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View Full Version : Object of the Week, May 17, 2015 - M101, NGC 5457, Arp 26



Howard B
May 19th, 2015, 03:14 AM
Spiral galaxy – SAB(rs)cd
Ursa Major
RA 14h 03m 12.6s
DEC +54° 20′ 57″
Magnitude: 7.86v
Apparent Size: 28′.8 × 26′.9
Distance: ~21 million light years
Discovered by: Pierre Méchain on March 27, 1781

M101 is not only a beautiful face on galaxy it also has the largest grouping of NGC objects in such a small apparent space - ten. Eleven, if you count M101 itself as NGC 5457. But that’s not the beauty of M101 to me, its real magic is seeing its stretched out grand design spiral arms and tidal tail all in one grand view as shown in this DSS image:

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But make no mistake, just because M101 is a Messier object doesn't mean it’s bright or easy to see in a suburban sky. You’ll need a truly dark and transparent sky to fully appreciate its overall shape and subtle details.

At first glance, M101 looks to have a fairly round overall shape, which is much different than what we see in its photos. It takes a great sky for its two faint trailing arms and overall asymmetry to become visible. You might also see the tidal tail stretching off the brightest arm, near NGC 5462, on a night like this. There are other knots and dark areas spread throughout M101 other than the ten NGC objects as shown in the chart below that you might see as well.

By the way, each of the NGC objects that belong to M101 are made up of multitudes of separate objects that we see grouped together because of their small apparent size and individual faintness. Only by being bunched near each other do we see them as the knots that embellish the spiral arms of M101. This seems to be generally true of HII regions we see in distant galaxies.

My favorite area in M101 is the pair NGC 5447 and NGC 5450, which are close enough together to often appear as one object. The best nights will show them as two separate and rather large nebulous patches with ragged edges, and they're placed near the end of the two fainter trailing arms of M101. I'm still trying to get a good look at the dark lane near the core, but I suspect that will take a nearly perfect night with my 28 inch.

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My drawing was built up by six hours of sketching at the eyepiece, spread out over several nights and two years, and every second was a joy.

NGC 5471: ~ 74 known HII regions, star clusters, SNR’s, x-ray and UV sources and compact galaxy cluster.
NGC 5462: ~ 117 known HII regions, star clusters, SNR’s, x-ray and UV sources and compact galaxy cluster.
NGC 5461: ~ 50 known HII regions, star clusters, SNR’s, x-ray and IR sources.
NGC 5458: ~ 19 known HII regions, star clusters, SNR’s, x-ray and UV sources.
NGC 5455: ~ 14 known HII regions, star clusters, SNR’s, x-ray, radio and UV sources.
NGC 5453: ~ 25 known HII regions, star clusters, SNR’s and UV sources.
NGC 5451: ~ 27 known HII regions, UV source and galaxies.
NGC 5450: ~ 40 known HII regions, star clusters and x-ray sources.
NGC 5449: ~ 27 known HII regions, compact galaxy cluster and galaxies.
NGC 5447: ~ 21 known HII regions, star clusters and x-ray sources.

Region A: ~ 14 known HII regions, star clusters, x-ray sources and compact galaxy cluster.
Region B: ~ 20 known HII regions, star clusters, SNR, IR and x-ray sources.
Region C: ~ 26 known HII regions and star clusters.
Region D: ~ 8 known HII regions and star clusters.
Region E: ~ 72 known HII regions, star clusters, SNR, and x-ray sources.
Region F: ~ 10 known HII regions, star clusters, x-ray and UV sources.
Region G: ~ 9 known HII regions, star clusters and UV sources.
Region H: ~ 67 known HII regions, star clusters x-ray and UV sources.

Sources: Aladin, Simbad and NED

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GIVE IT A GO AND LETS US KNOW!

Jimi Lowrey
May 19th, 2015, 03:24 PM
Nice OOTW Howard!

Do you have more info on the tidal tails? I was not aware of it.

Howard B
May 19th, 2015, 07:35 PM
Hi Jimi,

M101's distorted shape was probably cause by an encounter with NGC 5474, an oddly shaped galaxy about a degree to the south, which is opposite the direction I would expect that it could pull out a tidal tail, but that's only a guess on my part. I'd be interested to find out more about it too.

Also, I'm posting the correctly labeled drawing that corresponds with the list above.

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Ivan Maly
May 21st, 2015, 03:28 AM
Here is an old sketch I did with 16" shortly before the supernova. The SN was observed late that year under terrible conditions. I have revisited this galaxy just recently with 20"; there is lots more to see in it.

http://2.bp.blogspot.com/-RDgqT3UTdNI/Tcny7RcQm0I/AAAAAAAAABc/dISJEtf866U/s400/M101+verified+only+%D0%BA%D0%BE%D0%BF%D0%B8%D1%8F. jpg

Uwe Glahn
May 21st, 2015, 09:01 PM
Very realistic sketch Howard, wow! M 101 is always worth a look.

My first attempt to separate the arms was with my old 5" Binoscope. Unfortunately I only had a power of 20x but it was possible to pick up some arm structures. I remember the observation as very hard but I think the structures are real. For the brighter but smaller HII region the power was seemingly to small. Nice byproduct were the surrounding galaxies in the field of 3° around M 101.

20x125, fst 6m5+, field: 3°
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Next attempt was much more successful. I don't notice my observing/sketching time but it was nearly as long as your time.

14,5", 83x-141x, fst 6m5+
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Last result was a very detailed observation of the brightest HII region NGC 5471. I'm very happy with the result, because I was surprised how much detail this HII shows. The seeing was very good.

27", 1172x, fst 6m5+, Seeing II
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Howard B
May 23rd, 2015, 12:27 AM
Wow, awesome drawing of 5471 Uwe - the seeing must have been nearly perfect to see that much detail!