Sirius

Sirius is the brightest star in the night sky. It is found in the southern constellation of Canis Major. Its name comes from the Ancient Greek word Σείριος (Latin script: Seirios; lit. 'glowing' or 'scorching'). The star has the designated name α Canis Majoris, which is Romanized as Alpha Canis Majoris, and it is shortened to α CMa or Alpha CMa. Sirius looks very bright, with a visual apparent magnitude of −1.46. It is almost twice as bright as Canopus, the next brightest star.

Sirius is a binary star, so it is made of two stars that circle each other. The brighter one is a main-sequence star of spectral type A0 or A1, called Sirius A. The other is a faint white dwarf companion of spectral type DA2, called Sirius B. The two stars are between 8.2 and 31.5 astronomical units apart, and they orbit each other about every 50 years.

Sirius looks bright for two reasons. It gives off a lot of light, and it is close to the Solar System. At 2.64 parsecs (8.6 ly) away, the Sirius system is one of Earth's nearest neighbours. Sirius is slowly moving closer to us and should get a little brighter over the next 60,000 years. Around the year 66,270 AD, it will become the southern Pole Star, coming within 1.6 degrees of the south celestial pole because of axial precession and its own movement across the sky. It will stay the brightest star in Earth's night sky for about the next 210,000 years. Then Vega, another A-type star that gives off more light than Sirius, will become the brightest.

Long ago, the Sirius system was two bright bluish stars. The more massive of these, Sirius B, ran out of hydrogen fuel and became a red giant. It then lost its outer layers and shrank into its current state as a white dwarf around 120 million years ago. Sirius A is about twice as massive as the Sun (M_☉) and gives off 25 times as luminous as the Sun. It is still much less luminous than other bright stars such as Canopus, Betelgeuse, and Rigel.

Sirius is often called the "Dog Star", because it stands out in the constellation Canis Major, the Greater Dog. For the ancient Greeks, the heliacal rising of Sirius marked the "dog days" of summer. In Ancient Egypt, it marked the flooding of the Nile. The Polynesians, mostly in the Southern Hemisphere, used the star to mark winter. It was also an important reference for their navigation around the Pacific Ocean.

Etymology

The name "Sirius" comes from the Latin Sīrius. The Latin word came from the Ancient Greek word Σείριος (Seirios, meaning "glowing" or "scorcher"). The Greek word may have come from another place before the Archaic period. One expert links it with the Egyptian god Osiris. The name was first written down in the 7th century BC, in Hesiod's poem Works and Days. In 2016, the International Astronomical Union started a Working Group on Star Names (WGSN) to make a list of star names and make them standard. Its first list, from July 2016, included Sirius for the star α Canis Majoris A. The name is now in the IAU Catalog of Star Names.

Sirius has more than 50 other names. In Geoffrey Chaucer's essay Treatise on the Astrolabe, it is called Alhabor and is shown as a hound's head. This name was widely used on medieval astrolabes from Western Europe. It comes from an Arabic phrase meaning "the one who crossed over". In Sanskrit, it is called Mŕgavyādh ("deer hunter") or Vyādh ("hunter"). As Mrgavyadha, it stands for Rudra (Shiva). In Malayalam, it is called Makarajyoti and is important to the pilgrim center Sabarimala. In Scandinavia, the star was known as Lokabrenna ("Loki's torch"). In the astrology of the Middle Ages, Sirius was a Behenian fixed star, linked with beryl and juniper. Its astrological symbol was listed by Heinrich Cornelius Agrippa.

Observational history

See also: Winter Triangle

As the brightest star in the night sky, Sirius appears in some of the earliest star records. It rises almost exactly every 365.25 days, matching the solar year. It rises at Cairo on 19 July, just before the annual flooding of the Nile in ancient times. The ancient Egyptians worshipped it as the goddess Sopdet, and they saw Orion appear with it. The ancient Greeks thought the star brought hot, dry summers. They noticed that it twinkled more in early summer. People on the island of Ceos made offerings to Sirius and Zeus to bring cooling breezes. The Polynesians used bright stars to find their way across the Pacific Ocean. The Māori and people of Hawaii also used the star to mark the seasons. Its name was also used in other Polynesian languages, such as in the Marquesas Islands.

In 1717, Edmond Halley found that Sirius and other bright stars had moved. In 1868, Sir William Huggins measured the star's speed. This was the first time anyone had done this for any star. In 1698, Christiaan Huygens estimated its distance. Later measurements were made to find the distance more exactly. In 1844, Friedrich Wilhelm Bessel suggested that Sirius had an unseen companion. In 1862, Alvan Graham Clark first saw this faint companion while testing a telescope for Dearborn Observatory. It is now called Sirius B. In 1915, Walter Sydney Adams at Mount Wilson Observatory showed that Sirius B was a white dwarf. In 2005, the Hubble Space Telescope helped measure its size.

Sirius has also been described as red in some old records. Claudius Ptolemy wrote about this. Thomas Barker raised the question at the Royal Society in 1760. Chinese records describe Sirius as white (China). Scientists think the red colour may come from the terrestrial atmosphere, much like the colour of the Sun at sunrise and sunset.

