Alpha Crucis
Encyclopedia
Acrux is the brightest star in the constellation
Crux
, the Southern Cross, and, at a combined visual magnitude 0.77, is the twelfth brightest star in the night sky. Acrux is the southernmost first-magnitude star, just a bit more southerly than Alpha Centauri
.
located 321 light years
from the solar system
. Only two components are visually distinguishable, α1 and α2, separated by 4 arcseconds. α1 is magnitude 1.40 and α2 is magnitude 2.09, both hot class B
(almost class O
) stars, with surface temperatures of about 28,000 and 26,000 K
respectively. Their luminosities are 25,000 and 16,000 times that of the Sun. α1 and α2 orbit over such a long period that motion is only barely seen. From their minimum separation of 430 astronomical unit
s, the period is at least 1,500 years, and may be much longer.
α1 is itself a spectroscopic binary star, with its components thought to be around 14 and 10 times the mass of the Sun
and orbiting in only 76 days at a separation of about 1 AU. The masses of α2 and the brighter component of α1 suggest that the stars will someday explode as supernova
e. The fainter component of α1 may survive to become a massive white dwarf
.
Another class-B4 subgiant, Alpha Crucis C or alpha-3 Crucis, lies 90 arcseconds away from triple Acrux and shares Acrux's motion through space, suggesting it may be gravitationally bound to Acrux. However, if it is indeed located near Acrux, Alpha Crucis C is under-luminous for its class. It is probably just an optical double star
, lying beyond the gravitationally bound stars of Acrux in the background—most probably at double the distance i.e. 640 ly. This star will move further away from the others with time and proper motion
and will probably end up as a white dwarf star.
Since Acrux is at −63° declination, the southermost first-magnitude star, it is only visible south of latitude 27°N. Therefore, it barely rises from cities such as Miami, Florida
, or Karachi
, Pakistan
(both around 25°N) and not at all from New Orleans, Louisiana
, or Cairo
, Egypt
(both about 30°N). Because of Earth's axial precession, however, the star was visible to ancient Hindu astronomers in India who named it “Tri-shanku”.
In Chinese
, , meaning Cross, refers to an asterism consisting of α Crucis, γ Crucis
, β Crucis
and δ Crucis. Consequently, α Crucis itself is known as .
This star is become Estrela de Magalhães (Star of Magalhães) in Portuguese.
Acrux is represented in the flags of Australia
, New Zealand
and Papua New Guinea
as one of five stars which comprise the Southern Cross. It is also featured in the flag of Brazil
, along with 26 other stars, each of which represents a state. Acrux represents the State of São Paulo
.
's disk occulted it.
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....
Crux
Crux
Crux is the smallest of the 88 modern constellations, but is one of the most distinctive. Its name is Latin for cross, and it is dominated by a cross-shaped asterism that is commonly known as the Southern Cross.-Visibility:...
, the Southern Cross, and, at a combined visual magnitude 0.77, is the twelfth brightest star in the night sky. Acrux is the southernmost first-magnitude star, just a bit more southerly than Alpha Centauri
Alpha Centauri
Alpha Centauri is the brightest star in the southern constellation of Centaurus...
.
Physical properties
Acrux is a multiple starMultiple star
A multiple star consists of three or more stars which appear from the Earth to be close to one another in the sky. This may result from the stars being physically close and gravitationally bound to each other, in which case it is physical, or this closeness may be merely apparent, in which case...
located 321 light years
Light Years
Light Years is the seventh studio album by Australian recording artist Kylie Minogue. It was released on 25 September 2000 by Parlophone and Mushroom Records. The album's style was indicative of her return to "mainstream pop dance tunes"....
from the 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...
. Only two components are visually distinguishable, α1 and α2, separated by 4 arcseconds. α1 is magnitude 1.40 and α2 is magnitude 2.09, both hot class B
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...
(almost class O
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...
) stars, with surface temperatures of about 28,000 and 26,000 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
respectively. Their luminosities are 25,000 and 16,000 times that of the Sun. α1 and α2 orbit over such a long period that motion is only barely seen. From their minimum separation of 430 astronomical unit
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
s, the period is at least 1,500 years, and may be much longer.
α1 is itself a spectroscopic binary star, with its components thought to be around 14 and 10 times the mass of the Sun
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
and orbiting in only 76 days at a separation of about 1 AU. The masses of α2 and the brighter component of α1 suggest that the stars will someday explode as supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
e. The fainter component of α1 may survive to become a massive white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
.
Another class-B4 subgiant, Alpha Crucis C or alpha-3 Crucis, lies 90 arcseconds away from triple Acrux and shares Acrux's motion through space, suggesting it may be gravitationally bound to Acrux. However, if it is indeed located near Acrux, Alpha Crucis C is under-luminous for its class. It is probably just an optical double star
Double star
In observational astronomy, a double star is a pair of stars that appear close to each other in the sky as seen from Earth when viewed through an optical telescope. This can happen either because the pair forms a binary star, i.e...
