Despite the abundance of faint stars and rather dim constellations, the beautiful Milky Way and several attractive star clusters make for lovely sights on a midautumn night. And with a bright moon out of the way, this is the week to go exploring with binoculars or a small telescope.
Crisp, cold fall evenings, like the ones the northeast and midwestern United States experienced last weekend, provide good opportunities to explore the celestial sights of midautumn. These include the Milky Way galaxy, which spans the sky from west to east this week at around 8:30 p.m. local time, passing directly overhead. The moon is currently confined to the predawn morning hours and is a diminishing ("waning") crescent, which should allow viewers far from city lights to get a good glimpse of this broad, almost tattered band of faint light.
The bright star clouds in Sagittarius that mark the center of our Milky Way galaxy have now sunk below the southwestern horizon, but in the west, the portion of the Milky Way that runs through the constellations of Cygnus (the swan) and Aquila (the eagle) is relatively conspicuous. [How to Photograph the Milky Way in Light Pollution (Photos)]
In living color
While primarily a summertime star pattern, Cygnus, also known as the Northern Cross, can now be found nearly overhead soon after darkness falls and is still halfway up in the western sky by around 9 p.m. Within its boundaries is one of the most beautiful double stars, Albireo, located in the swan's beak, or foot of the cross. A good pair of 7-power binoculars should easily resolve it into two stars that exhibit a beautiful color contrast of diamond-blue and orange. That binocular view is proof for those who doubt that stars show color; so many people insist they are white and nothing else. Certainly, star colors are not easy to see, chiefly because our eyes' color sensors — the cones of the retina — are quite insensitive to dim light. At night, the rods take over, but they are effectively color-blind. Only the brightest stars can excite the cones, unless you use binoculars or a telescope to intensify a star's light. Color perception is aided further by the close juxtaposition of a contrasting pair of stars, as in Albireo.
Continuing up the western sky, and then over toward the east, the Milky Way becomes dimmer and narrower as it crosses a corner of Cepheus (the king) and the famous W figure of Cassiopeia (the queen). Farther to the east, the diffuse girdle of light remains faint as it extends through Perseus (the hero), the southern part of Auriga (the charioteer) and across portions of Taurus (the bull) and Gemini (the twins). The 2nd-magnitude star El Nath marks the approximate anti-center of our galaxy — that is, the sky point diametrically opposite to the galactic center in Sagittarius. If we could imagine the Milky Way as a sprawling metropolitan area, we are looking "in" toward Sagittarius, which could be considered "downtown" and accounts for why the Milky Way is so bright. In contrast, when we look in the direction of El Nath, we are looking "out" toward the rural "suburbs"; the anti-center half of the Milky Way looks fainter because we are looking toward the outermost fringes of our galactic system rather than toward the star-rich middle. [Best Night Sky Events of November 2017 (Stargazing Maps)]
The November Milky Way is notable for the many open star clusters in and near it that invite inspection in binoculars. This week, you should be able to find a hazy patch of light in the Milky Way, high in the northeast between Cassiopeia and Perseus. There are actually two concentrations here, called h Persei and Chi Persei, more popularly known as the Double Cluster in Perseus. The ancient astronomers Hipparchus and Ptolemy mentioned it as far back as 150 B.C., referring to it as a cloudy spot. With his crude telescope, Galileo first noted they were really clusters of stars. They contain very massive blue stars and some of the brightest red stars known. The blue stars are extremely hot, on the order of 100,000 degrees Fahrenheit (55,500 degrees Celsius), blazing out energy so fast that they cannot keep it up for more than 10 million years, changing to red as they age. H Persei is 6,800 light-years away, with Chi 800 light-years beyond. Together, they appear larger than the apparent width of the full moon and are exquisite sights in binoculars or a low-power telescope.
About halfway up in the northeast sky in the mid-evening hours, the constellation Auriga can be found. Its highlights include yellowish Capella, the second-brightest star currently above the horizon, and a row of three clusters of stars that are readily visible with only the slightest optical aid. Messier 36, Messier 37, and Messier 38 look quite similar to each other in binoculars — hazy gray glows lurking in a field rich with tiny stars. A more careful look reveals differences among them. Messier 36, in the middle of the row, is smaller and more concentrated than the other two. Messier 37 and Messier 38 are about equal in size, but Messier 37 is brighter. All three clusters are roughly the same distance from the sun, in the range of 4,000 to 4,500 light-years. Their estimated ages, however, differ widely: 25 million years for Messier 36, 220 million for Messier 38 and 300 million for Messier 37. Another binocular highlight nearby is a pattern known to some as the Leaping Minnow, a little group of 5th- and 6th-magnitude stars.
For a similar — albeit larger and brighter — cluster, check out nearby Messier 35 in Gemini. It's easy to locate by virtue of its closeness to the 3rd-magnitude star Eta Geminorum, sometimes known as Propus. [Seeing the Treasures of Messier's List with Mobile Astronomy Apps]
The Hyades and Pleiades
The last two open clusters on our tour are both in Taurus, to the north of the Milky Way. The first is the Hyades, marking the famous V shape of the bull's face. A cluster of this type is called an open cluster, because it has no obvious organization or symmetry. The stars move together through space like a family on a hike, seemingly going both across the sky and away. The paths of the stars converge on a "vanishing point" far to the east, near the bright star Procyon in Canis Minor (the little dog).
The resulting geometry allows us to determine the cluster's distance with some accuracy — 151 light-years, which means that its age is about 625 million years. Higher in the sky, the Pleiades, popularly called the Seven Sisters, resemble a little dipper. About 400 light-years away and some 20 light-years across, the group may be no older than 20 million years and contains hundreds of stars. Several stars in the cluster seem to be enveloped in clouds of dust, perhaps left over from the stuff from which they formed.
Depending on sky conditions and light pollution, most people can see between four and six naked-eye Pleiads. Some people with more acute vision can count many more. One such person is Allen Seltzer, who claims to have seen as many as 19 Pleiads with his unaided eyes while observing under pristinely dark skies from rural Arizona. Seltzer, who, four decades ago, served as education director at New York's Hayden Planetarium, is blessed with unusually keen vision, which he once demonstrated to me by reading The New York Times from across a nearly 20-foot room!
A word about binoculars
Low-power binoculars (up to 7 x 50's), such as those that would be used for these views of open clusters, make a very good first instrument for amateur observers. In my nearly 16 years of writing astronomy columns for Space.com, I have said on many occasions that, while some might think binoculars are a bit of a downgrade from a telescope, for certain aspects of sky watching, they are the best instrument to use. [The Best Binoculars for Earth and Sky]
Binoculars are easy to handle and relatively inexpensive, and they are helpful for learning the constellations and getting acquainted with many attractive deep-sky objects. And when held steadily, they also give you a glimpse of the craters of the moon, the crescent of Venus and the moons of Jupiter. Plus, because they don't give an inverted (upside-down) view, binoculars are convenient for comparing a sky map with the stars themselves. The practical experience they give will enable beginners to get more satisfaction from their first telescope. Of course, you'll want to consider all these things as we approach the holiday gift-giving season.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmer's Almanac and other publications, and he is also an on-camera meteorologist for Verizon Fios1 News, based in Rye Brook, New York. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.
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Joe Rao is Space.com's skywatching columnist, as well as a veteran meteorologist and eclipse chaser who also serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications. Joe is an 8-time Emmy-nominated meteorologist who served the Putnam Valley region of New York for over 21 years. You can find him on Twitter and YouTube tracking lunar and solar eclipses, meteor showers and more. To find out Joe's latest project, visit him on Twitter.