夜空中最亮的恒星——天狼星Sirius

Sirius (/ˈsɪriəs/, designated α Canis Majoris (Latinized to Alpha Canis Majoris, abbreviated Alpha CMaα CMa)) is the brightest star in the night sky. Its name is derived from the Greek word Σείριος Seirios “glowing” or “scorching”. With a visual apparent magnitude 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 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 units as they orbit every 50 years.[24]

天狼星(Sirius,/,称为αCanis Majoris(拉丁化为Alpha Canis Majoris,缩写为Alpha CMa,αCMa))是夜空中最亮的恒星。它的名称源自希腊文ΣείριοςSeirios的“ glowing”或“ scorching”。天狼星的视在视星等值为-1.46,几乎是第二亮的恒星Canopus的两倍。天狼星是一颗双星,由光谱类型为A0或A1的主序星(称为天狼星A)和光谱类型为DA2的微弱白矮星(称为天狼星B)组成。两者之间的距离在8.2至31.5天文单位之间变化它们每50年绕轨道运行一次。[24]

Sirius appears bright because of its intrinsic luminosity and its proximity to the Solar System. At a distance of 2.64 parsecs (8.6 ly), the Sirius system is one of Earth’s nearest neighbours. Sirius is gradually moving closer to the Solar System, so it will slightly increase in brightness over the next 60,000 years. 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 the next 210,000 years.[25]

天狼星由于其固有的光度和与太阳系的接近而显得明亮。天狼星系统距离地球2.66秒差距(8.6光年),是地球最近的邻居之一。天狼星正在逐渐靠近太阳系,因此在未来60,000年内,它的亮度将略有增加。在那之后,它的距离将开始增加,并且将变得越来越微弱,但是在接下来的210,000年里,它将继续是地球夜空中最明亮的恒星。[25]

Sirius A is about twice as massive as the Sun (M) and has an absolute visual magnitude of +1.42. It is 25 times more luminous than the Sun[12] but has a significantly lower luminosity than other bright stars such as Canopus or Rigel. The system is between 200 and 300 million years old.[12] It was originally composed of two bright bluish stars. The more massive of these, Sirius B, consumed its resources and became a red giant before shedding its outer layers and collapsing into its current state as a white dwarf around 120 million years ago.[12]

天狼星A的质量大约是太阳(M☉)的两倍,并且绝对视觉大小为+1.42。它的发光度是太阳的25倍[12],但其发光度却比其他明亮的恒星(例如Canopus或Rigel)低。该系统已有200到3亿年的历史。[12]它最初由两个明亮的蓝星组成。其中规模较大的小天狼星B消耗了资源变成了红巨星,在大约1.2亿年前脱落了外层并塌陷目前白矮星的状态,。[12]

Sirius is known colloquially as the “Dog Star“, reflecting its prominence in its constellationCanis Major (the Greater Dog).[18] The heliacal rising of Sirius marked the flooding of the Nile in Ancient Egypt and the “dog days” of summer for the ancient Greeks, 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.

天狼星俗称“狗星”,反映出它在其星群“Canis Major”(大狗)中的突出地位。[18]天狼星的升起标志着古埃及尼罗河的泛滥和古希腊人夏天的“狗日”,而波利尼西亚人(主要在南半球)则将该星标为冬天,这是他们航行在太平洋周围的重要参考。

观察历史Observational history

The brightest star in the night sky, Sirius is recorded in some of the earliest astronomical records. Its displacement from the ecliptic 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. This rising occurs at Cairo on 19 July (Julian), placing it just prior to the onset of the annual flooding of the Nile during antiquity.[26] Owing to the flood’s own irregularity, the extreme precision of the star’s return made it important to the ancient Egyptians,[26] who worshipped it as the goddess Sopdet (Ancient EgyptianSpdt, “Triangle”;[a] GreekΣῶθιςSō̂this), guarantor of the fertility of their land.[b] 

天狼星是夜空中最亮的星星,它被记录在一些最早的天文记录中。 与其他恒星相比,它从黄道的位移使它的螺旋上升明显规律,大约365.25天的时间使它相对于太阳年保持恒定。 这种上升发生在7月19日(朱利安)的开罗,恰好是古代尼罗河年度洪水爆发之前的时间。[26] 由于洪水本身的不规则性,恒星返回的极高精确度使其对于古埃及人来说非常重要,[26]他们将其视作女神Sopdet(古埃及语:Spdt,“三角形”; [a]希腊语:Σῶθις,Sō̂this ),保证其土地的肥沃性。[b]

The Egyptian civil calendar was apparently initiated to have its New Year “Mesori” coincide with the appearance of Sirius, although its lack of leap years meant that this congruence only held for four years until its date began to wander backwards through the months. The Egyptians continued to note the times of Sirius’s annual return, which may have led them to the discovery of the 1460-year Sothic cycle and influenced the development of the Julian and Alexandrian calendars.

