MONTHLY OBSERVER’S CHALLENGE
Compiled by:
Roger Ivester, North Carolina
&
Sue French, New York
February 2020
Report #133
NGC 1931 Bright Nebula and Cluster in Auriga
Sharing Observations and Bringing Amateur Astronomers Together
Introduction
The purpose of the Observer’s Challenge is to encourage the pursuit of visual observing. It’s open to everyone who’s interested, and if you’re able to contribute notes, and/or drawings, we’ll be happy to include them in our monthly summary. Visual astronomy depends on what’s seen through the eyepiece. Not only does it satisfy an innate curiosity, but it allows the visual observer to discover the beauty and the wonderment of the night sky. Before photography, all observations depended on what astronomers saw in the eyepiece, and how they recorded their observations. This was done through notes and drawings, and that’s the tradition we’re stressing in the Observer’s Challenge. And for folks with an interest in astrophotography, your digital images and notes are just as welcome. The hope is that you’ll read through these reports and become inspired to take more time at the eyepiece, study each object, and look for those subtle details that you might never have noticed before.
NGC 1931
NGC 1931 is a small emission and reflection nebula with an involved cluster. The brightest part of the nebula has a trapezium system at its heart. Somewhat at odds with their name, trapezium systems can consist of more than four stars, and they don’t have to be arrayed in a trapezoidal shape. The term was initially coined to mean “a multiple star system whose pairwise separations are of the same order.” Some later researchers include groups whose stars may not be gravitationally bound. NGC 1931 is roughly 7500 light-years away from us.
William Herschel discovered this NGC 1931 in 1793. His journal entry reads, “Very bright, irregularly round, about 4 or 5′ diameter. Seems to have one or two stars in the middle or an irregular nucleus. The chevelure diminishes very gradually
February Observer’s Challenge Final: Click on the Following link:
February 2020 OBSERVERS CHALLENGE _ NGC 1931
February: NGC 1931 – Bright Nebula and Cluster – Auriga; Mag. V= 10.1; Size 6′
RA: 05h 31m Dec. +34º 14′
The embedded Stars in NGC 1931: by Sue French
Don’t be surprised about not seeing the stars in NGC 1931. Folks get very mixed results. Go to http://www.deepsky-archive.com/ and type NGC 1931 in the Designation box, then look at everyone’s sketches.
Here’s a long-ago Amastro post from Brian Skiff that gives the magnitudes of the trapezium system embedded in the brightest part of the nebula. I see one of these stars, or a blend of them, in the 105mm scope. The 10-inch at 213× gives me six stars in the brightest part of the nebulosity plus several mag 11-13½ stars scattered to the south. I’ve pasted an image below Brian’s data and labeled the stars on it. The image is in infrared so that the nebulosity doesn’t blot out the stars. Below the Aladin image is a WEBDA chart showing which stars I saw in the main group through the 10-inch..
While cleaning up some star-lists, I collected data for stars in the nebulous open cluster NGC 1931 in Auriga. The group contains a faint trapezium system, ADS 4112, that might be of interest to ‘amastro’ folks. The specs for the group are shown below. The V magnitudes for the stars come from modern photoelectric or CCD studies. The separations derive from positions in the 2MASS catalogue, which should be better than the 100-year-old visual micrometry, but which in any case match the old data to within a few tenths of an arcsecond. The brighter trio is straightforward in a small telescope; in 1989 I was able to make out the fourth ‘E’ component very faintly in my 6-inch refractor at 200x. The very faint, close ‘D’ companion to star ‘B’ doubtless requires a very large aperture. The data quoted for it is from S. W. Burnham’s work; the magnitude is possibly too bright. The 2MASS coordinates are listed at the bottom. Star ‘D’ does not appear in any astrometric catalogue. The spectral types for the three brightest stars, by the way, are B0V, B0.5V, and B1V. Thus the object should contain some emission, although there must be a substantial reflection component, since filters do not provide much contrast enhancement.
