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SYFY WIRE Bad Astronomy

Astrophoto: The Big Dipper over Chile ... barely.

By Phil Plait

One of my favorite types of night-sky photograph is one showing a familiar sight in an unfamiliar way. Maybe an interesting foreground object, or an unusual set of filters to display colors oddly.

Or maybe by taking a long trip across our round ball of a planet and let spherical geometry do the trick for you.

Big Dipper

The photo above is from astrophotographer and friend-of-the-BA-blog Yuri Beletsky. It shows the famous Big Dipper, familiar to anyone who lives in the northern hemisphere. As soon as I saw it, I had to chuckle. I knew Yuri had traveled south of the equator to capture it. How?

Because the two stars at the end of the bowl of the Dipper, called Dubhe and Merak, point along the opening of the Dipper’s bowl toward Polaris, the north star. Polaris enjoys a somewhat unique position in our sky: The Earth’s axis in space points almost right at it. So, as the Earth spins, the star appears nearly motionless in the sky, neither rising nor setting. If you can measure its angle above the horizon, you can determine your latitude: If it’s straight overhead, you’re at the north pole, 90° north latitude. If it’s on the horizon, it’s 0° above the horizon and you’re at 0° latitude.

But look at the pointers. If they point toward Polaris, it must be below the horizon! That’s only possible from the southern hemisphere of the Earth (you can think of it as a negative latitude), and, sure enough, Yuri was in Chile when he took the shot.

It’s beautiful. It’s a testament to the dark skies he had, too: It’s a stack of two images (adding them together helps reduce noise in the image) totaling less than a minute exposure! Yet you can see a lot of stars.

The Big Dipper is more than just a celestial boreal icon. Except for Dubhe and the last star in the tail, called Alkaid, the other stars are all at about the same distance from us, and physically associated with each other. They’re called the Ursa Major Association (or cluster), after the constellation the Big Dipper is part of. Those stars are all about 80 light years from us (Alkaid is about 100, and Dubhe 120). And technically, they don’t form a constellation — the 88 official groupings of stars in the sky — but, instead, are what’s called an asterism, a pattern of stars forming some recognizable shape.

From my house in Colorado, when the Dipper is upside down like this —with the bowl pointing at the horizon — it’s above Polaris and a good 70° over the horizon. I have to crane my head up to see it. In Yuri’s photo though, from Chile, it skims the horizon, and for most of the night it’s below. Yuri chose his site well; the mountains are gorgeous, and the reflection of the Dipper in the water is simply ethereal.

I’ll add that another astrophotographer BA friend, Rogelio Bernal Andreo, also took an amazing shot of the Dipper, which I featured on the blog back in 2014:

Big Dipper

This shot, while not as terrestrially picturesque as Yuri’s, is far deeper, showing much fainter stars and other objects. He even captured incredibly faint dust that exists between the stars, called Integrated Flux Nebulae, or, galactic cirrus. The first term indicates that the dust isn’t illuminated by a single star, but instead feebly reflects the light from the collected stars in the galaxy around it, and the second just means it’s dust in our galaxy that’s feathery. There’s a lot more to see in that shot, so I suggest you read my post about it. Trust me, it’s worth your time.

I love all this. One incredibly deep photo, another shallower in exposure but no less shallow in its beauty, or what it shows us about our world and our sky.

If you are reading this in the late spring and early summer, then may I suggest going outside after sunset, just as the sky is getting fully dark? Face north, and turn your gaze upward. You’ll see the Big Dipper there, faithfully pointing out Polaris (and you can also “follow the arc to Arcturus”; letting your eyes slide down the tail and maintaining the curve to the bright orange star). You may be familiar with it already, but really, just how familiar is enough? There is always more to see, no matter where you search in the sky.