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  • Writer's pictureLuke

You Can-opus be Sirius!?

Looking south in at night it would be hard to miss the brightest star in our night sky Sirius. Now, I say in our “night” sky because of course the sun is a star and by a large margin the brightest one in our sky. So, let’s give the sun credit: sun, you are the brightest star that lights up our life, wouldn’t be here without you.

The name Sirius comes from Greek meaning scorching as it has been the brightest star in the night sky for all of written history. Sirius will continue to be the brightest star for some 200,000 more years when the distance between between the solar system and Sirius slowly increases and Sirius fades. Sirius is also called the “dog star” as it is located in the constellation Canis Major (the big dog) who is Orion’s trusty companion following right behind him across the night sky.

They say Orion has taken a liking to the seven sisters you can watch him chase them across the night sky. However Zeus (or Jupiter if you ask the Romans) put a bull (Taurus) in between them to protect the girls. Orion is being followed as well by his two best friends his little and big dog.

On occasion you might hear hot days referred to as “dog days” this actually comes from Sirius the “dog” star and dates back to ancient Egypt. When Sirius was seen rising in the morning with the sun it was the hottest days and coincided with the flooding of the Nile. because of this Egyptians would watch for it in the mornings to know to prepare for the coming floods. It was thought that the sun and Sirius shining together is what made the heat so it was natural to call it a “dog day”.

This last week I got to visit my grandfather in southern Arizona and while doing so I had the chance to see an old friend/favorite of the night sky Canopus. Canopus is the second brightest star in the night sky, and located in the constellation Carina (the giant ship). Canopus is not visible in much of the united states. Only in the southern most states can Canopus be viewed for a brief amount of time, a few nights a year. This is because Canopus is located at a declination of 52° S meaning that anyone above the latitude 38°N (90°N-52°S=38°N) will be unable to see Canopus in their night sky. Living in Boulder which is 40°N (Baseline Rd is actually called such because it follows the 40th parallel) means I never get to see Canopus unless I travel to see it.

Table 1 – The top 10 brightest stars. The brightness has been adjusted from normal units to be with respect to Vega, so it is easier to compare. 4 being 4 times as bright as Vega (or 400% the brightness if you rather) and .64 being just under 2/3rds as bright (or 64% as bright) as Vega. I choose Vega not only because it’s my favorite but also between Arturus, Vega and Capella, which are all about the same a comparative example should be in the sky year-round. Capella is currently right over head in the evening sky making it a good comparison right now. Also of mention: stars can vary in brightness these are based on commonly accepted averages.

Demonstrated quite well on this table is the difference between absolute and apparent magnitude. If you look at Sirius and Canopus, Sirius is twice as bright as Canopus sure but, Canopus is 310 light years away where Sirius is only 8.6 ly away. Comparing Canopus to Alpha Centauri you will see Alpha Cen is slightly dimmer but at 4.4 ly even closer than Sirius. This all tells us Canopus must be bright! While I was living in Australia Canopus was common in the sky. My mentor Greg (#SpaceGandalf) would say “Respect” when ever talking about Canopus. To be the second brightest star in the night sky and 310 ly away beating a star that is 70 times closer it must be waaaaay brighter! RESPECT indeed.

This is the difference between absolute and apparent magnitude. Absolute is how bright the star would be if we compared stars from the same distance and apparent is how it appears at the distance we see it in our sky. Sirius is our brightest neighbor it would be like the street light at the end of our block. Canopus is the brightest thing in our part of our galaxy it would be like a stadium 36 blocks away.

The difference between absolute and apparent magnitude.
The difference between absolute and apparent magnitude.

To put this yet another way; if you travel even 50ly from the sun you wouldn’t even be able to see it without a telescope. Our star (the sun) is actually really dim when compared to these stars, even if you traveled to Alpha Cen (4.4 ly) it wouldn't even be on the list at a brightness of 61% that of Vega.

If you want to see a pair of stars like this in our Colorado night sky take a look at Procyon and Rigel. Procyon is the brightest star in Canis Minor (the little dog) which is just above Sirius and Canis Major. Rigel is Orion’s bottom left foot. Both Rigel and Procyon are about the same brightness however, Procyon is 11 light years away and Rigel is 860 light years away.

Procyon Vs Rigel, both these stars look the same brightness however one is 849 light years further than the other

If you want to get an up close look at Sirius and learn more about our night sky consider booking an AstroTour

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