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VY Canis Majoris -- the largest known star

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#1 sgottlieb


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Posted 28 February 2011 - 03:39 PM

Conditions were superb on Saturday night at Lake Sonoma, nestled in the vineyards of Sonoma county, despite the mostly crummy weather we've had in the San Francisco bay area this winter.

Satellite images during the day showed there might be a clear window on Saturday, but I was still a bit pessimistic as observing conditions right after a front has passed are often poor with high water vapor content cutting down transparency and turbulent seeing conditions. But I took a look at the Trapezium and knew we in for a good night as the 5th and 6th star were widely resolved and tack sharp. Later I checked the seeing by resolving an 0.8" double in Leo Minor. The sky was as dark as it generally gets at Lake Sonoma (edge of blue/green light-pollution zones) with consistent SQM readings of 21.46 to 21.52.

I started off with a look at VY Canis Majoris, an orange-red hypergiant star that varies from mag 7.4-9.6 and surrounded by an envelope of ejected material forming a reddish reflection nebula. It also holds the current record for the largest known star in the galaxy (1800–2100x the solar diameter) as well as one of the brightest (500,000x the solar luminosity).

At 175x it appeared slightly fuzzy or soft, like a brighter star that wouldn't focus in poor seeing. At 285x unfiltered, a very small non-stellar orange disc was clearly visible surrounding a brighter center. More surprisingly a short, slightly curving "tail" extended from the glow to the west. At 325x, the central star was cleanly resolved within a very small, 6" round halo. The nebulous tail or filament (ejecta from the star) swept ~8" to the WNW. The small reddish reflection nebula is ejected material from previous outbursts.

I spent awhile working through two dozen members of the Cancer I galaxy cluster. This is a relatively nearby (~220 million light years), loose cluster spread out over three degrees in Cancer with an unusually high percentage of spirals. One interesting spiral/elliptical merger? is UGC 4332. Check out the SDSS image at tinyurl.com/4t4smk9 . Unfortunately there was no sign of the striking dust lane in my scope. Most of the fainter members of Cancer I are in the mag 15-16 range, but here are some of the brighter NGC galaxies --

NGC 2556
08 19 00.9 +20 56 13
V = 14.4; Size 0.6'x0.3'; PA = 138d
very faint, very small, round, very small brighter core, sharp stellar nucleus. A wide pair of mag 13.14 stars are within 2' SSW and a mag 10.5 star is 3.5' SE. Located 24' WSW of N2563.

NGC 2557
08 19 10.8 +21 26 09
V = 13.2; Size 1.2'x1.0'; Surf Br = 13.3; PA = 55d
fairly faint to moderately bright, fairly small, round, 30" diameter, gradually increases to a small brighter core. Forms the eastern vertex of a small isosceles triangle with two mag 13.5 stars 2.5' NW and 2.7' SW. Also forms a larger isosceles triangle with mag 7.8 HD 69698 situated 5' NW and a mag 11 star 5' SSW. IC 2293 lies 5.6' SE. This fainter galaxy appeared faint, small, round, 20" diameter, low even surface brightness.

NGC 2558
08 19 12.8 +20 30 38
V = 13.0; Size 1.7'x1.3'; Surf Br = 13.7; PA = 160d
fairly faint to moderately bright, fairly small, elongated 4:3 or 3:2 NNW-SSE, 32"x24", sharply concentrated with a 15" bright core.

NGC 2560
08 19 51.9 +20 59 06
V = 13.3; Size 1.4'x0.3'; Surf Br = 12.4; PA = 93d
fairly faint, fairly small, elongated 5:2 or 3:1 E-W, 0.8'x0.3', sharply concentrated with a small bright core and fainter extensions. Located 1.5' E of a mag 10.4 star and 11' SW of N2563.

NGC 2562
08 20 23.7 +21 07 53
V = 12.9; Size 1.0'x0.7'; Surf Br = 12.3; PA = 3d
moderately bright, fairly small, elongated 4:3 N-S, 30"x24", contains a small bright core that gradually increases to the center. Forms a nice pair with brighter N2563 4.7' SE. N2560 lies 11' SW and UGC 4332 is a similar distance W.

NGC 2563
08 20 35.7 +21 04 04
V = 12.2; Size 2.1'x1.5'; Surf Br = 13.3; PA = 80d
moderately to fairly bright, moderately large, round, 1.0' diameter, pretty sharply concentrated with a bright, 20" core. With direct vision, the core increases to brighter stellar nucleus. Slightly brighter and larger than N2562 4.7' NW. Located 20' NNE of mag 5.8 HD 69994. This is the brightest galaxy in the core region of the Cancer I cluster.

NGC 2569
08 21 21.1 +20 52 03
V = 14.3; Size 0.6'x0.5'; Surf Br = 12.8; PA = 119d
faint, very small, round, 15" diameter, very small bright core in a fainter halo. A thin, distinctive triangle of mag 13 stars (separations roughly 12", 30", 30") is 1.5' following. Forms a pair with N2570 2.6' N. Located 16' NE of mag 5.8 HD 69994.

NGC 2570
08 21 22.6 +20 54 37
V = 14.5; Size 1.1'x0.6'; Surf Br = 13.9; PA = 70d
very faint, small, slightly elongated ~E-W, 0.4'x0.3', very low even surface brightness, no noticeable core or zones. Appears larger than N2569, located 2.6' S, but more difficult due to a lower surface brightness. Located 14.5' SE of N2563 (brightest galaxy in the region) and 17' NE of mag 5.8 HD 69994.

