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Cosmic Challenge: NGC 6445, The Box Nebula

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Cosmic Challenge: NGC 6445, The Box Nebula


August 2020

Phil Harrington

This month's suggested aperture range:

Medium scopes


6- to 9.25-inch (15-23cm)











NGC 6445


17h 49.3m

-20° 00.6'





The sky is full of weird sights. And among planetary nebulae, NGC 6445 is one of the strangest. Discovered by William Herschel on May 28, 1786, NGC 6445 shines at 11th magnitude. That's bright enough to be seen even through giant binoculars. Although visible in smaller apertures, it takes a 6-inch telescope for NGC 6445's true, if bizarre, nature to shine through. The nebula's brighter central shell looks like a dented rectangle. Nature rarely creates an amorphous form with sharp edges, and indeed, the peculiar appearance of NGC 6445 is due largely to our perspective as well as its age. But the look is very odd nonetheless. No wonder NGC 6445 has been nicknamed the Box Nebula.


Above: Summer 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



As the material that will ultimately form a planetary nebula is expelled by its star, it takes on a cylindrical shape. If we look along its axis, we see the classic smoke-ring effect of the Ring Nebula, M57. But then, over time, gravitational influences from other sources, such as an outflowing stream of particles from the progenitor star, as well as from the gravity of companion stars or perhaps a family of planets, contort the shell into bizarre, distended shapes. Studies show that NGC 6445 is one of the sky's oldest planetary nebulae at an estimated 3,300 years, so there has been plenty of mixing time. From its full size, some 3'x1', and great distance, 4,500 light years, these same studies conclude the NGC 6445 is also one of the sky's largest, spanning perhaps 4 light years. Deep photos reveal its true asymmetrical bipolar structure, with a bright central ring surrounded by fainter nebulous tendrils. It is believed that most, if not all, planetary nebulae show bipolar tendencies owing to highly energetic streams of particles that flow from their progenitor stars. These streams, called bipolar outflows, are focused into cones of gas by the star's magnetic fields or perhaps by binary companions.


When examined visually under high magnification, the nebula's disk expands into a strange specter floating amid a very rich field of stars. The sketch below shows the scene through my 8-inch reflector at 112x. I can readily see that the nebula is not only rectangular, but also that it has what appears to be a hollow center, akin to a withered version of M57. Closer scrutiny also discloses several brighter patches stitched within the nebula's outer edges, with the most prominent knots seen toward the eastern and southern limits.



Above: NGC 6440 (top) and NGC 6445 (bottom) as sketched through the author's 8-inch (20cm) reflector.


Above: NGC 6445 by PanSTARRS. Image from WikiCommons.


One of the challenges presented by NGC 6445 is simply finding it among the star-rich fields of the Sagittarius Milky Way. The easiest way to starhop there is to begin at the bright open cluster M23, itself a wonderful target at low power. Slide 1° to the south-southwest toward 7th-magnitude SAO 160868, the brightest of five stars in an arc that curves away from M23 toward the west-southwest. From there, hop southwest for about 45 arc-seconds to an 8th-magnitude point. Through your telescope, this last star is joined by two fainter companions that collectively form a right triangle that points right at our quarry.


Will the real Box Nebula please stand up? Yes, there are two. NGC 6445 here as well as NGC 6309 in Ophiuchus. To my eye, NGC 6445 appears bigger and boxier than NGC 6309.  Some readers might prefer referring to it by another nickname, the Little Gem Nebula. Problem is, there are two little gems. NGC 6818, also in Sagittarius, goes by that moniker, as well. There seems no consensus as to who is who. The decision is left to you, dear reader.


Before we go, there's bonus object, globular cluster NGC 6440, nestled just a third of a degree south of our planetary challenge. Both will squeeze into a wide-field eyepiece's field, but high magnification is needed to enjoy the cluster, as it is with the planetary. Even at 284x, my 8-inch gives no hint of resolution within this packed swarm of ancient stars. Instead, NGC 6440 is simply a small, faint, diffuse ball of fuzz surrounding a somewhat brighter central core. The stars in the globular shine no greater than 16th magnitude, bringing them just within the range of giant backyard scopes.


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 2020 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.


  • John O'Hara, Procyon and Sasa like this


John O'Hara
Aug 01 2020 04:41 PM

I observed this object around midnight spanning 7/30 and 7/31/11 using my 7" Starmaster reflector.  I used both 74x and 219x for the observation made from my club's old Bortle 4 site on a night of 7/10 seeing and 8/10 transparency.  It appeared much as in Phil's drawing, with the planetary elongated NW-SE and boxed shaped.  I also saw the faint star off the NW edge and remember the distinct pair of stars about midway between NGC 6445 and 6440.  I didn't note whether or not I saw detail within the nebula, so now I wish I'd looked closer.  I plan to be at Cherry Springs State Park in north-central Pennsylvania, and hope to try it under darker skies.

    • PhilH likes this
John O'Hara
Aug 01 2020 04:42 PM

BTW, I made the observation using Phil's book as my guide.  Credit must be given where's it's due!

BTW, I made the observation using Phil's book as my guide.  Credit must be given where's it's due!

Excellent, John.  And you're welcome.  waytogo.gif

    • John O'Hara likes this

I had a 16" scope out on August 7 to view the shadow of Io and the GRS.  I decided to give this pair a try even though there was smoke from California wildfires causing a brighter than usual sky background here at the edge of the Las Vegas valley.  At 200x, NGC 6445 could be seen, and the addition of a nebula filter noticeably enhanced the view.  The shape of the nebula was definitely boxy.  After removing the nebula filter, I viewed NGC 6440 which was more subtle. I needed 270x to spread out the skyglow.just to see that something was there. 