埃及的公历显然是为了使新的“Meson”与天狼星的出现相吻合,尽管它缺乏闰年,这意味着这种一致性只持续了四年,随后它的日期开始向后漂移了几个月。 埃及人继续注意到天狼星(Sirius)每年回归的时代,这可能导致他们发现了1460年的苏特循环(Sothic cycle),并影响了朱利安历法和亚历山大历法的发展。

The ancient Greeks observed that the appearance of Sirius heralded the hot and dry summer and feared that it caused plants to wilt, men to weaken, and women to become aroused.[28] Due to its brightness, Sirius would have been seen to twinkle more in the unsettled weather conditions of early summer. To Greek observers, this signified certain emanations which caused its malignant influence. Anyone suffering its effects was said to be “star-struck” (ἀστροβόλητος, astrobólētos). It was described as “burning” or “flaming” in literature.[29] 

古希腊人观察到,天狼星的出现预示着炎热干燥的夏天,并担心它会导致植物枯萎,男人与女人的衰弱。[28] 由于其亮度,在初夏未定的天气条件下,Sirius会更闪烁。 对希腊观察家来说,这意味着某些恶性影响, 遭受其影响的任何人都被称为“star – struck”(astrobólētos,ἀστροβόλητος)。 在文学中被描述为“燃烧”。[29]

The season following the star’s reappearance came to be known as the “dog days”.[30] The inhabitants of the island of Ceos in the Aegean Sea would offer sacrifices to Sirius and Zeus 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.[29] The Romans celebrated the heliacal setting of Sirius around April 25, 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.[31]

恒星重新出现后的那个季节被称为“狗日”。[30] 爱琴海的Ceos岛上的居民会为Sirius和Zeus献祭牺牲品,希望能带来凉风,并在夏天等待恒星的再现。 如果它清楚地升起,那将预示着好运。 如果有雾或微弱,则预示着瘟疫。 [29]从公元前3世纪从岛上回收的硬币中,特征是狗或具有放射光线的星星,突出了天狼星的重要性。[29] 罗马人在4月25日前后庆祝小天狼星(Sirius)的医疗环境,向女神罗比戈(Robigo)牺牲了一只狗以及香,酒和一只绵羊,以免当年星辰的散发会对小麦作物造成小麦锈病。[31]

Ptolemy of Alexandria mapped the stars in Books VII and VIII of his Almagest, in which he used Sirius as the location for the globe’s central meridian. He depicted it as one of six red-coloured stars (see the Colour controversy section below). The other five are class M and K stars, such as Arcturus and Betelgeuse.[32]

亚历山大·托勒密(Alexanderia · Ptolemy)绘制了《阿尔玛格斯特》(Almagest)第七和第八册中的星星图,其中他以小天狼星(Sirius)为地球中央子午线的位置。 他将其描述为六颗红色的星星之一(请参阅下面的“颜色争议”部分)。 另外五颗是M和K类恒星,例如Arcturus和Betelgeuse。[32]

Bright stars were important to the ancient Polynesians for navigation between the many islands and atolls 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 night.[33] 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.[34] 

明亮的星星对于古代波利尼西亚人而言对于在太平洋的许多岛屿和环礁之间航行至关重要。 它们还用作纬度标记; Sirius的纬度与斐济群岛在17°S的纬度相匹配,因此每天晚上直接经过岛屿。[33] 天狼星是“大鸟”星座的主体,其名称为“ Manu”,Canopus是南部的翼尖,Procyon是北部的翼尖,将波利尼西亚的夜空分为两个半球。[34]

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 was marked by celebration in Hawaii, where it was known as Ka’ulua, “Queen of Heaven”. Many other Polynesian names have been recorded, including Tau-ua in the Marquesas IslandsRehua 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.[35] 

正如天狼星在早晨的天空中出现标志着希腊的夏天一样,它标志着毛利人进入冬天,毛利人的名字塔库鲁(Takurua)既描述了这颗星星又描述了季节。 它在冬至达到高潮时,在夏威夷举行了庆祝活动,被称为“天堂皇后” Ka’ulua。 许多其他波利尼西亚人的名字也被用来称呼这颗星星,包括Marquesas群岛的Tau-ua,新西兰的Rehua和Ta’urua-fau-papa“原始高酋长的节日”和Ta’urua-e-hiti-i-te- 塔拉泰菲艾(Tara-te-feiai),塔希提岛的“以祈祷和宗教仪式兴起的节日”。[35]

The Hawaiian people had many names for Sirius, including Aa (“glowing”),[36] Hoku-kauopae,[37] Kau-ano-meha (also Kaulanomeha), “Standing-alone-and-sacred”,[37][38] Hiki-kauelia or Hiki-kauilia (the navigational name), Hiki-kau-lono-meha (“star of solitary Lono”, the astrological name),[39] Kaulua (also Kaulua-ihai-mohai, “flower of the heavens”),[40] Hiki-kaueliaHoku-hoo-kele-waa (“star which causes the canoe to sail”, a marine navigation name),[41] and Kaulua-lena (“yellow star”).[40] The people of the Society Islands called Sirius variously Taurua-fau-papaTaurua-nui-te-amo-aha, and Taurua-e-hiti-i-tara-te-feiai.

