebulae in the night sky.

In the following figure, we see an example of an emission nebula, the Omega Nebula or Swan Nebula, which is between 5 and 6 thousand light years away from Earth and has an average diameter of 15 light years . Inside, there is a star cluster of 35 stars.

  • dark nebulae

Dark or opaque nebulae are quite visible even from Earth . This type of nebula produces dark regions that contrast with the brightness of the Milky Way’s stars.

Dark nebulae are called molecular clouds, since, unlike other types of nebulae, they contain large amounts of molecular gases, such as the molecules H 2 and He 2 . These gases cause the opacity of these nebulae to block light from other nearby stars and nebulae.

One of the most famous dark nebulae is the Horsehead Nebula, located 1500 light-years from Earth and which is part of the constellation Orion . Its name comes from its characteristic shape that resembles the silhouette of an equine.

  • reflection nebulae

Reflection nebulae, also known as diffuse nebulae, are not hot enough to cause the ionization of the gas that composes them, so they are only visible when reflecting light incident on them .

It is common for reflection nebulae to form in the vicinity of emission nebulae. In these nebulae, it is common to find traces of heavier elements, such as carbon and even iron .

The bluish colored nebula shown in the figure is known as the Mayan or Pleiades Nebula. Inside this reflection nebula, there are 35 stars spread over 35 light-years.

  • planetary nebulae

Planetary nebulae are emission nebulae whose shape confused some of the early astronomers using rudimentary observing instruments. This type of nebula originates from the mass ejection of intermediate-mass red giant stars. These stars are not massive enough to produce supernovae, so at the end of their lives, they start to fuse the element helium in their core, giving rise to carbon atoms and oxygen .

The core of these stars becomes increasingly compressed and hot. This causes it to emit a large amount of radiation, and the star begins to eject a large part of its mass in the form of stellar winds, giving rise to the initial shape of the planetary nebula.

The visibility of planetary nebulae is guaranteed from the moment the star’s core heats up to the point of emitting ultraviolet radiation. This type of nebula is of great importance for galactic enrichment : its occurrence produces heavier atoms, essential for the formation of minerals and other molecules.

Planetary nebulae, such as the Ring Nebula, shown in the previous figure, are very small compared to other types of nebulae, in addition, they are quite symmetrical and emit little light.

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