A nebula is a huge cloud of gas and interstellar dust that plays an important role in the life cycle of stars. Nebulas come in various shapes and sizes and create fascinating cosmic vistas. These objects have different compositions based on how and where they are formed.

Most of the nebulae are very large and sometimes their diameter reaches hundreds of light years. Many of the fascinating images of the Hubble and James Webb telescopes are of nebulae, such as the “Pillars of Creation”.

Nebulas are usually known for their beautiful and eye-catching designs, which are made of a variety of interstellar materials. These clouds made of gas, dust, and plasma are remnants of stellar processes such as hydrogen fusion in stars, stellar winds, and supernova explosions.

The composition of nebulae varies according to their age, position, and other physical conditions. For example, some nebulae may be dominated by hydrogen, while others may contain large amounts of helium, carbon, nitrogen, and oxygen. Also, the gas and dust inside the nebulae can be ionized, in such a way that it loses or gains a large amount of electrons, and this process leads to the emission of light in different wavelengths and the production of different colors and patterns in the nebulae.

In general, the composition and structure of nebulae is an attractive research topic in the fields of astrophysics and astronomy; Because they contain important clues about the history and evolution of the world.

Star birth nebulae are divided into two main categories. Reflection nebulae and diffusion nebulae. A reflection nebula is a stellar nursery that reflects light instead of emitting light. A reflection nebula can emit light, however its density usually prevents light from escaping into space. This type of nebula actually reflects the light of nearby stars. Because a reflection nebula reflects light instead of emitting it, its composition is hard to guess. When astronomers create a spectrum of a reflection nebula, the spectrum reveals the composition of the reflected light and thus the composition of objects near the nebula. By looking at the color of the nebula, you can see that it is reflective. These nebulae usually emit blue light and most of them are blue in color.

Emissive nebulae are the exact opposite of their reflective counterparts. These nebulae are often the place where stars are born, and the amount of energy released by the stars being born causes the atoms inside the nebula to ionize. High-energy photons collide with nearby atoms, causing electrons to jump to a higher energy level. When the electrons return to their original energy level, they release their stored energy in the form of photons and emit light. Therefore emission nebulae can produce their own light. For this reason, astronomers can accurately reveal the composition of emission nebulae using spectroscopic methods. A diffusion nebula can also be identified based on its color. Excited hydrogen atoms usually cause the red color of these nebulae.

Some nebulae are the result of the end of life of stars that have released their material into space. The remnant nebulae of stars are divided into two categories: planetary nebulae and supernova nebulae.

Every star begins its life with the gravitational collapse of a cloud of hydrogen gas. As the masses of hydrogen accumulate and form huge clouds, the temperature of hydrogen increases. By establishing the conditions for the fusion of the hydrogen nucleus into the helium nucleus, the process of star birth begins. With the occurrence of hydrogen fusion in the core of the star, the energy produced is opposite to the star’s gravity, and in this way, a state of equilibrium is established in the star. However, all stars have a limited supply of hydrogen. Eventually, the star’s energy source runs out. If the star is similar to our sun, the depletion of the hydrogen source leads to the formation of helium in the core of the star.

When the conditions for helium fusion are not possible, the star’s gravity will prevail and the star’s collapse process will begin. As the star collapses, its temperature and density increase. Thus, the star converts helium into heavier elements such as carbon and oxygen.

As the heavier elements burn, the core’s energy overcomes the star’s gravity. The star expands beyond its original size. As the star grows, its surface heat spreads over a larger area. The result of this process is the gradual cooling of the outer layers of the star and its red color. At this stage, the star turns into a so-called red giant. This process of contraction and expansion occurs several times until different atoms are formed and burn in the core of the star. As the star expands beyond its original size, its mass decreases. As the star expands further, its gravity becomes weaker and is no longer able to maintain the star’s components. The outer layers of the star gradually evaporate. Eventually, the star loses most of its material, leaving only a shell of stellar material that becomes a planetary nebula.

Massive stars die differently than low-mass stars. Since the mass of these stars is very high, the gravitational collapse of the star will be more intense and lead to the composition of heavier elements. When iron forms in the core of a massive star, its fate is decided. Although most massive stars can convert iron into heavier elements, the energy absorbed in the iron fusion process is much greater than the energy released.

