Observing the Milky Way, Part III Perseus to Puppis & Beyond: the winter Milky Way is faint and vague, but it's adorned with some of the finest clusters and nebulae in the sky.
Around the times and dates listed in the all-sky map on page 44, observers at mid-northern latitudes can see the Milky Way as a great bow of pale haze. It sweeps out of Cepheus, low in the north, up through Cassiopeia and Perseus in the northwest to Auriga, which is more or less overhead. From Auriga, the winter Milky Way descends between Gemini and Orion, passes through faint Monoceros, streams over the back of Canis Major, and finally is lost to horizon haze in Puppis, low in the southeast. During the next few hours, Puppis rises a little higher in the south, and the rest of the Milky Way arch descends slowly toward the western horizon.
But you might not be able to see this clearly unless you have very dark skies. Unlike the bright Milky Way of summer and early fall, the winter Milky Way is just a pale, glowing band. Moreover, it has very ambiguous edges and displays no conspicuous bright star clouds or dark dust features. At best you can see areas of slightly enhanced glow, the brightest being in central Auriga, central Monoceros, and Puppis.
The essential reason for the different appearance of the Milky Way in summer and winter is that in summer we look inward toward our galaxy's dust- and star-rich central region. In winter, by contrast, we look away from the central regions, through the thin outer disk of our spiral galaxy. The galaxy's center (galactic longitude and latitude 0[degrees]) is in extreme western Sagittarius, and the anticenter (longitude 180[degrees], latitude 0[degrees]) is located on the Taurus-Gemini border 3 1/2[degrees] east of Beta ([beta]) Tauri, the star also known as Elnath. By amazing coincidence, the galactic anticenter is just 6[degrees] west of another very special spot in the sky: the northernmost point on the ecliptic.
At the sidereal time shown on our all-sky map, the galactic anticenter is near the zenith for mid-northern observers. So if you stand facing west, galactic longitude 90[degrees], which is close to Deneb in Cygnus, will be on your right, near the north point of your horizon. This is the direction of galactic rotation; the stars in the Sun's neighborhood are heading toward Deneb as they orbit the galaxy's center. (But Deneb is orbiting in roughly the same direction, so the Sun won't catch up with it anytime soon.)
On your left as you face west, near the south point of the horizon, is galactic longitude 270[degrees]. This is the direction from which the Sun and its neighbors have come. So in terms of galactic rotation, when you look north from Elnath along the winter Milky Way, you're looking forward as well as out of our galaxy. And when you scan south along the winter Milky Way, you're looking backward and out.
Our Orion-Cygnus Spiral Arm
Our solar system is currently passing through one of our galaxy's lesser spiral arms, which is called (among many other aliases) the Orion-Cygnus Arm, Local Arm, Orion Arm, or Orion Spur. We're located near this arm's inner edge. So when we view the winter Milky Way, we're looking out through the core of the Orion-Cygus Arm, which lies between us and the galactic rim. That's why so many relatively nearby open clusters, stellar associations, and emission nebulae adorn the Milky Way from Perseus to Canis Major.
Beginning with Perseus, which is currently high in the northwest, we find the Alpha ([alpha]) Persei Cluster, which lies some 550 light-years away in the center of the Perseus OB3 Association. The Zeta ([zeta]) Persei Association (also called Perseus OB2) and its neighbor the California Nebula (NGC 1499) are roughly 1,000 light-years distant. In Taurus, above the west-northwest horizon, are the Pleiades and Hyades clusters, 400 and 150 light-years from us, respectively. To the west-southwest is the mighty Orion Association, centered some 1,500 light-years away. Low in the southwest, perhaps 2,500 light-years distant, is the Canis Major Association (Collinder 121), which includes 2nd-magnitude Delta ([delta]) and Eta ([eta]) Canis Majoris as well as 3rd- and 4th-magnitude [Omicron.sup.1] ([[omicron].sup.1]), [Omicron.sup.2] ([[omicron].sup.2]), and Sigma ([sigma]) Canis Majoris.
Except for the Alpha Persei Cluster, all of these objects lie well below the hazy stream of the winter Milky Way itself and therefore seem pendant from it. Sprinkled among these clusters and associations, along the same band from Perseus to Canis Major, are a large number of 2nd- to 4th-magnitude, blue, early-B stars--both main-sequence and giants--that are all several hundred light-years distant. They include Delta and Epsilon ([epsilon]) Persei; Zeta and Lambda ([lambda]) Tauri; Gamma ([gamma]) Orionis; Beta Monoceroti; and Beta, Epsilon, and Zeta Canis Majoris. This suggests that these stars, and the clusters and associations behind and among them, have some actual astronomical connection. And so they do.
