Puppis’ Best Open Clusters
By Paul Markov, December 1999.

Canada is not the best place to be "doing" astronomy in the wintertime. The cold winter temperatures make our hobby unpleasant, difficult, and sometimes dangerous. Rather than packing your telescope away until next Spring, why not do some observing from your own backyard in the city, where warmth and comfort are only steps away? Fortunately, the winter sky is host to several bright deep sky objects that can be viewed from the city. In this installment I will review some of the brighter objects in the constellation Puppis.

Puppis is a constellation located in the heart of the winter Milky Way, and can be found to the south and east of Canis Major. Don’t let some witty stargazer fool into thinking Puppis is the constellation of the "cute and cuddly little dog"! Puppis actually represents a ship’s stern, and is a fragment of the ancient constellation Argo Navis, the ship. The other two modern constellations that originated from Argo Navis are Carina, the keel, and Vela, the sail.

Due to its southerly declination, Puppis spends little time above the horizon as seen from Toronto. In late January, for example, Puppis rises in the east around 8 p.m., and by 4 a.m. it has nearly set. Compare that to Leo, which also rises at about the same time, but by 4 a.m. it has only just crossed the meridian, and sets around 10 a.m. The bottom line is that stargazers have relatively little time to observe objects in this constellation, so if you let Puppis slip by in the next couple of months, you’ll have to wait until next year.

Of the 148 deep sky objects listed in the Saguaro Astronomy Club database (free download at http://www.saguaroastro.org/), 112 are open clusters. This should come as no surprise because Puppis lies within the star-abundant boundaries of the Milky Way. Once again we are in luck because if we happen to be observing from urban areas, open clusters are the least affected by light pollution. Typically, deep sky objects that present themselves as point sources of light, as opposed to diffuse and extended sources of light, will be affected by light pollution the least. To prove this to yourself, try observing a galaxy and an open cluster from the city, both having a magnitude of eight - I can assure you the galaxy will be rather difficult to see, while the cluster will be fairly obvious.

Still, be aware that if an open cluster is small, compact, and relatively faint, it will take on the appearance of a "fuzzy blob", rather than several points of light that is typical of an open cluster, and light pollution will affect it considerably. Therefore, if you are planning a city observing session, choose clusters that are relatively bright, larger, and scattered.

All sixteen open clusters listed below are relatively bright, however some may not be visible from the city due to their low altitude above the horizon. The compounding of atmospheric extinction and light pollution may prove to be too much to overcome for the fainter "horizon hugging" clusters. Try them all anyway, and save the ones you cannot find for a more transparent night or for your next outing to a dark sky. The best time to observe these clusters in late January is between 11 p.m. and 1 a.m., and in late February is between 9 p.m. and 11 p.m.

Object R.A. DEC

Mag

Size

Uranometria Map #

Sky Atlas Map #

Collinder 135 07 17.0 -36 50

2.1

50.0'

361

19

Bochum 5 07 30.9 -17 04

7

11.0'

319

12

Bochum 4 07 31.0 -16 57

7.3

23.0'

319

12

NGC 2422 (M47) 07 36.6 -14 30

4.4

30.0'

274

12

NGC 2423 07 37.1 -13 52

6.7

19.0'

274

12

NGC 2439 07 40.8 -31 39

6.9

10.0'

362

19

NGC 2437 (M46) 07 41.8 -14 49

6.1

27.0'

274

12

NGC 2447 (M93) 07 44.6 -23 52

6.2

22.0'

320

19

NGC 2451 07 45.4 -37 58

2.8

45.0'

362

19

NGC 2477 07 52.3 -38 33

5.8

27.0'

362

19

NGC 2482 07 54.9 -24 18

7.3

12.0'

320

19

Collinder 173 08 04.0 -46 00

0.6

370'

396

20

NGC 2527 08 05.3 -28 10

6.5

22.0'

362

20

NGC 2539 08 10.7 -12 50

6.5

22.0'

275

12

NGC 2546 08 12.4 -37 38

6.3

41.0'

362

20

NGC 2571 08 18.9 -29 44

7

13.0'

362

20

Collinder 135 (Cr 135) will definitely be a pleasant sight in a rich field telescope or binoculars. At nearly one degree in size, this cluster contains only a few stars brighter than tenth magnitude, but you will not miss it as long as you spot p (Pi) Puppis, a magnitude 2.7 star on the southern edge of this cluster. Because Cr 135 never rises more than ten degrees above the horizon (as seen from Toronto) you may have trouble seeing many of its fainter members.

