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Treebeard's Stumper Answer
11 January 2002

Winter Stars

DMS students have my assignment to go outside at night this week and count how many stars they can see in the "bowtie" of the constellation of Orion. It's been cloudy, but I hope it clears for the dark new moon this weekend. Look southeast after sunset and view the bright planets Jupiter and Saturn flanking mighty Orion, along with the Milky Way and many other brilliant stars and constellations nearby. Summer nights are more comfortable for stargazing, but why does the winter night sky seem so much more dramatic? (I can think of several different reasons!)

I took this picture of Orion during the great Leonid Meteor shower on November 18, 2001. I set up my Olympus 2040Z digital camera on a tripod pointed at Orion, with manual exposure set to the max of f/1.8 @ 16 seconds, hoping to catch something. I took over 100 photos, and I was lucky to get this one keeper.

Note there are two parallel meteor streaks in the original photo. Look for the faint one about half-way up Orions upper-left torso. It's easier to see in the enhanced negative image (middle below). There are better Leonid pictures here.

Digital camera CCD ("charge-coupled device") sensors get "hot pixels" during long exposures. Serious digital astro-photography fans use special CCD sensors and keep them near freezing to minimize digital noise during long exposures. That's not recommended with a commercial digital camera! My original photo was speckled with red, green, and blue hot pixels (left). I used the freeware HotPixel Eliminator program and then used Paint Shop Pro to fiddle with contrast and gamma and histograms to make a "clean" image. But I'm pretty sure some of those "stars" are just enhanced hot pixels that got by the filters. I'll use the freeware star chart program Cartes du Ciel (right) to make sure.

My school assignment is to:

  • Find Orion at about 8:00 P.M. using the January star chart from SkyMaps.com;
  • Sketch what you see;
  • Count the stars within the "bowtie" of the main body of Orion; and
  • Try reading a simple "Distance Visual Acuity (E Game)" eye chart (in the dark) that I found on the Web.
Of course city lights will make a difference. That's the point. I'll report our results in my answer. It would be more difficult to do this assignment in the summer. Why is the winter night sky so much more dramatic?


The winter stars are so dramatic for many reasons. Winter storms clear the air. We're outside in the dark more because of short days. All those bright stars around Orion are our close neighbors, and we're looking away from the center of our galaxy so they stand out. It helps that Jupiter and Saturn are near Orion this year. That will change, but even the moon and planets are higher in the winter sky when the sun is low, because they are on opposite sides of the ecliptic plane. It's a fine view. The more you learn, the more you'll notice all through the year!

Notes:

I've read that there are about 6,000 visible stars in the sky, and you can see about 2,000 of them at a time on a very dark night. But it's hard to get consistent results counting stars, so I don't trust these numbers. It depends on how you look.

It can take hours for your eyes to fully adapt to the dark, and one glance at a bright light can ruin your night vision. Serious star-gazers learn to use averted vision, because you can see many more stars if your look out the corner of your eye instead of looking straight at them. This is because of the way our eyes are made with a light sensing retina of (at least) two different kinds of cells, rods and cones. Cone cells are concentrated in the center of the retina, and are best for sensing colors and fine detail. Rod cells throughout the retina are optimized for peripheral vision and dim light, but they can't sense color. Flashlights are for cone cells, with a small central focus and everything else is black. Turn the light off, and you really can see a lot more at night, but with a different kind of detail. Averted vision makes use of those rod cells.

I think this is what makes the Seven Sisters (aka the Pleiades or M45) so interesting. There are only six bright stars, so the name itself is a stumper. Stare at any of them, and you will catch a glimpse of a "new" star to the side. Shift your gaze to that star and it disappears, but a new one appears somewhere else. The effect is that the seventh sister keeps moving around. This explains some classic myths, like this one I found on the Web:

According to the ancient Greeks, the Pleiades were seven sisters. In Greek, the word "pleiades" means "doves." Their parents were Pleione and Atlas who was condemned by Zeus to support the Heavens on his shoulders. One day, the Pleiades were traveling with their mother and met the hunter Orion.

