Star Spy: Brightest not closest

21:23, Nov 21 2012
Brightest stars
Brightest stars from Earth.
Brightest Stars
Brightest stars from Earth
Light Year
Light Year

The Solar Eclipse

Here’s hoping that all of you had as clear a sky for the solar eclipse as we did down in the middle of the Mainland. Fraser was able to capture and animate the entire eclipse. Here in Lake Tekapo the maximum coverage was about two thirds. 

Several of us from the South Canterbury Astronomers Group shared the event with the students from Opihi College in Temuka.

It was exciting for all of us, and for many it was the first time they had ever really seen the Sun, sunspots and solar flares and all.

The next solar eclipse will occur on May 11 next year. Meanwhile, if you can access a proper solar viewer, the Sun is quite interesting to look at, even when there is no eclipse.   


The brighest stars

As we’ve already seen, you can’t tell by just looking at an object in the sky, how big it is, or even how bright it really is. 

The Sun and Moon are an excellent example. A casual observer could be forgiven for thinking that they are the same size. 

In fact, until you saw the Moon moving across in front of the Sun, you might propose that the Sun could be the smaller of the two, and much closer to us. 

This explanation would make as much sense, and would explain why the Sun appears so much brighter than the Moon in the sky.  It is the same with the stars. Depending on your location, on a dark, clear night you may be able to see thousands of them, some very dim and twinkling in and out of your vision, some very bright. 

In fact the stars can be easily confused with planets, which clearly are very much smaller, and very much closer to us than any stars, apart from the Sun.

We watched Mars and Saturn travel with the star Spica for several months during the year. Without a program, or knowledge of their respective colours, you might have found it quite difficult to know which was which.

We now have the three brightest stars in Earth’s night sky together in our southern sky for spring and summer. 

They are Sirius, the brightest star, Canopus, second brightest, and the third brightest, Alpha Centauri.    

Actually Canopus and Alpha Centauri are in our Lake Tekapo sky all year around, but Sirius is farther from the South Celestial pole so rises in spring and sets at the end of summer.

Their distances from us may surprise you. We’ve already seen that Alpha Centauri is the closest star to us at 4.3 light years. So you might logically expect it to be the brightest, not the third brightest star in the sky. 

In fact Sirius, the brightest star, is almost exactly twice as far away from us, 8.6 light years, as Alpha Centauri. 

But Canopus is the one that comes as a real surprise. The second brightest star in our sky, it is more than 70 times farther away from us than the third brightest star – 313 light years away! You can see that its trail is brighter than that of Alpha Centauri.

Canopus is not as well known as the other two, but readers of science fiction may recall that Arrakis, the fictional desert planet in the Dune series, was located in the Canopus planetary system. 

The three brightest stars are not too difficult to find. Through spring and summer, Sirius, also known as the Dog Star, follows not far behind Orion the Hunter. It is part of Orion’s large hunting dog, Canis Major (also upside down). 

As in the above wide angle photo, Canopus will be above and to the right of Sirius. Unlike Sirius, south of Auckland Canopus can be seen all year round, circling the South Celestial Pole as in the star trails photo above.

Alpha Centauri is outside the image, to the right (south) of Canopus, and throughout New Zealand it can be seen year-round circling the South Celestial Pole.