Jul 01

What’s Up in the Sky

What’s Up in the Sky – July, 2013

Stars, Of Course

I feel sorry for stars. After all, when it comes to astronomy, they should be the stars of the show, so to speak. When people see me loading my telescope into the car they will often ask if I’m going out looking at the stars. In fact many amateur astronomers are affectionately referred to as “stargazers”.

But the truth of the matter is we hardly ever use the telescope to look at individual stars. We observe star clusters, exploded stars, and double stars, but through a telescope, most stars look the same with slight variations in color and vast differences in brightness, both of which can be seen with the naked eye.

That’s why I think the fun of looking at the stars lies in knowing what you are looking at. And this month’s sky has quite the variety.

There are a number of internet videos comparing the sizes of stars, but that’s just the beginning. For example, facing east, locate the Summer Triangle formed by the stars Vega, Deneb and Altair Deneb, the star farthest to the north (left), is sixty thousand times as luminous as the sun, which is why it’s a good thing it’s sixteen hundred light years away. In fact, its enormous intrinsic brightness makes it one of the most remote stars visible to the naked eye.

The star at the top of the triangle, Vega, appears to be the brightest, but is only about sixty times as luminous as the Sun. It appears bright because it’s close – a mere 25 light years away. So there’s your lesson in apparent vs absolute brightness:: Nearby Vega only appears bright but far distant Deneb is absolutely a thousand times brighter.

Deneb is also very large, about two hundred times the diameter of the sun. But the biggest star in our sky is Antares, in the constellation Scorpius. With a diameter almost nine hundred times that of the Sun, if it were located at the center of our solar system, its outer surface would extend beyond the orbit of Mars!

Remember last month’s brightest star, Arcturus, the trivia question answer? Though not as big as Antares, it is an orange giant whose color can easily be seen. To the naked eye it appears slightly reddish, but if you view it through a telescope slightly out of focus, its color is more apparent.

Arcturus has a large proper motion, or sideways drift across the sky. And it is not moving in the general direction of most stars in the disc of our galaxy, but is actually cutting perpendicularly through it.
So you see, this month there are more than just fireworks up in the sky.

This month in history:
July 4: Deep impact collides with comet – 2005
July 7: Mars rover, Opportunity is launched – 2003
July 10: First transatlantic TV signals made possible by launch of Telstar – 1962
July 18: India launches its own satellite, becoming the seventh nation to do so – 1980
July 20: Humans walk on Moon for the first time – 1969
July 28: First photo of total solar eclipse – 1851