What Is A Nebula: The Cradle And Tomb Of The Stars

Looking up at the night sky, we see thousands of stars surrounded by the black emptiness of space. But this emptiness is deceptive. In fact, space contains a huge number of gas and dust particles, which scientists call the interstellar medium. These particles can coalesce into giant clouds — nebulae (from Latin, mist or cloud). Nebulae not only create a stunning spectacle in the night sky but also play a key role in the life cycle of stars and the evolution of galaxies. In this article, we will discuss what is a nebula, describe their types and everything related to them.

What Is a Nebula In Simple Terms?

In simple terms, a nebula is a giant cloud of dust and gas in the interstellar medium that stands out against the general background of the sky due to its radiation or absorption of radiation. Previously, any extended stationary astronomical object was called a nebula, but in the 1920s, it became clear that there are many galaxies among these clouds (for example, the Andromeda Nebula). After that, the definition of what is a nebula in space acquired its current meaning.

What Does a Nebula Look Like?

Nebulae are, without exaggeration, some of the most beautiful objects in the Universe. They often have unusual textures with areas varying in density and brightness, which gives them a three-dimensional appearance, especially in high-quality images. Some look like fuzzy clouds or mist, while others have more distinct structures.

For example, the “Pillars of Creation” in the Eagle Nebula appear as huge columns of gas. The Orion Nebula appears as a hazy patch in the constellation Orion. And a supernova remnant, the Crab Nebula, appears as a complex network of threads.

What Is a Nebula Made Of?

Nebulae are composed primarily of hydrogen and helium, which are found in the interstellar medium, as well as stardust and other elements. Hydrogen makes up about 90% of its mass, and helium makes up about 10%. Besides those, what elements are in a nebula? Carbon, nitrogen, oxygen, sulfur, potassium, magnesium, calcium, and iron. They make up less than 2%.

Is The Milky Way a Nebula?

No, Milky Way it is not. It is a galaxy 105,000 light years in diameter, which includes billions of stars, planetary systems, and several nebulae, such as Orion, Eagle, Cat’s Eye, and others. This also answers the question: Are nebulae bigger than galaxies? No. They are much smaller (up to several hundred light years) and simpler in structure.

Nebula Research Facts

The first clouds of interstellar matter were observed by ancient Greek astronomers. However, a more systematic study of these mysterious objects began in the 17th century, with the advent of telescopes. The first to discover the Orion Nebula, in 1610, without knowing exactly he what had discovered, was the French scientist Nicolas-Claude Fabri de Pieresc. After observing the cloud in a refracting telescope, de Peiresc referred to it as a cloudy nebulosity. The same object was later independently discovered by Christiaan Huygens in 1656, who published his findings in 1659.

One of the first astronomers to draw the public’s and scientific community’s attention to nebulae was Edmond Halley. In his 1715 paper, he claimed that nebulae were self-luminous cosmic objects. In 1784, William Herschel singled them out as a separate class of astronomical objects and proposed the term “planetary nebulae.” However, such objects have nothing in common with planets. Herschel called them so because they resembled gas giants when observed through a telescope.

In the 18th century, Charles Messier compiled a catalogue of stationary, diffuse objects similar to comets, which included both nebulae and galaxies. In the 19th century, William Huggins became the first astronomer to obtain spectra of planetary nebulae, which allowed us to better understand their nature.

Modern astronomers continue to investigate nebulae using telescopes and satellites. They study their composition, structure, and evolution. For example, the Hubble Space Telescope has been able to detect complex and unique structures in many planetary nebulae. Astronomers also analyse the optical spectra of nebulae to better understand the processes occurring in them.

The Relationship Between Nebula And Stars

Nebulae are often called both the cradle and tomb of the stars. Why so? It’s about the way they form. Scientists have found that some nebulae are formed from gas and dust ejected by dying stars. Others, on the contrary, serve as areas for the formation of new stars, which is why they are called star forming nebula or “star nurseries.” Let’s look at each case in more detail.

How Do Stars Form in Nebulae?

The dust and gas particles in the interstellar medium are very rarefied, with an average density of about 1 atom per cubic centimetre. However, over time, this rarefied matter can begin to coalesce into denser knots. What is the force that causes dust particles to stick together in a nebula? It is gravity. Under its influence, the gas and dust begin to compress, and the material in the centre of the knot gradually heats up. This process can take millions of years.

