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by Jerry Oltion



In their last Science column (in the November/December 2017 issue), Pat Murphy and Paul Doherty wrote about dark adaptation, how the eye adjusts over time to low light levels and enables you to see objects that are thousands of times dimmer than what you perceive by daylight. They ended with the exhortation to go out at night and use your night vision to explore the nocturnal world you seldom see.

I'd like to expand on that with an exhortation of my own: While you're out there, look up!

I've been an amateur astronomer for many years. The lure of the night sky keeps me out late on clear nights, to the point where the stars are more familiar to me than my own neighborhood. I take a telescope with me when I go out, but I often just look at the sky with my naked eyes and take in the amazing glory of it all without enhancement. There's plenty up there to see.


"My God, It's Full of Stars"

The first thing you'll notice when you look up at night with dark-adapted eyes, assuming it's not cloudy, is stars. How many you see depends on where you live, not because the stars are different over different parts of the world but because the sky over most cities is so polluted with stray light. If you live east of the Mississippi or anywhere in Europe there are precious few areas where you can see truly dark night sky, and even in the American West it's becoming a scarce commodity. If you live in a big city, only a couple dozen of the brightest stars might make it through the glare.

Maximize your chances of seeing stars by finding the darkest area you can. Get as far from street lights, yard lights, headlights, etc. as possible. On a moonlit night, put the Moon behind a tree or a building. Let your eyes adapt to the dark, which will take about five minutes for the first stage and half an hour for full adaptation.

Stay safe while you're doing this. Depending on the circumstances, go out in a group or to a place you know is not risky. Carry a flashlight; just don't use it unless you need it! White light will knock out your night vision and you'll have to start all over. (Likewise, don't use your phone unless you need to, and turn down the screen brightness if you do.)

As your eyes adapt to the dark, you will see more and more stars. The first thing you'll notice about them is that they're twinkling. And the closer to the horizon they are, the more they twinkle. Why is that? It's because the air is in constant motion, with warm currents rising off rooftops, parking lots, fields, rock outcrops, and so on. Anything that stored up heat during the day will be releasing it at night, and as the warm air rises and cool air falls to take its place, the boundaries between these "cells" of air create rough prisms that deflect the light just enough to shift the star's apparent position. The air is in constant motion, so these boundary prisms are constantly shifting around, which makes the stars shift around, too. It's a neat effect, and often produces flashes of color with the brightest stars.

This time of year, look for the bright star Sirius rising in the southeast. On a turbulent night Sirius will flash like a police light.



Next, look for patterns. The stars aren't evenly distributed. There are clumps of bright ones and gaps in between them. The clumps often create patterns that suggest familiar shapes—and thus constellations are born.

People have grouped the stars into hundreds of constellations over the millennia. In 1922 the International Astronomical Union divided the sky into the 88 most common or most obvious patterns, and standardized their boundaries. You've probably heard of at least some of them: Orion, Ursa Major (a.k.a. the Big Dipper), Camelopardalis. Okay, I'm kidding. Even I can't find Camelopardalis without a map. Fortunately there are plenty of star maps, and it's great fun to trace out the stick figures of the legendary luminaries like Perseus and Andromeda and Cassiopeia.*

If you're into astrology, you can look for the twelve constellations of the zodiac. Well, you can look for the ones that are up this time of year. Pisces is setting over in the west, with Aries, Taurus, Gemini, Cancer, and Leo strung out across the sky from west to east. Don't expect Pisces to look like a pair of fish or Aries a ram, but Taurus does kind of look like the head of a bull and Gemini definitely looks like two stick-figure people side by side. And below and between them, Orion the mighty hunter really looks like...well, like a butterfly. It's about to land on Lepus, the (yeah, maybe it looks a little like a) rabbit.

The cool thing about pareidolia, the tendency to see patterns in random data, is that your patterns don't necessarily match other people's patterns. So make up your own constellations! You'll remember them much better than the classical ones. I've always seen Lyra (a summer constellation) as a dog, not a musical instrument. A Scottie dog, to be precise, jumping up to sniff someone's crotch. That'll be the last constellation I forget when I grow old and senile.


Star Clusters

Some groups of stars aren't random. Some groups are actual groups. There are two obvious ones in the spring sky: The Hyades and the Pleiades. The Hyades is the big scattering of stars that makes up the head of Taurus, the bull. You'll notice way more bright stars in that fist-sized patch of sky than elsewhere. That's because it's a real group of stars that were formed together and have stayed close to one another ever since, and are currently very close to us (only 150 light-years away) as we all whirl around the center of the galaxy together. Ironically, the brightest of the lot, reddish Aldebaran, is not part of the cluster. It's a foreground star that's even closer to us (67 light years).