夏威夷人民对天狼星有很多称呼,包括Aa(“发光”),[36] Hoku-kauopae,[37] Kau-ano-meha(也称Kaulanomeha),“站立而神圣”,[37] [38] Hiki-kauelia或Hiki-kauilia(航海名称),Hiki-kau-lono-meha(“孤罗诺之星”,占星术名称),[39] Kaulua(又称Kaulua-ihai-mohai,“花 [40] Hiki-kauelia,Hoku-hoo-kele-waa(“使独木舟航行的星”,一种海洋航行名称),[41]和Kaulua-lena(“黄星”) 。[40] 社会群岛的人民称小天狼星为Taurua-fau-papa,Taurua-nui-te-amo-aha和Taurua-e-hiti-i-tara-te-feiai。

Other names for Sirius included Palolo-mua (Futuna), Mere (Mangaia), Apura (Manihiki), Taku-ua (Marquesas Islands), and Tokiva (Pukapuka).[37] In the cosmology of the Tuamotus, Sirius had various names, including Takurua-te-upuupu,[37] Te Kaha (“coconut fibre”),[42] Te Upuupu,[43] Taranga,[44] and Vero-ma-torutoru (“flaming and diminishing”).[45]

Sirius的其他名称包括Palolo-mua(Futuna),Mere(Mangaia),Apura(Manihiki),Taku-ua(Marquesas Islands)和Tokiva(Pukapuka)。[37] 在Tuamotus的宇宙学中,Sirius有不同的名字,包括Takurua-te-upuupu,[37] Te Kaha(“椰子纤维”),[42] Te Upuupu,[43] Taranga,[44]和Vero-ma- torutoru(“燃烧和减少”)。[45]

The indigenous Boorong people of northwestern Victoria named Sirius as Warepil.[46]

维多利亚州西北部的土著布隆人将天狼星称为Warepil。[46]

运动学Kinematics

In 1717, Edmond Halley discovered the proper motion of the hitherto presumed “fixed” stars[47] after comparing contemporary astrometric measurements with those from the second century AD given in Ptolemy’s Almagest. The bright stars AldebaranArcturus and Sirius were noted to have moved significantly; Sirius had progressed about 30 arc minutes (about the diameter of the Moon) to the southwest.[48]

1717年,埃德蒙·哈雷(Edmond Halley)在将当代的天文测量数据与托勒密的《阿尔玛格斯特》中公元二世纪的测量结果进行了比较之后,发现了迄今为止推测为“固定”恒星的适当运动[47]。 注意到Aldebaran,Arcturus和Sirius的明亮恒星运动明显; 天狼星向西南方向前进了大约30弧分(大约相当于月球的直径)。[48]

In 1868, Sirius became the first star to have its velocity measured, the beginning of the study of celestial radial velocities. Sir William Huggins examined the spectrum of the star and observed a red shift. He concluded that Sirius was receding from the Solar System at about 40 km/s.[49][50] Compared to the modern value of −5.5 km/s, this was an overestimate and had the wrong sign; the minus sign (−) means that it is approaching the Sun. It is possible that Huggins did not account for the Earth’s orbital velocity, which would cause an error of up to 30 km/s.

1868年,天狼星成为第一颗测量出其速度的恒星,这是研究天体径向速度的开始。 威廉·哈金斯爵士检查了这颗恒星的光谱并观察到红移。 他的结论是,天狼星以大约40 km / s的速度远离太阳系。[49] [50] 与-5.5 km / s的现代值相比,这是高估了,并且方向错了; 减号(-)表示它正在接近太阳。 哈金斯有可能没有考虑到地球的轨道速度,这将导致高达30 km / s的误差。

距离Distance

In his 1698 book, CosmotheorosChristiaan Huygens estimated the distance to Sirius at 27664 times the distance from the Earth to the Sun (about 0.437 light years, translating to a parallax of roughly 7.5 arcseconds).[51] There were several unsuccessful attempts to measure the parallax of Sirius: by Jacques Cassini (6 seconds); by some astronomers (including Nevil Maskelyne)[52] using Lacaille‘s observations made at the Cape of Good Hope (4 seconds); by Piazzi (the same amount); using Lacaille‘s observations made at Paris, more numerous and certain than those made at the Cape (no sensible parallax); by Bessel (no sensible parallax).[53]