Gould's Belt: The Foreground of Our Spiral Arm
Let's establish some terminology before discussing the structure of the nearby Milky Way. Our galaxy's spiral arms lie in a thin, spinning disk. The galactic equator--the nearly straight line across the star chart above--traces the central plane of that disk. Looking at right angles to that plane, along the galaxy's spin axis, we reach the north galactic pole, which lies just east of the Coma Berenices Star Cluster, and the south galactic pole, which is in Sculptor 2[degrees] southeast of the spectacular galaxy NGC 253.
The nearby associations, clusters, and nebulae from Perseus to Canis Major lie "below" the plane of the Milky Way, on the galactic-south side of the equator.
But the nearest stellar association of all, the 70[degrees]-long Scorpius-Centaurus Association, lies "above" the Milky Way plane, on the galactic-north (Coma Berenices) side of the equator. That's particularly obvious from the Southern Hemisphere, where the entire Scorpius-Centaurus Association is in plain view for much of the year. North of the tropics, parts of this association are never visible at all, and the rest can be seen only low in the summer sky.
So when we look toward the galactic interior from Scorpius to Centaurus, most of the luminous nearby stars appear above the plane. And when we look away from the galactic center, between Perseus and Canis Major, they appear below the galactic plane. This fact was noticed in the 19th century, first by the great English astronomer John Herschel during his stay in South Africa and then by the great American astronomer Benjamin Gould from his station in Argentina. As both astronomers noted, it hints that these stars lie in a thin sheet or ring that's tilted slightly with respect to the plane of the Milky Way--a structure now known as Gould's Belt.
The star chart above shows the approximate centerline of Gould's Belt as an orange line stretching from Perseus to Crux; you can see it across the entire Milky Way on the star chart in the center of the July 2013 issue.
Gould's Belt is tilted about 17[degrees] with respect to the galactic equator, crossing it in Crux and Cassiopeia. It's a small part of the Orion-Cygnus Arm--a local feature. The arm also contains some more distant clusters, associations, and dust clouds, which tend to lie close to the galactic equator.
The true space center of Gould's Belt is in the vicinity of the Alpha Persei Cluster. The Sun is within the belt about halfway between the Alpha Persei Cluster and the belt's interior edge near the Rho ([rho]) Ophiuchi complex. Radial velocity and proper motion studies have found that the objects around the periphery of Gould's Belt are moving outward from a common center at about 6 miles (10 km) per second. Given the belt's present size, this suggests that the expansion began some 70 million years ago, consistent with the estimated age of the Pleiades and Alpha Persei clusters, both of which are near the belt's center and therefore probably among its oldest clusters.
One hypothesis is that roughly 70 million years ago a supernova exploded in the vicinity of the dust cloud that was to become the Alpha Persei and Pleiades clusters. That event initiated star formation there, and the process has been expanding outward in a ring ever since. At present the most vigorous star formation in Gould's Belt is occurring in the Orion Nebula, the IC 348 nebula in the Zeta Persei Association, and the Rho Ophiuchi complex near the Scorpius-Ophiuchus border.
Beyond Gould's Belt
Thus when we look toward the winter Milky Way proper, along the galactic equator from Auriga to Puppis, we are in fact looking "over" Gould's Belt, which is a foreground structure within our Orion-Cygnus Arm. Beyond the belt, along the winter Milky Way itself, is a series of young stellar associations and open clusters with involved emission nebulae and dark dust clouds that can be thought of as tracing the core of the Orion-Cygnus Arm. They're all rather distant, from 3,000 to 5,000 light-years away, so they're not naked-eye objects. But some of these clusters and nebulae are easily visible in binoculars.
Tracing the winter Milky Way down from the zenith, the first bright Orion-Cygnus complex that we encounter is the Gemini OB1 Association, which is on the Gemini-Orion border between Castor's feet and the end of Orion's club. Gemini OB1 is about 5,000 light-years away and includes [Chi.sup.2] ([[chi].sup.2]) Orionis and the emission nebula NGC 2174, an object easily found with 10x50 binoculars 2[degrees] southwest of Eta Geminorum.
The next Orion-Cygnus Arm tracer down along the winter Milky Way is NGC 2264, the "Christmas Tree Cluster," which is in extreme northern Monoceros 3[degrees] south and slightly west of Xi ([xi]) Geminorum. This cluster is 20' long and is an easy 10x50 binocular target--though its south-pointing arrowhead shape can be a bit tricky to pick out from its rich star field. The 4.5-magni-tude blue giant variable S Monoceroti is on the north end of the cluster, marking the Christmas Tree's trunk. NGC 2264 is part of the Monoceros OB1 association, which is centered about 2,500 light-years from us.
