If you stand at night when the air is clear without clouds and fog then try to observe the universe, you will see a celestial ball decorated with millions or even billions of heavenly bodies, both those that emit light and those that receive and reflect light. These heavenly bodies are generally divided into three main groups, namely stars, solar system, and nebulae.
The nebula is one of the billions of objects in space that are quite interesting to be observed and studied. The word nebulae comes from Latin which means “cloud”. Although it means clouds, the nebula is not like a cloud seen in the sky of Earth. Then what is actually a nebula?
The Nebula is an interstellar cloud consisting of dust, hydrogen, helium, plasma and other ionized gases which are believed to be nurseries for new stars. In the beginning, the nebula was the name for all astronomical objects that seemed to spread, including all galaxies outside the Milky Way galaxy. For centuries ago, humans had thought of a distant galaxy as a nebula.
For example, the Andromeda galaxy. In the past, this galaxy was once called the Andromeda nebula. We know that the Andromeda galaxy is an example of spiral galaxy forms and therefore, ancient scientists also referred to all spiral galaxies as spiral nebulae. However, along with the development of the times, the properties or characteristics of actual galaxies were successfully observed and confirmed in the early 20th century by astronomer Vesto Slipher, Edwin Hubble and colleagues so that ultimately between nebulae and galaxies have different meanings.
Starting from the conceptualization of nebulae and galaxies, astronomers have now realized that space is not completely vacuumed (empty) as a whole. Rather it consists of gas and dust particles which are collectively known as Interstellar Medium (ISM) or interstellar medium. ISM consists of gas, which is about 75% hydrogen gas and 25% helium. The interstellar gas is largely maintained on neutral atoms (neutrons) and molecules, and charged particles (plasma) such as ions, protons, and electrons. This gas is very runny with density around 1 atom/cm3. As a comparison material, the Earth’s atmosphere has a density of around 3 x 1013 molecules / cm3.