The stars that we can see (except one)

The night sky is full of stars and some of the very brightest we see have some very interesting characteristics. Next time you’re looking at Sirius or Canopus you’ll be able to appreciate just how big they are compared to our very own star - the Sun.

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Look up on any given night, assuming it’s not cloudy, like it always seems to be in Wellington, and you’ll see a bunch of stars. If you’ve got good eyesight then you’ll see stars down to a magnitude of about 6. That’s about 6000 individual stars that you’ll be able to see in a nice dark sky location. If you’re in a town or suburbs of a city that number might be only 800-1000 stars. The stars that we see are not all the same and a lot of the stars that are close to us are not actually visible because they simply are not bright enough. Some of the brightest stars in the night sky are late in their lives and many of the most fabulous clusters we can see are full of hot young stars that will burn through their hydrogen cores in only a few hundred million years or so. A good place to start when looking at stars is our very own Sun (but don’t go looking at it unless you have the right equipment).

The Sun is a pretty unremarkable star as far as stars go. It’s a truely remarkable star for us though as we wouldn’t be here without it. It’s dimensions are huge compared to what we are used to, about 1,300,000 Earths could fit inside it (though they’d do some very interesting things to the Sun if that happened!). The surface temperature is hot, very hot – about 5700 degrees K and in it’s core the temperature is a huge 15 million K. The Sun weighs as much as 333,000 Earths and it’s this huge mass which keeps it fusing the hydrogen in it’s core. Despite it’s huge size and the enormous ferocity of what is happening in its core, our Sun is a pretty unremarkable star. In the distribution of stars in the universe it’s about average – which is just as well because if it was a bit bigger things would have been quite different for us and if it was smaller then it’d probably be a whole lot less stable and continually bathe us in higher energy particles from its constant solar storms – good thing our Sun is 1 Solar Mass.

The next brightest star in the night sky is Sirius. This is about twice as bright as Canopus, the next brightest star after Sirius. This star is about 8.6 light years away so is still relatively close to us and about the twice the distance of the nearest star to our Sun, Proxima Centauri. It’s also significantly more brighter than our Sun being about 25 times more luminous, that means if you took measurements of the luminosity of the Sun and Sirius from the same distance, the measurement from Sirius would be 25 times that of the Sun. As far as its dimensions go, Sirius is about twice as big as the Sun and about twice the mass. This means that it’s huge mass is squashing the core far greater than our Sun can so burning its fuel much more quickly. This makes the star a lot hotter on the surface at about 9500 degrees K. Astronomers have determined that Sirius is still burning hydrogen it the core so is still in the main sequence. Though it’s size means it will burn through the hydrogen a lot quicker than in our Sun. Sirius is actually a binary star and has a small white dwarf companion.

Sirius and the white dwarf companion (Credit: Hubble/NASA)

The next brightest star in our night sky is Canopus (see the featured image, Canopus taken from the ISS). This star is about eight times the mass of the Sun and is well advanced through it’s life. Canopus is 10000 times more luminous than the sun and if it was in our Solar System it would nearly extend to the orbit of Mercury. It’s surface temperature is about 7000K. Canopus has been a bit of a problem for astronomers to understand properly. Distance estimates have ranged from 96 to 1200 light years and were only confirmed by the Hipparcos satellite which put it at 310 light years. This allowed astronomers to work out its size but they are still a bit unclear on where Canopus is on it’s life cycle, whether it is running out of hydrogen in it’s core and is starting to expand, eventually into a red supergiant, or if it’s past the red supergiant phase and is settling down into a helium fusing star. Either way, it’s an impressive star that is always visible in the night sky from Wellington (when it’s not cloudy).

The next most visible star in the night sky is Alpha Centauri which almost the closest star to our own Sun. Proxima Centauri is the closest to our Sun and is in the same system as Alpha Centauri. Like our Sun, Alpha Centauri is an average type star, it’s a bit bigger and brighter but is very similar to our Sun in many ways. It has a couple of close neighbours, one imaginatively called Alpha Centauri B which is a little bit smaller than our Sun and Proxima Centauri which is a small red dwarf star only about 0.12 the mass of the Sun. These tiny stars live a long long time. They fuse the hydrogen in their cores for many billions of years and Proxima Centauri will long outlive our Sun.

New shot of Proxima Centauri, our nearest neighbour
Proxima Centauri (Credit: Hubble/NASA)

Look up at night and you’ll see a massive array of stars – many of them far more turbulent and angrier than our Sun, which is a good thing that they are so far away.