Updated: Jul 1
No new image this week due to the late winter storms and the profusion of clouds cynically blocking any access to the sky. It's been that way for at least the last 3 weeks and shows no interest in moving on anytime soon. So, given that Spring actually starts on Monday the 20th 2023, (See last weeks blog about that here: https://www.astrocrescent.com/post/the-four-seasons ) I thought that it would be interesting to have a look at a star that to me and many other astronomers heralds the Vernal Equinox. That star is the mighty Arcturus in the constellation of Bootes. And no, that is not the very unfashionable bootees worn by elderly people. It's pronounced "bo-OH-teez", which comes from the Latin for "herdsman or ox-driver".
Arcturus has an apparent visual magnitude of −0.05 and is the third brightest of the individual stars in the night sky, and the brightest in the Northern Celestial hemisphere. Located relatively close at 36.7 light years from the Sun, Arcturus is a single red giant of spectral type K1.5III. It is an aging star around 7.1 billion years old that has used up its core of hydrogen and has evolved off the main sequence. It is about the same mass as our Sun, but has expanded to 25 times its original size and is around 170 times as luminous. Its diameter is around 35 million kilometres. Arcturus is moving rapidly (about 270,000 mph) relative to our Sun, and is now almost at its closest point to the Sun. Closest approach will happen in about 4,000 years, when the star will be a few hundredths of a light-year closer to Earth than it is today. It will then continue its journey towards the constellation of Virgo and will actually fade below naked eye visibility in about 500,000 years as seen from Earth.
Of course, Arcturus has been known and admired since ancient times, with different cultures and a wide variety of meanings attached to it. It has been known as the "Watcher or Guardian". Arabic scholars knew it as "the lance bearer or the keeper of heaven". It has sometimes been called "Job's star" from the reference to it in the Book Of Job, which of course appears in the Bible. The English astronomer William Henry Smyth asserted that Arcturus was the first star on record to be observed in the daytime with a telescope. This was accomplished by the French astronomer Jean-Baptiste Morin in 1635 with a very small refractor. Arcturus became quite popular in the Spring of 1933 at the "Century of Progress" Exhibition in Chicago. Arcturus's light was employed in the mechanism used to open the exhibition. The star was chosen as it was thought that light from Arcturus had started its journey at about the time of the previous exhibition in 1893 (at 36.7 light-years away, the light actually started in 1896). Here is a comparison of Arcturus's size as compared to our Sun.
Wow, what a monster unleashed! Arcturus is rising round about 7.00pm in the Eastern sky at the present time (March 18th 2023). So, if we do get a clear night sometime soon, and you are about, see if you can find it. Its very bright and has a notable orange tint to it. Its not easy to mistake either as there is no other bright object close to it. I hope you have enjoyed this blog and biography of Arcturus, and if there is any star you are interested in particularly, then let me know via the comments and I will gladly do a write up on it. As usual, all comments good or bad are welcome. Thanks for reading! See you next time...