Nebulae: Shapes in the
Other Nebula Types
A nebula is a cloud in
interstellar space – a cloud of dust particles and gases that
occurs either at the start or end of a star’s evolution. Its
name comes from the Latin term for “cloud” and the cataloguing
of nebulous forms dates back to Ptolemy (AD 90-168).
The Two Main Types
Most nebulae are one of two main types: diffuse nebulae or
Diffuse nebulae are the type most commonly found. These nebulae
are large and have no well-defined boundaries. Along with bright
nebulae and reflection nebulae, diffuse nebulae comprise H II
regions, where star formation takes place.
Planetary nebulae on the other hand occur at the end of a star’s
evolution. When a star enters its red giant phase, it ejects
shells of ionized gas and plasma. This volatile combination
emerges as a nebula. Since they are dependent on such a singular
event, planetary nebulae tend to have shorter lifetimes. They
burn bright and then fade (in a few tens of thousands of years).
In addition, there are more specific types of nebulae, such as
emission nebulae and dark nebulae. These nebulae are all defined
by a certain characteristic, either in their appearance or
• Emission nebulae: Clouds of high-temperature ionized gas that
gives off light and color. Young stars are the source for the
ionized charge. An example is the Omega Nebula (Messier 17).
• Reflection nebulae: These clouds give off light as well, but
rather than be charged with a glow, they simply reflect the
light of a nearby star. In general, these nebulae often appear
• Dark nebulae: Dense and cold, these clouds of dust block light
from the stellar objects behind it. An example is the Horsehead
Nebulae (Barnard 33).
• Pulsar Wind Nebulae: When a star goes “supernova,” it blasts
out much of its stellar material. The result is a supernova
remnant (SNR); pulsar wind nebulae are often found inside SNR
Did you know:
Did you know that
the term “nebula” was once used to describe any extended
astronomical object? It’s true. For instance, the galaxies found
outside of the Milky Way were at a time referred to as
It wasn’t until
Edwin Hubble proved the existence of other galaxies (1925) that
the term “nebula” began to take on its stricter definition.