Also in northern Monoceros, about 5[degrees] south-southwest of NGC 2264 and 8[degrees] east-southeast of Betelgeuse, is the Rosette Nebula, whose brightest components have three separate NGC numbers: 2237, 2238, and 2246. It's surprisingly easy to spot with 10x50 binoculars under a dark sky.
At the center of the Rosette is the 20'-long rectangle of the open cluster NGC 2244. Although this 4.8-magnitude cluster is visible to the naked eye as a cloudy smudge, it can be difficult to identify through telescopes and binoculars despite its distinctive rectangular shape (with 6th-, 7th-, or 8th-magnitude stars at each corner, and in the middle of both long sides). It's not particularly populous and can be lost in its rich Milky Way star field. Many of these field stars are members of the Rosette's association, Monoceros OB2, centered about 4,500 light-years away. Others are members of Monoceros OB1, which is much closer in space but overlaps Monoceros OB2 in the sky.
Some 20[degrees] further southeast along the winter Milky Way from the Rosette, past the central Monoceros Milky Way glow, is the 4,500-light-year-distant Canis Major OB1 Association, which includes the populous open cluster NGC 2353 and the Seagull Nebula, IC 2177, both visible in giant binoculars. The easy binocular open cluster M50, just to the northwest of the Canis Major OB1 complex, is only 3,000 light-years away, so it must be an unrelated foreground object.
Finally, the compact open cluster NGC 2362 lies 15[degrees] due south of the Canis Major OB1 Association and 2 1/2[degrees] northeast of Delta Canis Majoris. Its brightest star is 4.4-magnitude Tau ([tau]) CMa. NGC 2362 is a very young cluster, just a few million years old, and is therefore a true Orion-Cygnus Arm tracer. It is 4,500 light-years away, about twice the distance of Delta Canis Majoris, which is the brightest member of the Canis Major Association, a Gould's Belt object. Because in this direction we look toward about galactic longitude 240[degrees], NGC 2362 can be thought of as "following" the Canis Major Association in terms of galactic rotation.
The Perseus Arm in the Winter Milky Way
The major Orion-Cygnus Spiral Arm tracers from Auriga down to Canis Major and Puppis are fairly widely spaced. This reflects the fact that the interstellar dust clouds from which young open clusters and stellar associations condense are more widely spaced in the outer galaxy than they are in the galactic interior toward the summer Milky Way. The dust is very thin between the complexes of stars and nebulae, giving us long, relatively clear views beyond the Orion-Cygnus arm into the outer galaxy. Large dust-free windows lie in central Auriga, central Monoceros, and central Puppis. In these windows, the Milky Way glow is distinctly brighter to the unaided eye, and binoculars reveal rich fields of faint stars sparkling on the Milky Way haze.
The Auriga, Monoceros, and Puppis windows allow us to trace the Perseus Spiral Arm far out toward the galaxy's rim. The Perseus Arm, which is so conspicuous in the Cassiopeia Window of the autumn Milky Way, is lost to view east of the Perseus Double Cluster, hidden behind the Gould's Belt dust clouds of southern Perseus and northern Taurus. However, we pick up the Perseus Arm once again in central Auriga with the complex containing the open cluster NGC 1893 and the emission nebula IC 410. Although they're at least 10,000 light-years away, the combined glow of NGC 1893 and IC 410 is visible in 10x50 binoculars as a very pale patch of haze about 20' across.
Past Auriga, the Perseus Arm arcs out toward the galaxy's rim, so the Perseus Arm tracers in central Monoceros and central Puppis are increasingly distant and faint. Most are invisible through binoculars and small telescopes. One exception is the nebula NGC 2467 (shown on the facing page), about 1? southeast of Omicron Puppis. This object, though more than 15,000 light-years away on the outer arc of the Perseus Arm, can be seen in 10x50 glasses as a small but moderately bright disk of haze.
The Far-Southern Milky Way
The far-southern Milky Way contains some very famous and spectacular objects, including the Southern Cross, the Coalsack, the Jewel Box Cluster (NGC 4755), and the incomparable Eta Carinae Nebula (NGC 3372). Unfortu-nately, this stretch of the Milky Way never rises above the horizon for observers significantly north of the tropics. But on spring evenings--or after midnight in February--it's possible for a mid-northern observer to get some idea how the far-southern Milky Way links up with the section that's visible from the Northern Hemisphere.
The key to "observing" the invisible far-southern Milky Way is the conspicuous spring constellation Corvus, the Crow, which is directly north of Crux, the Southern Cross, about halfway between Crux and the celestial equator. Thus as Corvus transits your meridian, Crux will be directly below the south point of your horizon by a distance that depends upon your latitude. At the same time, the southernmost constellations of the Northern Hemi-sphere's winter Milky Way--Monoceros, Canis Major, and Puppis--will be setting in your southwest. And Scorpius, the southernmost constellation of our summer Milky Way, will be rising in your southeast.
