Pi Herculis
Encyclopedia
Pi Herculis is a fourth-magnitude star
in the constellation
Hercules
. As one of the four stars in the Keystone asterism
(see yellow quadrangle
) is one of the more easily recognized in the constellation.
star
with a stellar classification
listed in the SIMBAD
astronomical database as K3Iab. The star is enormous compared to the Sun
, having a mass that is 4.5 times solar and a radius approximately 60 times depending on which wavelength
the star's angular diameter
is measured at. Due to limb darkening
, all giant
and supergiant
stars present unique challenges when measuring their photosphere
. Though its apparent magnitude
is only 3.156, this orange giant shines with a luminousity that is 1,330L☉, yielding an absolute magnitude
of -2.10, brighter in fact than the most of the hot B stars in the Pleiades
open star cluster
. The Hipparcos
satellite
mission estimated its distance at roughly 112 parsec
s from Earth, or just under 370 light years away.
es (27 times the mass of Jupiter
, probably a brown dwarf
) and 3 astronomical unit
s away from the bright primary. A substellar companion is only one of several hypotheses to explain the star's behaviour.
With a luminosity more than 1,000 times that of the Sun
, an orbit where a planet could be habitable would be located 37 AU
away from Pi Herculis—in Solar System terms, halfway between Neptune
and Pluto
's orbits. On the other hand, a putative companion would orbit in a scorching region and would be as hot as a planet would at 0.08 AU
around a Sun-like star. In any case it's likely that it would soon be swallowed up by the expanding giant.
Star
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...
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....
Hercules
Hercules (constellation)
Hercules is a constellation named after Hercules, the Roman mythological hero adapted from the Greek hero Heracles. Hercules was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations today...
. As one of the four stars in the Keystone asterism
Asterism (astronomy)
In astronomy, an asterism is a pattern of stars recognized on Earth's night sky. It may form part of an official constellation, or be composed of stars from more than one. Like constellations, asterisms are in most cases composed of stars which, while they are visible in the same general direction,...
(see yellow quadrangle
Quadrilateral
In Euclidean plane geometry, a quadrilateral is a polygon with four sides and four vertices or corners. Sometimes, the term quadrangle is used, by analogy with triangle, and sometimes tetragon for consistency with pentagon , hexagon and so on...
) is one of the more easily recognized in the constellation.
Properties
Pi Herculis is an orange bright giantBright giant
The luminosity class II in the Yerkes spectral classification is given to bright giants. These are stars which straddle the boundary between giants and supergiants, and the classification is in general given to giant stars with exceptionally high luminosity, but which are not sufficiently bright...
star
Star
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...
with a stellar classification
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...
listed in the SIMBAD
SIMBAD
SIMBAD is an astronomical database of objects beyond the Solar System...
astronomical database as K3Iab. The star is enormous compared to the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
, having a mass that is 4.5 times solar and a radius approximately 60 times depending on which wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
the star's angular diameter
Angular diameter
The angular diameter or apparent size of an object as seen from a given position is the “visual diameter” of the object measured as an angle. In the vision sciences it is called the visual angle. The visual diameter is the diameter of the perspective projection of the object on a plane through its...
is measured at. Due to limb darkening
Limb darkening
Limb darkening refers to the diminishing of intensity in the image of a star as one moves from the center of the image to the edge or "limb" of the image...
, all giant
Giant star
A giant star is a star with substantially larger radius and luminosity than a main sequence star of the same surface temperature. Typically, giant stars have radii between 10 and 100 solar radii and luminosities between 10 and 1,000 times that of the Sun. Stars still more luminous than giants are...
and supergiant
Supergiant
Supergiants are among the most massive stars. They occupy the top region of the Hertzsprung-Russell diagram. In the Yerkes spectral classification, supergiants are class Ia or Ib . They typically have bolometric absolute magnitudes between -5 and -12...
stars present unique challenges when measuring their photosphere
Photosphere
The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...
. Though its apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
is only 3.156, this orange giant shines with a luminousity that is 1,330L☉, yielding an absolute magnitude
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 -2.10, brighter in fact than the most of the hot B stars in the Pleiades
Pleiades (star cluster)
In astronomy, the Pleiades, or Seven Sisters , is an open star cluster containing middle-aged hot B-type stars located in the constellation of Taurus. It is among the nearest star clusters to Earth and is the cluster most obvious to the naked eye in the night sky...
open star cluster
Open cluster
An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy, and many more are thought to exist...
. The Hipparcos
Hipparcos
Hipparcos was a scientific mission of the European Space Agency , launched in 1989 and operated between 1989 and 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky...
satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
mission estimated its distance at roughly 112 parsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
s from Earth, or just under 370 light years away.
Planetary system?
Low-amplitude radial velocity variations with a period of 613 days in the bright giant have suggested the possible presence of a substellar companion. If this is really due to a low-mass object, such a companion would be as small as 0.027 Solar massSolar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
es (27 times the mass of Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
, probably a brown dwarf
Brown dwarf
Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...
) and 3 astronomical unit
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
s away from the bright primary. A substellar companion is only one of several hypotheses to explain the star's behaviour.
With a luminosity more than 1,000 times that of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
, an orbit where a planet could be habitable would be located 37 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
away from Pi Herculis—in Solar System terms, halfway between Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
and Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...
's orbits. On the other hand, a putative companion would orbit in a scorching region and would be as hot as a planet would at 0.08 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
around a Sun-like star. In any case it's likely that it would soon be swallowed up by the expanding giant.
External links
- http://stars.astro.illinois.edu/sow/piher.html
- http://www.dibonsmith.com/her_p.htm