Sirius is the
brightest star
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History
Th ...
in the
night sky
The night sky is the nighttime appearance of celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon.
Natural light sources in a night sky inc ...
. Its name is derived from the
Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group.
*Greek language, a branch of the Indo-European language family.
**Proto-Greek language, the assumed last common ancestor ...
word , or , meaning 'glowing' or 'scorching'. The star is designated α Canis Majoris,
Latinized to Alpha Canis Majoris, and abbreviated Alpha CMa or α CMa. With a visual
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 −1.46, Sirius is almost twice as bright as
Canopus, the next brightest star. Sirius is a
binary star consisting of a
main-sequence star
A star is an astronomical object comprising a luminous spheroid of plasma held together by its gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but their immense distances from Earth make ...
of
spectral type A0 or A1, termed Sirius A, and a faint
white dwarf companion of spectral type DA2, termed Sirius B. The distance between the two varies between 8.2 and 31.5
astronomical unit
The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbi ...
s as they orbit every 50 years.
Sirius appears bright because of its intrinsic
luminosity
Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object over time. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a s ...
and its proximity to the
Solar System
The Solar System Capitalization 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 ...
. At a distance of , the Sirius system is one of Earth's
nearest neighbours. Sirius is gradually moving closer to the Solar System, so it is expected to increase in brightness slightly over the next 60,000 years, reaching a peak magnitude of −1.68. After that time, its distance will begin to increase, and it will become fainter, but it will continue to be the brightest star in the Earth's night sky for approximately the next 210,000 years, before
Vega
Vega is the brightest star in the northern
Northern may refer to the following:
Geography
* North, a point in direction
* Northern Europe, the northern part or region of Europe
* Northern Highland, a region of Wisconsin, United Sta ...
, another A-type star and more luminous than Sirius, becomes the brightest star.
Sirius A is about twice as massive as the
Sun () and has an
absolute visual magnitude of +1.43. It is 25 times as luminous as the Sun,
but has a significantly lower luminosity than other bright stars such as Canopus,
Betelgeuse, or
Rigel. The system is between 200 and years old.
It was originally composed of two bright bluish stars. The initially more massive of these, Sirius B, consumed its hydrogen fuel and became a
red giant before shedding its outer layers and collapsing into its current state as a white dwarf around years ago.
Sirius is known colloquially as the "Dog Star", reflecting its prominence in its
constellation,
Canis Major (the Greater Dog).
The
heliacal rising
The heliacal rising ( ) or star rise of a star occurs annually, or the similar phenomenon of a planet, when it first becomes visible above the eastern horizon at dawn just before sunrise (thus becoming "the morning star") after a complete orbit of ...
of Sirius marked the
flooding of the Nile
The flooding of the Nile has been an important natural cycle in Egypt since ancient times. It is celebrated by Egyptians as an annual holiday for two weeks starting August 15, known as ''Wafaa El-Nil''. It is also celebrated in the Coptic Church b ...
in
Ancient Egypt and the "
dog days" of summer for the
ancient Greeks
Ancient Greece ( el, Ἑλλάς, Hellás) was a northeastern Mediterranean civilization, existing from the Greek Dark Ages of the 12th–9th centuries BC to the end of classical antiquity ( AD 600), that comprised a loose collection of cult ...
, while to the
Polynesians, mostly in the Southern Hemisphere, the star marked winter and was an important reference for their navigation around the Pacific Ocean.
Observational history
The brightest star seen from Earth, Sirius is recorded in some of the earliest astronomical records. Its displacement from the
ecliptic
The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic agai ...
causes its heliacal rising to be remarkably regular compared to other stars, with a period of almost exactly 365.25 days holding it constant relative to the
solar year
A tropical year or solar year (or tropical period) is the time that the Sun takes to return to the same position in the sky of a celestial body of the Solar System such as the Earth, completing a full cycle of seasons; for example, the time ...
. This rising occurs at
Cairo
Cairo ( ; ar, القاهرة, al-Qāhirah, ) is the capital of Egypt and its largest city, home to 10 million people. It is also part of the largest urban agglomeration in Africa, the Arab world and the Middle East: The Greater Cairo met ...
on 19July (
Julian
Julian may refer to:
People
* Julian (emperor) (331–363), Roman emperor from 361 to 363
* Julian (Rome), referring to the Roman gens Julia, with imperial dynasty offshoots
* Saint Julian (disambiguation), several Christian saints
* Julian (give ...
), placing it just before the onset of the
annual flooding of the
Nile
The Nile, , Bohairic , lg, Kiira , Nobiin language, Nobiin: Áman Dawū is a major north-flowing river in northeastern Africa. It flows into the Mediterranean Sea. The Nile is the longest river in Africa and has historically been considered ...
during antiquity.
