Virgo Stellar Stream
Encyclopedia
The Virgo Stellar Stream, also known as Virgo Overdensity, is the proposed name for a stellar stream
in the constellation
of Virgo
which was discovered in 2005. The stream is thought to be the remains of a dwarf spheroidal galaxy
that is in the process of merging with the Milky Way
. It is the largest galaxy
visible from the Earth
, in terms of the area
of the night sky
covered.
The stream was discovered from photometric
data from the Sloan Digital Sky Survey
, which was used to create a three dimensional
map of the Milky Way, using the color
s and brightness
of certain characteristic types of stars
to estimate their distance (a method known as "photometric parallax"). The first suggestion of a new galaxy in Virgo was made in 2001 from data obtained as part of the QUEST survey
, which used the 1.0 metre
Schmidt telescope at the Llano del Hato National Astronomical Observatory
in Venezuela
to look for RR Lyrae
variable star
s. Five were found in a clump with a right ascension
near 12.4 hours, and the astronomers speculated that this clump was part of a small galaxy being "cannibalised" by the Milky Way.
The stream covers over 100 square degree
s, and possibly as much as 1,000 square degrees (approximately 5% of the hemisphere
visible at any one time, or 5,000 times the area of the full moon
). Despite its proximity to the solar system
and the solid angle
that it consequently covers, the stream contains only a few hundred thousand stars. The low surface brightness
of the galaxy (possibly as low as 32.5 mag
/arcsec²) may have militated against its detection in surveys before SDSS. The number of stars in the stream is not greatly in excess of a star cluster
, and it has been described by a member of the team that discovered it as "a rather pathetic galaxy", in comparison to the Milky Way. Many of the stars have been known for centuries and thought of as normal Milky Way stars, although they have a lower metallicity
than normal Population I stars in the Milky Way.
The stream lies within the Milky Way, approximately 10 kiloparsecs (30,000 light-year
s) from the Sun, and extending over a region of space at least 10 kpc across in three dimensions. It is close on the plane of the sky to the Sagittarius Dwarf Elliptical Galaxy
, which was found in 1994 through a similar photometric analysis of a star survey. The Sagittarius Dwarf is another small galaxy which is also in the process of merging with the Milky Way; however, it is approximately 4 times further away than the stream, so the two are unlikely to be physically related, although it is possible that the Virgo Stellar Stream is a remnant left behind by the disruption of the Sagittarius Dwarf as it had orbited around the Milky Way. The Virgo Stellar Stream also resembles the Monoceros Ring
, found in 2002, which has similarly been attributed to the Canis Major Dwarf Galaxy
merging with the Milky Way.
Stellar stream
This is a list of stellar streams. A stellar stream is an association of stars orbiting a galaxy that was once a globular cluster or dwarf galaxy that has now been torn apart and stretched out along its orbit by tidal forces...
in 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....
of Virgo
Virgo (constellation)
Virgo is one of the constellations of the zodiac. Its name is Latin for virgin, and its symbol is . Lying between Leo to the west and Libra to the east, it is the second largest constellation in the sky...
which was discovered in 2005. The stream is thought to be the remains of a dwarf spheroidal galaxy
Dwarf spheroidal galaxy
Dwarf spheroidal galaxy is a term in astronomy applied to low luminosity galaxies that are companions to the Milky Way and to the similar systems that are companions to the Andromeda Galaxy M31...
that is in the process of merging with 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 the largest 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...
visible from the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, in terms of the area
Area
Area is a quantity that expresses the extent of a two-dimensional surface or shape in the plane. Area can be understood as the amount of material with a given thickness that would be necessary to fashion a model of the shape, or the amount of paint necessary to cover the surface with a single coat...
of the night sky
Night sky
The term night sky refers to the sky as seen at night. The term is usually associated with astronomy, with reference to views of celestial bodies such as stars, the Moon, and planets that become visible on a clear night after the Sun has set. Natural light sources in a night sky include moonlight,...
covered.
The stream was discovered from photometric
Photometry (astronomy)
Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation...
data from the Sloan Digital Sky Survey
Sloan Digital Sky Survey
The Sloan Digital Sky Survey or SDSS is a major multi-filter imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project was named after the Alfred P...
, which was used to create a three dimensional
Three-dimensional space
Three-dimensional space is a geometric 3-parameters model of the physical universe in which we live. These three dimensions are commonly called length, width, and depth , although any three directions can be chosen, provided that they do not lie in the same plane.In physics and mathematics, a...
map of the Milky Way, using the color
Color
Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
s and brightness
Absolute magnitude
Absolute magnitude is the measure of a celestial object's intrinsic brightness. it is also the apparent magnitude a star would have if it were 32.6 light years away from Earth...
of certain characteristic types of stars
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...
to estimate their distance (a method known as "photometric parallax"). The first suggestion of a new galaxy in Virgo was made in 2001 from data obtained as part of the QUEST survey
Quasar Equatorial Survey Team
The Quasar Equatorial Survey Team is a joint venture between Yale University, Indiana University, and Centro de Investigaciones de Astronomia to photographically survey the sky. It uses the 48 inch The Quasar Equatorial Survey Team is a joint venture between Yale University, Indiana University,...
, which used the 1.0 metre
Metre
The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...
