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Denver Observer

February Skies 2019

Image of M37, an open cluster.

Open cluster M37, by Joe Gafford.


by Zachary Singer

Some of our favorite planetary targets, Venus and Jupiter, are up in the pre-dawn sky this month, and Mercury appears in the evening, as we’ll see in “The Solar System,” below. In “Stars and Deep Sky,” we’ll take a look at two notable open clusters in Auriga, M36 and M37.

The Solar System

Mercury starts off February still lost in the solar glare, but begins to reappear after the first week of the month. It’s still difficult on the 10th, but the party is just beginning—just a few days later, on Valentine’s Day, you’ll see Mercury glowing at magnitude -1.2; look for it low in the west, just less than 5° above the horizon, 30 minutes after sunset (binoculars are a good idea). A telescopic image would show an almost-full disk 5.5” across.

Just two weeks later, Mercury will have increased its separation from the Sun, standing a full 9° up, in a darker sky 45 minutes after sunset. At that point, the planet will have dimmed slightly, to magnitude +0.6, but it will have grown to 8.5” across and will appear as a waning crescent. Take your observing opportunities while you can—Mercury moves fast in its orbit.

Venus was at maximum elongation, appearing farthest from the Sun in our sky, early last month. Now, every new morning brings it closer to the Sun’s glare, from our Earth-based point of view. In early February, the brilliant planet sits nearly 20° above the southeastern horizon, 40 minutes before sunrise; in telescopes, it appears just past half-lit. By month’s end, Venus’s altitude is down to just 12°. The planet is slowly circling to the far side of its orbit, relative to our position, so encroaching sunlight will become more and more of an issue, as the weeks progress.

Venus will line up just over 1° from Saturn on the morning of the 18th. The pair will be about 12° above the southeast horizon—and climbing—at 5:45 AM. At that time, sunrise will still be over an hour away.

Mars remains high enough for good observing conditions all month—but its apparent size continues to shrink, from 6” at the beginning of February, to 5” at the end. Still, while planetary detail will be lacking, Mars will still be easy to see as a disk, even in small ’scopes. Look for it about 45° above the western horizon, about an hour after sunset.

Mars has a close conjunction with Uranus on February 12th, when the two planets appear just under 1° apart. (They’ll also be almost that close a day before and after.) If your telescope has a wide field, the pairing will make a beautiful sight, and if not, try binoculars! For those of you without go-to systems, this is also an especially good opportunity to find Uranus—when you center Mars, magnitude +5.8 Uranus will be the next close, bright object, in your finderscope.

Jupiter fans, your ship has arrived—the planet is now about 22° up at around 6:30 AM, about 40 minutes before sunrise. As February progresses, Jupiter’s altitude in the pre-dawn sky will increase, to about 26° at the end of the month. (You’ll have to get up a little earlier, though, as “40 minutes before sunrise” will mean a 5:54 AM observation.) Jupiter climbs still higher in March. The Moon (or most of it) will share a 2° field with Jupiter around 5:30 AM on the 27th (watch early, to see the Moon’s separation from Jupiter lessen).

The two bodies will draw closer and closer, but sunlight will make observing more difficult. Still, a wide telescopic view should show both objects in daylight; by 7:30 AM, Jupiter will be only 1½° from the Moon’s center, and due south. (Since both will be only slightly past the Meridian, Jupiter will also lie directly below the Moon as well, making finding the planet in daylight straightforward.)

One extra note about this event: At approximately 5:41 AM that morning, the Moon will occult (pass in front of) 4th-magnitude Xi (ξ) Ophiuchi, along the Moon’s southeast limb. While it will be more challenging than the recent series of Aldebaran occultations, you should still be able to see it if you don’t mind getting up early.

Saturn’s “not quite there yet” for good telescopic observing this month, but it’s on the way. Though only 8° up, 40 minutes before sunrise in early February, that jumps to 18° by the end of the month.