I've viewed these objects over the years with telescopes ranging in size from 10" to 24" under suburban and dark skies.  They are on many and my own best objects lists.

Last night was much clearer so I imaged the pair with a 6" f/4 and OSC camera.

Here's a cropped part of a 24 minute exposure.  Both objects were visible on 15 second subs.

    • PhilH, John O'Hara and clusterbuster like this
Dave Mitsky
Aug 09 2020 06:15 PM

I haven't observed NGC 6445 in quite some time.  If all goes well, I'll take another look at it tonight using the 17" f/15 classical Cassegrain at the Naylor Observatory.

Will the real Box Nebula please stand up? Yes, there are two. NGC 6445 here as well as NGC 6309 in Ophiuchus. To my eye, NGC 6445 appears bigger and boxier than NGC 6309.  Some readers might prefer referring to it by another nickname, the Little Gem Nebula. Problem is, there are two little gems. NGC 6818, also in Sagittarius, goes by that moniker, as well. There seems no consensus as to who is who. The decision is left to you, dear reader.

DSO nicknames have really gotten out of hand in recent years. wink.gif

    • PhilH, John O'Hara and optinuke like this
Matt Lindsey
Aug 10 2020 09:46 AM

DSO nicknames have really gotten out of hand in recent years. wink.gif

Indeed.  SkySafari calls NGC 6445 the "Crescent Nebula".  I thought that was NGC 6888 in Cygnus.

    • Dave Mitsky and John O'Hara like this
Dave Mitsky
Aug 10 2020 01:34 PM

Last night I observed the planetary nebula NGC 6445 in Sagittarius for the first time in quite a while using the 17" f/15 classical Cassegrain housed in the French Dome at the Astronomical Society of Harrisburg's Naylor Observatory.  The transparency was marginal but I had some fairly good views at 170x (38mm Agena SWA), 216x (30mm Explore Scientific 70 degree), and 259x (25 mm Explore Scientific 70 degree).  I also used a no-name narrowband filter belonging to ASH with the 30 and 38mm eyepieces.  The best view was with the filtered 38mm Agena SWA.  NGC 6445 was rather dim and was fairly large.  It appeared to be roughly rectangular in shape.


I also took a look at NGC 6818, the original Little Gem Nebula, which was an easy target.


    • Matt Lindsey and John O'Hara like this
Dave Mitsky
Aug 11 2020 07:37 PM

On Monday night, I observed NGC 6440, the globular cluster that was mentioned in Phil's article, using the 17" classical Cassegrain at the Naylor Observatory.  It's another DSO that I haven't looked at for quite some time.  The transparency was rather poor close to the horizon.  It was not much more than a dim circular glow.  I used magnifications of 170 and 216x.

    • John O'Hara likes this

NGC 6445 and 6440 are favorites of mine. I stumbled on this pair a number of years ago when wandering around with my 12.5 inch.  Such serendipitous discoveries are often favorites.


I tend to look at this pair quite often, it's about 2° or so west of M23 and that whole area is just a wonder.


I've seen it from my San Diego backyard with both the 10 inch and 13.1 inch. I was probably using an O-lll of UHC filter..


I'll be checking it out one of these nights ...



    • John O'Hara likes this

I was out again in my backyard with the 16" LX200 the night of August 14 watching the double shadow event on Jupiter.  The night was much darker and transparent than when I visually observed NGC 6445 and NGC 6440 on August 7.  At 270x and using a nebula filter, NGC 6445 revealed brighter north and south edges and had a darker center.

I could not resolve NGC 6440 at 200x nor at 270x.  After going to 400x and blocking extraneous light, I was able to see a few stars using averted vision, but it took a while at the eyepiece and was definitely a challenge.

    • John O'Hara likes this

Last night I had my Astro-Tech AT-80LE out. I thought I'd give NGC6445 a try.  Skies measured about 21.3 mpsas.


I started looking about 20x, I know the path to this pair well, starting at M23.  The brighter 6440 was readily seen but it took some more magnification to pick up, 6445.  At 40-100x I could see it but I did not any more structure than a tiny box.. 


At least that's what came to mind. I think of it using Sky Safari's name of the Crescent Nebula.. 



    • John O'Hara likes this

A couple of observations --


24-inch f/3.7: this highly structured planetary was observed using 500x.  NGC 6445 has an unusual rectangular shape, elongated 3:2 NW-SE, with dimensions ~45"x30".  The planetary is brighter in fairly narrow strips along the four sides, creating an annular appearance.  The short northwest facing side is slightly brighter and contains a bright knot or section near the middle.  In addition, a faint knot is at the north vertex.  Another bright knot is at the east vertex and either a faint knot or very faint star is at the south vertex.  The short southeast facing side is sharply defined and quite straight. A very low surface brightness glow can be seen outside (east) of the eastern vertex.  In addition, with careful viewing an extremely faint outer shell or loop is outside the long southwest facing side, connected at the two vertices along this side.  A mag ~15.5 star is just outside the midpoint of the loop.  The outer shell or loop on the northeast side was not seen.


18-inch f/4.3: this fascinating planetary was viewed at 435x.  The overall shape is rectangular with the longer sides oriented NW to SE.  Both ends are noticeably brighter giving an annular appearance.  The NW end is slightly brighter and irregular in surface brightness with a brighter spot or two.  The rim at the northwest end is slightly bowed out and has a well-defined boundary edge in the interior.  The rim at the southeast end of the planetary is unusually straight and well-defined.  With averted vision it appeared to extend slightly beyond the main body of the planetary.  The bright lobe at this end is bar-shaped and extended SW to NE in the direction of the minor axis.

    • Dave Mitsky, John O'Hara and optinuke like this

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