To the upper right of the Hyades is another beautiful cluster. The Pleiades is smaller and dimmer because it's farther away (440 light years), but it's stars are actually brighter than the Hyades. They're also younger, burning hotter and bluer. To the naked eye under reasonably dark sky, they look distinctly actinic.

The Pleiades are also known as the Seven Sisters because you can supposedly see seven distinct stars without optical aid. That depends on how sharp your vision is. Some people just see a blur of stars, while some can pick out ten or eleven. Almost nobody sees seven, but most people see six distinctly and can tell that there's more, so that's probably where the idea of seven sisters came from.

If you drive a Subaru, check out the hood emblem: the Pleiades! Sure enough, "Subaru" is what this cluster is called in Japan.


Seeing Double

For a classic test of vision, try Mizar and Alcor. Mizar is the middle star in the handle of the Big Dipper, where the handle kinks. Look carefully at this star, and if you have sharp eyes you'll see a dimmer companion very close to the bright one. It helps to look slightly to the side so you can get the rods instead of the cones of your eyes involved. They're more sensitive to dim light, and Alcor will stand out a little better that way. Legend has it that whether or not you could split Mizar and Alcor was a test of visual acuity in the Roman army. If you could see Alcor, you could be an archer; otherwise you were infantry.


The River of Light

If you have dark enough sky, you might be able to see the Milky Way, the misty cloud of light that stretches across the sky from southeast to northwest. It's not a cloud, though; it's the combined light of billions of stars so distant and so numerous that you can't make out individuals. Why are they confined to a narrow band across the sky? Because they're part of a thin disk-shaped galaxy, an enormous cluster of stars shaped like a pancake seen edge on. We're part of that pancake, so we see it all around us, but it's thicker in one direction than the other because we're looking toward the dense center of the disk in the Sagittarius direction and out toward the periphery in the Orion direction.

You'll notice that the Milky Way isn't evenly illuminated even on the small scale. It's mottled, with big dark gaps and bright knots in it. That's because the galaxy is also filled with dark lanes of interstellar gas and dust that block out the starlight in certain directions, leaving the galaxy to shine through the gaps.


How Far Can You See?

Almost everything in the night sky is part of our own galaxy, which is about 100,000 light-years across. Anything farther away suffers from two factors: The more distant a light source is, the dimmer it looks and the smaller it looks. The universe is full of galaxies (at least as many billions of them as there are stars in our own galaxy), but most of them are way too far away to be visible to the naked eye. There are two conspicuous exceptions to that rule: the Large and Small Magellanic Clouds. Those are satellite galaxies to our own, and are only a couple hundred thousand light-years away. Both are easily visible as faint patches of light, but alas, they're only visible in the southern hemisphere. If you live very far north of the equator, you're out of luck. There's just one galaxy beyond our own that you can see from the northern hemisphere, and that one is just at the edge of visibility under a good, dark sky.

The Andromeda Galaxy is a near-twin to the Milky Way. Both are great spiral swirls of stars, flat as pancakes, with bright cores and dust lanes outlining their spiral arms. Andromeda is tilted about thirteen degrees from our line of sight, so we see it as a big oval rather than a straight line. If it were closer to us, it would be as bright or brighter than the Milky Way, but it's far enough away to be one of the dimmest objects we can see by naked eye. You'll need dark sky, well dark-adapted eyes, and a star chart to spot it. When you do, you'll just see a dim but surprisingly large smudge of gray. Remember to avert your vision a few degrees off to the side to engage the more sensitive part of your eyes.

How far is it? 2.5 million light- years. The light you're seeing left the Andromeda Galaxy about the time humanity was getting its start. That's how far you can see by naked eye: all the way to the beginning of human history.

Your children won't be able to see quite as far. Why not? Most everything in the universe is moving away from everything else due to the expansion of space that's still going on after the Big Bang, but the Andromeda Galaxy is moving toward us. So next year it will be about two billion miles closer. In about four billion years, it will collide with the Milky Way and…slip right through. No kidding. Despite their hundreds of billions of stars, galaxies are mostly empty space. Stars are tiny specks separated by vast amounts of nothing. When galaxies collide, which they frequently do, their mutual gravitation distorts the heck out of their shapes, but stars almost never hit each other. The Milky Way and the Andromeda Galaxy will pass through one another, reverse course and pass through one another again, and will finally settle into one enormous galaxy that we've already named: Milkomeda.

It's surprising how much and how far we can see when we turn off the lights and let our eyes do what they do best. Give it a try. There's a whole universe out there just waiting for you to discover it.


Jerry Oltion has been a science nut since he was old enough to spell "curious." He has written science fiction almost as long, and has done astronomy somewhat less. He writes a regular column on amateur telescope making for Sky & Telescope magazine, and spends many, many nights a year out under the stars.

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