Just as it did last month, Uranus appears close to 4th-magnitude Omicron (ο) Piscium, making this planet relatively easy to find. Center Omicron in your finderscope, and you’ll see Uranus there, too; look for it just 1½° and just a little east of north from the star in early January. By the end of the month, that gap will widen to just over 2°, with Uranus shifting to appear northeast of Omicron. Don’t forget the Uranus-Mars conjunction on the 12th—see Mars, above.

Uranus will begin sinking into the sunset in March. Before then, those of you without go-to systems or setting circles should make sure to take advantage of Uranus’s current position, because the planet’s retrograde motion, which kept it close to Omicron, is over—as Uranus resumes its eastward track, it will get increasingly harder to find. (Retrograde will of course reoccur, starting in the fall, but Uranus won’t come much less than 5° from Omicron next time, and after its departure then, we won’t get another easy landmark for years.)

Neptune is still technically observable in early February, standing about 18° above the horizon, an hour after sunset. But it won’t be a great target, and it will sink noticeably day by day, losing itself in solar glare well before this month is out.

Stars and Deep Sky

This month, we have two bright, well-known open clusters in the constellation Auriga, the Charioteer. Clusters of this sort are pronounced groupings of stars which formed inside huge clouds of gas and dust, such as the Orion Nebula, M42; after forming within the clouds, the stars’ radiation blows away the dust and gas, revealing the cluster.

There are two big ideas here: The first, as we’ve seen in past articles, is that not all clusters are alike! This month’s targets, M36 and M37, appear quite different from each other, even though they’re of roughly equal brightness as seen from Earth, and that’s a product of many factors, including the number of stars (well, no prizes for guessing that one), and the clusters’ ages. The second is that it’s not a coincidence that you can roughly draw a line through our two targets that also connects adjacent M38 (another beauty in Auriga), M35 (a gorgeous cluster in nearby Gemini), as well as many other clusters we’ve looked at in Cassiopeia and Canis Major (like M52 and M41), and many more beyond.

Chart of Denver Sky in Mid-February, wide view.

The Milky Way (marked in gray), as seen under dark skies near Denver, viewing southeast in mid-February at 9:00 PM. The chart’s center, marked by the Telrad circles, is about 60° up. The field of view covers about 145°, so the constellations may appear more compressed than usual. **Note the preponderance of open clusters, like M37, and their arrangement along the Milky Way (they’re marked as small dotted circles)—since these objects are part of our galaxy, it makes sense to find them mostly lying along the galaxy’s arms (i.e., along the Milky Way). Object positions, constellation and meridian lines charted in SkySafari, and then enhanced. (Click on image for enlarged version.)

Let’s take the second big idea first—as mentioned briefly above, clusters like these form from vast clouds of gas and dust. These clouds are associated with galaxies—although we can find them elsewhere, such clouds (and therefore, open clusters) are especially common in the arms of spiral galaxies, where astrophotographs show them glowing in bright pink shades. Astronomers also refer to these clouds as H II regions, and you can find them easily in Hubble Space Telescope images of galaxies like Andromeda (M31) or Triangulum (M33).

The arms of our own Milky Way, a spiral galaxy itself, are no different—they’re home to these star-factory H II regions, too—and to the clusters that are made within them. As you’d expect, then, looking along the arms of the Milky Way should be a good way to find open clusters (and H II regions), and just as expected, there they are!

Now let’s look again at our first big idea—open clusters aren’t all the same. With respect to our targets this month, they’re both roughly “next to each other” in space, lying in more-or-less the same direction, and at generally comparable distances (4,100 and 4,600 light-years, for M36 and M37, respectively). So, the differences we see between these two clusters have more to do with their actual characteristics than with, say, differences in distance (which would make otherwise-identical objects appear to differ in size and brightness), or with clouds of intervening dust blocking light from one object and not the other. M36 and M37 look different from each other, because they are different from each other.

