star
A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...
in the northern
constellation
A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object.
The first constellati ...
of
Hercules
Hercules (, ) is the Roman equivalent of the Greek divine hero Heracles, son of Jupiter and the mortal Alcmena. In classical mythology, Hercules is famous for his strength and for his numerous far-ranging adventures.
The Romans adapted the Gr ...
. With an
apparent visual magnitude
Apparent magnitude () is a measure of the brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction of the object's light ca ...
of 8.4 it is too faint to be viewed with the naked eye, but it can be seen with even a small telescope. The distance to this object has been determined directly using the
parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...
technique, yielding a value of .
This star has the
stellar classification
In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction gratin ...
of a K2 dwarf, with approximately 77% of the mass of the Sun and about 72% of the Sun's diameter. While they are on the
main sequence
In astronomy, the main sequence is a classification of stars which appear on plots of stellar color index, color versus absolute magnitude, brightness as a continuous and distinctive band. Stars on this band are known as main-sequence stars or d ...
, lower mass stars like this generate energy much more slowly than the Sun. As a result, this star is radiating only 23% of the Sun's bolometric luminosity. HD 156668 is emitting this energy from its outer atmosphere at an
effective temperature
The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperature ...
of around 4850 K, giving it the cool orange glow of a
K-type star
In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...
. It is slightly more enriched in
iron
Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...
compared to the Sun and is rotating at a leisurely rate of once every 51.5 days. Although much older than the Sun, this star is only middle-aged at about 8.6 billion years.
HD 156668 exhibits a stellar activity cycle with a period about 10 years.
Observations
From 1911 to 1915, Photographic Stellar Spectrum surveys were carried out by Annie Jump Cannon and Edward Charles Pickering. Anna Mary Palmer who was Henry Draper's wife financed the study, it was also her husband who was responsible for the star's designation in the Henry Draper Catalogue with subsequent extension (HD) although it was first designated as BD+29 2979 in the original catalogue that was published in 1863 by
Friedrich Wilhelm Argelander
Friedrich Wilhelm August Argelander (22 March 1799 – 17 February 1875) was a German astronomer. He is known for his determinations of stellar brightnesses, positions, and distances.
Life and work
Argelander was born in Memel in the Kingd ...
due to its position and brightness of 324,198 stars between +90° and −2° declination in the span of an 11-year observation. A number of proper motion surveys was published in 1957 where 9,867 stars in the Southern hemisphere where high proper motion was detected. The name BD was taken from a catalogue of the Bonner Durchmusterung (Bonn Survey) where observations are abbreviated as BD. Latter study expansions were carried out by the Cape Photographic Durchmusterung observatory in
Córdoba, Argentina
Córdoba () is a city in central Argentina, in the foothills of the Punilla Valley, Sierras Chicas on the Primero River, Suquía River, about northwest of Buenos Aires. It is the capital of Córdoba Province, Argentina, Córdoba Province an ...
and further extensions by Henry Lee Giclas who designated the star as G 181-34 in the Giclas catalogues. From 1990 to 1993, HD 156668 was observed at least twice and was featured at the 6th annual catalog of the Tokyo Photoelectric Meridian Circle (PMC) where it is one of 6649 stars observed.
Planetary system
Andrew Howard announced the discovery of a
Super-Earth
A super-Earth is a type of exoplanet with a mass higher than Earth, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17.1 times Earth's, respectively.
The term "super-Earth" refers only to t ...
in orbit around HD 156668. This planet is designated HD 156668 b. The announcement was made at the 215th American Astronomical Society meeting on January 4 to 7, 2010 in
Washington D.C.
Washington, D.C., formally the District of Columbia and commonly known as Washington or D.C., is the capital city and federal district of the United States. The city is on the Potomac River, across from Virginia, and shares land borders with ...
The planet orbits its star in only 4.6 days with a distance approximately 0.05 AU away from the parent star. The researchers used the wobble method where the resulting spectrum showed color shifts which was used to approximate the mass of the astronomical object. The observation revealed data that the exoplanet was at least 4.15 Earth masses. Later observations revealed that the
minimum mass
In astronomy, minimum mass is the lower-bound calculated mass of observed objects such as planets, stars, binary systems, nebulae, and black holes.
Minimum mass is a widely cited statistic for extrasolar planets detected by the radial velocit ...
is now 3.1 Earth masses.
Another planet, HD 156668 c, was discovered in 2021, and presence of additional long-period object is suspected.