Archive for the ‘Star hopping’ Category

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Mission 21: Nova in Sagittarius

July 7, 2012

Mission Objective: Nova

Equipment: Binoculars, Magnifying Finder, Telescope

Required Time: 10 minutes

Related Missions: Not Everyone’s Pot of Tea

Just got this via email from Sky & Telescope:

A fairly bright nova has been discovered in Japan by Koichi Nishiyama (from Kurame) and Fujio Kabashima (Miyaki). They made their find on July 7, 2012, using a 105-mm f/4 camera lens and CCD, describing the nova as magnitude 7.8. It is located in Sagittarius roughly between the “lid” and “spout” of the Teapot stars, at right ascension 18h 20.5m, declination -27° 44′ (equinox 2000.0). A number of other observers in Japan have made confirming sightings.

The discovery was announced on CBET 3166, issued July 7th by the Central Bureau for Astronomical Telegrams in Cambridge, Massachusetts. This circular assigns the star the official designations Nova Sagittarii 2012 No. 4 and also PNV J18202726-2744263.

The American Association of Variable Star Observers (AAVSO) is requesting observations of this nova. The AAVSO collects magnitude determinations by skilled observers to aid in the study of novae and other variable stars, and their site offers guidance on the techniques involved.

Whether or not you go as far as submitting observations to the AAVSO, the nova should be fun to track down and a relatively easy catch. And, hey, since you’ll be in Sagittarius anyway, you’ll have plenty of other pretty things to look at after you find the nova.

Here’s a widefield finder chart I knocked up in GIMP, using this photo from Wikipedia. The green lines mark out the “teapot” asterism in Sagittarius–if you don’t know how to find this, check here. The box is the field shown in the closeup finder chart below. The nova is indicated by the white cross. The red arrow is not the nova, it’s a magnitude 4.7 field star that will help you orient the closeup finder chart.

And here’s the closeup finder chart, courtesy of the AAVSO’s free oonline chart generator. I added the arrow to indicate the same field star as shown in the above image (the arrow points the same way, too). This is rotated 180 degrees from the sky (for Northern Hemisphere observers looking south) and from the widefield finder chart above, so you can use it as-is in a Newtonian reflector or straight-through finderscope, or flip it around for binoculars or a right-angle correct-image (RACI) finder. If you have a refractor or Cassegrain scope and you’re using a star diagonal, you can use the AAVSO plotter to make a rightside-up but reversed chart to match what you’ll see at the eyepiece. Copy and paste the nova’s designation into the box and select the options you want: PNV J18202726-2744263.

And now I’m going to post this and go look for the nova myself. Happy hunting!

Update the next day (July 8): I found it. I had to do a lot of flipping back and forth between chart and telescope to make sure I had the right object–it looks just like another 7th magnitude star in the eyepiece. And actually I used the finder rather than the scope. A 10×50 finder (or just about any pair of binoculars) collects enough light to show the nova easily, and gives a wide enough field to make it easy to find. I was using the SkyWatcher 90mm Mak, which has a maximum true field of about 1.5 degrees–too narrow to show most of the surrounding stars I needed to use to orient myself (imagine plotting a cross-country trip on Google Maps if you couldn’t zoom out and only saw a 10-mile stretch at a time).

Anyway, in all of the back-and-forthing I figured out a couple of geometric relationships that helped me figure out which 7th magnitude star was the nova. The next two charts are modifications of the AAVSO chart that show these relationships. I stripped out all the stars fainter than 8th magnitude, and all of the clutter of magnitude numbers, and rotated the charts 180 degrees to match the view in binoculars or a RACI finder (from the Northern Hemisphere).

First, the 4th magnitude star (red arrow) that I use as my “anchor” in this stretch of Sagittarius sits at the top of a north-south chain of 6th and 7th magnitude stars, and makes a diagonal with the second-brightest star in the field. The nova is about midway along that diagonal line, just above and to the left.

That helped, but as usual I was nervous about whether the star I thought was the nova actually was, so I used another relationship to double check.

The “anchor” star forms a right triangle with the two closest semi-bright (~6th-7th magnitude) stars, and the nova is almost straight down from the left-hand (eastern) point of the triangle.

If you haven’t yet mastered star-hopping, that’s basically all there is to it–working from the chart to the finder and eyepiece, you notice little geometric patterns and work from big, bright anchors to fainter, dimmer targets. After a while, it gets to be reflexive. You learn to build in double-checks to keep from wasting your time on chance alignments of stars that are close to–but not exactly–what you’re looking. Your internal monologue might run like so:

Looking at chart: Okay, there should be an arc of four stars like an eyebrow about two degrees left of [bright star].

