6 Attachment(s)
Object of the Week May 3, 2015 - NGC 3521 A Flocculent Galaxy
Object of the Week May 3, 2015 - NGC 3521 A Flocculent Galaxy
NGC 3521, UGC 6150, PGC 33550, MCG 0-28-30
Type: SAB(rs)bc
RA: 11h 05m 48.8s
Dec: -00° 02’ 06”
Size: 11.0’ x 5.1’
Mag: 9.0V 9.9B
Mean Surface Brightness: 22.1 mag/arcsec2
I recently came across a thread on another popular astronomy site where the discussion centered on contemporary print and electronic resources focused on deep sky observing. It was no surprise to me to find Deep Sky Forum (DSF) among those resources mentioned by several posters. However what did surprise me as one person’s comment that the many of the deep sky objects discussed here on DSF “require large telescopes” and “are not visible with modest aperture.” Others then chimed in to state that on the contrary many of the objects discussed here are indeed visible in 8” to 10” telescopes.
By conducting a non-scientific and non-statistical visual scan through the almost three and half years of Object of the Week (OOTW) postings there appears to be a slight bias toward apertures of 18” and greater. It is a fact that several of the long-time OOTW contributors do observe with telescopes in the range of 18” through 28”, and in one case, a 48” telescope. Perhaps this is the source of the slight bias.
Regardless, do not let these “large telescope” observation reports by certain members scare you off. Forget about whether size matters and instead focus on the tag line that concludes every OOTW article – “GIVE IT A GO AND LET US KNOW”
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Size Matters Not - Courtesy Uncle Rod (Mollise)
“GIVE IT A GO” This is the challenge you are given for each OOTW. I suggest you step up to this challenge each and every time you go searching for a deep sky object (DSO) to observe. Large aperture certainly helps, but there are many other factors that will contribute to or hinder a successful observation. A few examples are the sky and weather conditions at your observing site, and your selection of eyepieces and filters. Believe it or not your physical and mental acuity plays a huge role in the quality of your observation. The list goes on but I’ll not belabor the point further beyond suggesting that the road you take and whether or not you arrive at your destination, i.e., the target DSO, must begin with knowing your destination. Quite simply, if you don’t know where you are going, how are you going to get there? This I believe is the primary reason that many amateurs observe the Messier objects over and over and over. I know. I’ve been there and done that.
When DSF members share an exotic DSO they will typically note if the observation requires some minimum aperture. But even that isn’t and shouldn’t be written in stone. Just because a description may originate from an observation with a 25” scope doesn’t mean you shouldn’t try chasing the target with a 10” scope. That is the challenge – GIVE IT A GO – try it! Maybe you will surprise even yourself.
Now don’t forget the second half of the tag line – LET US KNOW. Share your experiences and observations. Tell us what could be seen and what couldn’t? How does the observation change if you take that 10” scope to a much darker site, or higher elevation? Share your results even if the target DSO wasn’t visible.
Regardless of the outcome remember to think about and enjoy the absolutely most awesome, cool and incredible deep sky objects that you have come to learn. Here are just a few of them from past months to whet your appetite - extragalactic messes, train wreck galaxies, superthin galaxies, mystical open clusters, a rare Hoag-type galaxy, a dissolving planetary, and a discordant quasar. And now to add to that list – a flocculent galaxy.
I used a new approach to select this week’s OOTW – NGC 3521. Using my recently acquired copy of Interstellarum Deep Sky Atlas, I picked a chart of Leo (#58) and then using the atlas’ unique visibility encoding, I randomly selected a DSO “that is visible in a 4” (100mm) scope under reasonably dark skies.” In fact after observing this beautiful spiral galaxy you will question why Charles Messier never cataloged it.
It was only in the year that Messier published the final version of his catalogue, 1784, that William Herschel discovered NGC 3521 early on in his more detailed surveys of the northern skies. Through his larger, 31-inch telescope, Herschel first observed NGC 3521 on February 22 [1784]. His discovery log entry (H I-13) reads, “Very bright, much extended in the direction of the meridian of the nebula, suddenly much brighter middle, 7’ or 8’ in length.” Herschel classified the object as a bright nebula.
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A negative image courtesy the NGC/IC Project
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Image by Dale Liebenberg
Today we know that NGC 3521 is a spiral galaxy seen at an oblique angle. At a distance of 23 million light-years, it is a large object measuring 72,000 light-years in true physical extent. With a mass equivalent to some 150 billion Suns, NGC 3521 is equal to or nearly as bright as many of the Messier galaxies in Leo, Virgo, and Coma Berenices. It is a full magnitude brighter than M108 and a half magnitude fainter than M82.
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In this ESO picture from the Very Large Telescope (VLT), colourful, yet ill defined, spiral arms replace Herschel’s “nebulosity”. Older stars dominate the reddish area in the center while young, hot blue stars permeate the arms further away from the core.
NGC 3521 is a mixed spiral system, having characteristics of both a barred spiral and a normal spiral galaxy. It has a morphological classification of SAB(rs)bc which indicates that it is a spiral galaxy with a trace of a bar structure (SAB), a weak inner ring (rs), and moderate to loosely wound arm structure (bc). Photographs show that NGC 3521’s spiral structure is choppy and disjointed. Systems with this type of patch fragmentation and discontinuous spiral arms are known as flocculent spiral galaxies. A few other well known flocculent galaxies are NGCs 2841, 4414, and 7793 as well as M101.
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Image courtesy SDSS-3
Use this chart to find NGC 3521 in southern Leo.
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In The Night Sky Observer’s Guide Volume 2 by George Kepple and Glen Sanner, you find this description of NGC 3521 in 8/10” telescopes at 100x: “NGC 3521 is 30’ east of 62 Leonis (m5.9) and 11’ NW of an 8th magnitude star. This fine galaxy shows a well concentrated oval core containing bright stellar nucleus surrounded by a much fainter somewhat diffuse 5’ x 2’ NNW-SSE halo.”
In their Observing Handbook and Catalogue of Deep-Sky Objects, authors Christian Luginbuhl and Brian Skiff call the galaxy beautiful and describe the view through a 12-inch telescope: “The ends are ragged, the core and halo mottled. The oval core is roughly centered, but the brightest parts become progressively more eccentric to the [western] edge, where a dark lane 20” wide passes.”
Again from The Night Sky Observer’s Guide Volume 2 you find this description of NGC 3521 in 16/18” telescopes at 150x: “NGC 3521 is a fabulous object for large instruments! The halo is bright, elongated 8’ x 3’ NNW-SSE, and contains a large, bright, extended core with a stellar nucleus. Both the core and halo are mottled. The halo’s periphery is irregular in brightness and extends further west then east from the galaxy’s center. With averted vision a patchy dark lane can be seen along the halo’s west edge. A 14th magnitude star is 3’ NW of the galaxy’s center with a 13th magnitude star just beyond.”
Now it’s your turn. Regardless of your telescope’s aperture…
“GIVE IT A GO AND LET US KNOW”
I look forward to reading everyone’s observation reports and seeing their sketches of the beautiful, flocculent spiral galaxy, NGC 3521.
GOOD LUCK AND GREAT VIEWING!
