Sirius Star

What is the Sirius star?

The name of Sirius comes from the Greek word Seirios, which means glowing or scorching. Sirius is easily the brightest star in the night sky visible in the constellation Canis Major; with an apparent magnitude of -1.46 it is almost twice as bright as Canopus, the second brightest star visible in the night sky. It appears so bright because of both its high intrinsic luminosity and its proximity to the Solar System, only about 8.6 light-years from Earth which is one of the closest stars.

Sirius is not just a single star, but is actually a binary star system consisting of Sirius A, a main sequence star of spectral type A0-A1, and Sirius B, a faint white dwarf star of spectral type DA2. These two stars orbit each other about every 50 years at a distance which varies from roughly 762.6 million miles to 2.93 billion miles.

For the next 60000 years Sirius will slowly move closer to our Solar System, so its brightness in the night sky will increase somewhat. After that it will slowly start moving away from Earth, but will remain the brightest star in the night sky for another 210000 years.

Sirius Facts

What are the facts about Sirius?

Sirius A is about twice as massive as the Sun with about 25 times the luminosity, but with significantly less luminosity than Canopus and Rigel. The Sirius System is only about 200 to 300 million years old, originally consisting of two bright blue-colored stars. Sirius B was the more massive of these two stars – it consumed its fuel much more quickly, becoming a giant red star when it entered the carbon fusion phase. Eventually it sheded its outer layers and collapsed into the white dwarf star we see today about 120 million years ago.

Sirius A Facts

Sirius A is a main sequence star(hydrogen fusion phase) with a spectral type of A0-A1 and an apparent magnitude of -1.47 and an absolute magnitude of +1.42. It has a U-B color index of -.05 and a B-V color index of +0.00. Sirius A has a mass of approximately 2.063 solar masses with a radius of about 1.711 times that of the Sun(as measured from the boundary of the Suns photosphere).

It rotates at about 9.94 miles per second around its axis with a luminosity of about 25.4 times that of the Sun. Sirius A has a surface temperature of about 17430 degrees Fahrenheit with a metallicity ratio of .50 – the metallicity in a star describes the relative abundance of elements heavier than hydrogen or helium. Sirius A is currently thought to be about 237 million years old.

Sirius B Facts

Sirius B is a white dwarf star with a spectral type of DA2, a stellar core which is composed mostly of electron-degenerate matter, and which is very dense compared to main sequence stars like the Sun and Sirius A. It has an apparent magnitude of +8.44 and an absolute magnitude of +11.18 with a U-B color index of -1.04 and a B-V color index of -.03. The mass of Sirius B is about 1.018 solar masses and its radius is about .0084 times that of the Sun, roughly 1/120th as much.

The rotational velocity has not been accurately established at this time. Sirius B has a surface temperature of about 44500 degrees Fahrenheit with a luminosity of only about .056 times that of the Sun. The metallicity of Sirius B has not been established at this time and it is believed to be about 228 million years old.

Sirius History

Sirius the dog star in the night sky

Sirius is known as the dog star, which comes from the fact that it can be seen in the constellation Canis Major, meaning ‘greater dog’. For the ancient Egyptians, the annual rising of Sirius in the eastern sky just before sunrise marked the flooding of the Nile river, for the ancient Greeks it was associated with the ‘dog days’ of summer, and for the Polynesians its visibility in the Southern Hemisphere marked the beginning of winter.

The Dogon people of Mali, West Africa seem to have known about some astronomical facts about Sirius well before the use of telescopes by western astronomers, including the 50 year orbital period of Sirius A and Sirius B. The Dogon people also claim to have had contact with intelligent extraterrestrial beings from Sirius in their past, although this is somewhat controversial.

Sirius Movement

Edmond Halley, an English astronomer, mathematician, and physicist, discovered the movement of some of the brightest stars, including Sirius, Aldebaran, and Arcturus, by comparing astronomical measurements made from Ptolemy’s Almagest(2nd century Greek mathematical and astronomical treatise on the relative motions of the stars) to contemporary star measurements of the time. He found that these stars had moved significantly, with Sirius moving about 30 arcseconds, the width of the full moon, to the southwest.

In 1868 Sirius became the first star to have its velocity measured when William Huggins, an English astronomer, examined the spectrum and observed a red shift associated with Sirius and made the conclusion that it was moving away from Earth at the velocity of about 25 miles per second. This estimate was way off from the modern value of today which is about 3.4 miles per second, possibly because Huggins did not take into account the Earth’s orbital velocity around the Sun, which is about 18.6 miles per second and would make up most of the error.

Sirius Distance

In 1698 Christiaan Huygens estimated that the distance of Sirius from the Earth was about 27664 times the distance of the Earth from the Sun, or about .437 light years. Later on around 1839 the Scottish astronomer Thomas Henderson used his own observations of Sirius along with the observations of South African Astronomer Thomas Maclear to determine that the parallax of Sirius was probably significantly less than .5 arcseconds. Today we know that the value of the parallax of Sirius is .3792 arcseconds, giving a value of 8.6 light-years for the distance of Sirius from Earth, within the range of Thomas Henderson’s estimate.

