Sirius, the Brightest Star in the Sky!

February 18, 2021
Sirius A and Sirius B bottom left

Sirius, the brightest star in the sky.

 

Sirius, also known as Alpha Canis Majoris or the dog star, gets its name from a Greek word meaning “sparkling” or “Scorching”.  At this time of the year, we are watching it in all its glory.  Usually, one of the first celestial objects we can see after sunset as it is the brightest star in the sky.  In fact, the fifth brightest object in the Sky after the Sun, the Moon, Venus and Jupiter, Sirius is fifth!

Sirius is actually a binary star, comprising of two star types.  The brightest of the two is main sequence star Termed Sirius A and its companion is an exceedingly small white dwarf named Sirius B.  The two are locked in orbit around each-other varying in distance from 8.2 Astronomical Units to 31.5AU.  It takes a lot longer than you might think for the pair to orbit, passing each other every 50 years!

So why is Sirius so bright?  Well, it is one of our nearest neighbours at just 8.6LY.  Closer stars are often brighter.  In fact, it will be gradually getting closer to our solar system for the next 60,000 years slightly getting brighter over that time, until it eventually starts to move away from us.  We estimate it will remain the brightest star in the sky, however, for the next 210,000 years!

Sirius A is the brighter component of the Binary, a blue-white star 25.4 times more luminous than our sun and with a diameter 1.71 times greater.  Sirius B was once a big bright blue star itself, but quickly burnt out its energy source and transformed into a Super Red Giant before collapsing back to the white Dwarf it is today around 120 million years ago.  It is quite possible that over many billions of years Siris A will turn into a Super Red Giant and eventually start to deposit mass on Sirius B creating a spectacular light show as a super nova.   But by that time, it will be receding from our own solar system and will be so far away any harmful gamma rays will not affect us!

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, placing it just before the onset of the annual flooding of the Nile 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 “Triangle”, guarantor of the fertility of their land.   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.

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.   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 emanations that caused its malignant influence. Anyone suffering its effects was said to be “star-struck”.  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 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 were 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 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.

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 night.   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 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 Islands, 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.

Discovery of Sirius B

In 1844, the German astronomer Friedrich Bessel deduced from changes in the proper motion of Sirius that it had an unseen companion.   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”.  This happened during testing of an 18.5-inch (470 mm) aperture great refractor telescope for Dearborn Observatory, which was one of the largest refracting telescope lenses in existence at the time, and the largest telescope in the United States.   Sirius B’s sighting was confirmed on March 8 with smaller telescopes.

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 ridiculously 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.   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.  When we look with our 8.5 inch scope, we are unable to see the Earth sized Sirius B let alone another 3rd smaller star.

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.  This led astronomers to conclude that it was a white dwarf—the second to be discovered.  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.   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.

When we show Sirius through the scope, we call it the Diamond star.  It often comes with a warning for our gentlemen customers who’s partners look at it in ore and proclaime on a nightly basis “I want one”!  For a chance to see Sirius in all its glory, the BOOK A TOUR with us and come and see the wonders of Tenerife’s Night Skies!

At this time of year (winter), we also see another bright star.  This one lies very close to the southern horizon and would never be visible in mainland Europe or the UK.  Most people know that Sirius is the brightest star in the sky, and it’s visible to anyone on the globe. The second-brightest star, Canopus, presents more of a challenge to observers.

If you are a member, in the next section we will talk about Canopus!  If not, then this is it for you right now, until you sign up as a member of our Moon Light club, it’s totally free, so why not sign up NOW!

If you have any questions, then please leave a comment or get in touch via email.

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