Schmidt telescope at the Llano del Hato National Astronomical Observatory
Llano del Hato National Astronomical Observatory
The Llano del Hato National Astronomical Observatory is an astronomical observatory in Venezuela, run by the Centro de Investigaciones de Astronomia...
in Venezuela
Venezuela
Venezuela , officially called the Bolivarian Republic of Venezuela , is a tropical country on the northern coast of South America. It borders Colombia to the west, Guyana to the east, and Brazil to the south...
to look for RR Lyrae
RR Lyrae
RR Lyrae is a variable star in the Lyra constellation. It is the prototype of the RR Lyrae variable class of stars. It has a period of about 13 hours, and oscillates between apparent magnitudes 7 and 8. Its variable nature was discovered by the Scottish astronomer Williamina Fleming at Harvard...
variable star
Variable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
s. Five were found in a clump with a right ascension
Right ascension
Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:...
near 12.4 hours, and the astronomers speculated that this clump was part of a small galaxy being "cannibalised" by the Milky Way.
The stream covers over 100 square degree
Square degree
A square degree is a non-SI unit measure of solid angle. It is denoted in various ways, including deg2, sq.deg. and ². Just as degrees are used to measure parts of a circle, square degrees are used to measure parts of a sphere. Analogous to one degree being equal to π /180 radians, a...
s, and possibly as much as 1,000 square degrees (approximately 5% of the hemisphere
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...
visible at any one time, or 5,000 times the area of the full moon
Full moon
Full moon lunar phase that occurs when the Moon is on the opposite side of the Earth from the Sun. More precisely, a full moon occurs when the geocentric apparent longitudes of the Sun and Moon differ by 180 degrees; the Moon is then in opposition with the Sun.Lunar eclipses can only occur at...
). Despite its proximity to the solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
and the solid angle
Solid angle
The solid angle, Ω, is the two-dimensional angle in three-dimensional space that an object subtends at a point. It is a measure of how large that object appears to an observer looking from that point...
that it consequently covers, the stream contains only a few hundred thousand stars. The low surface brightness
Surface brightness
The overall brightness of an extended astronomical object such as a galaxy, star cluster, or nebula, can be measured by its total magnitude, integrated magnitude or integrated visual magnitude; a related concept is surface brightness, which specifies the brightness of a standard-sized piece of an...
of the galaxy (possibly as low as 32.5 mag
Absolute magnitude
Absolute magnitude is the measure of a celestial object's intrinsic brightness. it is also the apparent magnitude a star would have if it were 32.6 light years away from Earth...
/arcsec²) may have militated against its detection in surveys before SDSS. The number of stars in the stream is not greatly in excess of a 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...
, and it has been described by a member of the team that discovered it as "a rather pathetic galaxy", in comparison to the Milky Way. Many of the stars have been known for centuries and thought of as normal Milky Way stars, although they have a lower metallicity
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...
than normal Population I stars in the Milky Way.
The stream lies within the Milky Way, approximately 10 kiloparsecs (30,000 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) from the Sun, and extending over a region of space at least 10 kpc across in three dimensions. It is close on the plane of the sky to the Sagittarius Dwarf Elliptical Galaxy
Sagittarius Dwarf Elliptical Galaxy
The Sagittarius Dwarf Elliptical Galaxy is an elliptical loop-shaped satellite galaxy of the Milky Way Galaxy. It consists of four globular clusters, the main cluster being discovered in 1994...
, which was found in 1994 through a similar photometric analysis of a star survey. The Sagittarius Dwarf is another small galaxy which is also in the process of merging with the Milky Way; however, it is approximately 4 times further away than the stream, so the two are unlikely to be physically related, although it is possible that the Virgo Stellar Stream is a remnant left behind by the disruption of the Sagittarius Dwarf as it had orbited around the Milky Way. The Virgo Stellar Stream also resembles the Monoceros Ring
Monoceros Ring
The Monoceros Ring is a ring of stars around the Milky Way which is proposed to consist of a stellar stream torn from the Canis Major Dwarf Galaxy as it merges with the Milky Way over the course of billions of years...
, found in 2002, which has similarly been attributed to the Canis Major Dwarf Galaxy
Canis Major Dwarf Galaxy
The Canis Major Dwarf Galaxy is located in the same part of the sky as the constellation Canis Major. The galaxy contains a relatively high percentage of red giant stars, and is thought to contain an estimated one billion stars in all....
merging with the Milky Way.
See also
- Local GroupLocal GroupThe Local Group is the group of galaxies that includes Earth's galaxy, the Milky Way. The group comprises more than 30 galaxies , with its gravitational center located somewhere between the Milky Way and the Andromeda Galaxy...
- List of nearest galaxies
External links
- Scientists find a galaxy joining ours (International Herald TribuneInternational Herald TribuneThe International Herald Tribune is a widely read English language international newspaper. It combines the resources of its own correspondents with those of The New York Times and is printed at 38 sites throughout the world, for sale in more than 160 countries and territories...
, 10 January 2006) - New Milky Way neighbor (AstronomyAstronomy (magazine)Astronomy is a monthly American magazine about astronomy. Targeting amateur astronomers for its readers, it contains columns on sky viewing, reader-submitted astrophotographs, and articles on astronomy and astrophysics that are readable by nonscientists....
, 12 January 2006)