Looking through finder: Got it.

Back to chart: Hmm, if I follow that arc to right, I should find a triangle of faint stars that points to [target].

Looking through finder: Sweeping right…hmm. There’s a triangle of faint stars, but is it the right one? I should find [target] right over here, but…nothing there.

Back to chart: Okay, now I see that the triangle of faint stars I want is not just right of the “eyebrow”, but right and a bit down, and it has a pair of stars like eyes just to the south.

Looking through finder: Here’s the triangle I thought was the right one, but–nope, no pair of stars like eyes. Let me back up to the “eyebrow” and try again. Now this time I go right and a bit down…yep, all right, there’s another triangle of stars, there’s the pair of stars like eyes. Hmm. Still don’t see the target, but it’s a small galaxy, so maybe it’s just not showing up in the finder. It should be right about…here. [put finder cross-hairs where target should be]

Looking through eyepiece: Well, hello beautiful!

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Mission 19: Cross asterism near NGC 2281

March 4, 2010

Mission Objectives: Asterism, Open cluster

Equipment: Binoculars, Telescope

Required Time: 3 minutes

Related Missions: Diamonds from the Ring of Fire

An asterism is just a pattern of stars that grabs someone’s attention. Asterisms differ from constellations in that they don’t have any official standing, although some like the Big Dipper (which is only part of the constellation Ursa Major) are better known than their host constellations and have been recognized for far longer. Since asterisms don’t have to meet anyone’s standards for asterism-hood, anyone can point one out, and stargazers have been discovering them for as long as humans have watched the skies.

I noticed this one for the first time a few weeks ago when I was hunting down NGC 2281 with my 15×70 binoculars. It’s an easy catch–just find Capella, trace a line to Menkalinan the next star counter-clockwise in the ring of stars that marks the constellation Auriga, and extend the line an equal distance in the same direction. Might as well take in NGC 2281 while you’re there–it’s just southwest of the right arm of the cross.

NGC 2281 is a nice little open cluster for either telescopes or binoculars–another glittering diamond in the celestial Ring of Fire. But in this case, I like the asterism better than the cluster! It’s well worth seeking out, and definitely better in binoculars than in telescopes.

The cross asterism seems really obvious, but I haven’t found any other mentions of it so far. Does anyone know if it has been noted or discussed before? I’ll be grateful for any info.

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Mission 18: Diamonds from the Ring of Fire

February 27, 2010

Mission Objectives: Bright stars, Open clusters, Messier objects, Star hopping

Equipment: Sky map, Binoculars, Telescope

Required Time: 5-10 minutes per window

Related Missions: Ring of Fire

Introduction: Here on Earth, diamonds are found in the magma pipes that fed long-extinct volcanoes. Sometime in the distant future, the volcanic provinces of the Pacific “Ring of Fire” will be prime diamond-hunting territory. So it’s fitting that winter’s Ring of Fire is also full of diamonds, in the form of open clusters that decorate the winter Milky Way.

All of the clusters described here are within reach of 50mm binoculars, although most won’t show much detail at 7-10x. Even the smaller ones will start to differentiate in 15×70 binoculars, and all of them are stunning in telescopes of any size.

Instructions for M41, M93, M46 and M47: Go outside after dark, face south, and find Sirius. Use it to trace the “doggy” shape of Canis Major. M41 is the biggest and brightest of the clusters in this area, and it’s an easy catch right in the heart of the dog. I usually find it by centering Sirius in the field of view and then just sweeping down (south) through the constellation. It never fails to swim into view. If you’re having a hard time, M41 makes one corner of an elongated triangle with Sirius and a trio of brightish stars along the dog’s back.

After M41, the rest of the Messier clusters around Canis Major may seem a bit anticlimactic, but each has its own charm and they are all well worth tracking down. And there are even better clusters to come.

Although it is nowhere near as brilliant as M41, M93 is one of my favorites. It is small but fairly dense, and at low magnification its irregular shape makes me think of a silvery flame burning in the night sky. To get there, trace your way down the dog’s back to the bright stars Wezen, Adhara, and Aludra, which mark the dog’s hindquarters and tail. If you’re in doubt about which is which, note that these three stars form a right triangle with Aludra at the south end. From Aludra, a loose chain of bright stars trails east into the constellation Puppis. Sweep over and up, over and up, and you’ll see M93. If you get to a star as bright as Aludra, you’ve hit Rho Puppis (looks like ‘p Pup’ in the map above) and gone too far.

