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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)

 

 

 

Target

Type

RA

DEC

Const.

Mag

Size

NGC 6445

Planetary
Nebula

17h 49.3m

-20┬░ 00.6'

Sagittarius

11.2

44"x30"

 

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 and Sasa like this


3 Comments

Photo
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
Photo
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


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