, lying beyond the gravitationally bound stars of Acrux in the background—most probably at double the distance i.e. 640 ly. This star will move further away from the others with time and proper motion
Proper motion
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...
and will probably end up as a white dwarf star.
Etymology
"Acrux" is simply a contraction of the A in Alpha plus Crux.Since Acrux is at −63° declination, the southermost first-magnitude star, it is only visible south of latitude 27°N. Therefore, it barely rises from cities such as Miami, Florida
Florida
Florida is a state in the southeastern United States, located on the nation's Atlantic and Gulf coasts. It is bordered to the west by the Gulf of Mexico, to the north by Alabama and Georgia and to the east by the Atlantic Ocean. With a population of 18,801,310 as measured by the 2010 census, it...
, or Karachi
Karachi
Karachi is the largest city, main seaport and the main financial centre of Pakistan, as well as the capital of the province of Sindh. The city has an estimated population of 13 to 15 million, while the total metropolitan area has a population of over 18 million...
, Pakistan
Pakistan
Pakistan , officially the Islamic Republic of Pakistan is a sovereign state in South Asia. It has a coastline along the Arabian Sea and the Gulf of Oman in the south and is bordered by Afghanistan and Iran in the west, India in the east and China in the far northeast. In the north, Tajikistan...
(both around 25°N) and not at all from New Orleans, Louisiana
Louisiana
Louisiana is a state located in the southern region of the United States of America. Its capital is Baton Rouge and largest city is New Orleans. Louisiana is the only state in the U.S. with political subdivisions termed parishes, which are local governments equivalent to counties...
, or Cairo
Cairo
Cairo , is the capital of Egypt and the largest city in the Arab world and Africa, and the 16th largest metropolitan area in the world. Nicknamed "The City of a Thousand Minarets" for its preponderance of Islamic architecture, Cairo has long been a centre of the region's political and cultural life...
, Egypt
Egypt
Egypt , officially the Arab Republic of Egypt, Arabic: , is a country mainly in North Africa, with the Sinai Peninsula forming a land bridge in Southwest Asia. Egypt is thus a transcontinental country, and a major power in Africa, the Mediterranean Basin, the Middle East and the Muslim world...
(both about 30°N). Because of Earth's axial precession, however, the star was visible to ancient Hindu astronomers in India who named it “Tri-shanku”.
In Chinese
Chinese language
The Chinese language is a language or language family consisting of varieties which are mutually intelligible to varying degrees. Originally the indigenous languages spoken by the Han Chinese in China, it forms one of the branches of Sino-Tibetan family of languages...
, , meaning Cross, refers to an asterism consisting of α Crucis, γ Crucis
Gamma Crucis
Gamma Crucis , often called Gacrux, is a red giant star approximately 88 light-years away in the constellation of Crux. Among Portuguese-speaking peoples it is also named "Rubídea" , in reference to its color. The star is the third-brightest star in Crux and one of the brightest stars in the...
, β Crucis
Beta Crucis
Mimosa or Becrux is the second brightest star in the constellation Crux and is one of the brightest stars in the night sky....
and δ Crucis. Consequently, α Crucis itself is known as .
This star is become Estrela de Magalhães (Star of Magalhães) in Portuguese.
Acrux is represented in the flags of Australia
Flag of Australia
The flag of Australia is a defaced Blue Ensign: a blue field with the Union Flag in the canton , and a large white seven-pointed star known as the Commonwealth Star in the lower hoist quarter...
, New Zealand
Flag of New Zealand
The flag of New Zealand is a defaced Blue Ensign with the Union Flag in the canton, and four red stars with white borders to the right. The stars represent the constellation of Crux, the Southern Cross....
and Papua New Guinea
Flag of Papua New Guinea
The flag of Papua New Guinea was adopted on July 1, 1971. In the hoist, it depicts the Southern Cross; in the fly, a raggiana bird of paradise is silhouetted. The designer of the flag was 15 year old schoolgirl Susan Huhume who won a nationwide competition for a new flag design in 1971.Red and...
as one of five stars which comprise the Southern Cross. It is also featured in the flag of Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
, along with 26 other stars, each of which represents a state. Acrux represents the State of São Paulo
São Paulo (state)
São Paulo is a state in Brazil. It is the major industrial and economic powerhouse of the Brazilian economy. Named after Saint Paul, São Paulo has the largest population, industrial complex, and economic production in the country. It is the richest state in Brazil...
.
Notable observations
On 2008 October 2, the Cassini–Huygens spacecraft resolved three of the components (A, B and C) of the multiple star system as SaturnSaturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
's disk occulted it.