Alpha Herculis (α Herculis, abbreviated Alpha Her, α Her), also designated Rasalgethi and 64 Herculis, is a multiple star system in the

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 ...

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 ...

. Appearing as a single point of light to the naked eye, it is resolvable into a number of components through a telescope. It has a combined

apparent magnitude Apparent magnitude () is a measure of the Irradiance, 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 (astronomy), ...

of 3.08, although the brightest component is variable in brightness. Based on

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 ...

measurements obtained during the

Hipparcos ''Hipparcos'' was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions and distances of ...

mission, it is approximately 360

light-year A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astr ...

s (110

parsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (AU), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, and ...

s) distant from the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

. It is also close to another bright star Rasalhague in the vicinity.

System

Alpha Herculis is a triple star system. The primary (brightest) of the three stars, designated α¹ Herculis or α Herculis A, is a pulsating variable star on the

asymptotic giant branch The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) lat ...

(AGB). The primary star forms a visual binary pair with a second star, which is itself a spectroscopic binary. Alpha Herculis also forms the A and B components of a wider system designated WDS J17146+1423, with two additional faint visual companions designated WDS J17146+1423C and D. The two fainter stars are far more distant than the triple system.

Nomenclature

''α Herculis'' ( Latinised to ''Alpha Herculis'') is the system's

Bayer designation A Bayer designation is a stellar designation in which a specific star is identified by a Greek alphabet, Greek or Latin letter followed by the genitive case, genitive form of its parent constellation's Latin name. The original list of Bayer design ...

; ''α¹'' and ''α² Herculis'', those of its two visible components. ''64 Herculis'' is the system's Flamsteed designation. WDS J17146+1423 is the wider system's designation in the Washington Double Star Catalog. The designations of Alpha Herculis' main components as ''Alpha Herculis A'' and ''B'' and the wider system's four components as WDS J17146+1423A, B, C and D, together with the spectroscopic pair - ''Alpha Herculis Ba'' and ''Bb'' - derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the

International Astronomical Union The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...

(IAU). Alpha Herculis bore the traditional name ''Rasalgethi'' or ''Ras Algethi'' ( 'Head of the Kneeler'). 'Head' comes from the fact that in antiquity Hercules was depicted upside down on maps of the constellation. In 2016, the IAU organized a

Working Group on Star Names The International Astronomical Union (IAU) established a Working Group on Star Names (WGSN) in May 2016 to catalog and standardize proper names for stars for the international astronomical community. It operates under Division C – Education ...

(WGSN) to catalog and standardize proper names for stars. The WGSN approved the name ''Rasalgethi'' for the component Alpha Herculis A (α¹) on 30 June 2016 and it is now so included in the List of IAU-approved Star Names. The term ''ra's al-jaθiyy'' or ''Ras al Djathi'' appeared in the catalogue of stars in the ''Calendarium'' of Al Achsasi al Mouakket, which was translated into

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

as ''Caput Ingeniculi''. In

Chinese astronomy Astronomy in China has a long history stretching from the Shang dynasty, being refined over a period of more than 3,000 years. The Ancient China, ancient Chinese people have identified stars from 1300 BCE, as Chinese star names later categori ...

, Alpha Herculis is called 帝座,

Pinyin Hanyu Pinyin, or simply pinyin, officially the Chinese Phonetic Alphabet, is the most common romanization system for Standard Chinese. ''Hanyu'' () literally means 'Han Chinese, Han language'—that is, the Chinese language—while ''pinyin' ...

: Dìzuò, meaning 'Emperor's Seat'. The star is seen as marking itself, and stands alone in the center of the ''Emperor's Seat'' asterism, Heavenly Market enclosure (see:

Chinese constellations Traditional Chinese astronomy has a system of dividing the celestial sphere into asterisms or constellations, known as "officials" ( Chinese ''xīng guān''). The Chinese asterisms are generally smaller than the constellations of Hellenisti ...

). 帝座 (Dìzuò) was westernized into ''Ti Tso'' by R.H. Allen, with the same meaning

Properties

Alpha Herculis A and B are more than 500 AU apart, with an estimated

orbital period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...

of approximately 3600 years. A presents as a relatively massive red bright giant, but

radial velocity The radial velocity or line-of-sight velocity of a target with respect to an observer is the rate of change of the vector displacement between the two points. It is formulated as the vector projection of the target-observer relative velocity ...

measurements suggest a companion with a period of the order of a decade. B's two components are a primary yellow

giant star A giant star has a substantially larger radius and luminosity than a main-sequence (or ''dwarf'') star of the same surface temperature. They lie above the main sequence (luminosity class V in the Yerkes spectral classification) on the Hertzsp ...

and a secondary, yellow-white

dwarf star A dwarf star is a star of relatively small size and low luminosity. Most main sequence stars are dwarf stars. The meaning of the word "dwarf" was later extended to some star-sized objects that are not stars, and compact stellar remnants that are ...

in a 51.578 day orbit. Alpha Herculis A is an

(AGB) star, a luminous red giant that has both hydrogen and helium shells around a degenerate carbon-oxygen core. It is the second nearest AGB star to the Sun. Its radius pulsates between 264 and 303 solar radii. At its minimum, the

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 ...

is of and the luminosity is of 7,200 solar luminosities, while at its maximum the temperature is of and the luminosity is of 9,330 solar luminosities. If Alpha Herculis were at the center of the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

its radius would extend past the orbit of

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

at 1.231.4 AU but not quite as far as the orbit of

Mars Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...

or the

asteroid belt The asteroid belt is a torus-shaped region in the Solar System, centered on the Sun and roughly spanning the space between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies called asteroids ...

. The red giant is estimated to have started its life with about . The primary has been specified as a standard star for the spectral class M5 Ib-II. Like most type M stars near the end of their lives, Alpha Herculis is experiencing a high degree of stellar mass loss creating a sparse, gaseous envelope that extends at least 930 AU. It is a

semiregular variable In astronomy, a semiregular variable star, a type of variable star, is a Red giant, giant or supergiant of intermediate and late (cooler) spectral type. It shows considerable periodicity in its light changes, accompanied or sometimes interrupted b ...

with complex changes in brightness with periods ranging from a few weeks to many years. The most noticeable variations occur at timescales of 80–140 days and at 1,000 - 3,000 days. The strongest detectable period is 128 days. The full range in brightness is from magnitude 2.7 to 4.0, but it usually varies over a much smaller range of around 0.6 magnitudes.

References

External links

*An Atlas of the Universe
Multiple Star Orbits
*Upside down Hercules showing Alpha Herculisethi as the head

{{DEFAULTSORT:Alpha Herculis Double stars Hercules (constellation) Herculis, Alpha Herculis, 064 Triple star systems Rasalgethi M-type bright giants G-type giants F-type main-sequence stars 6406 7 084345 156014 5 BD+14 3207 Semiregular variable stars M-type supergiants Asymptotic-giant-branch stars