Hoag's Object is an unusual ring galaxy in the constellation of Serpens Caput. It is named after

Arthur Hoag Arthur Allen Hoag (January 28, 1921 - July 17, 1999) was an American astronomer most famous for his discovery of Hoag's Object, a type of ring galaxy, in 1950. Biography Hoag was born January 28, 1921, in Ann Arbor, Michigan. The son of Lynne ...

, who discovered it in 1950 and identified it as either a

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 unrelate ...

or a

peculiar galaxy A peculiar galaxy is a galaxy of unusual size, shape, or composition. Between five and ten percent of known galaxies are categorized as peculiar. Astronomers have identified two types of peculiar galaxies: ''interacting galaxies'' and ''active ga ...

. The galaxy has approximately eight billion

star A star is an astronomical object comprising a luminous spheroid of plasma held together by its gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but their immense distances from Earth make ...

s, and is roughly 120,000 light years across.

Characteristics

A nearly perfect ring of young hot blue

s circles the older yellow nucleus of this ring galaxy c. 600 million

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 101 ...

s away in the constellation

Serpens Serpens ( grc, , , the Serpent) is a constellation in the northern celestial hemisphere. One of the 48 constellations listed by the 2nd-century astronomer Ptolemy, it remains one of the 88 modern constellations designated by the International ...

. The ring structure is so perfect and circular that it has been referred to as "The most perfect ring galaxy". The diameter of the 6

arcsecond A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The na ...

inner core of the galaxy is about () while the surrounding ring has an inner 28″ diameter of () and an outer 45″ diameter of (). The galaxy is estimated to have a mass of 700 billion suns. By comparison, the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked ey ...

galaxy has an estimated diameter of 150-200 kly and consists of between 100 and 500 billion stars and a mass between 800 billion and 1.54 trillion suns. The gap separating the two stellar populations may contain some

star cluster Star clusters are large groups of stars. Two main types of star clusters can be distinguished: globular clusters are tight groups of ten thousand to millions of old stars which are gravitationally bound, while open clusters are more loosely clus ...

s that are almost too faint to see. Though ring galaxies are rare, another more distant ring galaxy (SDSS J151713.93+213516.8) can be seen through Hoag's Object, between the nucleus and the outer ring of the galaxy, at roughly the one o'clock position in the image shown here. Noah Brosch and colleagues showed that the luminous ring lies at the inner edge of a much larger neutral hydrogen ring.

History and formation

Even though Hoag's Object was clearly shown on the

Palomar Star Survey Palomar may refer to: Places * Any of several locations in San Diego County, California: ** Palomar Mountain ** Palomar Observatory, located on Palomar Mountain ** Palomar College in San Marcos, California ** Palomar Medical Center in Escondido, ...

, it was not included in either the '' Morphological Catalogue of Galaxies'', the '' Catalogue of Galaxies and Clusters of Galaxies'', or the '' catalogue of galactic planetary nebulae''. In the initial announcement of his discovery, Hoag proposed the hypothesis that the visible ring was a product of gravitational lensing. This idea was later discarded because the nucleus and the ring have the same

redshift In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and simultaneous increase in f ...

, and because more advanced telescopes revealed the ring's knotty structure, which would not be visible if the galaxy were a product of gravitational lensing. Many of the galaxy's details remain mysterious, foremost of which is how it formed. So-called "classic" ring galaxies are generally formed by the collision of a small galaxy with a larger disk-shaped galaxy, producing a density wave in the disk that leads to a characteristic ring-like appearance. Such an event would have happened at least 2–3 billion years ago, and may have resembled the processes that form

polar-ring galaxies A polar-ring galaxy is a type of galaxy in which an outer ring of gas and stars rotates over the poles of the galaxy. These polar rings are thought to form when two galaxies gravitationally interact with each other. One possibility is that a mat ...

. However, there is no sign of any second galaxy that would have acted as the "bullet", and the likely older core of Hoag's Object has a very low velocity relative to the ring, making the typical formation hypothesis quite unlikely. Observations with one of the most sensitive telescopes have also failed to uncover any faint galaxy fragments that should be observable in a collision scenario. However, a team of scientists that analyzes the galaxy admits that "if the carnage happened more than 3 billion years ago, there might not be any

detritus In biology, detritus () is dead particulate organic material, as distinguished from dissolved organic material. Detritus typically includes the bodies or fragments of bodies of dead organisms, and fecal material. Detritus typically hosts commu ...

left to see." Noah Brosch suggested that Hoag's Object might be a product of an extreme "bar instability" that occurred a few billion years ago in a

barred spiral galaxy A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in about two thirds of all spiral galaxies, and generally affect both the motions of stars and interstellar gas within spiral galaxies ...

. Schweizer ''et al'' claim this is an unlikely hypothesis because the nucleus of the object is spheroidal, whereas the nucleus of a barred spiral galaxy is disc-shaped, among other reasons. However, they admit evidence is somewhat thin for this particular dispute to be settled satisfactorily. A few other galaxies share the primary characteristics of Hoag's Object, including a bright detached ring of stars, but their centers are elongated or barred, and they may exhibit some spiral structure. While none matches Hoag's Object in symmetry, these galaxies are known to some as Hoag-type galaxies.

Notes

1974 O'Connell paper assumes a

Hubble constant Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther they are, the faster they are moving ...

of 75; this figure adjusts for 2013's .

References

External links

*
Hubble Space Telescope image of Hoag's Object.

Galactic Rings – Fund. Cosmic Physics, 1996. Vol. 17, pp. 95–281
A Wheel within a Wheel (09/05/2002)
News Release Number: STScI-2002-21 – HubbleSite

{{Portal bar, Astronomy, Stars, Outer space Ring galaxies Serpens Peculiar galaxies 54559 ?

Characteristics

History and formation

See also

Notes

References

External links