The Scorpius-Centaurus Association extends from Scorpius on the northeast through Crux on the southwest. So when Corvus is at its highest, you can get some idea of the extent and shape of the Scorpius-Centaurus Association by imaginatively tracing it from Scorpius in your southeast to Crux beneath the south point of your horizon.
In Crux is the famous Coalsack dark cloud, which is about 5[degrees] across. It lies 10[degrees] past the southern end of the Great Rift, which runs down the center of the Milky Way from Cygnus to Centaurus. Nonetheless, the Coalsack seems to be a Great Rift outlier. It's some 550 light-years distant and 60 light-years across. Despite its apparent density, no star formation is currently taking place within it. But the Coalsack is not quite as dense as it looks. It actually obscures the stars behind it by a rather modest 2 1/2 magnitudes, but it looks virtually opaque because it's silhouetted upon a particularly bright star cloud.
The Scorpius-Centaurus Association and Great Rift mark the galactic interior edge of our Orion-Cygnus Spiral Arm. Beyond their eastern end we look across an interarm gap, poor in dust and open clusters, at the next spiral feature in from ours: the Sagittarius-Carina Arm, which extends from the Scutum Star Cloud on the northeast to the Eta Carinae region on the southwest. The star clouds around the Eta Carinae Nebula are exceptionally bright and rich for two reasons. First, the Sagittarius-Carina Arm here arcs out toward the galactic exterior, giving us a long view down its heart. And second, much of the interstellar dust in this stretch of the Sagittarius-Carina Arm seems to have been used up in star formation, providing us with an exceptionally clear view along it.
Several windows through the dust clouds of the Sagittarius-Carina Arm give us glimpses of the next spiral feature toward the galactic bulge: the Norma Spiral Arm (sometimes called the Scutum-Centaurus Arm). The northeast edge of the Norma Arm is at the Scutum Star Cloud, where it and the Sagittarius-Carina Arm seem to merge as they curve in around the galactic bulge. The Small Sagittarius Star Cloud, M24, is probably another stretch of the Norma Arm. The arm's name comes from the brilliant Norma Star Cloud in the southern part of that otherwise inconspicuous Milky Way constellation. Finally, the bright star clouds upon which the Coalsack is silhouetted probably lie on the southwest, outcurving edge of the Norma Arm.
One of the major unanswered questions about the local spiral structure of our galaxy concerns our own Orion-Cygnus Arm. Its course is clear in the neighborhood of the Sun, and it extends forward (in terms of galactic rotation) to the Cygnus Star Cloud, which lies between galactic longitudes 60[degrees] and 80[degrees].
But we look at a puzzle in the opposite direction, between galactic longitudes 240[degrees] and 260[degrees] in Puppis, Pyxis, western Vela, and western Carina. We would expect that in this direction we would be looking along the outcurving arc of our Orion-Cygnus Arm and would therefore see many young open clusters and emission nebulae--and perhaps another star cloud to match the one in Cygnus. In reality we see relatively few stars and just a smattering of open clusters, most of them quite old, and therefore not true spiral-arm tracers. There aren't any hidden star-forming regions; this is one of the most dust-free directions along the Milky Way plane, with as little as 3 magnitudes of absorption all the way out to the rim of the galaxy toward central Puppis.
But the plot thickens in Vela between galactic longitudes 260[degrees] and 270[degrees]. Here we would expect to see a dust-and cluster-poor interarm gap between the outcurving arcs of the Orion-Cygnus and Sagittarius-Carina arms. Instead, we find ourselves looking at a confusing clutter of dust clouds, young open clusters, and stellar associations. The nearest associations, which include the young, hyperluminous stars Zeta Puppis and Gamma Velorum, are well south of the galactic equator, so they are presumably features of Gould's Belt. But there are also clusters, associations, and nebulae at a wide range of distances right on the galactic equator.
One plausible explanation is that toward Vela we look along a bridge that joins our Orion-Cygnus Arm to the outcurving arc of the Sagittarius-Carina Arm. Or perhaps the entire Orion-Cygnus Arm is bending inward here. But for the moment all this remain uncertain.
Craig Crossen is a traveler, writer, and academic editor who calls Minnesota home. He is currently finishing books on the ancient Mesopotamian constellations and the history of archaeology in Iraq.
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|Title Annotation:||Galactic Depth Perspective|
|Publication:||Sky & Telescope|
|Date:||Feb 1, 2014|
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