TELESCOPE: Parks Astrolight 6" (15.2 cm) f/6 Newtonian Reflector
EYEPIECES/MAGNIFICATIONS: Parks Gold Series Plössls (30 mm, 20mm, and 7.5 mm), Parks Gold Series 2x Barlow (30x, 45x, 60x, 90x, 120x, 240x)
FILTERS: Parks ALP Filter, Lumicon UHC Filter, Lumicon OIII Filter
After a nearly three-month hiatus from my dark-sky site, another new moon weekend was approaching and (once again) cloudy weather and (unusually) rain were in the forecast. This set of circumstances drove me to a couple of mid-week trips to the back country in order to take advantage of excellent seeing conditions during the latter half of the week of February 19-25. The first of these excursions (on Thursday) begins where we left off in the previous session on 4 December 2005, as we continue to explore the deep-sky wonders of Taurus. This region extends from 04h 32m to 06h 08m in right ascension and from +17° to +27° in declination, carrying through Taurus and into the westernmost tip of Gemini.
NGC 1647 (H.VIII.8): To locate this large, sparse open cluster, center your scope on Aldebaran, the fiery-orange eye of Taurus and sweep 2° northward to the Cepheid variable star SZ Tauri (mag. 6.3-6.7), and then just over 2° east to reach the 6th-magnitude orange beacon (SAO 94112) that marks the southern edge of NGC 1647. Spanning ¾° in diameter, you'll need a low-power eyepiece to frame this large cluster. At 30x, this object is a sprawling, irregular mass of over 50 stars of magnitude 6 and fainter. Most of the cluster members seem to about 10th-11th magnitude, many of them arranged in pairs. A short arc of 9th magnitude stars springs northward from the double star AG 311 (8.9, 9.3; 33") just north of the cluster's center. NGC 1647 certainly lives up to its nickname, "The Crab Cluster" (Seronik, 2001). At high magnifications, this sparse group quickly loses its cluster-like appearance as it overflows the field of view. William Herschel discovered NGC 1647 on the night of 15 February 1784. In 1964 Efremov suggested that the Cepheid variable SZ Tauri (remember that star we passed on the way here) may actually be an outlying (or coronal) member of NGC 1647, sharing the same distance and radial velocity as the cluster stars (Gieren, 1987), (Turner, 1992). There are others, however, who dispute the membership of SZ Tauri in NGC 1647 (Geffert, 1996). NGC 1647 lies 1800 light years away and is believed to have formed 210 million years ago (Strong & Sinnott, 2000). It contains a total of 283 stars down to magnitude 15.54 (Pişmiş & Bozkurt, 1977).
NGC 1746: Sweep you scope 4° west of NGC 1647, then 2½° north to 5th-magnitude Iota (ι) Tauri, followed by another northward hop of 2¼° to reach a second large, sparse (and often overlooked) open cluster in Taurus. With an apparent diameter of 42', NGC 1746, like the previous cluster, is best appreciated in the low-power ocular. The view at 30x is one of subtle beauty, with over 100 stars scattered irregularly about the field of view. The fainter stars are concentrated toward the southeast side of the cluster, while the brighter members are more uniformly distributed. There are many interesting pairs, chains, and irregular groups to delight the observer and the background is powdered with countless faint stars at the threshold of visibility. A bright yellow star of 7th magnitude highlights the eastern edge of the cluster. The cluster labeled NGC 1746 on most atlases was noted by d'Arrest on 9 November 1863, though William Herschel documented two clusters here nearly 80 years previously on 17 December 1785. These are designated NGC 1750 (H.VIII.43) and NGC 1758 (H.VII.21). Some atlases (such as Uranometria) plot all three of these as overlapping clusters. This, of course, begs the question: How many clusters are present here, one, two or three? According to V. Straižys, et al, there is enough evidence to support the presence of open clusters 1750 and 1758 (at distances of 1700 and 2200 light-years, respectively), but little evidence to support the existence of NGC 1746 (Straižys, 1992). More recent studies (Galadí-Enríquez, 1998) have supported this conclusion.
As can be seen from the chart above, there are a significant number of double star targets in this region for those enjoy tracking these down. One of my favorites is 118 Tauri (Σ 716) (5.8, 6.6; 4.8"; 204°). This snug pair of blue and white stars may be located by sweeping 1° north of NGC 1746 to the 7th magnitude star SAO 76929 and then 5¾° east. Another favorite of mine is a pair of intense blue stars designated Σ 730 (6.0, 6.5; 9.6"; 141°) and located 8° south of 118 Tau (but only 1½° south of 4th magnitude 119-CE Tauri, if you'd prefer to start your search there). This pair lies in a particularly rich region, so be sure to sweep around with the low-power eyepiece a bit, before (or after) zeroing in on your target.
