If you enjoy Jeff's monthly star blog, you'll love Stelliferous Live! 
The Director of our Bishop Planetarium will present a live star talk followed by a discussion on topics to be covered this blog. Please join Jeff on the 4th Wednesday of most months for this new program.

Upcoming Events:
Wednesday, April 25, 7 p.m.
Wednesday, May 23, 7 p.m.

Stelliferous Live! events are free to attend, but donations are appreciated.
Please RSVP to Amara C. Nash: 941-746-4131, ext. 11 or
click here to register online for the Feb. 22 event.
 

May 2012

If you look toward the eastern horizon after sunset you’ll see Arcturus. I know that you’ll see it because it will practically be jumping up and down, waving its arms to get your attention. Well, maybe it won’t be doing jumping jacks, but it will be twinkling, or scintillating, like mad. When a bright star is near the horizon its light is refracted by gas in our atmosphere, so we see the light broken up as if by a prism, flashing from blue to green to red.

This effect surprises a lot of people, mainly I think because most people don’t think much about the molecules of gas in our atmosphere. Air seems so insubstantial, after all. But oh, it’s there…

Let’s take a broader perspective.

You live in what we like to call The Universe. That may be a bit “–centric”, since possibility of other universes exists. But we won’t worry about that at the moment. Our universe, even if we just deal with the part that we can see, gives us plenty of room to work with. The part that we can see is what we call the Observable Universe. You can imagine the Observable Universe as a sphere with a diameter of 28 billion light-years, or 150,000,000,000,000,000,000,000,000 (150 septillion) miles (remember, a light-year is a measure of distance equal to about six trillion miles). Within that sphere there’s a lot of stuff , like the 150 billion or so galaxies we’ve counted so far, for instance. But overall it’s a pretty empty place, with an average density of 0.0000000000000000000000000003 grams per cubic centimeter.

But that’s an average, and the matter in our universe isn’t spread out evenly. Gravity clumps matter together. Galaxies are clumps of billions of stars, with some gas and dust in the Interstellar Medium, the space in between stars. In the Interstellar Medium, we find on average one half of an atom per cubic centimeter. Sparse, but downright crowded compared to the universe as a whole.

We happen to live in a slightly unusual part of the Milky Way galaxy. We live in one of the spiral arms of the galaxy. But within that arm we find ourselves in the Local Bubble, a sphere some 300 light-years in diameter that is a bit emptier than the surrounding parts of the arm. In the Local Bubble, there are only 0.05 atoms per cubic centimeter of space. Where’d the other stuff go? Best guess is that a supernova explosion many millions of years ago blew our local stuff away.

But, within the Local Bubble, we find our sun and some neighboring stars inside the Local Fluff (properly – the Local Interstellar Cloud), a cloud inside the Bubble that’s about 30 light-years in diameter. The Local Fluff has a density of about 0.1 atoms per cubic centimeter of space.

So why do I bother with all of these examples of how empty the space between stars in our galaxy can be? Well, consider that in Earth’s atmosphere there are 27,000,000,000,000,000,000 (27 quintillion) atoms packed into every cubic centimeter. Who’s insubstantial now?

It’s these atoms that cause starlight to scintillate. Imagine that you are a photon of light from Arcturus travelling through the mostly empty space between the stars of our galaxy, on your way toward Earth. It’s a 36 year trip, and it’s pretty smooth sailing until you run into our atmosphere. Then you are bumped and deflected and jostled about everywhere you turn. That’s how the atmosphere works like a prism.

OK, so why do stars low on the horizon scintillate more? The starlight has to travel through more atmosphere to get to your eyes. In the process of running this longer gauntlet, the starlight is simply jostled more.

So go out early tonight and find the Big Dipper. Follow the sweeping curve of the Dipper’s Handle and it’ll bring your right to Arcturus (follow the arc to Arcturus). Take a few minutes to watch. I promise you that it’ll put on a good show.

A note about star charts: If you are looking at this star chart, you may have noticed that EAST and WEST are reversed, compared to a regular map. That’s because the chart shows the sky above your head. Imagine picking up your computer monitor (don’t really pick it up, please) and holding it up over your head. North would still be north, South would still be south, but now East and West are in their proper places. Don't worry, you'll get the hang of it in no time.