M37 has many more stars visible within it than M36 does—that’s partly because it has about double the mass of M36 (so more stars, if they’re all of the same type, and thus equally massive), but also because M37 is older. The latter might not make sense at first—intuitively, we tend to think of older things “breaking down”—and that’s actually correct as far as M37 goes. What’s happening is that at M37’s age of 300 million years or so, the hottest, brightest stars (classes O and B) have already burned out. Without the glare of the hotter stars interfering, we see the more numerous, longer-lived, cooler, and somewhat dimmer, class-A stars carrying the torch, as it were.

The individual stars of M37 are no match for the brighter, class-B stars of M36—but M37 makes up for it with far greater numbers of stars overall. M37 must’ve been really incredible when it was newer and its hottest stars were still aglow, but its type-A stars are no slouch, and there are lots of them!

(Just above, we compared the masses of the two clusters, saying there would naturally be more stars in M37 because of that cluster’s greater bulk, if the stars in the two clusters “were all of the same type”—they’re not. M37’s class-A stars are actually less massive than the class B stars of M36, meaning that there are even more of them than an “even-Steven” comparison would suggest.)

As far as observing goes, our first target, M37, at 05h 54m, +32° 33’, is one of my favorites—it’s easy to find, and it’s a beauty. I’ve seen this cluster in binoculars, in small telescopes, and in my 12-inch Newtonian—and it looks great in all of them. Unlike some clusters, including adjacent M36, M37 teems with stars; my notes for M37 often mention something like “stars spilled all over the field.” The first observations I can remember of this object were through my 6-inch Newtonian under dark skies, with so-so eyepieces; though I made no notes at that time, the view left a deep impression.

Since then, it’s become one of my “gotta-see” objects, even in the city. You might not be surprised to hear that city views in the 12-inch look good, but I’ve enjoyed city-based observations through my 6-inch Newtonian—and smaller ’scopes, too. On the extreme side, a view through a 4-inch Mak last summer, with skies of pea-soup transparency, and darkening from widespread fires raging at the time, nonetheless showed the cluster, if dimly. A more recent winter observation, under less-ridiculous but still only so-so city skies with a 5-inch Mak, showed the cluster easily at 60x, using a standard 25mm Plössl; the cluster was beautiful at 125x.

Under dark skies in an 8-inch Schmidt-Cassegrain (SCT), M37 is wonderful in a 25mm Plössl at 80x, showing most of the cluster. (If you want to see the whole thing with a ’scope like this, either a wider-field 25mm eyepiece or a 32mm Plössl should do the trick.) Still, I like the view better at 15mm or 135x—the field’s edge crops into the cluster and lets you feel like you’re looking deep into it, as well as showing dim stars that aren’t immediately obvious at lower power—it’s a mesmerizing view.

At 220x in the 8-inch, the view is quite narrow but very deep—more of the cluster’s dimmer stars, as well as background stars, become visible at higher power. It’s fascinating to slew around the cluster at this magnification—if your scope isn’t on a tracking mount, just aim at the western end of the cluster and let it drift through your eyepiece…

In contrast, M36, at 05h 38m, +34° 09’, also sometimes called the Pinwheel Cluster, seems more compact and concentrated. As we discussed above, M36 has fewer stars than M37 does, but the individual stars are noticeably brighter—in the 5-inch Mak in the city, M36’s stars were easily visible at 60x, and they remained so at 125x (though at that point, the field of view was a bit tight).

My notes for that 5-inch observation say that “Only a few ‘streams’ of stars were visible, but it was still satisfying.” Other sources, around the ’net and in books, also often note that M36’s stars appear in long streams or tendrils, making this both an interesting object, and a contrast to M37. (When you see these streams for yourself, you’ll understand why they call M36 the Pinwheel.) Because its fewer-but-brighter stars are more able to punch through atmospheric haze and light pollution (as with the observation through the 5-inch, when I noted transparency as just “3 out of 10”), M36 is arguably a better target for poor sky conditions than M37.