Thus, the star’s gravity takes full control and its final collapse becomes inevitable. In this way, the pressures of the nucleus increase so much that the atoms are compressed together and even the electrons and protons are mixed together to form neutrons. In this way, the core of the star is made entirely of neutrons and forms a neutron star . The collapse of the outer layers affects the forming neutron star and it explodes in a huge explosion called a supernova. The energy released from such an explosion can outshine all the surrounding stars in the galaxy. In this way, the material of the star is ejected into space and forms the remnants of the supernova.

A dark nebula is a cloud of gas and dust that is revealed by bright regions of stellar material and background stars. The nebula appears as a shadow against the bright background, forming fascinating shapes and structures.

There are also opaque nebulae that do not emit visible light and do not have a bright background but block their own background light. Dark nebulae, like reflective and emission nebulae, are sources of infrared rays due to their dust.

Among the dark nebulae, we can refer to the Horsehead Nebula and the CoalSack Nebula. These nebulae consist of thick clouds of dust and block the light of the gases behind them.

Some objects in the night sky are a combination of nebulae. The trifid nebula is a typical example of a composite nebula. This nebula consists of a diffusion nebula, a reflection nebula, and a dark nebula and has a unique and complex structure.

The Horseshoe Nebula, also known as the Swan Nebula and the Omega Nebula, is located in the constellation Sagittarius. Philippe Louis Louis de Chazo discovered this nebula in 1745 and Charles Messier classified it in 1764. This nebula is between 5000 and 6000 light-years away from Earth and has a diameter of 15 light-years. Horseshoe is one of the brightest and heaviest star formation regions in the Milky Way galaxy.

The Eagle Nebula, or Messier 16, is a young cluster of stars located in the constellation Serpent and was discovered by Jean-Philippe de Chezo in 1745. This nebula contains several active gas and dust regions of star birth, including the famous Pillars of Creation region. The distance between the Eagle Nebula and the Earth is 5700 light years.

The Southern Ring Nebula, or NGC 3132, is a famous and bright planetary nebula in the constellation Sails. The distance of this nebula from the Earth is approximately 2000 light years, and for this reason, researchers have conducted many studies on it. The Southern Ring Nebula was one of the objects selected for observation by the James Webb Space Telescope at the time of its inception.

Images of the Southern Ring Nebula show two stars close to each other. The central star of this nebula is a white dwarf planet. The bright and hot central star is an A-type main sequence star that is at least 1,277 AU away from the white dwarf.

The Veil Nebula is a cloud of ionized and hot gas and dust located in the constellation of Pisces. The distance of this supernova remnant nebula from Earth reaches 2400 light years and a large part of it is made up of oxygen, sulfur, and hydrogen. William Herschel discovered this nebula in 1784.

The Crab Nebula is like the Veil Nebula of supernova remnants. This nebula is located in the constellation Taurus, at a distance of 6500 light years from Earth. The diameter of this nebula is 11 light years and in its center is a pulsar, a type of neutron star with a diameter of 28 to 30 km and a rotation speed of 30 times per second.

Carina Nebula is a type of diffusion nebula that contains a huge and complex region of dark and bright dust in the constellation of Shahtakhteh. This nebula is approximately 8500 light years away from Earth. The Carina Nebula was one of the five objects selected for initial observations by the James Webb Telescope. The James Webb telescope captured a detailed image of the star-forming region known as cosmic rocks.

The Orion Emission Nebula or Orion, also known as Messier 42 or NGC 1976, is located in the Milky Way galaxy and the Orion constellation. The Orion is one of the brightest nebulae that can be seen with the naked eye at magnitude 4 in the night sky. Orion is also the closest star-forming region to Earth. The length of this nebula reaches 24 light years and it is 1344 light years away from Earth.

The Eye of God planetary nebula, also known as the Helix and the Spiral Nebula, is located 650 light-years from Earth in the constellation of Aquarius. This nebula is the result of a medium-to-low-mass star that has ejected its outer layers into space at the end of its life. The gasses of the star form an eye-like structure. The central stellar core, which is called the central star of the planetary nebula, is a white dwarf.

The Marsh Nebula, also known as Messier 8 and NGC 6523, is a diffusion nebula in the constellation Sagittarius. This nebula is located at a distance between 4000 and 6000 light years from Earth and its dimensions are 110 x 50 light years. This nebula also has a tornado-like structure resulting from an O-type star’s ultraviolet light.

The Horsehead Nebula is a small dark nebula also known as Bernard 33. This nebula, along with its companion Flare Nebula, is located in the constellation Orion and is part of the larger Orion molecular cloud complex. The distance of this nebula to the earth is 1375 light years and its radius is 3.5 light years.