Owing to the flood's own irregularity, the extreme precision of the star's return made it important to the
ancient Egyptians,
who worshipped it as the goddess
Sopdet ( egy,
Spdt, "Triangle"; grc-gre, , ''Sō̂this''), guarantor of the fertility of their land.
The
ancient Greeks
Ancient Greece ( el, Ἑλλάς, Hellás) was a northeastern Mediterranean civilization, existing from the Greek Dark Ages of the 12th–9th centuries BC to the end of classical antiquity ( AD 600), that comprised a loose collection of cult ...
observed that the appearance of Sirius as the morning star heralded the hot and dry summer and feared that the star caused plants to wilt, men to weaken, and women to become aroused.
Owing to its brightness, Sirius would have been seen to
twinkle more in the unsettled weather conditions of early summer. To Greek observers, this signified emanations that caused its malignant influence. Anyone suffering its effects was said to be "star-struck" (, ''astrobólētos''). It was described as "burning" or "flaming" in literature.
The season following the star's reappearance came to be known as the "dog days".
The inhabitants of the island of
Ceos in the
Aegean Sea
The Aegean Sea ; tr, Ege Denizi (Greek: Αιγαίο Πέλαγος: "Egéo Pélagos", Turkish: "Ege Denizi" or "Adalar Denizi") is an elongated embayment of the Mediterranean Sea between Europe and Asia. It is located between the Balkans an ...
would offer sacrifices to Sirius and
Zeus
Zeus or , , ; grc, Δῐός, ''Diós'', label=genitive Boeotian Aeolic and Laconian grc-dor, Δεύς, Deús ; grc, Δέος, ''Déos'', label=genitive el, Δίας, ''Días'' () is the sky and thunder god in ancient Greek religion, ...
to bring cooling breezes and would await the reappearance of the star in summer. If it rose clear, it would portend good fortune; if it was misty or faint then it foretold (or emanated) pestilence. Coins retrieved from the island from the 3rd century BC feature dogs or stars with emanating rays, highlighting Sirius's importance.
The Romans celebrated the heliacal setting of Sirius around 25 April,
sacrificing a dog, along with incense, wine, and a sheep, to the goddess
Robigo so that the star's emanations would not cause
wheat rust on wheat crops that year.
Bright stars were important to the ancient
Polynesians for navigation of the Pacific Ocean. They also served as latitude markers; the declination of Sirius matches the latitude of the archipelago of
Fiji at
17°S and thus passes directly over the islands each
sidereal day.
Sirius served as the body of a "Great Bird" constellation called ''Manu'', with Canopus as the southern wingtip and
Procyon the northern wingtip, which divided the Polynesian night sky into two hemispheres.
Just as the appearance of Sirius in the morning sky marked summer in Greece, it marked the onset of winter for the
Māori, whose name ''Takurua'' described both the star and the season. Its culmination at the
winter solstice
The winter solstice, also called the hibernal solstice, occurs when either of Earth's poles reaches its maximum tilt away from the Sun. This happens twice yearly, once in each hemisphere (Northern and Southern). For that hemisphere, the winte ...
was marked by celebration in
Hawaii
Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only ...
, where it was known as ''Ka'ulua'', "Queen of Heaven". Many other Polynesian names have been recorded, including ''Tau-ua'' in the
Marquesas Islands
The Marquesas Islands (; french: Îles Marquises or ' or '; Marquesan: ' (North Marquesan) and ' (South Marquesan
Marquesan is a collection of East-Central Polynesian dialects, of the Marquesic group, spoken in the Marquesas Islands of ...
, ''Rehua'' in New Zealand, and ''Ta'urua-fau-papa'' "Festivity of original high chiefs" and ''Ta'urua-e-hiti-i-te-tara-te-feiai'' "Festivity who rises with prayers and religious ceremonies" in Tahiti.
Kinematics
In 1717,
Edmond Halley
Edmond (or Edmund) Halley (; – ) was an English astronomer, mathematician and physicist. He was the second Astronomer Royal in Britain, succeeding John Flamsteed in 1720.
From an observatory he constructed on Saint Helena in 1676–77, Hal ...
discovered the
proper motion of the hitherto presumed "fixed" stars
after comparing contemporary
astrometric
Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, the Milky Way.
Histor ...
measurements with those from the second century AD given in Ptolemy's ''
Almagest
The ''Almagest'' is a 2nd-century Greek-language mathematical and astronomical treatise on the apparent motions of the stars and planetary paths, written by Claudius Ptolemy ( ). One of the most influential scientific texts in history, it cano ...
''. The bright stars
Aldebaran,
Arcturus and Sirius were noted to have moved significa