Definition: Throughout this website I refer to different types of Nebulae.
A Nebula is named after the Greek word for “cloud”. Nebulae (plural) come in many shapes and sizes and have a way of captivating those that observe and photograph these deep sky objects in space.
Many nebulae are visible due to fluorescence caused by embedded hot stars. Other nebulae are so dispersed they can only be distinguished with long exposures and special filters.
Nebulae have varying compositions depending on how they were formed and where they are.
Most nebulae are enormous in size. Some are even hundreds of light-years in diameter. Nebulae do contain some mass.
They have a greater density than the space surrounding them, however many nebulae are less dense than any vacuum we have created on Earth.
A nebula is any of the various tenuous clouds of gas and dust that occur in interstellar space. All of the nebulae that we can see are in our Milky Way Galaxy.
Nebula Types: There are several different types of nebula.
Dark nebulae are very dense and cold molecular clouds.
Bright nebulae are comparatively dense clouds of gas within the diffuse interstellar medium including:
Reflection Nebulae, HII regions, Diffused ionized gas, Planetary Nebulae, and Supernova Remnants.
Emission Nebula / Star Forming Region: Emission Nebulae are those that emit radiation from ionized gas and are often called HII regions because they are largely composed of ionized hydrogen.
The Orion Nebula is an emission nebula and star-forming region.
Also known as “stellar nurseries”, these massive collections of hydrogen gas are pulled together by gravity to form incredible formations like the “Pillars of Creation” found within the Eagle Nebula. Stars form from contracting clouds of dust and gas.
The light generated by the newly formed stars ionize the surrounding gas cloud causing it to ‘glow’.
Planetary Nebula: Planetary nebulae are formed when a star dies and creates dramatic formations of radiating cosmic gas.
Some great examples of planetary nebulae in the night sky include the Ring Nebula, the Dumbbell Nebula, and the Helix Nebula. A planetary nebula is an expanding gas shell ejected from a red giant star late in its lives.
Nebula are relatively short lived lasting only thousands of years. Planetary nebula are usually very faint objects. None are visible to unaided eye.
Supernova Remnant: Massive stars end their lives by blasting much of their mass into interstellar space.
This happens in just a matter of minutes! What remains is a super dense neutron star or black hole.
A supernova remnant is a cosmic explosion that has spread the materials from a star across a huge expanse of space.
The remnants of this explosion have formed into a nebula, and this nebula type creates some of the most incredible formations in space.
The Crab Nebula (M1) is one such nebula. It can be seen with even small binoculars.
Dark Nebula: A Dark Nebula is a cloud of gas and dust that is revealed due to the bright areas of interstellar material and stars behind it.
The nebula is silhouetted against a bright background to create interesting shapes and formations.
Reflection Nebula: Reflection nebulae are clouds of interstellar dust which might reflect the light of a nearby star or stars.
The energy from the nearby stars is insufficient to ionize the gas of the nebula to create an emission nebula, but is enough to give sufficient scattering to make the dust visible.
Thus, the frequency spectrum shown by reflection nebulae is similar to that of the illuminating stars.
Among the microscopic particles responsible for the scattering are carbon compounds (e. g. diamond dust) and compounds of other elements such as iron and nickel.
The latter two are often aligned with the galactic magnetic field and cause the scattered light to be slightly polarized.