A nebula is an interstellar cloud of dust, hydrogen gas and plasma. It is the first stage of a star's cycle. Originally nebula was a general name for any extended astronomical object, including galaxies beyond the Milky Way (some examples of the older usage survive; for example, the Andromeda Galaxy was referred to as the Andromeda Nebula before galaxies were discovered by Edwin Hubble).

Nebulae often form star-forming regions, such as in the Eagle Nebula. This nebula is depicted in one of NASA's most famous images, the "Pillars of Creation". In these regions the formations of gas, dust and other materials 'clump' together to form larger masses, which attract further matter, and eventually will become big enough to form stars. The remaining materials are then believed to form planets, and other planetary system objects.

Many nebulae form from the gravitational collapse of diffuse gas in the interstellar medium or ISM. As the material collapses under its own weight, massive stars may form in the center, and their ultraviolet radiation ionizes the surrounding gas, making it visible at optical wavelengths. An example of this type of nebula is the Rosette Nebula or the Pelican Nebula. The size of these nebulae, known as HII regions, varies depending on the size of the original cloud of gas, and the number of stars formed can vary too. As the sites of star formation, the formed stars are sometimes known as a young, loose cluster.

Some nebulae are formed as the result of supernova explosions, the death throes of massive, short-lived stars. The material thrown off from the supernova explosion is ionised by the supernova remnant. One of the best examples of this is the Crab Nebula, in Taurus. It is the result of a recorded supernova, SN 1054, in the year 1054 and at the centre of the nebula is a neutron star, created during the explosion.

Other nebulae may form as planetary nebulae. This is the final stage of a low-mass star's life, like Earth's Sun. Stars with a mass up to 8-10 solar masses evolve into red giants and slowly lose their outer layers during pulsations in their atmospheres. When a star has lost a sufficient amount of material, its temperature increases and the ultraviolet radiation it emits is capable of ionizing the surrounding nebula that it has thrown off.

Reflection Nebula In 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 nebulae, 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 (Kaler, 1998). Edwin Hubble distinguished between the emission and reflection nebulae in 1922. Reflection nebulae are usually blue because the scattering is more efficient for blue light than red (this is the same scattering process that gives us blue skies and red sunsets).

Diffuse Nebula

Most nebulae can be described as diffuse nebulae, which means that they are extended and contain no well-defined boundaries. In astronomy, diffuse nebulae is the general term for illuminated nebulae. The three types of diffuse nebulae are reflection nebulae, emission nebulae and supernova remnants. They are diffuse as opposed to the non-diffuse dark nebulae, i.e. the particles have spread out.

In visible light these nebulae may be divided into emission nebulae and reflection nebulae, a categorization that depends on how the light we see is created. Emission nebulae contain ionized gas (mostly ionized hydrogen) that produces spectral line emission. These emission nebulae are often called HII regions; the term "HII" is used in professional astronomy to refer to ionized hydrogen. In contrast to emission nebulae, reflection nebulae do not produce significant amounts of visible light by themselves but instead reflect light from nearby stars.

The Horsehead Nebula, an example of a dark nebula. Dark nebulae are similar to diffuse nebulae, but they are not seen by their emitted or reflected light. Instead, they are seen as dark clouds in front of more distant stars or in front of emission nebulae. Although these nebulae appear different at optical wavelengths, they all appear to be bright sources of emission at infrared wavelengths. This emission comes primarily from the dust within the nebulae.

Planetary Nebula

Planetary nebulae are nebulae that form from the gaseous shells that are ejected from low-mass asymptotic giant branch stars when they transform into white dwarfs. These nebulae are emission nebulae with spectral emission that is similar to the emission nebulae found in star formation regions. Technically, they are a type of HII region because the majority of hydrogen will be ionised. However, planetary nebulae are denser and more compact than the emission nebulae in star formation regions. Planetary nebulae are so called because the first astronomers who observed these objects thought that the nebulae resembled the disks of planets, although they are not at all related to planets.