克里斯蒂安·惠更斯(Christiaan Huygens)在其1698年出版的《宇宙论》(Cosmotheoros)中,估计到天狼星的距离是地球到太阳的距离的27664倍(大约0.437光年,大约是7.5弧秒的视差)。[51] 有几次尝试测量天狼星视差的尝试均未成功:雅克·卡西尼(Jacques Cassini)(6秒); 一些天文学家(包括内维尔·马斯克林(Nevil Maskelyne))[52]利用拉卡耶在好望角的观测结果(4秒); 皮亚齐(相同金额); 使用拉卡耶(Lacaille)在巴黎所做的观察,比在海角(Cape)观察到的观察更多,更确定(没有明显的视差); 贝塞尔(没有明显的视差)。[53]

Scottish astronomer Thomas Henderson used his observations made in 1832–1833 and South African astronomer Thomas Maclear‘s observations made in 1836–1837, to determine that the value of the parallax was 0.23 arcseconds, and error of the parallax was estimated not to exceed a quarter of a second, or as Henderson wrote in 1839, “On the whole we may conclude that the parallax of Sirius is not greater than half a second in space; and that it is probably much less.”[54] Astronomers adopted a value of 0.25 arcseconds for much of the 19th century.[55] It is now known to have a parallax of 0.3792 ± 0.0016 arcseconds and therefore a distance of 1/0.3792 ≅ 2.637 parsecs, showing Henderson’s measurement to be accurate.

苏格兰天文学家托马斯·亨德森(Thomas Henderson)利用他在1832年至1833年所做的观察和南非天文学家托马斯·马克莱尔(Thomas Maclear)在1836年至1837年所做的观察,确定视差的值为0.23弧秒,并且视差的误差估计不超过四分之一。 1秒,或者就像亨德森(Henderson)在1839年所写的那样:“总体上,我们可以得出结论,天狼星的视差在太空中不大于半秒;并且它可能要小得多。” [54]天文学家将其取值为0.25 在19世纪的大部分时间里,都是弧秒。[55] 现在已知视差为0.3792±0.0016弧秒,因此距离为1 / 0.3792≅2.637帕秒,这表明亨德森的测量是准确的。

Discovery of Sirius B[edit]

Hubble Space Telescope image of Sirius A and Sirius B. The white dwarf can be seen to the lower left. The diffraction spikes and concentric rings are instrumental effects.

In 1844, the German astronomer Friedrich Bessel deduced from changes in the proper motion of Sirius that it had an unseen companion.[56] On January 31, 1862, American telescope-maker and astronomer Alvan Graham Clark first observed the faint companion, which is now called Sirius B, or affectionately “the Pup”.[57] This happened during testing of an 18.5-inch (470 mm) aperture great refractor telescope for Dearborn Observatory, which was the largest refracting telescope lens in existence at the time, and the largest telescope in the United States.[58] Sirius B’s sighting was confirmed on March 8 with smaller telescopes.[59]

The visible star is now sometimes known as Sirius A. Since 1894, some apparent orbital irregularities in the Sirius system have been observed, suggesting a third very small companion star, but this has never been confirmed. The best fit to the data indicates a six-year orbit around Sirius A and a mass of 0.06 M. This star would be five to ten magnitudes fainter than the white dwarf Sirius B, which would make it difficult to observe.[60] Observations published in 2008 were unable to detect either a third star or a planet. An apparent “third star” observed in the 1920s is now believed to be a background object.[61]

In 1915, Walter Sydney Adams, using a 60-inch (1.5 m) reflector at Mount Wilson Observatory, observed the spectrum of Sirius B and determined that it was a faint whitish star.[62] This led astronomers to conclude that it was a white dwarf, the second to be discovered.[63] The diameter of Sirius A was first measured by Robert Hanbury Brown and Richard Q. Twiss in 1959 at Jodrell Bank using their stellar intensity interferometer.[64] In 2005, using the Hubble Space Telescope, astronomers determined that Sirius B has nearly the diameter of the Earth, 12,000 kilometres (7,500 mi), with a mass 102% of the Sun’s.[65]

Colour controversy[edit]