When the material in the centre of the knot heats up to a certain temperature, a protostar is formed. The protostar continues to accumulate mass from the surrounding gas and dust until the temperature and pressure in its core become high enough for nuclear fusion to begin. Hydrogen atoms fuse to form helium and release enormous amounts of energy. This moment marks the birth of a new star. The new star begins to emit light and solar wind, which disperses the remaining gas and dust, clearing the area around the star.

How Do Nebulae Form From Stars?

When a massive star explodes as a supernova, it ejects a huge amount of matter into space, creating a nebulosity.

A smaller star like our Sun sheds its outer layers of gas and dust into space when it reaches the end of its evolution. This process can take several thousand years.

The remaining hot core becomes a white dwarf, emitting ultraviolet radiation and ionising ejected gases, which begin to glow. These processes form a planetary nebula. Such clouds typically persist for tens of thousands of years before dissipating into the interstellar medium.

Thus, nebulae serve both as “cradles” for new stars, providing the necessary material and conditions for their formation, and can also be a product of dying stars, which makes them a kind of “graves” for these stars.

What Are the Different Types of Nebulae?

Until recently, the subject of nebulae types was barely raised. Any object in the interstellar medium with a large extent and luminosity was called this way. However, with the development of observation technologies and astrophysical research, it became possible to study these mysterious clouds in more detail, and it turned out that their classification should be based on narrower criteria. So today, nebulae are divided into the following types:

• diffuse: emissive and reflective;
• Supernova remnant; 
• planetary;
• dark.

Each type plays an important role in the evolution of stars and galaxies, as well as in enriching the interstellar medium with heavy elements. Let’s consider each type separately.

What Is a Diffuse Nebula?

Diffuse nebulae emit or reflect light. Because of this, they are usually brighter than others and have blurred edges, which is how they got their name. Diffuse nebulae come in two main types: reflective and emissive.

• Reflective — do not emit their own light, but reflect light from nearby stars, which makes them visible.

Example: NGC 1999 in the constellation Orion, which is about 1,500 light-years from Earth. It is illuminated by the V380 Orionis star, residing inside it. This star has a high surface temperature of about 10,000 degrees Celsius and a mass 3.5 times that of the Sun. At the centre of NGC 1999 is the “Bok globule,” a dense cloud of gas and dust similar to a black hole that blocks the light behind it.

• Emissive (HII regions) — glow due to the ionisation of gas, usually hydrogen, from the ultraviolet radiation of the nearby hot stars.

What is an emission nebula example? The Orion Nebula (M42) is located in the constellation Orion, about 1,344 light-years from Earth. At its centre, is a young open cluster of stars called Orion Trapezium. These stars illuminate the nebula and ionise its material, making the cloud incredibly bright and visible to the naked eye. Orion is also reflective, as it contains dust clouds that reflect light from nearby stars.

What Is a Planetary Nebula?

These clouds of dust and gas form when a star that has become a red giant ejects its outer layers into space. Radiation from the star’s exposed hot core ionises the ejected shell, causing it to glow. For this reason, planetary nebulae usually have round shapes, but can also be more complex due to interactions with the interstellar medium and magnetic fields.

Example: Helix (NGC 7293), also known as the “Eye of God,” is located in the constellation Aquarius, about 650 light-years from Earth. Helix gets its name because of its spiral shape, making it appear as if we are looking down on it from above.

What Is a Supernova Remnant?

It is the structure left behind when a star explodes as a supernova. The explosion creates an expanding shock wave that traps and heats the surrounding interstellar material, forming a complex and often beautiful cloud.

Example: The Crab Nebula (NGC 1952) in the constellation Taurus, about 6,500 light-years from Earth. It was formed by a stellar explosion recorded by Chinese astronomers in 1054. The cloud is about 11 light years in diameter and continues to expand at a speed of about 1,500 kilometres per second! This cloud got its name from a drawing by astronomer William Parsons, who, in 1844, noticed its resemblance to a crab’s claw.

What Is a Dark Nebula?

It is a cold molecular cloud composed primarily of hydrogen molecules with some ammonia and carbon monoxide. It is usually very dense, so it absorbs visible light from emissive or reflective clouds, as well as from stars behind it.

