We are now at the midpoint of winter in the Northern Hemisphere, and on a clear and cold night the sky is especially transparent, and the faintest stars abound in great profusion. Careful skywatchers can pick out dim borderline stars and nebulas that cannot be seen at other seasons when the air is less clear.
Astro Bob: How to see the winter Milky Way
This wide-angle photo captures the southern half of the winter Milky Way from Sirius (bottom) to the Pleiades in Taurus (center right). (Bob King)
January 31, 2021 at 2:56 PM
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We often associate the Milky Way with summer nights, but it’s also present in winter. Many people don’t get to see the “colder half” because it’s not as bright, and chilly temperatures make us think twice about nighttime outings. But since we’re all pretty much used to the cold by February, now seems like a good time to leave the nest.
As the Earth revolves around the sun we face a different in space at different times of year. For example, in December and January, we look toward Orion at nightfall, but in May and June, Scorpius dominates. Likewise in summer we face toward the center of the Milky Way galaxy, while in winter the center is to our backs. The Earth’s orbital motion causes the stars in the eastern sky to rise four minutes earlier each night and those in the west to set four minutes earlier. Over days and weeks, the minutes accumulate, and the stars slowly drift westward. (Bob King)
As the Earth orbits the sun we face in different directions along the way just like a kid sees the carnival scenery change as she circles round and round on a carousel. At nightfall during the summer months, we face toward the central regions of the galaxy where the stars are more richly concentrated. In winter, we face the opposite direction toward the edge, where the stars thin out, and the galaxy gives way to a gaping emptiness lit only by the occasional rogue sun.
What exactly is the Milky Way? There are two definitions. The one we’re most familiar with is the hazy band of light that cuts an angled path across the constellations. To the unaided eye it looks like fog or mist. Only through binoculars or a telescope does its true nature as a carpet of stars become apparent.
This model of the Milky Way Galaxy is based upon decades of observations made by professional astronomers and data gathered from spacecraft. We live in a spiral galaxy with a prominent bulge of stars at its center called a bar. The sun and solar system are located about halfway between the galaxy’s center and edge. At nightfall during the northern hemisphere summer, we face inward toward the galactic center and outward during the winter. (NASA / JPL-Caltech / ESO / R. Hurt)
The Milky Way is also the name of the entire galaxy, the one we call home. The foggy band, along with every star in the sky including the sun, are part of it. Additionally, it contains billions of planets and hundreds of star clusters and glowing stellar nurseries called nebulae all packaged into an enormous, flattened disk about 100,000 light-years across.
Viewed from the side, the Milky Way looks something like a pizza but with a denser bulge of stars at its center (extra sausage?). If we could see the galaxy from above we’d be able to admire its multiple spiral arms that unwind from the bulge like octopus arms.
In this closer view we get a better look at the solar neighborhood and the names and locations of several spiral arms. (ESO with additions by the author)
Because the solar system orbits around the center of the galaxy inside the disk we can’t distinguish the arms directly. To map their shapes and distances astronomers use optical, infrared and radio telescopes equipped with spectrographs and cameras to determine the motions of stars and gas clouds across the Milky Way.
At left is a face-on view of the Milky Way showing its spiral shape. At right is the edge-on view with the sun’s position shown. When we look through the flat disk of the galaxy, the stars pile up across the distance to form the fuzzy band of light of the Milky Way. When we look above or below the disk the number of stars along our line of sight drops dramatically; instead of a band, we see random stars scattered across the sky. (NASA / Richard Powell with additions by the author)
The solar system is located about 27,000 light-years or halfway between the galaxy’s center and edge inside the Orion Arm, a minor spiral feature about 3,500 light-years wide and 10,000 light-years long. When we look directly through the disk of the galaxy, where the stars are most concentrated, they pile up across the light-years to form the familiar Milky Way band.
The difference in brightness and structure between the winter and summer Milky Way is obvious in these side-by-side photos. On winter nights, the Earth faces in the direction of Taurus and Auriga (left photo, top), while in summer we face toward Sagittarius and Scorpius (lower right). (Bob King)
The summer Milky Way band is comprised of stars within the spiral arms that wrap between us and the center. In winter, we basically turn our back on the center and face outward, where the stars thin out with distance. For this reason the Milky Way looks fainter and not as broad.
Free-floating in space without the Earth in the way, the Milky Way would encircle the entire sky, wrapping above and below us. (Stellarium)
Despite appearing as separate, isolated bands, the summer and winter Milky Ways are actually halves of a great circle. If we could depart the Earth and go for a spacewalk, say on a journey to the moon, we’d discover that the two bands join together to form a complete circle of hazy light, brighter and wider in the direction of the center and thinner and fainter in the anti-center direction. We can never see the complete circle from the Earth because it’s cut off by the horizon. In space there is no horizon. How liberating!
This is an all-sky view of the winter Milky Way which extends from the southeastern horizon through the zenith (highest point in the sky) and down to the northwestern horizon. Directions are indicated around the circle. Notice that the section from Auriga through Perseus is fainter than the two ends. (Stellarium)
The next two weeks, from Jan. 31 to Feb. 14, are ideal for seeing the winter half. Best viewing time is from 8 p.m. to 11 p.m. You only need a clear night and a dark sky. To help you find a good sky nearest your home, go to this Light Pollution Map and use your mouse’s drag and scroll functions to locate and zoom in on your city. Heavily light-polluted areas are colored red, purple and yellow. Darker regions are blue. Find a location within a reasonable distance and take flight on the next clear night.