Dolidze-Dzimselejsvili 3: While you're in the vicinity of 118 Tauri, you may want to have a look at a pair of rather sparse clusters lurking on the western edge of the vast (but invisible in my scope) supernova remnant Simeis 147. The first of these, DoDz 3 may found 1¾° NNE of 118. Look for an arc of four stars, magnitude 9-10, with a few fainter stars scattered around the immediate vicinity. Probably not the most exciting cluster you've ever seen, though.
Dolidze-Dzimselejsvili 4: The second cluster, DoDz 4, is a bit larger, but not much more impressive. Located 1½° NE of 118, this cluster consists of an elongated scattered group of about 20 stars, virtually indistinguishable from the surrounding star fields. The 6.5 magnitude star SAO 77310 lies near its eastern edge, and an 8th-magnitude reddish-orange star marks the western edge.
Messier 1 (NGC 1952): Sweeping 4° due south of DoDz 4 (or 1° northwest of Zeta (ζ) Tauri) brings us to this evening's showpiece object, Messier 1, the famous Crab Nebula supernova remnant. The Crab Nebula was bright and obvious as it crossed the southern margin of my field view, slewing southward from the previous object. I was reminded how easily this deep-sky wonder looses its luster under light-polluted or moonlit skies. Even at 30x its distinctively irregular profile and mottled surface were plainly evident as its minute form floated amid a panoply of glimmering stars. Increasing the magnification to 60x begins to give some form to the mottled, irregular face of the remnant. The characteristic S-shape becomes readily apparent, as does the dark notch in the eastern edge. Additional details play subtly across the face of the nebula, glimmering fitfully at the threshold of vision. At 120x, M1's low surface brightness begins to affect its appearance a little, but it is framed nicely in the field of view. The central part of the nebula appears a little darker, surrounded by a broken ellipse of brighter nebulosity (brightest to the southwest). A faint knot is glimpsed intermittently near the center of the ellipse. A faint outer envelope punctuated by a few slightly brighter wisps and patches of nebulosity surrounds the ellipse.
At this point I backed the magnification down to 90x and experimented a little with my meager filter collection. The Lumicon OIII and UHC filters gave similar views and helped pick out a few details with a little more certainty, but the view was a little too dark for my taste. The Parks ALP (anti-light-pollution) filter provided the superior view of the three filters, primarily by virtue of higher light transmission giving me a brighter view of the nebula while moderating the ambient skyglow. This was, without question, the best view I've had of Messier 1 through a six-inch scope.
As the prototypical supernova remnant, countless pages have appeared in the astronomical literature over the years, far more than I have the time or inclination to digest. Ken Hewitt-White (2006) gives an excellent overview of the Crab Nebula in a recent issue of Night Sky. It is without question one of the most intensely studied objects in the sky. On 4 July 1054 the supernova that generated the Crab Nebula blazed forth near Zeta Tauri and was bright enough to be seen in broad daylight according to the Chinese astronomers who documented the event. Interestingly, M1 was not discovered by Charles Messier, who included it as the first object in his famous catalog of non-comets; this honor goes to John Bevis, who observed the object in 1731, twenty-seven years before Messier's independent discovery on 12 September 1758. Messier 1 earned its nickname, the Crab Nebula, from a sketch made by Lord Rosse in 1844 while using a 36-inch reflector. As many have noted, the sketch looks more like a pineapple than a crab (Dewhirst, 1983). Lord Rosse repudiated the drawing in relatively short order, but the name stuck.
C. O. Lampland at Lowell Observatory first detected the expansion of the Crab Nebula in 1921. The nebula is expanding at a velocity in excess of 1600 kilometers per second and now measures nearly 6 light-years in diameter; it lies at an estimated distance of 6,000 light years (Strong & Sinnott, 2000). In 1954 Walter Baade discovered the strongly polarized nature of the light emitted by M1, implying a very strong magnetic field. At about this time, Dr. Baade also identified one of the two stars near the center of the nebula as the probable supernova remnant. The Crab Nebula was identified as a strong source of radio emission (among the 4 brightest radio sources in the entire sky) in 1948 by researchers at Jordell Bank in England (and is thus designated Taurus A by radio astronomers). X-ray emission was detected from the Crab Nebula by instruments aboard an Aerobee rocket in 1963; this emission was discovered (after several occultations by the Moon) to be confined to an area about 2' in size near the center of the nebula. The star, CM Tauri, at the heart of the Crab Nebula was identified as a pulsar in 1968. This pulsar spins at a rate of nearly 30 times per second (Burnham, 1978).
