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14th Apr

2010

Let’s Get Sirius!

Last week, Klara and I went for a nice walk around the Kazimierz district of Cracow and ate a delicious zapiekanka for 6 zloty (about 2 dollars). On our way home, we decided to stop by the Wisla river, which I live practically next to, and we took the remaining few bites of our meal while watching the last sliver of sun that remained disappear as we rotated away from it.

It was at that very moment when I recalled a blog post by astronomer Phil Plait, in which he said that in the subsequent few days it would be possible to see both Venus and Mercury next to one another after sun set. This was a perfect opportunity for us to see if that was true and we stuck around for a little bit to see the nearby planets.

Low and behold, not long after sunset appeared a bright “star” in the sky above where the sun had just been. I figured that it had to be Venus, since Venus is almost twice as close to us as Mercury and a lot bigger. Interestingly, Mercury is only 0.055 earth masses and is actually smaller than the moon Ganymede, which revolves around Jupiter. We waited for Mercury to show its hot little self, but couldn’t see it.

That’s when Klara pointed at another star in the southwest direction (we were looking west) and asked if that was it. I looked at the twinkling and bright star and told Klara I didn’t think that it was Mercury since I assumed that it would be closer to Venus in the sky. Realizing that I didn’t know much at all about astronomy, I decided to educate myself a bit about the stars I was looking at, especially this bright southwestern star because I recall always seeing it throughout my life as one of the brightest in the sky.

So the next day I downloaded a free program called Stellarium. It is basically a planetarium on your computer that allows you to manipulate visuals and settings at a whim. After plugging in your coordinates (find them on google maps) you can see which stars are visible from where you are, at any time you choose. It’s even possible to turn off the visibility of the ground to look ‘down’ through the earth and see what’s visible from the southern hemisphere.

Of course one of the first things I did using Stellarium was to see what the southwestern star was. It was Sirius, the brightest star in the sky. A quick wikipedia search (yes, I use wikipedia all the time for everything) explained that it is almost twice as bright as the next brightest star. It’s still not as bright as Venus and Jupiter, but they don’t count as stars.

There are other cool things about Sirius, besides it’s luminosity. For one, it’s a local neighbor of ours. It’s about 8.5 light years away making it the fifth closest star. There are no planets around Sirius, but that’s OK because it makes up for it by being a binary star system composed of two suns revolving close to one another.

One of the suns, Sirius A, is a lot larger than the other and its surface is almost twice as hot as our own sun’s: a steamy 10,000 degrees Kelvin. That’s three times the temperature required to boil iron.

Sirius B, on the other hand, is a lot smaller but used to be even larger than its brother. Around 120 million years ago (the Cretaceous period), when dinosaurs walked the planet, it collapsed from a red giant into a white dwarf. That means that some dinosaurs would see Sirius as nearly twice as bright as we do. Nowadays, Sirius B is much smaller, with a volume of earth’s but a mass of our own sun’s (astronomy is so freakin’ cool!).  Its surface is 25,000 degrees Kelvin, yet its small volume makes it thousands of times less bright than its counterpart.

Next time you’re out, see if you can spot Sirius to the left of where the sun set (at least in the northern hemisphere). It’s visible from pretty much everywhere in the world and bright enough to be seen during the day from some places so it shouldn’t be hard to spot. It will stay near the horizon for most of the time and it sets after the sun and rises before it. I’m glad I know now at least one star by name and I hope some of my readers will as well.

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Comments

starshine

2010-04-14 19:25:31

stellarium is a great prog

http://tinyurl.com/impact_shirt

Christoph K

2010-04-15 12:00:32

I remember trying to see this as well after i read about it on badastronomy.com
But i guess the light polution in this region might be to big to get a good look at mercury.
The red giant doesn’t really collaps. It much mor sheds it uter layers leaving just the core behind.
One small nitpick. It’s just Kelvin not degree kelvin.
Besides of that excellent post as always!

    Matt Roman

    2010-04-15 13:45:11

    Ahhh yes. Constructive criticism is much appreciated. That’s why I’m doing this: to smooth over my understanding of science and writing! 🙂

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