While in Cancer, I also decided to take a look at Abell Galaxy Cluster 671. While the Cancer I cluster is probably an unbound group, AGC 671 is a very tight knot of galaxies with a half-dozen packed into a 6' circle including IC 2374, IC 2376, IC 2378 and IC 2380. The brightest of these has a B mag of only 14.9 and I was also able to glimpse a few members as faint as B = 16.4.

IC 2374
08 28 22.1 +30 26 36
V = 15.3; Size 0.5'x0.45'; PA = 163d
extremely faint to very faint, very small, round, 15" diameter. Forms the NW vertex of an equilateral triangle with IC 2378 2.2' ESE and IC 2376 2.3' SSE. CGCG 149-028 lies 1.4' NNE.

IC 2376
08 28 26.1 +30 24 28
Size 0.7'x0.45'; PA = 65d
faint to very faint, very small, elongated SW-NE, 18"x12". Located 25" NW of a mag 13.7 star and 1.8' SW of IC 2378.

IC 2378
08 28 31.6 +30 25 53
V = 14.0; Size 1.1'x0.8'; PA = 30d
faint to fairly faint, fairly small, slightly elongated SW-NE, 45"x35", low surface brightness halo with only a broad weak concentration, then sharply concentrated with a very small brighter nucleus. This is the largest and brightest galaxy in the core of AGC 671 with a number of faint galaxies nearby including IC 2374 2.2' WNW, IC 2376 1.8' SW, IC 2380 3' SE, CGCG 149-28 and CGCG 149-35.

IC 2380
08 28 43.9 +30 24 16
V = 14.4; Size 0.75'x0.6'; PA = 169d
faint, very small, round, 20" diameter, weak concentration. Situated 1.4' NE of a mag 12 star and 3' SE of brighter IC 2378.

Later on in the evening I took a look at Porrima (Gamma Virginis), a striking mag 3.5/3.6 double. Back in the early '80's this was one of my favorite doubles with a separation of around 4". But it started closing and by 2002 the separation was down to 1". In 2004, with the separation down to 0.5", all I could see was an elongated disc (nearly a figure 8), so I could tell the star was double! It reached a minimum separation below 0.4" in 2005, when it was merged in most amateur scopes, though has been slowly increasing since. The separation is now back to 1.6" and the components were cleanly split at 280x. I'd recommend a look this spring if you haven't observed this bright pair.

#2 GeneT


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Posted 28 February 2011 - 04:48 PM

Congratulations--great viewing session!

#3 reiner


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Posted 01 March 2011 - 05:19 AM

Very interesting observation on VY CMa! I will give this one definitely a try. From your description, it resembles a bit R Aqr, though that one is a double star. What is the cause of the curved tail with VY CMa?

#4 HellsKitchen



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Posted 01 March 2011 - 06:00 AM

Great report. Your decription of VY CMa is fascinating. Must put it on my obs list!

#5 cliff mygatt

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Posted 01 March 2011 - 09:25 AM

Nice report. I can only wish for clear skies in the Pacific Northwest. I am so ready to get out for a few days and go deep.

#6 sgottlieb


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Posted 01 March 2011 - 03:00 PM

Great report. Your decription of VY CMa is fascinating. Must put it on my obs list!

The small nebula surrounding the star was discovered visually back in 1917 using a 4.8-inch scope at the Cordoba Observatory, yet I'm not aware of any modern catalogue designation as a (reflection) nebula. Here's the discovery description:

"Senor Guerin described the object as a nebula about 8" x 12", red, tending toward dark red or scarlet; as containing three nuclei, the preceding of which is the brightest and the point which he observed for position. The general aspect is that of a comet, the tail (which is excessively faint) extending to the east (Power 220)."

If the seeing is good, jack up the magnification and see if you can see multiple nuclei. Guerin described the "tail extending to the east", but I saw the tail as heading west or WNW.

#7 sgottlieb


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Posted 01 March 2011 - 03:08 PM

Very interesting observation on VY CMa! I will give this one definitely a try. From your description, it resembles a bit R Aqr, though that one is a double star. What is the cause of the curved tail with VY CMa?

Hi Reiner, here's a study (based on HST/WFPC2 images) on the knots and arcs surrounding VY CMa (the tail I described being the brightest).

#8 hbanich



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Posted 01 March 2011 - 03:30 PM

Great report. Your decription of VY CMa is fascinating. Must put it on my obs list!

Me too! Hopefully some clear, dark skies are on their way before VY gets too low in the southwest.

#9 rookie


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Posted 02 March 2011 - 02:21 PM

Here's a popular YouTube Video that was featured on APOD 2/22/11:
Star Size Comparisons

#10 nytecam


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Posted 05 March 2011 - 04:11 PM

Thanks Steve - excellent observation report and a timely reminder for me to try for a pic of VY CMa which I did last night very low in the southern murk from London ;) No filter used so this IR emitter records overexposed with a cyan fringe due to the effect of colour skewing. Field stars to ~m14 :rainbow:

Also tried for PNe in Orion eg Abell 12 [easy] and Abell 14 but not recorded @ mp18.2 ;)

#11 Yaar


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Posted 05 March 2011 - 08:01 PM

Thank you Steve for the detailed report and for the heads-up about the VY CMa nebula. I'll be sure to observe VY CMa with the 18" and hopefully also with the 24" next month when we have a 'rutjuga' - assuming clear skies, of course.


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