List of Planetary Nebulae

Protoplanetary Nebula

A protoplanetary nebula (PPN) is an astronomical object which is at the short-lived episode during a star's rapid stellar evolution between the late asymptotic giant branch (LAGB) phase and the subsequent planetary nebula (PN) phase.[4] A PPN emits strong in infrared radiation, and is a kind of reflection nebula. The exact point when a PPN becomes a planetary nebula (PN) is defined by the temperature of the central star.

Emission Nebula

Emission nebulae are clouds of high temperature gas. The atoms in the cloud are energized by ultraviolet light from a nearby star and emit radiation as they fall back into lower energy states (in much the same way as a neon light). These nebulae are usually red because the predominant emission line of hydrogen happens to be red (other colors are produced by other atoms, but hydrogen is by far the most abundant). Emission nebulae are usually the sites of recent and ongoing star formation.

Reflection nebulae are clouds of dust which are simply reflecting the light of a nearby star or stars. Reflection nebulae are also usually sites of star formation. They are usually blue because the scattering is more efficient for blue light. Reflection nebulae and emission nebulae are often seen together and are sometimes b oth referred to as diffuse nebulae.

Dark Nebula

Dark nebulae are clouds of dust which are simply blocking the light from whatever is behind. They are physically very similar to reflection nebulae; they look different only because of the geometry of the light source, the cloud and the Earth. Dark nebulae are also often seen in conjunction with reflection and emission nebulae. A typical diffuse nebula is a few hundred light-years across.

Supernova remnants

A supernova occurs when a high-mass star reaches the end of its life. When nuclear fusion ceases in the core of the star, the star collapses inward on itself. The gas falling inward either rebounds or gets so strongly heated that it expands outwards from the core, thus causing the star to explode. The expanding shell of gas form a supernova remnant, a special type of diffuse nebula. Although much of the optical and X-ray emission from supernova remnants originates from ionized gas, a substantial amount of the radio emission is a form of non-thermal emission called synchrotron emission. This emission originates from high-velocity and electrons oscillating within magnetic fields.

Nebulae From NASA

Nebulae take the form of magnificent celestial works of art.
Their names are a form of scrying or divination,
laced with metaphoric, mystical, and mythological content.


The Orion Nebula

The Orion Nebula (also known as Messier 42, M42, or NGC 1976) is a diffuse nebula situated south[b] 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 1,27076 light years and is the closest region of massive star formation to Earth. The M42 nebula is estimated to be 24 light years across. Older texts frequently referred to the Orion Nebula as the Great Nebula in Orion or the Great Orion Nebula. Yet older, astrological texts refer to it as Ensis (Latin for "sword"), which was also the name given to the star Eta Orionis, which can be seen close to the nebula from Earth.

The Orion Nebula is one of the most scrutinized and photographed objects in the night sky, and is among the most intensely studied celestial features. The nebula has revealed much about the process of how stars and planetary systems are formed from collapsing clouds of gas and dust. Astronomers have directly observed protoplanetary disks, brown dwarfs, intense and turbulent motions of the gas, and the photo-ionizing effects of massive nearby stars in the nebula.

Hubble celebrates 23 years on the job with a Horsehead of a different color  
MSNBC - April 19, 2013

Astronomers have come out with a Horsehead Nebula of a different color to celebrate the Hubble Space Telescope's 23rd birthday. The iconic nebula in the constellation Orion, about 1,500 light-years away, can be seen even through small telescopes. In visible light, it's a dark dust cloud in the shape of a horse's head, silhouetted against a backdrop of glowing hydrogen gas. But the Horsehead takes on a completely different look in the new view released Friday.

Herschel telescope revisits cosmic classic
  BBC - January 18, 2012

Europe's Herschel space telescope has produced a majestic new version of a classic astronomical target - the Eagle Nebula (also called M16). This dense region of gas and dust some 6,500 light-years from Earth hosts copious numbers of bright new stars. Radiation from these objects is sculpting the clouds of gas and dust, producing in places great columns and curtains of material. Look just below the centre of the image and you will see the columns that were famously dubbed the "Pillars of Creation" when they were pictured by the Hubble telescope in 1995.

