V391 Pegasi
Encyclopedia
V391 Pegasi, also catalogued as HS 2201+2610, is a blue-white subdwarf star
approximately 4,570 light-year
s away in the constellation
of Pegasus
. The star is clarified as an "extreme horizontal branch
star." It is small with only half the mass
and one-over-four-and-one-thirds the diameter of the Sun. It has luminosity 15.4 times the Sun
. It could be quite old, perhaps in excess of 10 Gyr. It is a pulsating variable star
of the V361 Hydrae type (or also called sdBVr type).
Subdwarf B stars such as V391 Pegasi are thought to be the result of the ejection of the hydrogen envelope of a red giant
star at or just before the onset of helium
fusion. The ejection left only a tiny amount of hydrogen on the surface - less than 1/1000 of the total stellar mass. The future for the star is to eventually cool down to make a low mass white dwarf. Most stars retain more of their hydrogen after the first red giant phase, and eventually become asymptotic giant branch
stars. The reasons for why some stars, like V391 Pegasi, lose so much mass are not well known.
planet
designated V391 Pegasi b
was found in orbit
by variable star timing method. This planet around "extreme horizontal branch" star provided clues about what could actually happen to the planets in our solar system
when our Sun
turns into a red giant within the next five billion years.
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...
approximately 4,570 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
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 Pegasus
Pegasus (constellation)
Pegasus is a constellation in the northern sky, named after the winged horse Pegasus in Greek mythology. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and remains one of the 88 modern constellations.-Stars:...
. The star is clarified as an "extreme horizontal branch
Horizontal branch
The horizontal branch is a stage of stellar evolution which immediately follows the red giant branch in stars whose masses are similar to the Sun's...
star." It is small with only half the mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
and one-over-four-and-one-thirds the diameter of the Sun. It has luminosity 15.4 times 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...
. It could be quite old, perhaps in excess of 10 Gyr. It is a pulsating 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...
of the V361 Hydrae type (or also called sdBVr type).
Subdwarf B stars such as V391 Pegasi are thought to be the result of the ejection of the hydrogen envelope of a red giant
Red giant
A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower...
star at or just before the onset of helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
fusion. The ejection left only a tiny amount of hydrogen on the surface - less than 1/1000 of the total stellar mass. The future for the star is to eventually cool down to make a low mass white dwarf. Most stars retain more of their hydrogen after the first red giant phase, and eventually become asymptotic giant branch
Asymptotic Giant Branch
The asymptotic giant branch is the region of the Hertzsprung-Russell diagram populated by evolving low to medium-mass stars. This is a period of stellar evolution undertaken by all low to intermediate mass stars late in their lives....
stars. The reasons for why some stars, like V391 Pegasi, lose so much mass are not well known.
Planetary system
In 2007, a gas giantGas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...
planet
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...
designated V391 Pegasi b
V391 Pegasi b
V391 Pegasi b, also known as HS 2201+2610 b, is an extrasolar planet orbiting the star V391 Pegasi approximately 4570 light-years away in the constellation of Pegasus. The planet was discovered by means of variable star timing, which measured anomalies in variability of the star caused by a planet...
was found in orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
by variable star timing method. This planet around "extreme horizontal branch" star provided clues about what could actually happen to the planets in our 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...
when our 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...
turns into a red giant within the next five billion years.