In the 8-inch SCT under a dark sky, M36 was quite bright at 80x. (Adding more magnification didn’t really improve the view.) Since it’s right next to M37 and makes a great comparison, M36 is definitely worth a look!

Directions to M37 and M36

If you’re reasonably familiar with Auriga—or at least, where to find it in the sky—skip ahead to “Finding M37,” below. To get the rest of us oriented, Auriga is a bright constellation, and its main stars are easily seen on a decent night in southern Denver. Auriga’s brightest star, Capella, is the second-brightest in the sky, with only brilliant Sirius outshining it until Vega rises around midnight. Since Sirius sits fairly close to our southern horizon, Capella is left unchallenged near the zenith, the highest point on the dome of our sky. Look for Capella almost straight overhead at 9 PM in early February; it shifts westward as the month progresses, but it will remain quite obvious.

Chart of sky around constellation Auriga.

The sky around the constellation Auriga, as seen under dark skies near Denver, viewing due south in mid-February at 9:00 PM. The chart’s center, marked by the Telrad circles, is 70° above the horizon. Object positions, constellation and meridian lines charted in SkySafari, and then enhanced. (Click on image for enlarged version.)

If you’re looking in the right place for Auriga, then you’ll see the constellation Orion roughly halfway between Auriga and the horizon. That south-looking view will also include the constellation Gemini “down and to the left”, or southeast, of Auriga; similarly, Taurus will lie “down and to the right,” or southwest, of Auriga. (See second chart, above.)

Finding M37 is straightforward once you’ve got a grip on Auriga and its distinctive, roughly pentagonal shape (note that on some charts, including this one, the “pentagon” outline has another star attached, bringing one part of the constellation’s outline to a sharp point.)

At the opposite side of Auriga from Capella is Elnath, aka Beta (β) Tauri, the next-brightest star in the constellation. (Technically at least, that’s not quite true—this star is now officially part of Taurus—but it used to be known as Gamma [γ] Aurigae!) Elnath sits right on the border between both constellations, and it’s easy enough to see as both a part of Auriga and as one of the tips of Taurus the Bull’s horns. For our purposes here, we’ll still call it part of Auriga.

If you start at Elnath, and glance toward the next bright “vertex” star in Auriga, moving clockwise around the pentagon, you’ll find Theta (θ) Aurigae, fairly bright at magnitude +2.7. Our two clusters lie about halfway between Elnath and Theta.

Slide your Telrad’s center along the line between these two stars, until it’s at the halfway point, and you should see the glow of both M37 and M36 in your finderscope, near the opposite edges of the field from each other. If you’re off a bit (or your finderscope has a narrow field) and M37 doesn’t show, try sliding the Telrad towards Pollux (in Gemini), until the Telrad’s trailing edge is on the line between Elnath and Theta. (That should put M37 near your finder’s crosshairs.)

As noted, from the Telrad’s initial position, centered between Elnath and Theta, you should also see M36 toward the edge of your finderscope’s field. If you need a more accurate positioning (say, to locate M36 directly when light pollution obscures it in the finderscope), then from that initial position, slew your ’scope perpendicularly to the Elnath-Theta line and away from Gemini. Stop when the imaginary Elnath-Theta line lies between the trailing edges of your Telrad’s outer (4°) and mid-sized (2°) circles—not quite at their midpoint, but a touch closer to the outermost one. M36 should be in your low-power eyepiece, or close to it; if it’s not visible, spiral gently around that point.

Since we’re looking into a rich part of the Milky Way, in an area known for open clusters and H II regions, you might think there are more objects lurking nearby. You’re right—M38, another famous open cluster, as easy in small ’scopes as our two targets, lies just over 2° northwest of M36—it’s on our star charts, along with nebulae and even more nearby clusters. I’ll leave you to explore these on your own…

—See you next month.

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