Observation

Sirius is the brightest star in the night sky. It is almost twice as bright as the second-brightest star, Canopus. Its apparent magnitude is −1.46. From Earth, Sirius always looks dimmer than Jupiter and Venus. At some times, it also looks dimmer than Mercury and Mars. Sirius can be seen from almost everywhere on Earth. The exception is far north, north of 73° N. From some northern cities, it stays low in the sky. From Saint Petersburg, it only gets 13° above the horizon. Because of its declination of about −17°, Sirius is a circumpolar star at latitudes south of 73° S. This means it never sets there. From the Southern Hemisphere in early July, Sirius can be seen in the evening, setting after the Sun. It can also be seen in the morning, rising before the Sun. Together with Procyon and Betelgeuse, Sirius makes up one of the three vertices of the Winter Triangle for people in the Northern Hemisphere. Sirius often flashes rainbow colours because of its twinkling.

Sirius can be seen in daylight with just your eyes, but only under the right conditions. The sky must be very clear. The observer should be high up, the star should pass overhead, and the Sun should be low on the horizon. These conditions are easiest to meet around sunset in March and April, and around sunrise in September and October. Viewing is better in the Southern Hemisphere, because Sirius is far to the south in the sky. Sirius is actually two stars that move around each other. Their distance in the sky changes, from 3 arcseconds at its closest to 11 arcseconds at its farthest. At the closest point, the small white dwarf is hard to tell apart from its brighter companion. A telescope with an opening of at least 300 mm (12 in) and very steady air is needed to see it. After a periastron in 1994, the two stars moved apart. Apoastron happened in 2019. From Earth, the greatest separation was seen in 2023, at 11.333″.

Location

Sirius is about 2.6 parsecs (8.6 light-years) away. Two of the eight nearest stars to the Sun are in the Sirius system. It is the fifth closest star system to the Sun. Sirius looks so bright mainly because it is close. Other nearby stars, such as Alpha Centauri, Procyon and Vega, also look bright for this reason. Very distant, very bright giant stars look different. Examples are Canopus, Rigel and Betelgeuse, although Canopus may be a bright giant. Even so, Sirius is about 25 times brighter than the Sun.

The closest large neighbouring star to Sirius is Procyon. It is about 1.61 parsecs (5.24 light-years) away. The Voyager 2 spacecraft was launched in 1977 to study the four giant planets in the Solar System. It is expected to pass within 4.3 light-years (1.3 parsecs) of Sirius in about 296,000 years.

Stellar system

Sirius is a binary star system. It has two white stars that circle each other. They are about 20 AU apart, which is roughly the distance between the Sun and Uranus. They take 50.1 years to orbit each other once. The brighter star, Sirius A, is a main-sequence star with a surface temperature of about 9,940 K. Its partner, Sirius B, has already become a white dwarf. It is now much dimmer, even though it was once the heavier of the two. The system is estimated to be 230 million years old. Sirius A is also called the Dog Star, and it is classed as an Am star.

Sirius B, sometimes called "the Pup", is one of the heaviest white dwarfs known. Its mass fits into a space about the same size as Earth. It has no inner heat source, so it will slowly cool over the next two billion years or so. Scientists have looked for other stars in the system. In 2017, the Hubble Space Telescope showed that there is no third star of normal size, though a smaller brown dwarf could still be possible. In 1909, Ejnar Hertzsprung first suggested that Sirius belonged to the Ursa Major Moving Group, but later studies made this less certain.

Main article: Gaia 1

Cultural significance

See also: Nommo

Main articles: Serer religion and Saltigue

Many cultures have given Sirius a special meaning, especially in connection with dogs. It is often called the "Dog Star" because it is the brightest star in Canis Major, the "Great Dog" constellation, which was shown as Orion's dog. The Ancient Greeks believed Sirius could make dogs act strangely during the hot "dog days" of summer. The Romans called it Canicula, which means "little dog." Homer described it in the Iliad as a star that brought heat and fevers. In Chinese astronomy, Sirius is the star of the "celestial wolf" in the Mansion of Jǐng. Many peoples of North America also linked it with dogs, including the Seri, Tohono Oʼodham, Blackfoot, Cherokee, Pawnee, and Inuit of the Bering Strait. In a Greek myth about the god of Sirius and the goddess Opora, the north wind god Boreas helped cool the earth.

In Persian mythology and Zoroastrianism, Sirius appears as Tishtrya, a god who brought rain. Sirius is mentioned in the Qur'an in Surah An-Najm, where it is called ash-shi‘rā, meaning "the Bright Star." The Dogon people of Mali were said by some researchers to know about Sirius's orbit, though other researchers have doubted this. In the religion of the Serer people, Sirius is called Yoonir and is one of the most sacred stars. Sirius also appears in modern life. It is on the coat of arms of Macquarie University, it gave its name to HMS Sirius of the Royal Navy, and it is on the flag of Brazil. Poets including Dante and John Milton have also written about the star.