Tuesday, April 21, 2009


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 ionises 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 NebulaIn 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 1,270±76 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.

Horsehead and Orion Nebulae

NASA - 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
NASA - October 23, 2008

The Electric Fires of Creation
Thunderbolts - July 22, 2008

Reflection Nebula in Orion
NASA - October 10, 2006

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

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

Flame Nebula in Infrared
NASA - July 13, 1999

Barnard's Loop - Emission Nebula in Orion

Horsehead Nebula in Orion

Wisps Surrounding the Horsehead Nebula
NASA - April 6, 2008

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

Horsehead Nebula - B33
NASA - March 21, 2005

The Colorful Horsehead Nebula
NASA - October 7, 2003

Horsehead Nebula Wikipedia

Eagle Nebula

M16 and the Eagle Nebula
NASA - July 19, 2008

Inside the Eagle Nebula
NASA - 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 NASA - May 13, 2004

Red Square Nebula Wikipedia

Cat's Eye Nebula

Cat's Eye Nebula
NASA - September 4, 2005

Cat's Eye Nebula
NASA - October 31, 1999

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

Cat's Eye Nebula Wikipedia

Spokes in the Helix Nebula
NASA - September 4, 2008

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

Helix Nebula Wikipedia

The Heart and Soul Nebulas
NASA - September 14, 2008

Soul Nebula Wikipedia

Light from the Heart Nebula
NASA - October 3, 2006

Heart Nebula Wikipedia

Long Stem Rosette
NASA - February 14, 2008

Rosette Nebula Wikipedia

Veil Nebula

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

The Veil Nebula Unveiled
NASA - December 6, 2005

Veil Nebula Wikipedia

Dumbbell Nebula

The Dumbbells
NASA - December 17, 2008

M76 Above and Below
NASA - November 21, 2008

M27: The Dumbbell Nebula
NASA - June 3, 2005

Dumbbell Nebula Wikipedia

Crab Nebula

Crab Pulsar Wind Nebula
NASA - December 27, 2008

Crab Nebula Mosaic
NASA - December 2, 2005

Crab Nebula Wikipedia

Bubble Nebula

The Bubble Nebula
NASA - January 24, 2009

Bubble Nebula in Cygnus
NASA - November 13, 2008

Bubble Nebula Wikipedia

Other Nebulae From NASA

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

The North America Nebula
NASA - October 28, 2008

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

Cocoon Nebula Wikipedia

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

Eta Carinae and the Homunculus Nebula
NASA - June 17, 2008

Carina Nebula Wikipedia

Homunculus Nebula Wikipedia

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

Trifid Nebula Wikipedia

A Beautiful Boomerang Nebula
NASA - December 28, 2007

Boomerang Nebula Wikipedia

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

Crescent Nebula Wikipedia

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

Eight-burst or Southern Ring Nebula Wikipedia

Tentacles of the Tarantula Nebula
NASA - August 22, 2007

Tarantula Nebula Wikipedia

The Merope Reflection Nebula
NASA - June 11, 2007

Merope Nebula Wikipedia

Carina Nebula Panorama from Hubble
NASA - April 30, 2007

Carina Nebula Wikipedia

Planetary Nebula NGC 2440
NASA - February 15, 2007

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

California Nebula Wikipedia

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

NGC 7635: The Bubble
NASA - October 18, 2006

Bubble Nebula Wikipedia

The Eskimo Nebula from Hubble
NASA - July 9, 2006

Eskimo Nebula Wikipedia

M57: The Ring Nebula
NASA - June 25, 2006

Ring Nebula Wikipedia

M8: The Lagoon Nebula
NASA - February 10, 2006

Lagoon Nebula Wikipedia

Tarantula Nebula
NASA - January 6, 2006

Tarantula Nebula Wikipedia

Snake in the Dark Nebula
NASA - May 21, 2005

Snake Nebula Wikipedia

MZ3 - Ant Nebula
NASA - May 1, 2005

MZ3 - Ant Nebula Wikipedia

The Fox Fur Nebula
NASA - March 14, 2005

Fox Fur Nebula Wikipedia

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

IC 418: The Spirograph Nebula Wikipedia

Ringed Nebulae
NASA - July 9, 2004

Cone Nebula
NASA - May 29, 2004

Cone Nebula Wikipedia

The Pencil Nebula Supernova Shockwave
NASA - June 9, 2003

Pencil Nebula Wikipedia

Egg Nebula
NASA - April 9, 2003

Egg Nebula Wikipedia

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

Little Ghost Wikipedia

MyCn18: An Hourglass Nebula
NASA - June 15, 2002

Hourglass Nebula Wikipedia

The Pipe Nebula
NASA - May 26, 2002

Pipe Nebula Wikipedia

Red Spider Nebula
NASA - July 24, 2001

Red Spider Nebula Wikipedia

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

Cat's Paw or Bear Claw Nebula Wikipedia

M2-9: Wings of a Butterfly Nebula
NASA - March 21, 1999

Wings of a Butterfly Nebula or Butterfly Nebula Wikipedia

In the News ...

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 2005

Trifid Nebula: Giant Incubator

Science Daily - January 17, 2005