I’ll second the Stellarium recommendation, as I mentioned it in a previous post without giving it enough props. Its an awesome piece of free software.
And if you have an iPhone, the Distant Suns app is a great program. There’s a “Lite” (free) version that has a little functionality removed…but is still quite useful for answering most of your “what is that?” questions. If you pay $10 for the full version–totally worth it if you ask me, given how much work the developer puts into making it better all the time–you get a lot of enhancements like compass support. That lets you point your phone at the sky and have the app slew around to follow you, like the Android app mentioned above. The full version also lets you change the date and time to see what the sky looks like at at another point in time.
I glossed right over that bit…oops & sorry I couldn’t count the hours I’ve spent with Stellarium zooming in/out and trying to keep centered on things really, really far away that zip off screen when viewed ‘live’. Gives great perspective, though.
Kudos, too, for those mentioning the iPhone apps as I was unaware of those. Great!
As several people suggested, Stellarium is a heck of a program. I was looking at Arctarus last night. It’s a heck of a star. I still don’t really understand why it seems to change color but that is what it is.
The 'dope comes through again! Where else in the world can someone just ask random questions like this and have answers within minutes?
No need to do that…if you press the spacebar after selecting an object, Stellarium will center the view on that object and track along with it as it moves.
If the sky is dark enough around where you are to be able to see the “Big Dipper”, try this:
follow the curve of the Big Dipper’s “handle”—if the arc it traces leads to your object, then it is, in fact, the star Arcturus.
For beginning astronomy buffs, the Big Dipper points the way to many interesting constellations and “asterisms”, such as the “pointer” stars in the bowl of the Big Dipper pointing to Polaris, the North Star, in the handle of the Little Dipper.
Stars are so far away that their apparent size is basically zero, so their light only hits one photoreceptor in your eye. Turbulences in the athmosphere distort the light (especially if the star is near the horizon), and the light hits other receptors. Because we have different receptors for blue, green and red, the color you see changes along with the brightness.
Planets are much closer, but also dimmer, so their light hits multiple receptors and they don’t twinkle, even though they seem very similar to stars otherwise.
Another iPhone app that is very cool for identifying stars (which I would try on the twinkly Christmas star, but it’s overcast) is called “Pocket Universe”. Seeing it on a friend’s phone tipped me over in the decision to get an iPhone.
It has nothing to do with your photoreceptors. The varying density of the atmosphere along the path of the star’s light acts something like a weak prism.
BTW, because the light’s path to each of your eyes is different, the appearance of the star to each eye is also different. You can see this by crossing your eyes slightly when you look at a star. This is a major component of the twinkling.
I’ve been watching Arcturus for years, and have always seen the color differences. When I first noticed it, many years ago, I trained my telescope on it, I could see it was a star, just a point. But I also noticed that the top edge was reddish, and the bottom edge was blue/greenish. Always. I’ve always claimed it had to be atmosphere scintillation. Just my observations.
I finally remembered, and it was clear, to go outside and look at the coloured star with my phone’s little astronomy app. The app says it’s Arcturus, too.
I couldn’t count the hours I’ve spent with Stellarium zooming in/out and trying to keep centered on things really, really far away that zip off screen when viewed ‘live’. Gives great perspective, though.
I still don’t really understand why it seems to change color but that is what it is.