Bochum 4 (Bo 4) and Bochum 5 (Bo 5) will definitely require a telescope and moderate power, especially from the city. Despite being slightly fainter, you will find Bo 4 easier to see than Bo 5 because it is twice the size and with about twenty members, it contains twice the number of stars. Bo 5 will be more difficult to observe relative to the other open clusters listed here because it has fewer stars and a strong central concentration, making it a nebulous object that is severely affected by light pollution.

M46, M47, and M93 are showpieces relative to these other clusters, and should all be visible from the city even with smaller instruments. If you are observing with a telescope, increase the magnification and try spotting NGC 2438, an eleventh magnitude planetary nebula located on the northern edge of M46.

I first viewed NGC 2423 from the city with a four-inch reflector and noted it to be small, faint and poor. However, if you are observing with an eight-inch telescope from darker skies, you may see up to twenty or thirty resolvable stars. NGC 2423 is located just half-a-degree north of M 47.

NGC 2439 is a small, rich, compact cluster that will not disappoint. Up to forty stars may be seen with an eight-inch telescope. At nearly seventh magnitude and a declination of minus thirty-one degrees, this cluster may be out of reach from light polluted skies.

At three-quarter degrees in size NGC 2451 is a large, bright open cluster that’s sure to impress even with a small instrument, given favourable observing conditions. This sparsely populated cluster contains about a dozen bright stars that can be seen from the city. However, most stars belonging to this cluster will be hidden from view by light pollution and atmospheric extinction due to its low altitude.

NGC 2477 is located just 1.5 degrees south-east of NGC 2451, and although not as bright as its neighbour, it will most likely be more impressive. Under favourable conditions, you should be able to see about 160 stars with an eight-inch instrument, most of them concentrated in the core of the cluster. Unfortunately NGC 2477 never rises higher than eight degrees as seen from Toronto, so it will be fainter than you would expect and you will see a lot fewer stars. At low power NGC 2477 may take on the appearance of a loose globular cluster.

NGC 2482 is located just over two degrees east of M93. At a magnitude of only 7.3 and a size of only twelve arc-minutes, this object may be a little more difficult to spot from light polluted skies, however it should still be visible even in a smaller instrument. Using an eight-inch telescope from dark skies I spotted this cluster easily at 62X and described as faint but fairly large. Twenty to thirty visible stars make up this cluster.

Collinder 173 (Cr 173) is an unusual open cluster. Immediately evident are its brightness, its size, and its very low altitude as seen from Toronto. Its status as a real open cluster is unclear to me, since it is listed as one in the Saguaro Astronomy Club database, but it is not listed or plotted in Uranometria and its Field Guide. Centered on 8 hr 4 min, minus 46 degrees, this cluster only shows about 60% of itself to people observing from Toronto, while the other 40% never rises above the horizon. Its enormous apparent size and brightness make it a perfect candidate for binocular viewing, still, Cr 173 will be a difficult object to observe. Look for at least a dozen stars ranging from fifth to sixth magnitude scattered across a six-degree field of view. You will need a completely unobstructed view of the southern sky, it must be a perfectly clear night free from haze, and you should have as little light pollution as possible. Atmospheric extinction may make this cluster more difficult to observe than you would expect. Don’t use a telescope to view this cluster, even if you have a rich-field telescope. Only binoculars or a finderscope will allow you to view what will look like a very poorly populated and scattered cluster.

NGC 2527 and NGC 2539 share identical physical data. You will find both of these fairly easy to spot as they are bright and large. With about forty stars, NGC 2527 is moderately rich with a displays a slight central concentration. NGC 2539 contains about fifty stars and is slightly scattered. There are several clumps of stars, many in pairs. If you were to draw an outline around this cluster, you would notice a slightly triangular shape. You can’t miss 19 Puppis, a magnitude 4.7 star on the southern edge of the NGC 2539.

NGC 2546 is five degrees due east of NGC 2451. If it were not for its low altitude this cluster would be quite spectacular. As seen from Toronto, it never rises higher than nine degrees from the horizon. This scattered cluster is two-thirds of a degree in size and will most likely fill your entire field of view with about forty faint stars under favourable observing conditions and an eight-inch telescope.

NGC 2571 is a modest cluster and may be a little hard to see from light polluted skies. Expect to see a small faint fuzz, with perhaps a few brighter stars. From darker skies you may see up to thirty stars at medium power.

Copyright (C) 1999 by Paul Markov


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