Horsehead and Orion Nebulae

APOD - March 10, 2009

Adrift 1,500 light-years away in one of the night sky's most recognizable constellations, the glowing Orion Nebula and the dark Horsehead Nebula are contrasting cosmic vistas. They appear in opposite corners of this stunning mosaic taken with a digital camera attached to a small telescope. The magnificent emission region, the Orion Nebula (aka M42), lies at the upper right of the picture. Immediately to its left is a prominent bluish reflection nebula sometimes called the Running Man. The Horsehead nebula appears as a dark cloud, a small silhouette notched against the long red glow at the lower left. Alnitak is the easternmost star in Orion's belt and is seen as the brightest star to the left of the Horsehead. Below Alnitak is the Flame Nebula, with clouds of bright emission and dramatic dark dust lanes. Pervasive tendrils of glowing hydrogen gas are easily traced throughout the region in this deep field image of the same region.


The Sword of Orion - M42, M43

Great Orion Nebula
  APOD - October 23, 2008

The Electric Fires of Creation
  Thunderbolts - July 22, 2008

Reflection Nebula in Orion
  APOD - October 10, 2006

LL Ori and the Orion Nebula
  APOD - January 20, 2006

Orion Belt Stars - Alnitak, Alnilam, Mintaka
  APOD- October 13, 2005

Flame Nebula in Infrared
  APOD - July 13, 1999

Barnard's Loop - Emission Nebula in Orion

Horsehead Nebula in Orion

Wisps Surrounding the Horsehead Nebula
  APOD - April 6, 2008

Horse Head Shaped Reflection Nebula IC 4592
  APOD - August 8, 2006

Horsehead Nebula - B33
  APOD - March 21, 2005

The Colorful Horsehead Nebula
  APOD - October 7, 2003

Horsehead Nebula   Wikipedia

Eagle Nebula

M16 and the Eagle Nebula
  APOD - July 19, 2008

Inside the Eagle Nebula
  APOD - February 26, 2006

The Eagle Nebula

Red Square Nebula

MWC 922: The Red Square Nebula
NASA - April 16, 2007

"Red Square" Nebula's Secrets Revealed   National Geographic - April 13, 2007

Rectangular Nebula is a Double Star
  MSNBC - May 11, 2004

Rungs of the Red Rectangle   APOD - May 13, 2004

Red Square Nebula   Wikipedia

Cat's Eye Nebula

Cat's Eye Nebula
  APOD - September 4, 2005

Cat's Eye Nebula
  APOD - October 31, 1999

Cat's Eye Wide and Deep
  APOD - June 29, 2007

Halo of the Cat's Eye
  APOD - May 9, 2010

Cat's Eye Nebula   Wikipedia

Helix Nebula

Spokes in the Helix Nebula
  APOD - September 4, 2008

NGC 7293: The Helix Nebula
  APOD - August 3, 2007

Helix Nebula   Wikipedia

Heart Nebula

Spiral Meteor through the Heart Nebula

The Heart and Soul Nebulas
  APOD - September 14, 2008

Soul Nebula   Wikipedia

Light from the Heart Nebula
  APOD - October 3, 2006

Heart Nebula   Wikipedia

Long Stem Rosette
  APOD- February 14, 2008

Rosette Nebula   Wikipedia

Veil Nebula

Pickering's Triangle from Kitt Peak
  APOD - July 1, 2008

The Veil Nebula Unveiled
  APOD - December 6, 2005

Veil Nebula   Wikipedia

Dumbbell Nebula

The Dumbbells
  APOD - December 17, 2008

M76 Above and Below
  APOD - November 21, 2008

M27: The Dumbbell Nebula
APOD - June 3, 2005

Dumbbell Nebula   Wikipedia

Crab Nebula

Crab Pulsar Wind Nebula
  APOD - December 27, 2008

Crab Nebula Mosaic
  NASA - December 2, 2005

Crab Nebula   Wikipedia

Watch the Video ... Listen to the tones.
  Fermi telescope spots 'superflares' in the Crab Nebula   PhysOrg - May 12, 2011
The famous Crab Nebula supernova remnant has erupted in an enormous flare five times more powerful than any flare previously seen from the object. On April 12, NASA's Fermi Gamma-ray Space Telescope first detected the outburst, which lasted six days.

