Hoag's Object
Encyclopedia
Hoag's Object is a non-typical galaxy
of the type known as a ring galaxy
. The appearance of this object has interested amateur astronomers as much as its uncommon structure has fascinated professionals. The galaxy is named
after Arthur Allen Hoag
who discovered it in 1950 and identified it as either a planetary nebula
or a peculiar galaxy
with 8 billion stars.
s circles the older yellow nucleus of this ring galaxy ~600 million light-year
s away in the constellation Serpens
. The diameter of the 6″ (seconds of arc) inner core of the galaxy is about while the surrounding ring has an inner 28″ diameter of and an outer 45″ diameter of , which is slightly larger than the Milky Way Galaxy. The gap separating the two stellar populations may contain some star cluster
s that are almost too faint to see. As rare as this type of galaxy is, another more distant ring galaxy can be seen through Hoag's Object, between the nucleus and the outer ring of the galaxy, at the one o'clock position in the picture to the right.
, the Catalogue of Galaxies and Clusters of Galaxies, or the Catalogue of Galactic Planetary Nebulae.
In the initial announcement of his discovery, Art 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
, and because more advanced telescopes revealed the knotty structure of the ring, something that would not be visible if the ring were the product of gravitational lensing.
Many of the details of the galaxy remain a mystery, 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. This collision produces a density wave in the disk which leads to a characteristic ring-like appearance. Such an event would have happened at least 2-3 billion years in the past, and may have resembled the processes that form polar-ring galaxies
. However, there is no sign of any second galaxy that would have acted as the "bullet", and the core of Hoag's Object has a very low velocity relative to the ring, making the typical formation hypothesis quite unlikely.
Noah Brosch suggested that Hoag's Object might be a product of an extreme "bar instability" which occurred a few billion years ago in a barred spiral galaxy
. Schweizer et al claim that 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. Interestingly, a few 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 match Hoag's Object in symmetry, this handful of galaxies are known to some as Hoag-type galaxies.
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
of the type known as a ring galaxy
Ring galaxy
A ring galaxy is a galaxy with a ring-like appearance. The ring consists of massive, relatively young blue stars, which are extremely bright. The central region contains relatively little luminous matter. Some astronomers believe that ring galaxies are formed when a smaller galaxy passes through...
. The appearance of this object has interested amateur astronomers as much as its uncommon structure has fascinated professionals. The galaxy is named
Galaxies named after people
A small number of galaxies or galaxy groups have been named after individual people. In most cases, the named individual was the person who discovered the object, who first brought attention to it, or who first studied it scientifically....
after Arthur Allen Hoag
Arthur Allen Hoag
Arthur Allen Hoag was an American astronomer most famous for his discovery of Hoag's object in 1950. He was the son of Harvard Medical School, Cornell, and University of Michigan faculty member Lynne Arthur Hoag and wife Wylma Wood Hoag. He had two sisters, Mary and Elizabeth, and a son named Tom...
who discovered it in 1950 and identified it as either a planetary nebula
Planetary nebula
A planetary nebula is an emission nebula consisting of an expanding glowing shell of ionized gas ejected during the asymptotic giant branch phase of certain types of stars late in their life...
or a peculiar galaxy
Peculiar galaxy
A peculiar galaxy is a galaxy which is unusual in its size, shape, or composition. Peculiar galaxies come about as a result of interactions between galaxies, and they may contain atypical amounts of dust or gas, may have higher or lower surface brightness than a typical galaxy, or may have...
with 8 billion stars.
Characteristics
A nearly perfect ring of young hot blue starStar
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s circles the older yellow nucleus of this ring galaxy ~600 million light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
s away in the constellation Serpens
Serpens
Serpens is a constellation of the northern hemisphere. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy and it remains one of the 88 modern constellations defined by the International Astronomical Union....
. The diameter of the 6″ (seconds of arc) inner core of the galaxy is about while the surrounding ring has an inner 28″ diameter of and an outer 45″ diameter of , which is slightly larger than the Milky Way Galaxy. The gap separating the two stellar populations may contain some star cluster
Star cluster
Star 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, more loosely clustered groups of stars, generally contain less than...
s that are almost too faint to see. As rare as this type of galaxy is, another more distant ring galaxy can be seen through Hoag's Object, between the nucleus and the outer ring of the galaxy, at the one o'clock position in the picture to the right.
History and formation
Even though Hoag's Object was clearly shown on the Palomar Star Survey, it was not included in either the Morphological Catalogue of GalaxiesMorphological Catalogue of Galaxies
The Morphological Catalogue of Galaxies ' or Morfologiceskij Katalog Galaktik, is a Russian catalogue of 30642 galaxies compiled by Boris Vorontsov-Velyaminov and V. P. Arkhipova. It is based on a close scrutiny of prints of the Palomar Sky Survey plates, and putatively complete to a photographic...
, the Catalogue of Galaxies and Clusters of Galaxies, or the Catalogue of Galactic Planetary Nebulae.
In the initial announcement of his discovery, Art 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
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...
, and because more advanced telescopes revealed the knotty structure of the ring, something that would not be visible if the ring were the product of gravitational lensing.
Many of the details of the galaxy remain a mystery, 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. This collision produces a density wave in the disk which leads to a characteristic ring-like appearance. Such an event would have happened at least 2-3 billion years in the past, and may have resembled the processes that form polar-ring galaxies
Polar-ring galaxy
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 material is tidally stripped from a passing galaxy to...
. However, there is no sign of any second galaxy that would have acted as the "bullet", and the core of Hoag's Object has a very low velocity relative to the ring, making the typical formation hypothesis quite unlikely.
Noah Brosch suggested that Hoag's Object might be a product of an extreme "bar instability" which occurred a few billion years ago in a barred spiral galaxy
Barred spiral galaxy
A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in approximately two-thirds of all spiral galaxies...
. Schweizer et al claim that 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. Interestingly, a few 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 match Hoag's Object in symmetry, this handful of galaxies are known to some as Hoag-type galaxies.
External links
- Hubble Space Telescope image of Hoag's Object.
- Accretion Rings 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
- ESA/Hubble image of Hoag's Object