Nebula

A nebula (Latin for 'cloud, fog'; pl. nebulae or nebulas) is a bright part of the space between stars, called the interstellar medium. It can be made of ionized, neutral, or molecular hydrogen and also cosmic dust. Nebulae are places where new stars are born, like the Pillars of Creation in the Eagle Nebula. In these areas, gas, dust, and other materials come together to form bigger clumps. These clumps pull in more matter and become heavy enough to form stars. The leftover material may then form planets and other objects in a planetary system.

Most nebulae are very big; some are hundreds of light-years wide. A nebula that we can see from Earth would look bigger but not brighter if we were close to it. The Orion Nebula, the brightest nebula in the sky and covering an area twice the size of the full Moon, can be seen without a telescope but was not noticed by early astronomers. Even though nebulae are denser than the space around them, they are still much less dense than any vacuum we can make on Earth (10⁵ to 10⁷ molecules per cubic centimeter) – a nebular cloud the size of the Earth would weigh only a few kilograms.

At first, the word "nebula" was used for any fuzzy astronomical object, including galaxies far from the Milky Way. The Andromeda Galaxy, for example, was once called the Andromeda Nebula (and spiral galaxies were called "spiral nebulae") before we learned what galaxies really are in the early 1900s, thanks to work by Vesto Slipher and Edwin Hubble. Edwin Hubble found that most nebulae are linked to stars and glow because of starlight. He also helped sort nebulae based on the kind of light they give off.

Observational history

Long ago, around 150 AD, a man named Ptolemy wrote about stars that looked fuzzy or cloudy. Later, in 964, a Muslim astronomer named Abd al-Rahman al-Sufi wrote about a small cloud where the Andromeda Galaxy is now seen. In 1054, bright lights in the sky, called supernovas, were seen by astronomers. This led to the creation of the Crab Nebula.

In 1610, a man named Nicolas-Claude Fabri de Peiresc used a telescope and found the Orion Nebula. Over time, more of these cloudy areas in the sky were found and recorded. By 1781, a man named Charles Messier had made a list of 103 of these cloudy spots. Later, William Herschel and his sister Caroline Herschel added many more to these lists. Scientists have kept studying these beautiful cloudy areas in space ever since.

Formation

Nebulae form in many ways. Some start from gas floating in space, called the interstellar medium. This gas cools and clumps together to make new stars. Other nebulae are created when stars end their lives in big explosions called supernovae. These explosions throw off material that glows in space.

One famous example is the Crab Nebula, which came from a supernova seen in the year 1054. Stars like our Sun can also create nebulae at the end of their lives. They puff off their outer layers and leave behind a small, dense core called a white dwarf.

Types

Nebulae are huge clouds of gas and dust in space, and they come in many shapes and sizes. Some famous examples are the Omega Nebula, a bright cloud of gas, and the Horsehead Nebula, a dark shape against the stars.

Nebulae can be sorted into a few main kinds. Emission nebulae shine because of energy from hot stars inside them. Dark nebulae are thick clouds that hide the light from stars behind them. Planetary nebulae are what is left after a star has ended its life, and supernova remnants are the clouds left after very big explosions of stars. Each kind helps in the life cycle of stars.

Examples

Some famous examples of nebulae include the Ant Nebula, Barnard's Loop, Boomerang Nebula, Cat's Eye Nebula, Crab Nebula, and Eagle Nebula. Nebulae such as the Orion Nebula and Carina Nebula are places where new stars are born.

These clouds of gas and dust are studied using many different catalogs, including the Gum catalog, RCW Catalogue, Sharpless catalog, Messier Catalogue, and Caldwell Catalogue. Other catalogs like the Barnard Catalogue focus on darker areas of space called dark nebulae.