The last two in this window, M46 and M47, make a nice contrasting pair. From Sirius, scan east to find the stars that make the back of the dog’s head. I imagine these stars forming one end of a shallow arc that includes several bright background stars and ends on the paired clusters. If that doesn’t work for you, use Stellarium or the atlas of your choice to pick out intermediate stars to use as waypoints. A word of caution: this is a rich region of the sky, with loads of tiny faint clusters that aren’t marked on any but the most detailed maps. More than once I have been looking for M46 and M47 and gotten hung up in the wrong place. If you have any doubt about whether the clusters you’re looking at are the right ones, they’re probably not. One way to recognize them for sure is to note the differences between them; M47 is very sparse with a handful of bright stars in an irregular pattern, whereas M46 has many more stars that are more even in brightness, although none of them are nearly as bright as the most prominent members of M47.

Instructions for M35-M38: Now go to the north end of the Ring of Fire, to the bright star Capella. Use it to trace the 5- or 6-sided (depending on how many stars you include) ring of the constellation Auriga. The side of the polygon opposite Capella is formed by the long line from Alnath (technically in the neighboring constellation Taurus) to the next star clockwise. The clusters M37 and M36 are on either side of that line at the halfway point. Extend the line from M37 to M36 on to the west with a slight bend to the north to find M38. As with M46 and M47, this trio of clusters make an interesting study in contrasts. Here are my notes from Messier Marathon night:

  • M37: compact, dense with faint stars, very rich but dim
  • M36: smallest but brightest of trio, dominated by a few brighter stars
  • M38: intermediate between the other two in both richness and brightness

The first time I observed these clusters, I found M37 and M36 easily and then spent almost an hour trying to locate M38. It just wasn’t there! Then I checked the descriptions of the clusters and realized that I had actually been looking at M36 and M38. I’d been extending the line in the wrong direction. I backtracked and picked up M37 easily–an illustration of why it is useful to know what things ought to look like, and not just where they are.

The Auriga trio are nice clusters, but the fourth and final M-cluster in this window blows them all away. To find M35, trace down the body of the western twin in Gemini, from bright Castor to the swooping arc of stars that marks the outside ‘foot’. Just above the toe of the boot, in a right triangle with the last two stars in the arc, you’ll find M35, a big, bright cluster that rivals M41 in either binoculars or telescopes. If you’ve got a telescope, you can get a twofer–the small, compact cluster NGC 2158 is right next to M35 in the same field. It’s a tough catch in binoculars unless you’re under dark skies, but almost any telescope ought to show it easily. There’s a nice photo of the pair here.

Instructions for M44: I saved the best for last. M44, also known as the Beehive Cluster or Praesepe (“the manger”), is probably the second best cluster in the sky after the Pleiades. But it’s not as easy to find. The Pleiades have enough bright stars to shine out even in suburban skies, but the Beehive is an aptly named swarm of smaller lights. To complicate matters, M44 is located in Cancer, which has no bright stars.

I usually get to the Beehive from Gemini. Here are some methods that might work for you. My usual path is to draw a line from the extended arm of the western twin, through Pollux, and on in the same direction for about the same distance. Right now that line also intersects Mars, so you could cut your travel time by just drawing a line from Pollux, through Mars, to Praesepe. But that method is only going to work for a few days, maybe a couple of weeks at most, because Mars is on the move (compare its position in the map above with this shot from just a few weeks ago). Finally, if your skies are really nasty, you might try drawing a triangle from Procyon, to Pollux, to Praesepe. It won’t be a perfect equilateral, but it’s close; M44 is just above the point of what would be a perfect equilateral.

Or you could do what I often resort to when I’m in  a rush: find the region between the Gemini twins and Regulus, in Leo (just off-screen to the lower left in the image above), and just sweep around with binoculars or your finder. It’s pretty low-fi, but it’s never failed me yet.

M44 is a true showpiece of the sky, with dozens of stars of even brightness seemingly arranged in a net or grid. It can be seen with the naked eye under dark skies, but it really shines in binoculars. As with the Pleiades, it usually looks better in binos than in telescopes, although a short focal length, rich-field scope might have a wide enough field to show the cluster with some surrounding sky for context.

Coda: The nine open clusters in this  mission are just the tip of the iceberg. This section of the winter sky is littered with hundreds more. There are plenty of bright NGC clusters that rival or exceed many Messiers. The region around the ‘feet’ of the Gemini twins is an especially rich area to sweep with binoculars or a telescope at low power, whether you’re hunting for specific targets or just soaking up the view.

Spring is coming. Although the constellations of winter are high overhead at sunset, they are already starting their long slide toward the western horizon. So get ‘em while you can.

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