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Cosmic Challenge: Polarissima

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Cosmic Challenge: Polarissima


April 2022

Phil Harrington

This month's suggested aperture range:

10-inch (25 cm) to 14-inch (36cm) telescopes













11h 47.2h

+89° 05.6'

Ursa Minor





In astronomy, as in real estate, we have the Three Ls: location, location, location.  NGC 3172 is a challenge for all seasons -- literally -- as its location keeps it above the horizon throughout the year no matter what time of night you are looking.  That's because NGC 3172 lies within 1° of the North Celestial Pole, closer than any other NGC object.


John Herschel discovered NGC 3172 during a deep-sky sweep with his 18.7-inch telescope in 1831.  He later christened his new "nebula" Polarissima Borealis, or simply Polarissima, for its distinctive location.


Above: Evening star map showing the location of this month's Cosmic Challenge.


Credit: Map adapted from Star Watch by Phil Harrington


Above: Finder chart for this month's Cosmic Challenge.


Credit: Chart adapted from Cosmic Challenge by Phil Harrington
Click on the chart to open a printable PDF version in a new window



Apart from its location near the top of the celestial sphere, NGC 3172 holds no distinguishing characteristics that set it apart from the throng of similar galaxies to its south.  Classified as a lenticular system, Polarissima shines at 14th magnitude and measures just 1' in diameter. It is estimated to be about 285 million light-years away and span about 85 thousand light-years across.


Twelve years ago, a supernova flared along the outer edge of NGC 3172. It was discovered by Tom Boles, an amateur astronomer in the UK who has built an observatory dedicated to finding extragalactic supernova. You can see his discovery image on his website here. According to the discovery announcement from the International Astronomical Union's Central Bureau for Astronomical Telegrams, the supernova, classified as SN2010af, attained a magnitude of 17.1 on March 4, 2010, before fading.


Polarissima is easiest to find by casting off from Polaris, so take aim at the pole star through your finder.  If you look carefully, you should see that it belongs to a circlet of faint stars that remind many of a heavenly engagement ring.  Polaris serves as the diamond, while fainter stars fill out the rest of the ring.  Two of the ring stars closest to Polaris, 6th-magnitude Lambda (λ) Ursae Minoris and 7th-magnitude SAO 1401, make handy reference stars, as NGC 3172 is located almost exactly halfway between the two.


While it has been seen through apertures as small as 6-inches, it still presents a formidable test for 10-inchers, especially under less-than-ideal conditions.  Regardless of aperture, NGC 3172 is an object that requires high magnification.  Through my 10-inch at 181x, Polarissima shows off a subtle, round disk peppered with a brighter core and is accompanied by a 13th-magnitude field star just 2' away.


Above: NGC 3172 as portrayed through the author's 10-inch (25cm) telescope.


Incidentally, if you think the area around the North Celestial Pole is a desolate part of the sky, think again. While it may appear empty visually, the region is fraught with clouds of interstellar dust. Take a look at this post from March 2022 started by CN'er edif300. Clearly, there is a lot going on around the NCP!


Have a favorite challenge object of your own?  I'd love to hear about it, as well as how you did with this month's challenge.  Contact me through my website or post to this month's discussion forum.


Until next month, remember that half of the fun is the thrill of the chase.  Game on!

About the Author:

Phil Harrington writes the monthly Binocular Universe column in Astronomy magazine and is the author of 9 books on astronomy.  Visit his web site at www.philharrington.net to learn more.

Phil Harrington's Cosmic Challenge is copyright 2022 by Philip S. Harrington.  All rights reserved.  No reproduction, in whole or in part, beyond single copies for use by an individual, is permitted without written permission of the copyright holder.


  • okiestarman56 and AlwaysHungry like this


Dave Mitsky
Apr 01 2022 12:16 PM

I haven't observed NGC 3172 for quite some time.  I'll have to take another look at Polarissima the next time I'm at a site dark enough for the task.


There's another image of SN 2010af posted at http://www.deep-sky....ve/N3172-SN.JPG


    • PhilH, John O'Hara and erick86 like this

Found it with my 6" GSO. I posted last month so I'll let someone else post this month. But here's a peek at it from last night. Would be better on a bigger scope. John Herschel, like his dad, was really on the mark.

    • Dave Mitsky, PhilH, John O'Hara and 1 other like this
John O'Hara
Apr 20 2022 09:24 AM

For years after reading about this object in Scotty's "Deep Sky Wonders" column, I tried to spy this one out with my 6" f/8 refractor from the dark skies of Cherry Springs State Park, PA.  However, whenever I thought of it, the Astro-Physics 706 GEM was always polar aligned, and I did not wish to shift it to alt-az mode in the middle of a dark observing night.  I'm not sure if I could have found it with my 6 inch or not, but the difficulty of using a GEM around the NCP certainly did not help.  I have seen it without problem in my 12.5" Dob.

    • Dave Mitsky and PhilH like this

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