Around the year 150 CE, the Greek astronomer of the Roman period, Claudius Ptolemy, described Sirius as reddish, along with five other stars, BetelgeuseAntaresAldebaranArcturus and Pollux, all of which are of orange or red hue.[66] The discrepancy was first noted by amateur astronomer Thomas Barker, squire of Lyndon Hall in Rutland, who prepared a paper and spoke at a meeting of the Royal Society in London in 1760.[67] The existence of other stars changing in brightness gave credibility to the idea that some may change in colour too; Sir John Herschel noted this in 1839, possibly influenced by witnessing Eta Carinae two years earlier.[68] Thomas Jefferson Jackson See resurrected discussion on red Sirius with the publication of several papers in 1892, and a final summary in 1926.[69] He cited not only Ptolemy but also the poet Aratus, the orator Cicero, and general Germanicus as calling the star red, though acknowledging that none of the latter three authors were astronomers, the last two merely translating Aratus’ poem Phaenomena.[70] Seneca had described Sirius as being of a deeper red than Mars.[71] Not all ancient observers saw Sirius as red. The 1st-century poet Marcus Manilius described it as “sea-blue”, as did the 4th century Avienus.[72] It was the standard white star in ancient China, and multiple records from the 2nd century BCE up to the 7th century CE all describe Sirius as white.[73][74]

In 1985, German astronomers Wolfhard Schlosser and Werner Bergmann published an account of an 8th-century Lombardic manuscript, which contains De cursu stellarum ratio by St. Gregory of Tours. The Latin text taught readers how to determine the times of nighttime prayers from positions of the stars, and Sirius is described within as rubeola – “reddish”. The authors proposed this was further evidence Sirius B had been a red giant at the time.[75] Other scholars replied that it was likely St. Gregory had been referring to Arcturus.[76][77]

The possibility that stellar evolution of either Sirius A or Sirius B could be responsible for this discrepancy has been rejected by astronomers on the grounds that the timescale of thousands of years is too short and that there is no sign of the nebulosity in the system that would be expected had such a change taken place.[71] An interaction with a third star, to date undiscovered, has also been proposed as a possibility for a red appearance.[78] Alternative explanations are either that the description as red is a poetic metaphor for ill fortune, or that the dramatic scintillations of the star when rising left the viewer with the impression that it was red. To the naked eye, it often appears to be flashing with red, white, and blue hues when near the horizon.[71]

观测Observation

Sirius (bottom) and the constellation Orion (right). The three brightest stars in this image – Sirius, Betelgeuse (top right), and Procyon (top left) – form the Winter Triangle.

With an apparent magnitude of −1.46, Sirius is the brightest star in the night sky, almost twice as bright as the second-brightest star, Canopus.[79] From Earth, Sirius always appears dimmer than Jupiter and Venus, as well as Mercury and Mars at certain times.[80] Sirius is visible from almost everywhere on Earth, except latitudes north of 73° N, and it does not rise very high when viewed from some northern cities (reaching only 13° above the horizon from Saint Petersburg).[81] Due to its declination of roughly −17°, Sirius is a circumpolar star from latitudes south of 73° S. From the Southern Hemisphere in early July, Sirius can be seen in both the evening where it sets after the Sun and in the morning where it rises before the Sun.[82]

Sirius, along with Procyon and Betelgeuse, forms one of the three vertices of the Winter Triangle to observers in the Northern Hemisphere.[83]

Due to precession (and slight proper motion), Sirius will move further south in the future. Starting around the year 9000, Sirius will no longer be visible from northern and central Europe, and in 14,000 its declination will be −67° and thus it will be circumpolar throughout South Africa and in most parts of Australia.

Sirius can be observed in daylight with the naked eye under the right conditions. Ideally, the sky should be very clear, with the observer at a high altitude, the star passing overhead, and the Sun low on the horizon.[84] These observing conditions are more easily met in the Southern Hemisphere, due to the southerly declination of Sirius.

The orbital motion of the Sirius binary system brings the two stars to a minimum angular separation of 3 arcseconds and a maximum of 11 arcseconds. At the closest approach, it is an observational challenge to distinguish the white dwarf from its more luminous companion, requiring a telescope with at least 300 mm (12 in) aperture and excellent seeing conditions. A periastron occurred in 1994 and the pair have since been moving apart, making them easier to separate with a telescope.[85]

At a distance of 2.6 parsecs (8.6 ly), the Sirius system contains two of the eight nearest stars to the Sun, and it is the fifth closest stellar system to the Sun.[86] This proximity is the main reason for its brightness, as with other near stars such as Alpha Centauri and in contrast to distant, highly luminous supergiants such as Canopus, Rigel or Betelgeuse.[87] It is still around 25 times more luminous than the Sun.[12] The closest large neighbouring star to Sirius is Procyon, 1.61 parsecs (5.24 ly) away.[88] The Voyager 2 spacecraft, launched in 1977 to study the four giant planets in the Solar System, is expected to pass within 4.3 light-years (1.3 pc) of Sirius in approximately 296,000 years.[89]

Stellar system[edit]

The orbit of Sirius B around A as seen from Earth (slanted ellipse). The wide horizontal ellipse shows the true shape of the orbit (with an arbitrary orientation) as it would appear if viewed straight on.

Artist’s impression of Sirius A and Sirius B. Sirius A is the larger of the two stars.

