NGC 7424
Encyclopedia
NGC
7424 is a barred spiral galaxy
located 37.5 million light-years away in the southern
constellation
Grus (the Crane)
. Its structure and diameter (about 100,000 light-year
s) make it similar to our own galaxy
, the Milky Way
.
It is called a "grand design
" galaxy because of its well defined spiral arms. One supernova and two ultraluminous X-ray sources have been discovered in NGC 7424.
while the bright blue color of the loose arms indicates the presence of ionised hydrogen
and clusters
of massive young stars
. NGC 7424 is listed as a member of the IC
1459 Grus Group of galaxies, but is suspected of being a "field galaxy
"; that is, not gravitationally bound to any group.
on the outer edge of NGC 7424 on 10 December 2001.
Type IIb supernovae (SNe) initially exhibit spectral lines of hydrogen (like typical Type II's
), but these disappear after a short time to be replaced by lines of oxygen, calcium and magnesium (like typical Type Ib's and Ic's
).
On 28 May 2002 Cambridge University astrophysicist Stuart Ryder et al. found what they believe is the binary companion to SN 2001ig. It is a massive O or B class star
that had an eccentric orbit
around the progenitor, a Wolf-Rayet star
. They believe that the companion periodically stripped the outer hydrogen-rich envelope of the progenitor, accounting for the observed spectral changes.
Princeton University fellow Alicia Soderberg et al. also believe that the progenitor was a Wolf-Rayet star
, but suggest that the periodic mass loss was a result of the intense stellar wind these stars produce.
discovered two Ultraluminous X-ray source
s (ULXs) with the Chandra X-ray Observatory
. ULXs are objects that emit tremendous amounts of X-rays (> 1032 watt
s or 1039 erg/s
), assuming they radiate isotropically (the same in all directions). This amount is larger than currently understood stellar processes (including supernovae) but smaller than the amount of X-ray
s emitted by active galactic nuclei
, which accounts for their alternate name, Intermediate-luminosity X-ray Objects (IXOs). The source designated ULX1 was found in a relatively empty interarm region, far from any bright clusters or star-forming complexes
, and showed a 75% increase in X-ray luminosity over the course of 20 days. ULX2 was found in an exceptionally bright young stellar complex
, and showed an order of magnitude increase over the same time period.
New General Catalogue
The New General Catalogue of Nebulae and Clusters of Stars is a well-known catalogue of deep sky objects in astronomy. It contains 7,840 objects, known as the NGC objects...
7424 is 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...
located 37.5 million light-years away in the southern
Celestial sphere
In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere. Projected upward from Earth's equator and poles are the...
constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
Grus (the Crane)
Grus (constellation)
Grus is a constellation in the southern sky. Its name is Latin for the crane, a species of bird. It was introduced in the late sixteenth century.-History:The stars that form Grus were originally considered part of Piscis Austrinus...
. Its structure and diameter (about 100,000 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) make it similar to our own galaxy
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...
, the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
.
It is called a "grand design
Grand design spiral galaxy
A grand design spiral galaxy is a type of spiral galaxy with prominent and well-defined spiral arms, as opposed to multi-arm and flocculent spirals which have subtler structural features. The spiral arms of a grand design galaxy extend clearly around the galaxy through many radians and can be...
" galaxy because of its well defined spiral arms. One supernova and two ultraluminous X-ray sources have been discovered in NGC 7424.
Characteristics
NGC 7424 is intermediate between normal spirals (SA) and strongly barred galaxies (SB). Other features include the presence of a central ring-like structure and a relatively low core brightness relative to the arms. The redder color of the prominent bar indicates an older population of starsMetallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...
while the bright blue color of the loose arms indicates the presence of ionised hydrogen
H II region
An H II region is a large, low-density cloud of partially ionized gas in which star formation has recently taken place. The short-lived, blue stars forged in these regions emit copious amounts of ultraviolet light, ionizing the surrounding gas...
and clusters
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...
of massive young stars
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...
. NGC 7424 is listed as a member of the IC
Index Catalogue
The Index Catalogue —also known as the Index Catalogue of Nebulae, the Index Catalogue of Nebulae and Clusters of Stars, IC I, or IC II— is a catalogue of galaxies, nebulae and star clusters that serves as a supplement to the New General Catalogue...
1459 Grus Group of galaxies, but is suspected of being a "field galaxy
Field galaxy
A field galaxy is a galaxy that does not belong to a larger cluster of galaxies, but is gravitationally alone. The vast majority of galaxies exist outside of clusters.Most low surface brightness galaxies are field galaxies....
