A nebula is an interstellar cloud of dust, hydrogen, helium and other ionized gases. Originally, nebula referred to any diffuse astronomical object, including galaxies beyond the Milky Way. The Andromeda Galaxy, for example, was once referred to as the Andromeda Nebula (and spiral galaxies in general as spiral nebulae) before the true nature of galaxies was confirmed in the early 20th century by Edwin Hubble.

Nebulae often form as a result of star formation, but they can also be created by the death of a star. Supernova remnants, for example, are clouds of debris left behind after a star explodes. The Crab Nebula, located in the constellation Taurus, is the remnant of a star that exploded in 1054 AD.

Nebulae come in a variety of shapes and sizes. Some, like the Crab Nebula, are elongated and filamentary. Others, like the Orion Nebula, are round and diffuse. Still others, like the Horsehead Nebula, are small and dense.

The term "nebula" is often used interchangeably with "cloud". However, a nebula is generally larger and more diffuse than a cloud. Is nebula a planet or star? Nebula is not a planet or star. It is a cloud of gas and dust in space. Is Earth in a nebula? No, Earth is not in a nebula. A nebula is a cloud of gas and dust in space, and Earth is not a cloud of gas and dust. Earth is a planet that is part of the Solar System, which is itself located within the Milky Way galaxy. Is a nebula a dead star? No, a nebula is not a dead star. A nebula is a cloud of gas and dust in space. Many nebulae are associated with the birth of stars, while others are the remains of dead stars. Is a nebula bigger than a galaxy? No, a nebula is not bigger than a galaxy. A nebula is a giant cloud of gas and dust in space, while a galaxy is a gravitationally bound system of stars, gas, and dust. How big is a nebula? There is no definitive answer to this question as nebula can vary greatly in size, from a few light-years across to hundreds of light-years across. However, on average, a nebula is thought to be around 10 light-years in diameter.