Chandra X-ray Observatory image of the Sirius star system, where the spike-like pattern is due to the support structure for the transmission grating. The bright source is Sirius B. Credit: NASA/SAO/CXC.

Sirius is a binary star system consisting of two white stars orbiting each other with a separation of about 20 AU[d] (roughly the distance between the Sun and Uranus) and a period of 50.1 years. The brighter component, termed Sirius A, is a main-sequence star of spectral type early A, with an estimated surface temperature of 9,940 K.[13] Its companion, Sirius B, is a star that has already evolved off the main sequence and become a white dwarf. Currently 10,000 times less luminous in the visual spectrum, Sirius B was once the more massive of the two.[90] The age of the system has been estimated at around 230 million years. Early in its life, it is thought to have been two bluish-white stars orbiting each other in an elliptical orbit every 9.1 years.[90] The system emits a higher than expected level of infrared radiation, as measured by IRAS space-based observatory. This might be an indication of dust in the system, which is considered somewhat unusual for a binary star.[88][91] The Chandra X-ray Observatory image shows Sirius B outshining its partner as an X-ray source.[92]

In 2015, Vigan and colleagues used the VLT Survey Telescope to search for evidence of substellar companions, and were able to rule out the presence of giant planets 11 times more massive than Jupiter at 0.5 AU distance from Sirius A, 6–7 times the mass of Jupiter at 1–2 AU distance, and down to around 4 times the mass of Jupiter at 10 AU distance.[93]

Sirius A[edit]

Comparison of Sirius A and the Sun, to scale and relative surface brightness.

Sirius A has a mass of 2 M.[11][12][94] The radius of this star has been measured by an astronomical interferometer, giving an estimated angular diameter of 5.936±0.016 mas. The projected rotational velocity is a relatively low 16 km/s,[15] which does not produce any significant flattening of its disk.[95] This is at marked variance with the similar-sized Vega, which rotates at a much faster 274 km/s and bulges prominently around its equator.[96] A weak magnetic field has been detected on the surface of Sirius A.[97]

Stellar models suggest that the star formed during the collapsing of a molecular cloud and that, after 10 million years, its internal energy generation was derived entirely from nuclear reactions. The core became convective and used the CNO cycle for energy generation.[95] It is predicted that Sirius A will have completely exhausted the store of hydrogen at its core within a billion (109) years of its formation. At this point, it will pass through a red giant stage, then settle down to become a white dwarf.

{\displaystyle \textstyle \left[{\frac {{\ce {Fe}}}{{\ce {H}}}}\right]=0.5}

Sirius A is classed as an Am star because the spectrum shows deep metallic absorption lines,[98] indicating an enhancement in elements heavier than helium, such as iron.[88][95] The spectral type has been reported as A0mA1 Va, which indicates that it would be classified as A1 from hydrogen and helium lines, but A0 from the metallic lines that cause it to be grouped with the Am stars.[7] When compared to the Sun, the proportion of iron in the atmosphere of Sirius A relative to hydrogen is given by {\displaystyle \textstyle \left[{\frac {{\ce {Fe}}}{{\ce {H}}}}\right]=0.5},[14] meaning iron is 316% as abundant as in the Sun’s atmosphere. The high surface content of metallic elements is unlikely to be true of the entire star; rather the iron-peak and heavy metals are radiatively levitated towards the surface.[95]

Sirius B[edit]

Comparison of Sirius B to Earth

Sirius B is one of the more massive white dwarfs known. With a mass of 1.02 M, it is almost double the 0.5–0.6 M average. This mass is packed into a volume roughly equal to the Earth’s.[65] The current surface temperature is 25,200 K.[12] Because there is no internal heat source, Sirius B will steadily cool as the remaining heat is radiated into space over more than two billion years.[99]

A white dwarf forms after a star has evolved from the main sequence and then passed through a red giant stage. This occurred when Sirius B was less than half its current age, around 120 million years ago. The original star had an estimated 5 M[12] and was a B-type star (roughly B4–5)[100][101] when it was still on the main sequence. While it passed through the red giant stage, Sirius B may have enriched the metallicity of its companion.

