After a truly horrible 2016, both in terms of the number and caliber of notable deaths, as well as the godawful election season we went through here in the US (I won’t even mention the election results), I bid 2016 an angry and well-deserved farewell, and none too soon. But not before taking a shot at the E and F stars of the Trapezium.
It was nice and clear last night, and Orion was riding high in the sky towards midnight. All the good Denverites were nestled all snug in their beds, while visions of ski lift tickets danced in their heads. Meaning that by this time of night, light pollution would be at a relative low because people were both off of the roads and asleep, and therefore not using their car headlights or house lights, just like it was when I bagged M1, M78, and M79 for the first time a couple of months ago.
The Trapezium is the tight little bundle of four young hot stars whose brilliance is the light behind the Orion Nebula – literally. The light from the Trapezium stars causes the nebula, the gas, to become aglow. They’re called the Trapezium because the four stars are in a trapezoid shape. As Orion was at its highest point in the sky for the night at this time, almost 45 degrees elevation, and because the sky conditions were pretty good (low humidity, little wind), I decided to give another one of my stellar nemeses a go – the E and F stars of the Trapezium. I’d been trying to view them for a couple of years now, without any luck.
Did I say that the Trapezium was four stars? It’s actually a few more than that. The four stars are just the relatively brighter ones that are “easy” to see. (Well, all four of them aren’t quite so easy to see if you’re trying to see them from Manhattan.) But there are a few other, dimmer stars there as well. Here’s a chart showing the stars, and their magnitudes.
The B star, at just shy of 8th magnitude, is the one that can be difficult at times, or at least it sometimes was back in Manhattan. But here in Denver, with less light pollution, the B star hasn’t been a problem at all. It’s the E and F stars, at just past 10th magnitude, that are the ones that I’ve been trying to see. And failing miserably.
The E and F stars are difficult to see for two reasons – first, and most importantly, it’s obviously because they’re dim. Even though my 5-inch Mak (which operates at an aperture of about 120mm due to an undersized primary mirror, and due to losses because of less than 100% reflectivity in its mirrors, transmits about as much light as a 103mm refractor) can theoretically get down to 12th magnitude, light pollution thoroughly reduces that.
The second reason is because the Trapezium itself is quite small. As you can see from the chart, it’s only about 20 or so arcseconds across from side to side. As the chart shows, the distance between the A and E stars is only about 4 arcseconds, and the F star looks to be even a smidge closer than that to the C star. You can easily see all four stars as being separate even at the lowest power available in the Mak – the 32mm Plossl giving 48x. But it still requires a great deal of magnification to separate the four main stars from each other to any degree. And with greater magnification comes a dimming of the field of view. As I pushed the magnification up to 193x with the TeleVue 8mm Plossl, even the B star itself started to get more and more difficult to see.
Back in Manhattan, the lowest magnitude I’ve ever been able to see through the Mak was 9.7, when I got just the glimpsiest glimpse of Rhea at 3am on Memorial Day weekend with averted vision. But that was under extraordinary “lack-of-light-pollution” conditions, due to everyone having left Manhattan for the three-day weekend. Normally, I could only just get down to about 9th magnitude in the Mak there. And if sky conditions weren’t good, and if I hadn’t sufficiently dark adapted, even the B star at 8th magnitude could be difficult.
Here in Denver, the light pollution is certainly less than it is in Manhattan. But how much less? I’ve been thinking that I could go almost a full magnitude deeper here off of my apartment building roof, down to about 4th magnitude. That would let my limiting magnitude in the Mak go down an equivalent amount, from say, magnitude 9.2 down to 10.2, and with luck, I could just glimpse E and F with averted vision. But perhaps not.
At this time of year, Gemini is a constellation that offers a good way to gauge how deep I can go. The arms and chests of the stick figure Twins are made up of stars going in a pretty straight line, from magnitude 3.55 (Kappa Gem) down to 4.4 (Tau Gem). Plus, Gemini is very close to the zenith now (about 70+ degrees elevation), so the effects of atmospheric extinction would be minimized, because that high up, there was that much less atmosphere up at the zenith to do any extincting.
Kappa (3.55) is the star all the way on the end of the left arm; Theta (3.6) is its counterpart on the end on the right. Both were visible almost as soon as I stepped out onto the roof from the brightly lit stairwell, albeit with averted vision. After about 10-15 minutes of dark adapting, the central star, Iota Gem, which forms a nice equilateral triangle with Castor and Pollux, and is where the two arms meet, also became visible, but also only with averted vision. Iota is at 3.75. I suspect that if I had stayed out for another 20 minutes or so, I might have dark adapted a bit more so that Iota would be visible directly, just like I was able to do with Albireo in the summer of 2015 when I watched the Perseids.
This puts my gain in limiting magnitude at about 0.7 magnitudes from Manhattan to Denver, to maybe 3.8 late at night after some serious dark adapting. Which is definitely a significant jump, but not quite enough to get down to the magnitude 10.2 and 10.3 that I need to see the E and F stars. I’d need a bit more than a full magnitude gain to do that.
At least, I can’t see them from here in the city. But from the DAS Dark Site, well, all bets are off. I can definitely go 2 magnitudes deeper there, if not a bit more. And that will be enough to finally bag the E and F stars! Now, to just get out there.
Meanwhile, the 24mm ES 68 continued to perform admirably, going between the various winter open clusters: M41 to M46, M47, M48, M44, and M67. M44, the Beehive Cluster, aka the Praespe, at 95 arcminutes across, was definitely too big to fit into the field of view all at one go. But because of how the stars are scattered, it wasn’t ridiculously overlarge, and still looked very nice at 64x.
Just before I packed up, I got a quick look at Jupiter, which had just risen at about 1:30am. At that point, it was only 8 degrees above the horizon and it looked positively awful. The equatorial bands were only “there” because I knew they were; it wasn’t as if I could really see them directly. Io was just about to give a shadow transit, but given the distortion from being so low on the horizon, I knew I wouldn’t have been able to see it anyway. In fact, it was so low that it gave off the same atmospheric dispersion that I saw in Mars this past spring. Yes, instead of Mars, this time it was Jupiter that was a patriotic red, white, and blue. But in just over a month or so, Jupiter season will begin in earnest, and it will be riding much higher in the sky at a much earlier hour. Hooray for Jupiter season!