This photo looking toward the center of the Milky Way galaxy covers an area about twice as wide as the full Moon and contains more than 180,000 stars. The image captures a portion of our galaxy about 220 by 110 light-years across. Photo courtesy of NASA
Some gorgeous skywatching rotates into life on July and August nights in eastern Maine, where we have some of the darkest skies in North America.
Just a few points of orientation are enough to find your bearings. Your eye can pick out roughly 6,000 individual stars in a black sky. About 300 of the brighter ones have names, like Polaris, Sirius and Vega. The rest are known to astronomers by numbers.
The names by and large are ancient, given mainly by Arab astronomers, and also Greeks and Romans. Altair, for example, which you can see on summer evenings, is called after the tag end of the Arabic phrase al-Nasr al-Tair, the flying eagle, which is applied to the whole constellation we call Aquila, the Latin word for eagle.
The first complete atlas of the sky that systematically identified the bright stars was the Uranometria published in 1603, in which Johann Bayer assigned Greek letters to the stars in each constellation. Altair is Alpha Aquila because it’s the brightest star in the constellation. In the 1700s, John Flamsteed used the brighter stars’ positions, rather than their brightness, to assign numbers.
The Bayer and Flamsteed designations are still basic identifiers today. And millions that were too faint for the early modern astronomers to spot, but that can be seen through our much more powerful telescopes, are now cataloged by number in compendiums like the Yale Bright Star Catalogue, the Smithsonian Astrophysical Observatory Star Catalog, and the Henry Draper Catalogue. Hundreds of billions more in our galaxy are unnamed.
An odd thing that happens after a few excursions among the visible 6,000 is that you begin to notice not all stars are alike. They differ the way snowflakes differ, or human fingerprints or irises. Each one has its own sort of character or personality. The astronomers from ancient times to now have spent a lot of time sorting out these differences, although nowadays they generally don’t use words like “personality” to describe a star’s qualities. They use phrases such as “spectral class,” “proper motion,” “luminosity” and “magnitude.”
You see just by looking at them, for example, that Vega is brighter than Polaris. Polaris is brighter than Thuban, which is just a skip from the edge of the bowl of the Little Dipper. The scale for describing the brightness of stars, which has been in use pretty much intact since ancient times, is called “magnitude.” The brighter a star appears, the lower its magnitude.
This seems counterintuitive, in a way, because you’d think the brighter the star, the higher its magnitude number would be. But the originators of the system centuries ago referred to the brightest stars as being of the first magnitude, the next brightest stars were of the second magnitude, and so on. So when numbers started being assigned, 1 was brightest, 2 was less bright, 3 less bright still, and so on into the telescope-only ranges: 7, 8, 9 and on. So the brightest star in the sky, Sirius, has a magnitude of minus 1.46. Vega, the fifth-brightest star in the sky, has a magnitude of .03. Polaris has a magnitude of 2.02, Thuban 3.65. Just about anybody can learn to estimate star magnitudes by eye, as it were, and binoculars.
Your eye can also pick up star colors. Betelgeuse atop Orion’s belt is noticeably red, and so is Arcturus. Rigel has a bluish tinge. Sirius seems to sparkle iridescently.
Color and brightness alone are enough to make meaningful distinctions, as the naked-eye astronomers (all of them before about 1610, when Galileo turned one of the first magnifying lenses on the sky) well knew and depicted. Modern astronomers, of course, have discovered a lot more star distinctions. As telescopes grew more powerful, the measurements of star positions became more precise, and it was noticed that while the stars seem fixed in place in their daily and annual motions across the sky, they’re not: Each individual star is a traveler. It has a “radial velocity,” which is a measure of its speed toward or away from us here on Earth, and a “proper motion,” which is a measure of its apparent annual change of position. All the stars are orbiting around the center of the galaxy; our solar system is thought to make one orbit in about 220 million to 250 million years.
These measurements are calculated from kaleidoscopic amounts of detail gathered from starlight. The fundamental method is to focus the light of a single star into a spectrometer, which splits the light into its component colors. To vastly oversimplify, the astronomers have figured out what the arrangements and intensities of the colors indicate about a star’s age, chemical makeup, absolute magnitude (as opposed to the apparent magnitude we perceive here on Earth), distance, output of energy, direction and speed of travel, and so on, including whether it has any close companion stars or planets.
Our own sun is a star of medium size and brightness, as stars go, classified as a dwarf star of spectral class G2. Because we’re so close to it, the sun’s apparent magnitude is figured at minus 26.7, far too bright to look at directly; its absolute magnitude is 4.83. Betelgeuse is an M1 red giant, 160 million times the sun’s volume (though just 20 times the sun’s mass), with an absolute magnitude of minus 5.6. Rigel is a B8 white supergiant with an absolute magnitude of minus 7.1.
You could spend a lifetime sorting out information on one star at a time, synthesizing the astronomers’ facts with your own night vision, and probably not make your way through the visible 6,000. Because what happens, at points, is that particular stars get your attention and you return to them, over and over. The more you look at them, and the more you absorb the starlight data, the more you see. And don’t see, as it were, through your glass, darkly.
Dana Wilde lives in Troy. You can contact him at naturalist1@dwildepress.net. His book about the stars and planets is “Nebulae: A Backyard Cosmography” available in print electronic editions from online book sellers. Backyard Naturalist appears the second and fourth Thursdays each month.
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