This star is primarily composed of a carbon–oxygen mixture that was generated by helium fusion in the progenitor star.[12] This is overlaid by an envelope of lighter elements, with the materials segregated by mass because of the high surface gravity.[102] The outer atmosphere of Sirius B is now almost pure hydrogen—the element with the lowest mass—and no other elements are seen in its spectrum.[103]

Apparent third star[edit]

Since 1894, irregularities have been observed in the orbits of Sirius A and B with an apparent periodicity of 6–6.4 years. A 1995 study concluded that such a companion likely exists, with a mass of roughly 0.05 solar masses- a small red dwarf or large brown dwarf, with an apparent magnitude of >15, and less than 3 arcseconds from Sirius A.[60]

More recent (and accurate) astrometric observations by the Hubble Space Telescope ruled out the existence of such an object entirely. The 1995 study predicted an astrometric movement of roughly 90 mas (0.09 arcseconds), but Hubble was unable to detect any location anomaly to an accuracy of 5 mas (0.005 arcsec). This ruled out any objects orbiting Sirius A with more than 0.033 solar masses orbiting in 0.5 years, and 0.014 in 2 years. The study was also able to rule out any companions to Sirius B with more than 0.024 solar masses orbiting in 0.5 years, and 0.0095 orbiting in 1.8 years. Effectively, there are almost certainly no additional bodies in the Sirius system larger than a small brown dwarf or large exoplanet.[104]

Star cluster membership[edit]

In 1909, Ejnar Hertzsprung was the first to suggest that Sirius was a member of the Ursa Major Moving Group, based on his observations of the system’s movements across the sky. The Ursa Major Group is a set of 220 stars that share a common motion through space. It was once a member of an open cluster, but has since become gravitationally unbound from the cluster.[105] Analyses in 2003 and 2005 found Sirius’s membership in the group to be questionable: the Ursa Major Group has an estimated age of 500 ± 100 million years, whereas Sirius, with metallicity similar to the Sun’s, has an age that is only half this, making it too young to belong to the group.[12][106][107] Sirius may instead be a member of the proposed Sirius Supercluster, along with other scattered stars such as Beta AurigaeAlpha Coronae BorealisBeta CraterisBeta Eridani and Beta Serpentis.[108] This would be one of three large clusters located within 500 light-years (150 pc) of the Sun. The other two are the Hyades and the Pleiades, and each of these clusters consists of hundreds of stars.[109]

A bust of Sopdet, Egyptian goddess of Sirius and the fertility of the Nilesyncretized with Isis and Demeter

Gaia 1[edit]

Main article: Gaia 1

In 2017, a massive star cluster was discovered only 10′ from Sirius. It was discovered during a statistical analysis of Gaia data. The cluster is over a thousand times further away from us than the star system.[110]

Etymology and cultural significance[edit]

See also: Winter Triangle

The proper name “Sirius” comes from the Latin Sīrius, from the Ancient Greek Σείριος (Seirios, “glowing” or “scorcher”).[111] The Greek word itself may have been imported from elsewhere before the Archaic period,[112] one authority suggesting a link with the Egyptian god Osiris.[113] The name’s earliest recorded use dates from the 7th century BC in Hesiod‘s poetic work Works and Days.[112] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[114] to catalog and standardize proper names for stars. The WGSN’s first bulletin of July 2016[115] included a table of the first two batches of names approved by the WGSN; which included Sirius for the star α Canis Majoris A. It is now so entered in the IAU Catalog of Star Names.[116]

Sirius - Agrippa.png

Sirius has over 50 other designations and names attached to it.[79] In Geoffrey Chaucer‘s essay Treatise on the Astrolabe, it bears the name Alhabor and is depicted by a hound’s head. This name is widely used on medieval astrolabes from Western Europe.[19] In Sanskrit it is known as Mrgavyadha “deer hunter”, or Lubdhaka “hunter”. As Mrgavyadha, the star represents Rudra (Shiva).[117][118] The star is referred as Makarajyoti in Malayalam and has religious significance to the pilgrim center Sabarimala.[119] In Scandinavia, the star has been known as Lokabrenna (“burning done by Loki”, or “Loki’s torch”).[120] In the astrology of the Middle Ages, Sirius was a Behenian fixed star,[121] associated with beryl and juniper. Its astrological symbol  was listed by Heinrich Cornelius Agrippa.[122]

Many cultures have historically attached special significance to Sirius, particularly in relation to dogs. It is often colloquially called the “Dog Star” as the brightest star of Canis Major, the “Great Dog” constellation. Canis Major was classically depicted as Orion‘s dog. The Ancient Greeks thought that Sirius’s emanations could affect dogs adversely, making them behave abnormally during the “dog days”, the hottest days of the summer. The Romans knew these days as dies caniculares, and the star Sirius was called Canicula, “little dog”. The excessive panting of dogs in hot weather was thought to place them at risk of desiccation and disease. In extreme cases, a foaming dog might have rabies, which could infect and kill humans they had bitten.[29] Homer, in the Iliad, describes the approach of Achilles toward Troy in these words:[123]

Sirius rises late in the dark, liquid sky
On summer nights, star of stars,
Orion’s Dog they call it, brightest
Of all, but an evil portent, bringing heat
And fevers to suffering humanity.