Recent studies of M1 with the Hubble Space Telescope have revealed that the nebulous filaments, first noted by Lord Rosse, are cloaked in glowing plasma. It is believed that these filaments form where high-speed plasma from the nebula's central regions interacts with a dense outer shell. Regions of magnetic instability have also been identified in areas where fingers of plasma fall inward toward the center (O'Meara, 1998). Several months of observations with the Chandra X-ray Observatory and the HST have been combined to produce movies illustrating dynamic changes in the Crab Pulsar's vicinity (these movies may be seen at: http://www.chandra.harvard.edu/photo/2002/0052/movies.html). These image sequences show a bright X-ray ring around the pulsar which astronomers characterize as an "unstable, quasi-stationary shock" formed by a wind of electrons and positrons streaming away from the pulsar's equator. As these particles slam into the ring, bright filaments are generated, which speed away from the pulsar at half the speed of light (Bobra, 2003), (Talcott, 2003).
Deep-Sky Challenge: [KC97c] G182.4+00.2
On cloudy nights (of which I've had plenty lately) I often get my astronomy fix prowling through old survey plates in The STScI Digitized Sky Survey. Recently I picked an area of the sky (pretty much at random) within the region covered by this observing report, and happened across this little "anonymous" patch of nebulosity in the northeastern corner of Taurus about 45' south-southwest of the 5th magnitude star 136 Tauri. SIMBAD identifies this as the HII (ionized) region [KC97c] G182.4+00.2. Its epoch 2000.0 coordinates are 05h 51m 52.0s, +27° 00' 42" and I estimate its diameter at about 7 arcminutes from the POSSII red image below (which measures 15' x 15'; north is up, east is left). I did not have much success with this nebula in my own scope, but it is prominent enough on the survey plates, that folks with larger instruments may have some success in spotting this object. I would love to hear from you if you manage to see it!
Digitized Sky Survey
We'll finish off this session with three small open clusters in the westernmost reaches of Gemini which make an interesting study in contrasts. From the Crab Nebula, find the bright blue giant star Zeta Tauri in your low-power eyepiece 1° to the southeast. Extending east-northeast from this star is a remarkable asterism consisting to two strings of stars, placed end to end. The westernmost string is almost perfectly straight, while the second string is also very straight but ends in a southward-pointing hook of stars at its eastern end. The whole thing measures nearly 2° in length. From the hooked end of this stellar walking stick, continue eastward another 3¼° to a point about halfway between the stars Chi-1 (χ1) and Chi-2 (χ2) Orionis (the two stars marking the top of Orion's club), and then slew northward 2.4° to reach the first of our three clusters:
NGC 2129 (H.VIII.26): This small, scattered group of stars is dominated by a pair of bright stars, 7th magnitude SAO 77842 and 8th magnitude SAO 77839. These bright stars are probably foreground objects, but they put a nice accent on an otherwise very faint assemblage of stars. William Herschel discovered NGC 2129 on 16 November 1784. There has been some debate concerning the reality of this cluster (Peña & Peniche, 1994), but the most recent studies (Carraro, 2006) seem to have reaffirmed the notion that NGC 2129 is indeed a young cluster of stars.
IC 2157: To locate this cluster, nudge your scope a mere 42' east to the 4th magnitude star 1 Geminorum, and then 46' north and slightly east. Here you'll find a faint, irregular grouping of stars, whose brightest members form a distinctive "V" with a few fainter stars scattered throughout. Look for a small circlet of faint stars just to the north of IC 2157, this is another cluster designated IC 2156. T. E. Epsin discovered both of these objects in 1900 (Archinal & Hines, 2003). IC 2157 lies on the outer edge of the Perseus arm of the Milky Way, nearly 8,500 light years from Earth (Kharchenko, 1997). IC 2156 has not been sufficiently studied to determine whether it is a true cluster or merely an asterism.
NGC 2158 (H.VI.17): Nudging your scope 35' eastward from IC 2157 will bring us to our final object for the evening, the tiny, but incredibly rich open cluster NGC 2158. This cluster resides in the same low-power field of view as the large and magnificent cluster M-35 and is usually given a quick look, almost as an afterthought, while viewing its brighter neighbor. It is worthy of a closer look on its own merits, however. At 30x, this distant cluster looks like a small, circular patch of grayish light, slightly brighter in the middle; the multitude of blazing stars which is M-35 glimmer seductively to the east – ignore them (for now). Increasing the magnification to 60x gives our little cluster a mottled appearance with many faint stars sparkling at the threshold of vision. A brighter star (about mag. 11) gleams on the southeast edge. The brighter central region seems to be elongated northwest-southeast and a little off-center to the southwest. Bringing the magnification up to 120x begins to resolve the brightest stars across the cluster's face; I was able to make out 15 stars with certainty while many others flickered in and out of view, particularly with averted vision. The brightest stars form a nearly-complete elliptical ring; the brighter limb of the ellipse runs through the center of the cluster, while the fainter limb delineates the southwestern edge. An elegant arrangement of field stars decorates the region northeast of the cluster.