Sirius B Discovery

The German astronomer Friedrich Bessel studied the motion of Sirius and in 1844 came to the conclusion that it had an unseen companion star. In early 1862 the American astronomer Alvan Graham Clark was the first to actually observe Sirius B while he was testing an 18.5 inch refracting telescope, which was the largest at the time.

In 1915 Walter Sydney Adams used a 60 inch reflecting telescope at Mount Wilson Observatory to observe the spectrum of Sirius B, determining that it was a white dwarf star. Robert Hanbury Brown, a British astronomer, and Richard Quintin Twiss, also a British astronomer, were the first to measure the diameter of Sirius A in 1959 with stellar intensity interferometry at the Jodrell Bank Observatory.

In 2005 the Hubble Space Telescope was used by astronomers to determine that Sirius B has a diameter almost as great as the Earth, around 7500 miles, with a mass actually slightly greater than the Sun at about 1.02 solar masses, showing the great relative density of white dwarf stars.

Sirius Color

There has been some discrepancies in the observed color of Sirius through the ages. Around 150 A.D. the Greek astronomer Claudius Ptolemy described Sirius as having a reddish color. Before that Seneca, a Roman philosopher who lived from around 4 B.C. to 65 A.D. described Sirius as having a deeper red color than Mars.

Not all ancient observers saw Sirius as having a red color – Marcus Manilius, a 1st century Roman astrologer, and Avienus, a 4th century Latin writer, both described the color of Sirius as sea blue. There are multiple records from ancient China, from the 2nd century B.C. to the 7th century A.D., which describes the color of Sirius as white.

A possible explanation for these discrepancies in the observed color of Sirius back in ancient times could be a variation in atmospheric conditions, as well as the relative position of Sirius in the sky(near the horizon with the light going through a thicker portion of the atmosphere, as opposed to near the zenith of the sky where the atmosphere the light passed through would be thinner), and the time of the year among other things. This controversy as to the difference in the observed color of Sirius in ancient times remains to this day.

Sirius Star System

The Sirius Star System

The Sirius Star System is currently believed to be a binary star system composed of Sirius A and Sirius B, although there may also be a possibility of a third star as well. The two stars orbit each other at a mean distance of about 20 astronomical units, about 1.86 billion miles roughly every 50.1 years. The Sirius Star System is thought to have formed some 230 million years ago.

Sirius A

Sirius A is a main sequence star about twice as massive as the Sun with a spectral type A0-A1 and a surface temperature of about 17430 degrees Fahrenheit. A weak magnetic field exists on the surface of Sirius A.

It is currently thought that Sirius A will completely exhaust it’s hydrogen fuel in about 1 billion years, after which it will become a red giant star and then eventually collapse into a white dwarf star, much as the phases which Sirius B has gone through.

Sirius B

Sirius B started off as a main sequence star much more massive than Sirius A, and because of this very large mass, quickly evolved(in stellar terms) to a red giant star, eventually collapsing to the white dwarf star that we see today. It is about 10000 times less luminous than Sirius A in the visible light spectrum.

Sirius B is also one of the more massive white dwarf stars known – it has a mass equal to about 1.02 solar masses, which compares to an average mass for white dwarfs of .5 to .6 solar masses, roughly double. As stated before, white dwarf stars are very dense – Sirius B packs a mass slightly greater than the Sun’s into a volume slightly less than the Earth’s!

Right now the surface temperature of Sirius B is about 44500 degrees Fahrenheit, but since there is essentially no internal heat source, Sirius B is expected to gradually cool over the next 2 billion years as the heat that is present radiates away into space.

Third Star

Since 1894 some irregularities have been observed in the orbits of Sirius A and Sirius B. In 1995 a study of the Sirius Star System came to the conclusion that a small red or brown star may exist less than 3 arcseconds from Sirius A, with a mass of less than .05 solar masses and a very faint apparent magnitude of less than 15.

Later on observations by the Hubble Space Telescope seem to contradict this – it has found no evidence that the hypothesized smaller third star exists, although this is still not conclusive – there is still some possibility that a very small brown dwarf star could exist in the Sirius Star System.

Star Cluster Member

It is currently thought that Sirius may be a member of a large star cluster with the proposed name of Sirius Supercluster. If this is the case it would be one of three large star clusters located within 500 light-years of the Sun, the other two being the Hyades star cluster and the Pleiades star cluster – both with hundreds of stars.

Observing Sirius

Observing Sirius

Observing Sirius in the night sky is very easy, especially in the Northern Hemisphere, although it is also visible to some extent in the Southern Hemisphere as well. Sirius actually refers to two stars, Sirius A(by far the brightest) and Sirius B(a relatively faint companion star), and being the brightest star in the night sky with an apparent magnitude of -1.46, a pair of binoculars can give a good view of Sirius, although even a modest telescope will give even better results.

The best way to find Sirius is to first locate the belt in the constellation Orion – Sirius is then easily located since it is by far the brightest star in the night sky. In the winter after about 9 pm central time is the best time to view Sirius in the Northern Hemisphere using the constellation Orion as a guide. Sirius is easily visible in the Southern Hemisphere as well most of the year, but mainly in the winter months in the Northern Hemisphere.

The small and much fainter companion star of Sirius A, the white dwarf Sirius B, can also be viewed with a modest telescope.

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