\Brian
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ADS 4112 = BD+34 1074: 5 31 27 +34 14.9 (2000)
V mags sep pa
AB 11.5,12.3 8″.1 239
AC 13.0 10″.5 310
AE 14.0 14″.6 17
BD (15.8) 2″.3 322
RA (2000) Dec
A 5 31 27.08 +34 14 49.6
B 5 31 26.54 +34 14 45.0
C 5 31 26.43 +34 14 56.3
E 5 31 27.50 +34 15 03.2
Sue French: Observer from New York
254/1494mm (10-inch f/5.8) Newtonian. Seeing: fair. Transparency: a little better than average.
43×: NGC 1931 is just a short hop westward from starfish-like M36. It presents a small hazy spot surrounding a star.
115×: The nebula spans about 3 arcminutes, and the star now appears triple. Several additional stars straggle south through west-southwest of the nebulous mass.
213×: Six stars are now buried in the nebulosity, three brightest members arranged in a little triangle.
The WEBDA cluster plot below marks the four trapezium-system stars viewed as well as two additional stars spotted within the nebula.
V-magnitudes of the trapezium stars according to WEBDA:
A=11.4, B=12.3, C=13.0, E=14.0. There is a component D in the trapezium system very close to B, but it’s thought to be magnitude 15.8 or dimmer and was not seen. The two arrowed stars were visible: the northern one shining at magnitude 14.1, and the southern one at magnitude 14.5.
After the star-plot is the sketch I made at 213× with the 10-inch scope on 17 February 2020 at 7pm EST. It was a pleasant night for February. The temperature was in the lower 20s and there was no wind. Unfortunately the seeing and transparency were both below average, and there was full snow cover on the ground. I couldn’t see the 14.5-magnitude star mentioned in the previous observation. I decided to sketch just the part of the nebula I could see and the four stars visible within it. My sketch looked pretty good to me, but a scanned image didn’t show the faintest parts, so I penciled over the original sketch to make it scan better, I hope without changing anything too much.
Pencil Sketch: Sue French:
North is up and west to the right
Mario Motta: Observer from Massachusetts
Taken with my 32-inch telescope, and SBIG STL 1001E camera. One hour of H-alpha, one hour of Sulfur S2 filters, and only 20 minutes of O3 filter as there was essentially no Oxygen signal.
Processed in PixInsight.
Roger Ivester: Observer from North Carolina
On the night of January 28, 2020, the transparency and seeing were very good. Using my 10-inch, f/4.5 reflector, NGC 1931 was very easy to locate and see at 57×, appearing as a star with a mostly round halo of nebulosity.
When increasing the magnification to 267×, using a 12mm eyepiece and a 2.8× Barlow, the bright nucleus revealed a tiny trio of faint stars, with a fourth, much fainter star, toward the WSW. This fourth star was extremely difficult, and could not be held constantly, but intermittently at best. The nebula was elongated, with a NE-SW orientation.
The first time I observed NGC 1931 was with poor seeing, on January 8, 1994, and could not see the trio of stars. My second attempt to see the trio of stars was in January, 2020, but again with poor seeing and transparency, and was unable to see any of the faint stars.
Pencil sketch, with the colors inverted:
James Dire: Observer from Illinois
NGC 1931 is a mixed emission and reflection nebula with an embedded star cluster found in the constellation Auriga. The nebula is an active star forming region. The complex is located about one degree west and a tad north of the star cluster M36. NGC 1931 also lies 5.75 degrees north and one degree east of the star Elnath. NGC 1931 measures roughly 3 arcminutes in size and lies 10,000 light years away. The nebula is estimated to be magnitude 10.
NGC 1931 contains myriad young, hot O and B stars whose radiation is responsible for the blue hues of the reflection nebula. Four stars in the center of the nebula form a trapezium similar to that in the Orion Nebula. Sometimes NGC 1931 is considered a mini version of the Orion Nebula. A much larger vast region of nebulosity known as IC 417 surrounds NGC 1931. The emissions from IC 417 are the characteristic red colors from H II ions.
My image of NGC 1931 was taken with an 8-inch Ritchey–Chrétien telescope operating at f/6.4 with the use of a focal reducer/field flattener. The camera used was an SBIG ST-2000XCM CCD cooled to -20°C. The exposure was 100 minutes. The bright white area in the center contains the trapezium. The exposure was not long enough to bright out the red emissions of IC 417, which would have filled most of the region captured in this image.
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