Let’s arc to Arcturus

A few days ago I blew the entire day in a cafe soaking up caffeine, soaking up air conditioning, and soaking up a book that I wanted to finish. It was just getting dark by the time I left. When I was in the parking lot I looked up to see if any stars were out yet. The only one that I could see was the red giant Arcturus.

It was a welcome sight. After being cloistered all day, Arcturus was like a cosmic handshake ushering me back into the real world.

Arcturus will be sliding from prominence as summertime progresses. Right now, though, the star is still visible in Saipan’s evening sky.

As we look for it we can use a saying uttered by many a scoutmaster, mariner, navigator, adventurer, and backyard astronomer: “Arc to Arcturus.”

With this is mind, if you look in the northern sky at 9pm Saipan time today, June 23, you’ll notice that the Big Dipper is standing up-and-down. Its bowl is way down near the horizon, and its handle is pointing up into the air. Yeah, the Big Dipper is doing a big dip.

The two end-stars on the Dipper’s handle, Alkaid and Mizar, point upward to Arcturus. If our eyes follow this line we’ll come to a really bright, reddish star that is about 80 degrees over the horizon in the northwest. This is Arcturus.

If my words don’t do the trick, the Web is full of diagrams that surely will. Of course, the Earth is always in motion, so the whole stellar scene is constantly moving as we perceive it, but a diagram will still fix the positions of the stars as they pertain to each other.

Arcturus is, by far, the brightest star in its patch of sky. That’s why it’s such a famous and widely-used guidepost. Right now, however, there’s a far brighter visitor trucking through the general vicinity: Jupiter.

Since Jupiter is a planet, it’s not fixed in its position relative to Arcturus. Jupiter is just passing through. Right now, Jupiter is closer to another bright star (but not as bright as Arcturus) called Spica; the two will be lower in the sky, and to the south, of Arcturus.

Arcturus is pretty much like our sun’s older brother, so when we look at Arcturus we’re seeing the future of the sun in a few billion years or so. You’ll notice that, while our sun is yellow, Arcturus is red, or at least reddish. That’s because it’s at a later stage in its stellar life. After middle age it bloated in size (just as our sun will) and it cooled down. This cooling results in the red color. The bloating results in more surface area. So although Arcturus and the sun are roughly the same mass, Arcturus is bigger and it emits about 180 times more light than the sun does.

If you’re wondering what’s going to happen to your garden when the sun starts bloating into a red giant billions of years down the road, well, for starters, the growing sun will boil off all the Earth’s oceans. Your garden is a goner. But look on the bright side: We’ll be able to roller skate to Guam and we can throw pennies into the Marianas Trench, taking bets on when we’ll hear them hit bottom.

Stars are giant nuclear reactors. When we look at them we’re seeing the results of their various chemistries, which result in their various burning temperatures and hence their various colors. Armed with some science, our eyes can serve as cosmic nuclear thermometers.

In the array of stars that we see with our bare eyes, as we go from the coolest to the hottest part of the scale, the star colors are red, orange, yellow, white, and then, at the hottest end of things, blue. Arcturus, to my eye, at least, is on the orange side of red, so I just call it reddish.

So how does this translate into actual surface temperatures? In very rough terms, and I’m just eye-balling some numbers off a graph here, we’re dealing with surface temperatures of about 3,000 C for the red stars to around 30,000 C for the bluest ones. Actually, astronomers use the “Kelvin” temperature scale, not Celsius, but that distinction is academic for our context since we just want some general perspective.

More perspective: On the galactic scale of things, Arcturus, at 37 light-years of distance, is very close to us. Incidentally, it’s in the kite-shaped constellation called Bootes, which has apparently never captured my attention because I don’t remember anything about it.

One final note: The phrase “Arc to Arcturus” has a second part, “Spike to Spica.” Spica, as already noted, is hanging out with Jupiter right now. So there’s quite a cosmic trio awaiting anyone who cares to look.

Now that we’ve connected with the cosmos, I’m going to arc myself to a cheeseburger. Then I’ll hit the cafe again so I can spike some more coffee.