Abell 39 (PN A66 39) is a low

surface brightness In astronomy, surface brightness (SB) quantifies the apparent brightness or flux density per unit angular area of a spatially extended object such as a galaxy or nebula, or of the night sky background. An object's surface brightness depends on its ...

planetary nebula A planetary nebula (PN, plural PNe) is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives. The term "planetary nebula" is a misnomer because they are unrelated to ...

in the constellation of

Hercules Hercules (, ) is the Roman equivalent of the Greek divine hero Heracles, son of Jupiter and the mortal Alcmena. In classical mythology, Hercules is famous for his strength and for his numerous far-ranging adventures. The Romans adapted the Gr ...

. It is the 39th entry in George Abell's 1966

Abell Catalog of Planetary Nebulae The Abell Catalog of Planetary Nebulae was created in 1966 by George O. Abell and was composed of 86 entries thought to be planetary nebulae that were collected from discoveries, about half by Albert George Wilson and the rest by Abell, Rob ...

(and 27th in his 1955 catalog) of 86 old planetary nebulae which either Abell or

Albert George Wilson Albert George Wilson (July 28, 1918 – August 27, 2012) was an American astronomer and a discoverer of minor planets. He was born in Houston, Texas. He received his Ph.D. in mathematics from Caltech in 1947; his thesis title was ''Axially ...

discovered before August 1955 as part of the

National Geographic Society - Palomar Observatory Sky Survey National may refer to: Common uses * Nation or country ** Nationality – a ''national'' is a person who is subject to a nation, regardless of whether the person has full rights as a citizen Places in the United States * National, Maryland, c ...

. It is estimated to be about 3,800

light-year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

s from earth and thus 2,600 light-years above the

Galactic plane The galactic plane is the plane on which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles. In actual usage, the terms ''galactic plane'' and ''galactic poles'' usuall ...

. It is almost perfectly spherical and also one of the largest known spheres with a radius of about 1.4 light-years.

Central star

Its central star is slightly west of center by about 2 or 0.1 light-years. This offset does not appear to be due to interaction with the

interstellar medium In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...

, but instead, it is hypothesized that a small asymmetric mass ejection has accelerated the central star. The mass of the central star is estimated to be about with the material in the planetary nebula comprising an additional . This planetary nebula has a nearly uniform

spherical A sphere () is a geometrical object that is a three-dimensional analogue to a two-dimensional circle. A sphere is the set of points that are all at the same distance from a given point in three-dimensional space.. That given point is the ce ...

shell. However, the eastern limb of the nebula is 50% more luminous than the western limb. Additionally, irregularities in the surface brightness are seen across the face of the shell. The source of the east–west asymmetry is not known but it could be related to the offset of the central star. The central star is classified as a subdwarf O star with surface temperature of 8900 K.

Structure and composition

The bright rim of the planetary nebula has an average thickness of about 10.1 or about 0.19 light-years. There is a faint halo that extends about 18 beyond the bright rim giving a complete diameter of around 190 under the assumption that this emission is uniform around the planetary nebula. This planetary nebula has been expanding for an estimated 11,000 years, based on an assumed expansion velocity between 32 and 37

km/s The metre per second is the unit of both speed (a scalar quantity) and velocity (a vector quantity, which has direction and magnitude) in the International System of Units (SI), equal to the speed of a body covering a distance of one metre in ...

and a 0.4 parsec radius. Background

galaxies A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...

are visible near the nebula, and some can be seen through the translucent nebula.

Oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

is only about half as abundant in the nebula as it is in our own

sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

References

External links

George Jacoby et al. created a spectacular image of Abell 39 in 1997.
* {{DEFAULTSORT:Abell 39 Planetary nebulae Hercules (constellation) 39