Observing Sirius – Our Wide Sky

Observing Sirius

January 27, 2018

Sirius is the brightest star on the night sky. It’s looking lovely overhead in the evening right now, and in a telescope or binoculars it sparkles in a black background.

Sirius is the brightest star in the night sky. Some people wonder if it is safe to view through a telescope. It is safe to look at Sirius and other bright stars. The Sun will damage your eyes, so don’t look at it. The Moon is bright through a telescope and can give your eyes an ache.

Looking at individual stars might seem like a boring pass time. Bright Stars are just stars aren’t they?

But bright stars are all different, they have a different story to tell, and a different relationship to human science and culture.

Let's take a look at Sirius

Sirius has a magnitude of -1.46 which makes it the brightest star in the night sky. (Remember that negative magnitudes are brighter). There are only 3 other stars with a negative magnitude. Canopus, Rigel Kent, and Arcturus.

Sirius is 8.6 light years away. In the scheme of things, this makes us pretty much neighbours. Its proximity is the reason we see it so brightly. Even though it is special to us, it’s really a pretty ordinary star.

Finding Sirius

Sirius is at 16 degrees south. This means that it is not far south of the Celestial Equator. It is visible to everyone except the very far North.

 If you live in the Northern Hemisphere, look to the South, and if you are in the Southern Hemisphere look up and to the North.

Sirius culminates (passes across the meridian, the line between north and south that passes over your head) around 9pm in Early February. This makes it easy to find. It is bright, and right in line with North and South at 9pm. 

How many stars?

Sirius is actually a star system. There are two stars in orbit around each other. Sirius A is about twice the mass of the Sun, and we see its light shining so brightly.

Sirius B is a white dwarf that’s about as massive as the Sun, but a lot dimmer. Its apparent magnitude is 8.44.

They orbit each other about every 50 years at an average distance of 20 times the distance from the Earth to the Sun.

Chandra Xray Telescope got a pretty good view:

Image: Chandra

The angular separation of the two stars maxes out at 11 arcseconds. This is 11 60ths of one 60th of a degree. (There are 360 degrees in a circle). As a reference the Moon is half of a degree in angular size. 

This is a challenge to split for even the biggest amateur telescopes. With a 12 inch telescope you’ll need a dark sky and good conditions. Sirius B was not even seen in a telescope until 1862.

This is part of a paper published in the Astronomical Journal in 1891, by C P Howard. It is the analysis of the orbit of Sirius A and B.

You can read the whole paper here.

Colours

Sirius has a reputation for twinkling and changing colour. The twinkling and colour change are more pronounced as the star approaches the horizon. It is commonly caused by turbulent atmospheric conditions refracting the white light from Sirius into its component colours. It has no relation to Sirius itself. It’s more pronounced than other stars because Sirius is brighter, and we can see the effect with out naked eye.

There are some stunning images of the colours of Sirius. They are all copyright so I have included links to places where they have been used with permission:

Camera Shake Sirius

The Colours of Sirius

More colours of Sirius

Humans and Sirius

Sirius is the brightest star in the constellation Canis Major, or the Greater Dog. Sirius is also known as the Dog Star. It is a marker of summer in the south and winter in the north.

Like many other bright stars, Sirius has also long been important as a navigational star for maritime, land and aviation navigation. Even space craft use stars, including Sirius, to calibrate their navigation.

Because of its brightness it has been important throughout history, and to many cultures. The ancient Egyptians worshiped it; the ancient Greeks blamed it for afflictions if the heat of summer. It has many names through the traditions of the world.

In the Ancient Greek Sirius means burning or glowing. In Sanskrit it is known as the Deer Hunter. Chinese astronomers called it the Celestial Wolf. First Nations of many countries had different names such as Moon Dog, Wolf Star. On Hawaii it has been known as ‘A‘a or Bright Star.

Modern Astronomy

It has also been important to astronomy because of its proximity. In 1868 William Higgins found a red shift in the stars spectrum and determined that the star was moving across the sky. This was the first star to have its velocity measured, though not quite accurately.

It is also one of the stars that can be measured by parallax. In the 1830s Scottish astronomer Thomas Henderson made an accurate measurement of the parallax of Sirius.

Humans to Sirius

8.6 light years is to far for any of us to travel to Sirius. But we can send our space craft.

Voyager 1 is now on a path to pass by Sirius. It will take 296,000 years.

Voyager

Originally Published on Quora.

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