NGC 691 is an
unbarred spiral galaxy
An unbarred spiral galaxyAstronomy Pictures(accessed 18 April 2010) is a type of spiral galaxy without a central bar, or one that is not a barred spiral galaxy. It is designated with an SA in the galaxy morphological classification scheme.
Ba ...
located in the constellation
Aries
Aries may refer to:
*Aries (astrology), an astrological sign
*Aries (constellation), a constellation of stars in the zodiac
Arts, entertainment and media
* ''Aries'' (album), by Luis Miguel, 1993
* ''Aries'' (EP), by Alice Chater, 2020
* "Aries" ...
. It is located at a distance of circa 120 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 1012 ...
s from Earth, which, given its apparent dimensions, means that NGC 691 is about 130,000 light years across. It was discovered by
William Herschel
Frederick William Herschel (; german: Friedrich Wilhelm Herschel; 15 November 1738 – 25 August 1822) was a German-born British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline H ...
on November 13, 1786.
NGC 691 features a multiple ring structure, with three rings recognised in the
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
, with diameters of 1.03, 1.67, and 2.79 arcminutes. When imaged in
H-alpha
H-alpha (Hα) is a specific deep-red visible spectral line in the Balmer series with a wavelength of 656.28 nm in air and 656.46 nm in vacuum; it occurs when a hydrogen electron falls from its third to second lowest energy level. H-alph ...
, the galaxy appears patchy. The total
rate of the galaxy is estimated to be about 0.6 per year. One
supernova
A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...
has been observed in NGC 691, SN 2005W. It was discovered by Yoji Hirose in unfiltered
CCD frames taken on Feb. 1.442 UT with a 0.35-m f/6.8
Schmidt-Cassegrain reflector. The supernova was located 56" east and 1" south of the center of NGC 691 and at the time of the discovery had an
apparent magnitude
Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's li ...
of 15.2. Spectrographic observations indicated it was a
type Ia supernova
A Type Ia supernova (read: "type one-A") is a type of supernova that occurs in binary systems (two stars orbiting one another) in which one of the stars is a white dwarf. The other star can be anything from a giant star to an even smaller white ...
about a week before maximum. The peak magnitude of the supernova was 14.3, on February 10.759.
NGC 691 is the foremost member of a
galaxy group
A galaxy group or group of galaxies (GrG) is an aggregation of galaxies comprising about 50 or fewer gravitationally bound members, each at least as luminous as the Milky Way (about 1010 times the luminosity of the Sun); collections of galaxies ...
known as the NGC 691 group. Other members of the group include
IC 163,
NGC 678,
NGC 680,
NGC 694,
IC 167
IC 167 is a barred spiral galaxy in the constellation of Aries. It was first reported by Bigourdan in 1891 and included in Dreyer's first ''Index Catalogue''.
Galaxy group information
IC 167 is a member of the NGC 697 group. There is some evid ...
, and
NGC 697.
Gallery
NGC691 - SDSS DR14.jpg, NGC 691 (SDSS DR14)
N691s.jpg, NGC 691 (top) and IC 167 (bottom) by the Mount Lemmon SkyCenter
NGC691 - HST - Potw2122a.tiff, Hubble Space Telescope
The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...
References
External links
*
NGC 691 on SIMBAD
{{DEFAULTSORT:NGC 691
Unbarred spiral galaxies
Aries (constellation)
0691
01305
06793
Astronomical objects discovered in 1786
Discoveries by William Herschel