Crab Nebula's gamma-ray flare mystifies astronomers   BBC - May 11, 2011
The Crab Nebula has shocked astronomers by emitting an unprecedented blast of gamma rays, the highest-energy light in the Universe. The cause of the 12 April gamma-ray flare, described at the Third Fermi Symposium in Rome, is a total mystery. It seems to have come from a small area of the famous nebula, which is the wreckage from an exploded star. The object has long been considered a steady source of light, but the Fermi telescope hints at greater activity. The gamma-ray emission lasted for some six days, hitting levels 30 times higher than normal and varying at times from hour to hour.

Bubble Nebula

The Bubble Nebula
  APOD - January 24, 2009

Bubble Nebula   Wikipedia

Bubble Nebula in Cygnus
  APOD - November 13, 2008

NGC 7635: The Bubble
  APOD - October 18, 2006

Bug Nebula

  Bug Nebula: Newly discovered star one of hottest in Galaxy  
Bug Nebula M51   APOD - December 26, 2009

The Butterfly Nebula from Upgraded Hubble M2-9
APOD - September 10, 2009

Butterfly Nebula   Wikipedia

Elephant Trunk Nebula

The Elephant's Trunk Nebula
  APOD - November 6, 2010
Sagitta Constellation

Elephant's Trunk Nebula   Wikipedia

The Elephant's Trunk in IC 1396
  APOD - December 27, 2008

Eta Carina Nebula

Eta Carina Nebula

Eta Carinae and the Homunculus Nebula

NGC 7008 -- APOD - June 17, 2008

Homunculus Nebula   Wikipedia

Eta Carinae: Jet Blasts
National Geographic - September 16, 2009

Eta and Keyhole in the Carina Nebula
  APOD - April 30, 2006

Keyhole Nebula   Wikipedia

The Keyhole Nebula
  APOD- May 23, 1999

Other Nebulae

Ou4: A Giant Squid Nebula   APOD - July 18, 2014

The Medusa Nebula   APOD - October 25, 2012

Medusa Nebula   Wikipedia

A Spiral Nebula Surrounding Star R Sculptoris  
APOD - October 16, 2012

R Sculptoris   Wikipedia

Seagull Nebula
  APOD - March 8, 2012

IC 2177   Wikipedia

HH-222: The Waterfall Nebula
  APOD - October 24, 2011

The California Nebula
APOD - March 2, 2011

The California Nebula   Wikipedia

The Iris Nebula
  APOD - November 12, 2010

The Iris Nebula   Wikipedia

The Necklace Nebula
APOD - November 3, 2010
It is found in the small and ancient constellation Sagitta.