Orion fell in love with Pleione and her charming daughters. He spent a great deal of time chasing after them, trying to win their affection. After several years, Zeus intervened and transformed the women into doves to help them escape. They flew into the sky to become the cluster of stars that today has their name.

However, only six stars are visible in the sky without a telescope. The ancient Greeks explained the absence of a seventh star with several different stories. According to one story, one of the Pleiades, Merope deserted her sisters because she was ashamed of having a mortal husband, who also happened to be a criminal.

Constantly shifting about out the corner of our eye is a good model of shame. I think many authors (eg here and here) have missed the point. Take a look with binoculars and you will see how many sisters there really are!

The Orion constellation is just as ambiguous. The first two pictures are from the freeware star chart program Cartes du Ciel using different databases. The last picture is a long-exposure photo. So how many stars can you see in Orion?!

Dunn Middle School kids did my assignment last week and counted between 7 and 54 stars. The averages with the eye chart came out as expected. (Line 1 is the biggest, line 4 is small; kids who could only read line 1 counted more stars.) City lights make a difference, but I'm sure some of the variation is also because it's so hard to count those stars we see out the corner of our eye. That's a flaw in any star counting project, but it's still worth doing just to get outside and looking at the universe. We'll try it again next week with a full moon.

Winter stars really are more dramatic, though I think they compete with the late summer view towards the center of the Milky Way with Capricorn and Scorpio. Consider this list of the 25 brightest stars in the Santa Barbara sky, with links to Chris Dolan's pages on The Constellations and their Stars:

Common Name Scientific Name (Abr.) Distance
(light years)
Apparent
Magnitude
Visible in our
winter sky?

Sirius Canis Major (CMa) 8.6 -1.46 Yes
Canopus Carina (Car) 74 -0.72 Maybe (south)
Rigil Kentaurus Centaurus (Cen) 4.3 -0.27 No (south)
Arcturus Bootes (Boo) 34 -0.04 Yes
Vega Lyra (Lyr) 25 0.03 No (summer)
Capella Auriga (Aur) 41 0.08 Yes
Rigel Orion (Ori) ~1400 0.12 Yes
Procyon Canis Minor (CMi) 11.4 0.38 Yes
Achernar Eridanus (Eri) 69 0.46 No (south)
Betelgeuse Orion (Ori) ~1400 0.50 (var.) Yes
Hadar Centaurus (Cen) 320 0.61 (var.) No (south)
Acrux Crux (Cru) 510 0.76 No (south)
Altair Aquila (Aql) 16 0.77 No (summer)
Aldebaran Taurus (Tau) 60 0.85 (var.) Yes
Antares Scopius (Sco) ~520 0.96 (var.) No (summer)
Spica Virgo (Vir) 220 0.98 (var.) Yes
Pollux Gemini (Gem) 40 1.14 Yes
Fomalhaut Piscis Austrinus (PsA) 22 1.16 No (summer)
Becrux Crux (Cru) 460 1.25 (var.) No (south)
Deneb Cygnus (Cyg) 1500 1.25 No (summer)
Regulus Leo (Leo) 69 1.35 Yes
Adhara Canis Major (CMa) 570 1.50 Yes
Castor Gemini (Gem) 49 1.57 Yes
Gacrux Crux (Cru) 120 1.63 (var.) No (south)
Shaula Scorpius (Sco) 330 1.63 (var.) No (summer)

We can see 12 of these brightist stars in winter, compared with only 6 in the summer. Eight of these bright winter stars are less than 100 light years away, so they really are our neighbors, at least as judged with a cosmic yardstick. But it's interesting that many of the closest stars are faint, dimmer than our Sun by factors of 100 to 10,000.

Here are some links for further research on the winter sky:

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Copyright © 2002 by Marc Kummel / mkummel@rain.org