Discovered on 16 November 1784 by William Herschel, NGC 2158 lies at an immense distance of 12,000 light-years, making it one of the most remote objects of its class accessible with amateur instruments. It is an old cluster (around 2 million years) whose brightest stars are red giants (French, 2003). Though it strongly resembles a globular star cluster (both on photographs and in the eyepiece) it is much younger than any known globular cluster in our galaxy, but despite this its constituent stars are remarkably metal-poor. It is postulated that this is because it formed on the outer edge of the Milky Way, where there is less concentration of the heavy elements. This makes NGC 2158 an important object of study for astronomers interested in the chemical composition of the galaxy (Carraro, 2002). More recently, NGC 2158 has been targeted by a team of researchers looking for planetary systems in star clusters (Mochejska, 2004); so far they've managed to identify 57 variable stars, most with low-amplitude magnitude fluctuations.
REFERENCES & ADDITIONAL READING
Archinal, Brent A. and Steven J. Hynes. 2003. Star Clusters. Willmann-Bell, Inc. Richmond, Virginia.
French, Sue. 2005. Spotlight On: A Study in Contrasts. Sky & Telescope. 109:2:32.
Galadí-Enríquez, D., et al. 1998. The overlapping open clusters NGC 1750 and NGC 1758. I. UBVRI-CCD photometry. Astronomy and Astrophysics. 333:471-478.
Galadí-Enríquez, D., et al. 1998. The overlapping open clusters NGC 1750 and NGC 1758. II. BVR photographic photometry and proper motions. Astronomy & Astrophysics Supplement Series. 131:239-258.
Galadí-Enríquez, D., C. Jordi and E. Trullols. 1998. The overlapping open clusters NGC 1750 and NGC 1758. III. Cluster-field segregation and clusters physical parameters. Astronomy and Astrophysics. 337:125-140.
Geffert, M., et al. 1996. The astrometric accuracy of "Carte du Ciel" plates and proper motions in the field of the open cluster NGC 1647. Astronomy & Astrophysics Supplement Series. 118:277-282.
Gieren, Wolfgang P. 1987. A Note on Several Galactic Cepheids in Open Clusters and Associations. Publications of the Astronomical Society of the Pacific. 100:262-265.
Hewitt-White, Ken. 2006. Science Spotlight: The Crab Nebula. Night Sky. 3:1:29-34.
Kaler, James B. 2005. Targets: Through Taurus to the Anticenter. Sky & Telescope. 109:2:90-94.
Kepple, George Robert and Glen W. Sanner. 1998. The Night Sky Observer's Guide. Vol. 1. Willmann-Bell, Inc. Richmond, Virginia.
Kharchenko, N, V. Andruk and E. Schilbach. 1997. Schmidt survey in the Galactic anticentre direction. 1. Investigation of open clusters. Astronomische Nachrichten. 318:5:253-266.
Luginbuhl, Christian B. and Brian A. Skiff. 1989. Observing Handbook and Catalogue of Deep-Sky Objects. Cambridge University Press, Cambridge, United Kingdom.
Mochejska, B. J., et al. 2004. Planets in Stellar Clusters Extensive Search. II. Discovery of 57 Variables in the Cluster NGC 2158 with Millimagnitude Image Subtraction Photometry. The Astronomical Journal. 128:1:312-322.
O'Meara, Stephen James. 1998. Deep-Sky Companions: The Messier Objects. Cambridge University Press, Cambridge, United Kingdom.
O'Meara, Stephen James. 2000. Observer's Log: My Favorite Deep-Sky Wonders. Sky & Telescope. 100:2:105-111.
Peña, J. H. and R. Peniche. 1994. uvby Photometry of Open Clusters. IV. NGC 1444, NGC 1662, NGC 2129, NGC 2169, and NGC 7209. Revista Mexicana de Astronomía y Astrofísica. 28:139-152.
Pişmiş, P., and Ş. Bozkurt. 1977. A Study of the Diameter and Luminosity Function of Open Clusters Based on Starcounts. Astronomy & Astrophysics Supplement Series. 30:81-87.
Seronik, Gary. 2001. Binocular Highlight: NGC 1647: The Crab Cluster. Sky & Telescope. 101:1:108.