An
emission nebula is a
cloudA cloud is a visible mass of droplets or frozen crystals suspended in the atmosphere above the surface of the Earth or another planetary body. A cloud is also a visible mass attracted by gravity, such as masses of material in space called interstellar clouds and nebulae...
of ionized
gasThis page is about the physical properties of gas as a state of matter. For the uses of gases, and other meanings, see Gas .A gas is one of four states of matter. Near absolute zero, a substance exists as a solid...
(
i.e. a
plasmaIn physics and chemistry, plasma is a partially ionized gas, in which a certain proportion of electrons are free rather than being bound to an atom or molecule. The ability of the positive and negative charges to move somewhat independently makes the plasma electrically conductive so that it...
) emitting light of various colors. The most common source for
ionizationIonization is the physical process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation ....
are high-energy
photonIn physics, a photon is an elementary particle, the quantum of the electromagnetic field and the basic "unit" of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s emitted from a nearby hot
starA star is a massive, luminous ball of plasma that is held together by gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible in the night sky, when they are not outshone by the Sun...
. Among the several different types of emission nebula are
H II regionAn H II region is a cloud of glowing low density gas and plasma, sometimes several hundred light-years across, in which star formation is taking place. Young, hot, blue stars which have formed from the gas emit copious amounts of ultraviolet light, ionizing and heating the gas surrounding them...
s, in which star formation is taking place and young, massive stars are the source of the ionizing photons; and
planetary nebulaA planetary nebula is an emission nebula consisting of an expanding glowing shell of ionized gas and plasma ejected during the asymptotic giant branch phase of certain types of stars late in their life...
e, in which a dying star has thrown off its outer layers, with the exposed hot core then ionizing them.
General information
Usually, a young star will ionize part of the same cloud from which it was born although only massive, hot stars can release sufficient energy to ionize a significant part of a cloud. In many emission nebulae, an entire
clusterStar clusters or star clouds are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars which are gravitationally bound, while open clusters, a more loosely clustered group of stars, generally contain less than...
of young stars is doing the work.
The nebula's color depends on its chemical composition and degree of
ionizationIonization is the physical process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation ....
. Due to the prevalence of
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
in interstellar gas, and its relatively low energy of ionization, many emission nebulae appear red due to the strong emissions of the
Balmer seriesThe Balmer series or Balmer lines in atomic physics, is the designation of one of a set of six different named series describing the spectral line emissions of the hydrogen atom....
. If more energy is available, other elements will be ionized and green and blue nebulae become possible. By examining the
spectraThe electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
of nebulae, astronomers deduce their chemical content. Most emission nebulae are about 90% hydrogen, with the remainder
heliumHelium is the chemical element with atomic number 2, and is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert monatomic gas that heads the noble gas group in the periodic table...
,
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
,
nitrogenNitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere.Many industrially important...
, and other elements.
Some of the most prominent emission nebulae visible from the
northern hemisphereThe Northern Hemisphere is the half of a planet that is north of the equator—the word hemisphere literally means 'half sphere'. It is also that half of the celestial sphere north of the celestial equator...
are the
North America NebulaThe North America Nebula is an emission nebula in the constellation Cygnus, close to Deneb . The remarkable shape of the emission nebula resembles that of the continent of North America, complete with a prominent Gulf of Mexico...
(NGC 7000) and
Veil NebulaThe Veil Nebula, is part of the Cygnus Loop, radio source W78, or Sharpless 103. Other parts of the loop include the 'Eastern Veil', the 'Western Veil' or 'Witch's Broom Nebula', and Pickering's Triangular Wisp. It is a large, relatively faint supernova remnant in the constellation Cygnus...
NGC 6960/6992 in
CygnusCygnus is a northern constellation. Its name is the Latinized Hellenic word for swan. One of the most recognizable constellations of the northern summer and autumn, it features a prominent asterism known as the Northern Cross...
, while in the south celestial hemisphere, the
Lagoon NebulaThe Lagoon Nebula is a giant interstellar cloud in the constellation Sagittarius. It is classified as an emission nebula and as an H II region....
M8 / NGC 6523 in
SagittariusSagittarius is one of the constellations of the zodiac. Its name is Latin for the archer, and its symbol is , a stylized arrow. Sagittarius is commonly represented as a centaur drawing a bow...
and the
Orion NebulaThe Orion Nebula is a diffuse nebula situated south of Orion's Belt. It is one of the brightest nebulae, and is visible to the naked eye in the night sky. M42 is located at a distance of 1,344±20 light years and is the closest region of massive star formation to Earth...
M42. Further in the southern hemisphere is the bright Carina Nebula NGC 3372.
Emission nebulae often have dark areas in them which result from
cloudsIn astronomy, the interstellar medium is the gas and dust that pervade interstellar space: the matter that exists between the star systems within a galaxy. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
of dust which block the light. The combination of emission nebula and dust cloud make for some beautiful objects, and many of these nebulae bear the name of objects that they early astronomers thought they resembled, such as the
North America NebulaThe North America Nebula is an emission nebula in the constellation Cygnus, close to Deneb . The remarkable shape of the emission nebula resembles that of the continent of North America, complete with a prominent Gulf of Mexico...
or the
Cone NebulaThe Cone Nebula is an H II region in the constellation of Monoceros. It was discovered by William Herschel on December 26, 1785, at which time he designated it H V.27. The nebula is located about 800 parsecs or 2,600 light-years away from Earth.The Cone Nebula is part of the nebulosity...
.
Many nebulae are made up of both
reflectionIn Astronomy, reflection nebulae are clouds of dust which are simply reflecting the light of a nearby star or stars. The energy from the nearby star, or 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...
and emission components such as the
Trifid NebulaThe Trifid Nebula is an H II region located in Sagittarius. Its name means 'divided into three lobes'...
.