Alpha Herculis (α Herculis, abbreviated Alpha Her, α Her), also designated 64 Herculis, is a
multiple star system
A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a '' star cluster'' or '' galaxy'', although, broadly speaki ...
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 origins of the e ...
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 ...
. 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 brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's li ...
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 lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby objects ...
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 of celestial obj ...
mission, it is approximately 360
light-year
A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...
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 center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
.
System
Alpha Herculis is a triple star system. The primary (brightest) of the three stars, designated α
1 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), and is the second nearest AGB star after
Mira
Mira (), designation Omicron Ceti (ο Ceti, abbreviated Omicron Cet, ο Cet), is a red-giant star estimated to be 200–400 light-years from the Sun in the constellation Cetus.
ο Ceti is a binary stellar system, consisting of a varia ...
. 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 or Latin letter followed by the genitive form of its parent constellation's Latin name. The original list of Bayer designations contained 1,564 stars. ...
; ''α1'' and ''α2 Herculis'', those of its two visible components. ''64 Herculis'' is the system's Flamsteed designation
A Flamsteed designation is a combination of a number and constellation name that uniquely identifies most naked eye stars in the modern constellations visible from southern England. They are named for John Flamsteed who first used them while co ...
. WDS J17146+1423 is the wider system's designation in the Washington Double Star Catalog
The Washington Double Star Catalog, or WDS, is a catalog of double stars, maintained at the United States Naval Observatory. The catalog contains positions, magnitudes, proper motions and spectral types and has entries for (as of June 2017) 141, ...
. 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 system
A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a '' star cluster'' or '' galaxy'', although, broadly speaki ...
s, and adopted by the International Astronomical Union
The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...
(IAU).
Alpha Herculis bore the traditional name ''Rasalgethi'' or ''Ras Algethi'' ( ar, رأس الجاثي ra‘is al-jāthī '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
Muḥammad al-Akhṣāṣī al-Muwaqqit ( ar, محمد الاخصاصي الموقت) was an Egyptian astronomer whose and catalogue of stars, ('Pearls of brilliance upon the solar operations'), was written at Cairo about 1650.
Al-Akhsasi was a s ...
, which was translated into Latin
Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
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 Chinese people have identified stars from 1300 BCE, as Chinese star names later categorized in the twe ...
, Alpha Herculis is called 帝座, Pinyin
Hanyu Pinyin (), often shortened to just pinyin, is the official romanization system for Standard Mandarin Chinese in China, and to some extent, in Singapore and Malaysia. It is often used to teach Mandarin, normally written in Chinese for ...
: 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
The Heavenly Market Enclosure (天市垣, ''Tian Shi Yuan''), is one of the ''San Yuan'' or Three enclosures
Traditional Chinese astronomy has a system of dividing the celestial sphere into asterisms or constellations, known as "officials" ( C ...
(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 Hellenistic t ...
). 帝座 (Dìzuò) was westernized into ''Ti Tso'' by R.H. Allen, with the same meaning Richard Hinckley Allen: Star Names — Their Lore and Meaning: Hercules
/ref>
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
A giant star is a star with substantially larger radius and luminosity than a main-sequence (or ''dwarf'') star of the same surface temperature.Giant star, entry in ''Astronomy Encyclopedia'', ed. Patrick Moore, New York: Oxford University Press ...
, but radial velocity
The radial velocity or line-of-sight velocity, also known as radial speed or range rate, of a target with respect to an observer is the temporal rate of change, rate of change of the distance or Slant range, range between the two points. It is e ...
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 is a star with substantially larger radius and luminosity than a main sequence, main-sequence (or ''dwarf'') star of the same effective temperature, surface temperature.Giant star, entry in ''Astronomy Encyclopedia'', ed. Patrick Moo ...
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 which are ...
in a 51.578 day orbit.
Alpha Herculis A is an 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) 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. The angular diameter of the star has been measured with an interferometer
Interferometry is a technique which uses the ''interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber op ...
as
34 ± 0.8 mas, or 0.034 arcseconds. At its estimated distance of 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 this corresponds to a radius of about 280 million kilometers (or 170 million miles), which is roughly or 1.87 AU.[To determine Rasalgethi's radius in terms of solar units, the calculations begin with the formula for ]angular diameter
The angular diameter, angular size, apparent diameter, or apparent size is an angular distance describing how large a sphere or circle appears from a given point of view. In the vision sciences, it is called the visual angle, and in optics, it is ...
as follows:
:
where ''δ'' equals Rasalgethi's angular diameter in arcseconds
A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The na ...
, ''dR'' the star's diameter in AU, and ''DR'' the Distance from Earth in parsecs. If one knows the angular diameter and the Distance, then one can solve for ''dR'' as follows:
:.
To obtain Rasalgheti's radius:
:.
Converting into Solar units, 1 AU = 149,597,871 km and the mean radius of the Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
= 696,000 km, hence the calculation:
: (rounded). 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 "Solar S ...
its radius would extend past the orbit of Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
at 1.5 AU but not quite as far as the asteroid belt
The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...
. The red giant is estimated to have started its life with about .
Alpha Herculis A 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 Stellar mass loss is a phenomenon observed in stars. All stars lose some mass over their lives at widely varying rates. Triggering events can cause the sudden ejection of a large portion of the star's mass. Stellar mass loss can also occur when a st ...
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 giant or supergiant of intermediate and late (cooler) spectral type showing considerable periodicity in its light changes, accompanied or sometimes interrupted by various irre ...
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.][
]
See also
*List of largest known stars
Below are lists of the largest stars currently known, ordered by radius and separated into categories by galaxy. The unit of measurement used is the radius of the Sun (approximately ).
The angular diameters of stars can be measured directly us ...
*Lists of stars
The following are lists of stars. These are astronomical objects that spend some portion of their existence generating energy through thermonuclear fusion.
By location
* Lists of stars by constellation
By name
* List of traditional star names
* L ...
in the constellation 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 ...
* Betelgeuse mass loss
* Spectral types F, G & M
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