Example: Barnard 68, located in the constellation Ophiuchus, about 400-500 light years from Earth. This cloud is very cold, about -260 degrees Celsius, and so dense that the stars behind it cannot be seen from Earth in visible light.

Barnard 68 is on the verge of gravitational collapse, which could lead to the formation of a new star within the next 200,000 years.

What Color Is a Nebula And What Does It Depend On?

Nebulae are truly a sight to behold, especially when viewed in true colour! Similar to aurorae, these giant clouds of gas and dust can appear in vivid hues due to the emission of light from excited atoms within them. The spectrum of colours depends on the composition of gas and dust, as well as the processes occurring within the nebulosity. Here are the main reasons why nebulae appear in different colours:

Gas Ionisation

When stars emit ultraviolet light, it ionises gas. Different elements emit light at different wavelengths, which determines the colour. For example, ionised hydrogen emits red light, while ionised oxygen emits green or blue-green light.

Light Reflection

Some clouds glow by reflecting light from nearby stars. They often have a blue tint because blue light is scattered better than red one.

Light Absorption

Dark nebulae do not emit light but absorb it, blocking the light from brighter objects behind them. These nebulae appear as dark spots against the starry sky.

Chemical Composition

Different chemical elements can emit light at different wavelengths. For example, the presence of sulfur can give the nebula a reddish hue, while nitrogen can give it a violet tint.

The combination of these factors creates a variety of colours and shades, allowing us to not only enjoy the stunning view but also learn some important facts about nebula.

What Is The Temperature Of a Nebula And How Is It Measured?

Nebulae temperature can vary greatly depending on their type and location. Some regions, especially molecular clouds, can be very cold, with temperatures as low as 10 Kelvin (about -263°C). Other regions, especially those near young, hot stars, can reach temperatures of tens of thousands of degrees Celsius. For example, the central stars in planetary nebulae can heat the surrounding gas to temperatures of 25,000 K to 200,000 K. 

But how do scientists measure this temperature? With spectroscopy and radiometry. Different elements emit light at specific wavelengths when heated, and by studying these emission lines with a spectroscope, scientists can determine the temperature. For cooler areas, such as dark nebulae, radio telescopes are used to detect emissions from molecules, for example carbon monoxide. The intensity and distribution of these emissions help determine the cloud temperature.

FAQ About Nebulae 

To wrap up, here are the answers to your most frequently asked questions about nebulae.

Why Are Nebulae Called Star Nurseries?

In some regions of interstellar space, gas and dust start compressing under the influence of gravity, forming dense regions that eventually become protostars. These protostars continue to compress and heat up until they begin to glow as full-fledged stars. Thus, nebulae can truly be compared to nurseries where young stars are “raised.”

What Is the Size of a Nebula?

Nebulae size can vary greatly. Some may be only a few light years in diameter, while others may span hundreds of light years. For example, one of the most famous nebulae, the Orion Nebula, is about 24 light years in diameter.

What Is the Difference Between a Stellar Nebula And a Planetary Nebula?

Stellar nebulae are the birthplaces of stars, while planetary nebulae are the remnants of stars that have reached the end of their life cycle.

What Is the Difference Between a Planetary Nebula And a Supernova?

A planetary nebula is a gentle shedding of the outer layers of a small star that has evolved into a red giant, while a supernova is a violent explosion marking the end of a massive star’s life.

From Cradle to Grave And Vice Versa

So, now that you know what nebula is, it will no longer be just a beautiful phenomenon in the night sky for you. It is a unique living environment in which old stars die and new ones are born.

References and Additional Information:

• Nebula https://www.britannica.com/science/nebula
• What’s the difference between a galaxy and a nebula https://www.unistellar.com/blog/whats-the-difference-between-a-galaxy-and-a-nebula/
• Stellar Nebula: cosmic cradle of creation  https://spaceyv.com/stellar-nebulae/
• Supernova vs Nebula (How Are They Different?) https://scopethegalaxy.com/supernova-vs-nebula/
• Difference between Nebula and Planetary Nebula https://www.differencebetween.info/difference-between-nebula-and-planetary-nebula
• The Crab Nebula Seen in New Light by NASA’s Webb https://www.nasa.gov/missions/webb/the-crab-nebula-seen-in-new-light-by-nasas-webb/
• Nebula Facts https://space-facts.com/nebulae/