In Iranian mythology, especially in Persian mythology and in Zoroastrianism, the ancient religion of Persia, Sirius appears as Tishtrya and is revered as the rain-maker divinity (Tishtar of New Persian poetry). Beside passages in the sacred texts of the Avesta, the Avestan language Tishtrya followed by the version Tir in Middle and New Persian is also depicted in the Persian epic Shahnameh of Ferdowsi. Due to the concept of the yazatas, powers which are “worthy of worship”, Tishtrya is a divinity of rain and fertility and an antagonist of apaosha, the demon of drought. In this struggle, Tishtrya is depicted as a white horse.[124][125][126][127]

In Chinese astronomy Sirius is known as the star of the “celestial wolf” (Chinese and Japanese: 天狼 Chinese romanization: Tiānláng; Japanese romanization: Tenrō;[128]) in the Mansion of Jǐng (井宿). Many nations among the indigenous peoples of North America also associated Sirius with canines; the Seri and Tohono O’odham of the southwest note the star as a dog that follows mountain sheep, while the Blackfoot called it “Dog-face”. The Cherokee paired Sirius with Antares as a dog-star guardian of either end of the “Path of Souls”. The Pawnee of Nebraska had several associations; the Wolf (Skidi) tribe knew it as the “Wolf Star”, while other branches knew it as the “Coyote Star”. Further north, the Alaskan Inuit of the Bering Strait called it “Moon Dog”.[129]

Several cultures also associated the star with a bow and arrows. The ancient Chinese visualized a large bow and arrow across the southern sky, formed by the constellations of Puppis and Canis Major. In this, the arrow tip is pointed at the wolf Sirius. A similar association is depicted at the Temple of Hathor in Dendera, where the goddess Satet has drawn her arrow at Hathor (Sirius). Known as “Tir”, the star was portrayed as the arrow itself in later Persian culture.[130]

Sirius is mentioned in SurahAn-Najm (“The Star”), of the Qur’an, where it is given the name الشِّعْرَى (transliteration: aš-ši‘rā or ash-shira; the leader).[131] The verse is: “وأنَّهُ هُوَ رَبُّ الشِّعْرَى”, “That He is the Lord of Sirius (the Mighty Star).” (An-Najm:49)[132] Ibn Kathir said in his commentary “that it is the bright star, named Mirzam Al-Jawza’ (Sirius), which a group of Arabs used to worship”.[133] The alternate name Aschere, used by Johann Bayer, is derived from this.[18]

In theosophy, it is believed the Seven Stars of the Pleiades transmit the spiritual energy of the Seven Rays from the Galactic Logos to the Seven Stars of the Great Bear, then to Sirius. From there is it sent via the Sun to the god of Earth (Sanat Kumara), and finally through the seven Masters of the Seven Rays to the human race.[134]

Dogon[edit]

See also: Nommo

The Dogon people are an ethnic group in Mali, West Africa, reported by some researchers to have traditional astronomical knowledge about Sirius that would normally be considered impossible without the use of telescopes. According to Marcel Griaule, they knew about the fifty-year orbital period of Sirius and its companion prior to western astronomers.[135][136] Robert Temple claims that the Dogon people have a tradition of contact with intelligent extraterrestrial beings from Sirius.[137] This has been the subject of controversy and speculation.

Yoonir, symbol of the universe in Serer religion.[138][139]

Doubts have been raised about the validity of Griaule and Dieterlein’s work.[140][141] In 1991, anthropologist Walter van Beek concluded about the Dogon, “Though they do speak about sigu tolo [which is what Griaule claimed the Dogon called Sirius] they disagree completely with each other as to which star is meant; for some it is an invisible star that should rise to announce the sigu [festival], for another it is Venus that, through a different position, appears as sigu tolo. All agree, however, that they learned about the star from Griaule.”[142]

Noah Brosch claims that the cultural transfer of relatively modern astronomical information could have taken place in 1893, when a French expedition arrived in Central West Africa to observe the total eclipse on April 16.[143]

Serer religion[edit]

Main articles: Serer religion and Saltigue

In the religion of the Serer people of Senegalthe Gambia and Mauritania, Sirius is called Yoonir from the Serer language (and some of the Cangin language speakers, who are all ethnically Serers). The star Sirius is one of the most important and sacred stars in Serer religious cosmology and symbolism. The Serer high priests and priestesses (Saltigues, the hereditary “rain priests”[144]) chart Yoonir in order to forecast rainfall and enable Serer farmers to start planting seeds. In Serer religious cosmology, it is the symbol of the universe.[138][139]

Modern significance[edit]

See also: Sirius in fiction

Sirius is a frequent subject of science fiction,[145] and has been the subject of poetry.[146] Dante and John Milton reference the star, and it is the “powerful western fallen star” of Walt Whitman‘s “Then Lilacs Last in the Dooryard Bloom’d”, while Tennyson‘s poem The Princess describes the star’s scintillation:[147]

…the fiery Sirius alters hue
And bickers into red and emerald.

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