"; that is, not gravitationally bound to any group.
Supernova 2001ig
SN 2001ig was a rare Type IIb supernova discovered by Australian amateur Robert EvansRobert Evans (astronomer)
Robert Owen Evans is a minister of the Uniting Church in Australia and an amateur astronomer who holds the all-time record for visual discoveries of supernovae ....
on the outer edge of NGC 7424 on 10 December 2001.
Type IIb supernovae (SNe) initially exhibit spectral lines of hydrogen (like typical Type II's
Type II supernova
A Type II supernova results from the rapid collapse and violent explosion of a massive star. A star must have at least 9 times, and no more than 40–50 times the mass of the Sun for this type of explosion. It is distinguished from other types of supernova by the presence of hydrogen in its spectrum...
), but these disappear after a short time to be replaced by lines of oxygen, calcium and magnesium (like typical Type Ib's and Ic's
Type Ib and Ic supernovae
Types Ib and Ic supernovae are categories of stellar explosions that are caused by the core collapse of massive stars. These stars have shed their outer envelope of hydrogen, and, when compared to the spectrum of Type Ia supernovae, they lack the absorption line of silicon...
).
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...
that had an eccentric orbit
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...
around the progenitor, a Wolf-Rayet star
Wolf-Rayet star
Wolf–Rayet stars are evolved, massive stars , which are losing mass rapidly by means of a very strong stellar wind, with speeds up to 2000 km/s...
. They believe that the companion periodically stripped the outer hydrogen-rich envelope of the progenitor, accounting for the observed spectral changes.
Princeton University fellow Alicia Soderberg et al. also believe that the progenitor was a Wolf-Rayet star
Wolf-Rayet star
Wolf–Rayet stars are evolved, massive stars , which are losing mass rapidly by means of a very strong stellar wind, with speeds up to 2000 km/s...
, but suggest that the periodic mass loss was a result of the intense stellar wind these stars produce.
Ultraluminous X-ray sources
In May and June 2002 Roberto Soria and his colleagues at the Harvard-Smithsonian Center for AstrophysicsHarvard-Smithsonian Center for Astrophysics
The Harvard–Smithsonian Center for Astrophysics is one of the largest and most diverse astrophysical institutions in the world, where scientists carry out a broad program of research in astronomy, astrophysics, earth and space sciences, and science education...
discovered two Ultraluminous X-ray source
Ultraluminous X-ray source
An ultra-luminous X-ray source is an astronomical source of X-rays that is less luminous than an active galactic nucleus but is more consistently luminous than any known stellar process , assuming that it radiates isotropically...
s (ULXs) with the Chandra X-ray Observatory
Chandra X-ray Observatory
The Chandra X-ray Observatory is a satellite launched on STS-93 by NASA on July 23, 1999. It was named in honor of Indian-American physicist Subrahmanyan Chandrasekhar who is known for determining the maximum mass for white dwarfs. "Chandra" also means "moon" or "luminous" in Sanskrit.Chandra...
. ULXs are objects that emit tremendous amounts of X-rays (> 1032 watt
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...
s or 1039 erg/s
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
), assuming they radiate isotropically (the same in all directions). This amount is larger than currently understood stellar processes (including supernovae) but smaller than the amount of X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
s emitted by active galactic nuclei
Active galactic nucleus
An active galactic nucleus is a compact region at the centre of a galaxy that has a much higher than normal luminosity over at least some portion, and possibly all, of the electromagnetic spectrum. Such excess emission has been observed in the radio, infrared, optical, ultra-violet, X-ray and...
, which accounts for their alternate name, Intermediate-luminosity X-ray Objects (IXOs). The source designated ULX1 was found in a relatively empty interarm region, far from any bright clusters or star-forming complexes
H II region
An H II region is a large, low-density cloud of partially ionized gas in which star formation has recently taken place. The short-lived, blue stars forged in these regions emit copious amounts of ultraviolet light, ionizing the surrounding gas...
, and showed a 75% increase in X-ray luminosity over the course of 20 days. ULX2 was found in an exceptionally bright young stellar complex
H II region
An H II region is a large, low-density cloud of partially ionized gas in which star formation has recently taken place. The short-lived, blue stars forged in these regions emit copious amounts of ultraviolet light, ionizing the surrounding gas...
, and showed an order of magnitude increase over the same time period.