Nebula IRAS 05437+2502: An Enigmatic Star Cloud from Hubble

APOD - August 9, 2010

M8: The Lagoon Nebula
  APOD - August 5, 2010

Lagoon Nebula   Wikipedia

Spiral Nebula M51
  APOD - December 26, 2009

Question Mark Spiral Nebula
It was soon followed by M99

The North America and Pelican Nebulae
  APOD - June 30, 2009

North America Nebula   Wikipedia

Pelican Nebula   Wikipedia

M97: The Owl Nebula
  APOD - May 15, 2009

Owl Nebula   Wikipedia

IC 443 Jellyfish Nebula
  APOD - May 14, 2009

IC 443 Jellyfish Nebula   Wikipedia

The North America Nebula
  APOD - October 28, 2008

North America Nebula   Wikipedia

C 5146: The Cocoon Nebula
  APOD - August 27, 2008

Cocoon Nebula   Wikipedia

NGC 7008: The Fetus Nebula
  APOD - August 19, 2008

NGC 7008   Wikipedia

In the Center of the Trifid Nebula
  APOD - June 30, 2008

Trifid Nebula   Wikipedia

Echoes from RS Pup
  APOD - February 12, 2008

A Beautiful Boomerang Nebula
  APOD- December 28, 2007

Boomerang Nebula   Wikipedia

NGC 6888: The Crescent Nebula
  APOD - November 11, 2007

Crescent Nebula   Wikipedia

NGC 3132: The Eight Burst Nebula
  APOD - October 14, 2007

Eight-burst or Southern Ring Nebula   Wikipedia

Tentacles of the Tarantula Nebula
  APOD - August 22, 2007

Tarantula Nebula   Wikipedia

The Merope Reflection Nebula
  APOD - June 11, 2007

Merope Nebula   Wikipedia

Planetary Nebula NGC 2440
  APOD - February 15, 2007

Planetary Nebula   Wikipedia

NGC 1499: The California Nebula
  APOD - October 24, 2006

California Nebula   Wikipedia

IC 4628: The Prawn Nebula
  APOD - October 20, 2006

The Eskimo Nebula from Hubble
  APOD - July 9, 2006

Eskimo Nebula   Wikipedia

Ring Nebula Deep Field
  APOD - November 6, 2009

M57: The Ring Nebula
  APOD - June 25, 2006

Ring Nebula   Wikipedia

M8: The Lagoon Nebula
  APOD - February 10, 2006

Lagoon Nebula   Wikipedia

Tarantula Nebula
APOD - January 6, 2006

Tarantula Nebula Wikipedia

Snake in the Dark Nebula
  APOD - May 21, 2005

Snake Nebula   Wikipedia

MZ3 - Ant Nebula
  APOD - May 1, 2005

MZ3 - Ant Nebula   Wikipedia

The Fox Fur Nebula
  APOD - March 14, 2005

Fox Fur Nebula   Wikipedia

IC 418: The Spirograph Nebula
  APOD - October 17, 2004

IC 418: The Spirograph Nebula   Wikipedia

Ringed Nebulae
  APOD - July 9, 2004

Cone Nebula
  APOD - May 29, 2004

Cone Nebula   Wikipedia

The Pencil Nebula Supernova Shockwave
  APOD - June 9, 2003

Pencil Nebula   Wikipedia

Egg Nebula
  APOD - April 9, 2003

Egg Nebula   Wikipedia

NGC 6369: The Little Ghost Nebula
  APOD - November 8, 2002

Little Ghost   Wikipedia

MyCn18: An Hourglass Nebula
  APOD - June 15, 2002

Hourglass Nebula   Wikipedia

The Pipe Nebula
  APOD - May 26, 2002

Pipe Nebula   Wikipedia

Red Spider Nebula
  APOD - July 24, 2001

Red Spider Nebula   Wikipedia

The Cat's Paw Nebula
  NASA - December 7, 1999

Cat's Paw or Bear Claw Nebula   Wikipedia

In the News ...

Massive Star Blows Fancy Hourglass Nebula   Wired - February 15, 2010

Hubble Image Showcases Star Birth in M83, the Southern Pinwheel   PhysOrg - November 5, 2009

Comets Clash at Heart of Helix Nebula
  PhysOrg - February 13, 2007

Doom for Hubble's iconic Pillars of Creation
  BBC - January 9, 2007

Pulsar in Crab Nebula Has Four Poles, Astronomers Suggest
  National Geographic - January 9, 2007

Double Helix Nebula Near Center of the Milky Way
  PhysOrg - March 16, 2006

Best Photo of Crab Nebula
  National Geographic - December 2, 2005

Magnetic Fields In The Central Stars Of Four Planetary Nebulae
Science Daily - January 18, 2005

Trifid Nebula: Giant Incubator
  Science Daily - January 17, 2005