What is a Nebula?

Types of Nebulae

1. Emission Nebula

   One of the most common types of nebulae is the emission nebula, primarily composed of ionized hydrogen gas – which emits light, creating a stunning display of luminosity. Emission nebulae are often “stellar nurseries” where new stars are born from the gravitational collapse of the dense clouds. Astronomers can analyse the spectrum of light and determine the composition, temperature, density and motion of the gas. An example of an emission nebula include the Orion Nebula and the Eagle Nebula. The Orion Nebula, located in the constellation Orion, is one of the brightest emission nebulae and can be seen with the naked eye!

2. Reflection Nebula

   Unlike emission nebulae, reflection nebulae do not emit their own light but instead reflect the light of nearby stars, illuminating the surrounding cosmic dust. Reflection nebulae are often found in regions of star formation, and can indicate the presence of young, hot stars. The Witch Head Nebula is an example of a reflection nebula.

3. Planetary Nebula

   These nebulae are remnants of low to intermediate-mass stars in the final stages of their evolution. As these stars run out of fuel, they shed their outer layers into space, forming a glowing shell of ionized gas around the stellar core – known as a White Dwarf star. Planetary nebulae are named as such due to the resemblance to the planets when viewed through early telescopes, but they have nothing to do with planets!

   The first planetary nebula discovered, the Dumbbell Nebula (M27), was observed by the French astronomer Charles Messier in 1764. He described it as having a “round and well-defined” shape, similar to the appearance of Uranus and Neptune through his telescope.

   The Cat’s Eye Nebula and the Ring Nebula are also examples of planetary nebulae.

4. Dark Nebula

   Dark nebulae, or absorption nebulae are dense clouds of dust and gas that obscure the light from background stars due to the dust grains absorbing and scattering visible light. Despite their apparent emptiness, dark nebulae play a crucial role in the lifecycle of a star. As the stars form, they can ionize the surrounding gas, turning the dark nebula into an emission nebula! The interiors of dark nebulae are very cold, with temperatures around 10-20 Kelvin (-263 to -253 degrees Celsius.) The Horsehead Nebula is a famous example of a dark nebula.

5. Supernova Remnants

   Perhaps the coolest of the nebulae, supernova remnants are the leftovers of massive stars that have exploded in a supernova at the end of their life. The explosion generates powerful shock waves that eject stellar material outward at high velocities, often thousands of kilometres per second. Left behind is a neutron star or a black hole!

   A famous example of this is the Crab Nebula, which is one of the most studied supernova remnants in the night sky. This supernova explosion was actually observed by Chinese astronomers in 1054 AD. They described it as a “guest star” as it suddenly appeared in the night sky and remained visible for several weeks, even during the daytime. When the star at the centre of the Crab Nebula exploded, it released an immense amount of energy, briefly outshining all the stars in the Milky Way. The event was so bright that it was even visible during the daylight for 23 days and remained visible to the naked eye in the night sky for about two years before fading from view.