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Cosmic Challenge: Globular Clusters in the Fornax Dwarf Galaxy


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Cosmic Challenge: Globular Clusters in the Fornax Dwarf Galaxy

 

December 2016

 

Phil Harrington

This month's suggested aperture range

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

 

Let's begin this challenge with a riddle. What's big and round, close at hand, and yet nearly impossible to see? If you answered "the Fornax Dwarf Galaxy," then you are correct! The Fornax Dwarf, a dwarf spheroidal system, covers a 17'x13' area of our late autumn sky and lies about 530,000 light years from the Milky Way. That's well within the confines of our Local Group of galaxies. And with a magnitude rating of 9.3, it sounds like it should be bright and easy to see. But when we look its way, it's not there. Even the best photos manage to record only an incredibly dim, elliptical haze peppered by some 19th-magnitude stars!

Above: Autumn star map.

Credit: 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

 

The Fornax Dwarf Galaxy is a paradox. Even though the galaxy itself is beyond the range of our telescopes even from the darkest observing locations, four of its six known globular clusters are within the grasp of 10-inch, or maybe 12-inch, telescopes.

Of those distant globulars, NGC 1049 is the brightest, so we will begin there. Interestingly, the Fornax Dwarf was only discovered in 1938 by Harlow Shapley, but NGC 1049 was found a century earlier by John Herschel as he cataloged the southern sky from the Cape of Good Hope. Of course, Herschel never realized the true location or distance of what he had found.

Part of the challenge posed by NGC 1049 is in locating it. Fornax is not an easy constellation to see. Your best bet is to start at the pentagon representing the tail of Cetus the Whale and drop about 35° southward along the Cetus-Eridanus border to 3rd-magnitude Beta (β) Fornacis. Binoculars will certainly help with the trip. Once at Beta, look for a small isosceles triangle just to its south formed by Eta-1 (η -1), Eta-2 (η -2), and Eta-3 (η -3) Fornacis. Follow the triangle's "point" (Eta-1) toward the northwest to Lambda-2 (λ -2) Fornacis. NGC 1049 is about ¾° northeast of Lambda-2. 

Although some observers claim to have seen NGC 1049 in telescopes as small as 6 inches in aperture, it's usually considered a difficult catch in 10 inchers when viewed through suburban skies. My old 13.1-inch f/4.5 Newtonian showed NGC 1049 as a round glow measuring only about 1 arc-minute across and shining at about 13th magnitude. At 125x, I could just make out a vague star-like central core. Its nucleus became a little more obvious at 214x, but there was little hope of seeing any individual stars, the brightest of which shine at magnitude 18.4.

Three of the Fornax Dwarf's other globular clusters also lie within range of large backyard instruments. The brightest of these, designated Fornax 5 is found 40' northeast of NGC 1049. In their book Observing Handbook and Catalogue of Deep-Sky Objects (Cambridge University Press, second edition 2003), authors Christian Luginbuhl and Brian Skiff report they have glimpsed both NGC 1049 and Fornax 5 as stellar points in a 6-inch telescope. Ahh, to live in Arizona! Through a 12-inch instrument, they feel that Fornax 5 may even be a little brighter than NGC 1049. Meanwhile, here on the East Coast, it struck me as a little smaller and a little fainter. What is your interpretation? 

Globular cluster Fornax 4 is smaller and fainter still. Look for a tiny diffuse disk about 7' east-southeast of an 8th-magnitude star and 18' southeast of NGC 1049. 

Although it appears the largest of the four, Fornax 2's exceedingly low surface brightness makes it difficult to confirm. Look for it about 37' southwest of NGC 1049. Luginbuhl and Skiff tell us it is visible in their 12-inch Cassegrainian reflector at 250x, but I was never able to duplicate this feat in my 13.1-inch Newtonian here on Long Island. Maybe I should consider relocating! 

This leaves us with two leftover globulars that defy detection through all but the largest apertures. Fornax 1 resides 23' due north of Lambda-2. Its 0.8' disk rates only magnitude 15.6. Fornax 6, superimposed near the center of its parent galaxy, is even dimmer and smaller. All are plotted on the chart above. Good luck spotting either of those!

Above: This is a Digitized Sky Survey 2 image of the dwarf galaxy in Fornax, along with its six globular clusters that make up this month's challenge. 

Credit: ESO/Digitized Sky Survey 2

 

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 test.  Contact me through my web site 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 2016 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, Procyon and Dr. Greg Bohemier like this


7 Comments

Interesting!

Thanks for posting this one Philip! I used your book last winter to find NGC 1049, Fornax 5, Fornax 4, and Fornax 2 with my 10-inch SCT from northern Arkansas. I felt NGC 1049 was definitely brighter than F5 while F4 was about the same as F5. Fornax 2 was by far the faintest of the bunch. I had never heard of this challenge until I read your book!

Thanks, both for your comment about this article as well as the book!  I'm hoping both will "force" people (happily) to look for new and interesting objects that might otherwise pass unnoticed.

 

Nice that you could make out several in your 10-inch SCT.  I once spotted NGC 1049 in my 8-inch Newt, but left the,others to my 18-inch.

Photo
Dr. Greg Bohemier
Dec 08 2016 07:39 PM

Nice article Phil, I've seen a few of these globulars with an 8" SCT in the 1980s from Monson, MA under moderately dark skies.  They became part of the globular 100 article I wrote for the Springfield Stars Club. The were Fornax 4 and 5. My observation notes listed them as stellar.  I also found an observation note from 1984, noting that Fornax 4 also was seen with my 15 cm (6") refractor. It was also seen as being stellar.

    • PhilH likes this

Not bad there, Greg, especially spotting F4 in that aperture!  Do you recall what chart or map you used to confirm?  Back that, info was sparse for this sort of thing.  I recall all the trouble I had first locating Stephan's Quintet, again for lack of a good reference.

 

Thanks for sharing. 

Photo
Dr. Greg Bohemier
Dec 10 2016 02:21 PM

Hello Phil,  Those observations were made a long time ago.  Nevertheless, I seem to recall finding a chart in a professional journal that was listed as a reference an old "Deep Sky" magazine.  The chart I used still may exist in a file cabinet back in Missouri at my "old home."  I am not able to access it now or provide a reference because I live in Tucson.  Yeah, I became a snow bird!    Anyway, I left almost all of my astronomy observation notes -- I just have a few that are computerized-- and sketches back in Missouri. I wish I didn't because I know I have some sketches of my observations of the Fornax globulars, which were made in the 1980s.  As you know, I'm now retired.  After decades it is nice to be back into having a personal observing program and writing about astronomy once again.  It has been a terribly long hiatus because for many MANY years the only observing I have been able to do involved showing the heavens to the astronomy classes I taught at the college. Now, having time to do personal deep sky observing has opened up for me once again!!!

Good to hear, Greg!

 

I just made a quick scan of the index of Deep Sky that's in the magazine's final issue, but could find mention of 1049.  But I looked quickly, so could have missed mention of the rest.  I know that I wrote an article aboutt he Fornax Galaxy Cluster way back in the Jan 1988 issue of S&T, but made no mention of these in that piece.



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