Sunday, 10 May 2020

Astrognome A-Z of Constellations # 14 Canis Major


Canis Major The Greater Dog
Canis Major is a stunning constellation with many bright stars and clusters, unfortunately for observers in Britain it is always low in the sky so we never see it at its best. It represents one of the two dogs the other being Canis Minor that follows at the heels of Orion the Hunter.
The brightest star in Canis Major, Sirius is also the brightest star in the sky. This is the dog star and sometimes known as the scorching one or the Nile star. Although low down in winter skies it is easy to find. If you do need a guide to find Sirius you can use the stars of Orion’s belt and draw a line down and to the left this will lead you to Sirius.
At magnitude -1.4 (it has a minus sign in front of the magnitude number because it is so bright, there are 3 stars with minus magnitudes in the night sky) Sirius outshines all the other stars in the sky but appearances can be deceptive because Sirius is very close to us being only 8.6 light years away. It is an A class star which means it is hotter than the Sun with a temperature of 9,500’C compared to 5,800’C of the Sun.
Sirius was very important in ancient Egypt because astronomers/priests would wait to see Sirius rise just before the Sun rose in the morning sky. They knew this meant the river Nile was about to flood, it was at this point that farmers had to make sure their fields were ready for the inrush of water and sediment that would help their crops to grow. This astronomical event is known as the Helical Rising.
Sirius has a companion star a white dwarf which was discovered by the American telescope maker Alvan Clark in 1862 when he was testing a 15 inch refractor. A white dwarf is a small, faintly lit object made of super dense stuff called degenerate matter. It is thought that most stars, including the Sun, will end their days as white dwarfs. A teaspoon full of white dwarf material would weigh many tons.
As Sirius is the Dog Star it seems appropriate that the small companion star which is officially known as Sirius B should be known unofficially as ‘The Pup’
We know that the brightest stars in a constellation are allocated a letter from the Greek alphabet with the brightest being alpha then beta etc., and we have also seen that the system often does not work, well here we go again.
The second brightest star is epsilon (5th letter of the Greek alphabet) or Adhara which means ‘Virgins’ and has a magnitude of 1.5 it is a B2 giant star with a whopping surface temperture of 22,500’C which makes our Sun seem very cool by comparison. Adhara is 440 light years away.
A line drawn down from Sirius and slightly to the left leads to delta or Wezen which means ‘Weight’ which has a magnitude of 1.8 and is a F8 supergiant and is slightly hotter than the Sun, Wezen lies about 1600 light years away. It is thought that in the next 100,000 years Wezen could destroy itself in a supernova explosion.
To the right of Sirius is beta or Murzim which means ‘Announcer’ and should be the 2nd brightest star in this constellation however it is in fact the 4th. Murzim is a magnitude 2.0 blue giant class star lying 500 light years away, its another very hot star with a surface temperature of 22,750’C.
Below Wezen and to the left is eta or Aludra the meaning is unknown and is B5 supergiant of magnitude 2.4 , with a temp of 14,750`. Aludra is about 2,000 light years away. It will also become a Supernova in the next few million years.
A line drawn to the right from epsilon will lead to zeta or Phurud meaning the ‘Bright Single Ones’. This is a magnitude 3.0 star, 362 light years away. Another B class star this time a B2 with a temperature of 18,400’ C.
To the left of Sirius is gamma or Muliphein whose meaning is unknown. Gamma is of magnitude 4.1 and is a B8 giant with a surface temperature of 13,100’C and lies 440 light years away. There is a mystery here because Gamma appears to have varied over a period of many 100s of years. It is recorded that in 1670 the Italian astronomer Montanari said that it disappeared from view. It was not observed for another 23 years.
Below Sirius is an open cluster, M41 which contains about 80 stars and shines at a magnitude of 4.5 it is about 2,100 light years away it was recorded as far as back as 325 BCE by Aristotle in Greece. In ‘modern time’ it was reported by the Italian astronomer Hodierna in 1654 and recorded by Messier in 1765.
NGC 2362
An open star cluster NGC 2362 was discovered by Hodierna in 1654 this cluster was not reported again until it was eventually found by William Herschel (discoverer of the planet Uranus in 1781) on March 4, 1783
NGC 2362 contains about 60 stars, and is of mag 4.1 and is only about 25 million years old. The brightest star in this cluster is tau which is of magnitude 4.39 and is a class O9 supergiant star with a surface temperature of an incredible 31,500’C, it is one of the most luminous supergiants known. The cluster is about 5,000 light years in distance from Earth.


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