The southern half of the winter Milky Way is easy to see from a dark sky. Let the trio of stars in Orion’s Belt be your guide. (Stellarium)
If you’re uncomfortable going alone bring a friend. Once you’ve arrived at a suitable spot (for me, that’s usually a dirt road), allow your eyes 10-15 minutes to adapt to the darkness.. Wow! What a view, right? The section from Sirius through Orion and Gemini is probably the most obvious. Above Gemini, the Milky Way narrows and fades as it climbs to the top of the sky. If you turn around and face northwest you’ll see that it brightens again in the W of Cassiopeia on its way to the Northern Cross at the western horizon.
Breathe in the cold air, lean back and fall into the galaxy’s thrall.
“Astro” Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob .
Messier 35
This bright open star cluster is beautiful through small binoculars and spectacular in a small telescope. It is located in the constellation Gemini, the twins, positioned above and to the right of the third-magnitude star Propus, off the trailing left foot of Castor. This week it stands high in the southern sky at around 9 p.m. local time. It is visible with the unaided eye under favorable dark sky conditions.
Garrett P. Serviss, who was a popularizer of astronomy at the turn of the 20th century, wrote in his book “Astronomy with an Opera Glass” that to the naked eye, “It is a nebulous speck.” The 19th-century English merchant and astronomer, William Lassell, said that this cluster is “a marvelously striking object. No one can see it for the first time without an exclamation,” according to “New Handbook of the Heavens” (McGraw Hill, 1948).
In low-power binoculars it may look like a dim, fairly large unresolved interstellar cloud, but look again. Even through light-polluted suburban skies, 7-power binoculars reveal at least a half dozen of the cluster’s brightest stars against the whitish glow of about 200 fainter ones, whose stars appear in curving rows, reminiscent of the bursting of a skyrocket.
While here, also look for NGC 2158, a smaller, fainter open cluster on the southwest edge of M35. It appears in small telescopes as a circular smudge, which has fooled many (like me) into thinking that they might have discovered a new comet. It lies about 16,000 light-years away from Earth — five times more distant than M35.
Messier 44
Situated in the middle of the dull constellation of Cancer, the crab appears a hazy patch of light to the naked eye. But it becomes a large and beautiful scattered cluster of stars in binoculars. This week, at 9 p.m. local time, it can be found halfway up in the east-southeast sky; highest in the southern sky before midnight.
Messier 44, or M44, is one of the sky’s finest open clusters, easy to see through binoculars even from most cities and with the naked eye from dark locations. Some sharp-eyed viewers can almost resolve it with just their eyes alone. If you’re blessed with a night that is dark and transparent you can see M44 with the naked eye as a dim patch of subtle light. But the slightest haze in the sky will wipe it out. Before light pollution existed, the ancients regarded M44’s disappearance as foretelling the approach of rain, according to “Burnham’s Celestial Handbook” (Dover, 1978).
Binoculars are ideal for examining this wide, interesting star field. To find it, trace an imaginary line from Sirius (the brightest star) to its upper left through the star Procyon. With binoculars, slowly continue along this line and eventually you should run across this cluster of stars.
Some astronomy texts refer to M44 as “Praesepe, the manger,” while others call it the “Beehive” cluster. A manger is defined as “a trough in which feed for donkeys is placed.” The cluster was apparently first called Praesepe 20 centuries ago; ancient Greek and Roman astronomers depicted two nearby stars of Cancer, Asellus Borealis and Asellus Australis — the northern and southern donkeys — as feeding from a manger, according to the astronomy book “A Year of the Stars” (Prometheus, 2003). Galileo first resolved the cluster into stars in 1610.
More than 200 stars can be seen in M44 using binoculars or a telescope. It contains a few orange stars and many bluish-white and yellow stars. The cluster’s relatively “new” moniker — “Beehive” — apparently evolved thanks to its central group of stars so arranged into rough rows that form a generally triangular shape, suggesting a beehive.
Double Star Cluster in Perseus
You might call this cluster “Beehive squared.” This week, at 7 p.m. local time, look to the northwest and about two-thirds of the way from the horizon to the point overhead, you’ll see the familiar zigzag row of five stars marking the constellation of Cassiopeia, the queen. Now going from top to bottom, the second and third stars are, respectively, Ruchbah and Gamma Cass. Stretch an imaginary line between these two stars and continue moving upward. The naked eye will see what appears to be a brighter patch of the Milky Way.
Unlike the Beehive, there are actually two concentrations of stars here, called h Persei and Chi Persei, popularly known as the Double Star Cluster of Perseus.
The ancient astronomers Hipparchus and Ptolemy described the Double Cluster as a cloudy spot. Galileo with his crude telescope first noticed they were really a double cluster of stars.
Astronomers estimate that the clusters’ apparent brightness is dimmed by roughly 1.7 magnitudes compared to their actual brightness, thanks to interstellar dust. When we take this into account, it turns out that the 10 brightest stars of this Double Cluster are actually comparable to such brilliant luminaries as Betelgeuse and Rigel in the constellation Orion. Each cluster contains 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 (56,000 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. Estimates place these star clusters at a distance of about 7,400 light-years from Earth. The two clusters appear to overlap and together, their apparent size in the sky exceeds that of the full moon. In binoculars or a small telescope they form one of the most impressive and spectacular objects in the heavens.
