Shallow Sky Object of the Month: Corona Australis

Original article appears in GuideStar August, 2012.

Object: Corona Australis
Class: Constellation
Magnitude: 4.1 (brightest star) — all the stars that comprise the crown are between 4th and 5th magnitude
RA: 18 h 34 m 24 s
Dec: -41 deg 29 min 24 sec
Size/Spectral: 128 degs2. 80th in size.
Distance: 6500 ly
Optics needed: Binoculars to see the constellation, a small telescope for the double stars and a larger telescope for the nebula.

Why this is interesting: Anytime that the teapot (asterism) in Sagittarius is visible we point our telescopes at all of the famous objects in and around the area. These objects are a telescope magnet, attracting unwary observers to them like a fly to honey.

I remember a night in the Texas Hill Country with only my binoculars at hand standing at the top of a hill and wondering what I could observe that would be new to me. My pocket sky guide had very little detail, but my eye was drawn to a constellation I had never observed before — Corona Australis, the southern crown.

We’re familiar with Corona Borealis, the northern crown but much less familiar with its southern cousin.

The southernmost star in the crown is 17.3 degrees above the horizon and the northernmost star in the crown is about 22.25 degrees above the horizon at midmonth at 30 degrees north latitude.

If you want to see some objects you’ve never seen before, this is the place to be. I’d venture a guess that you have never looked for and found this constellation before. I found it easy to pick out in the dark Hill Country skies.

Once you’ve found the constellation, what else is there to look for? Plenty as it turns out.

NGC6723 is a bright globular cluster just off the north edge of the crown (it actually sits in the constellation Sagittarius). Can you see it in binoculars on a dark night?

Just southeast of 6723 by the width of the moon (1/2 degree) is NGC6726/6727 and two young stars just joining, or just about to join the main sequence. This is a star forming region only 430 light years away.

Twelve arc minutes southwest of this is IC4812. If you are working on the Caldwell list, don’t miss the globular C78 (NGC6541) at the southwest corner of the constellation boundary, about 12 degrees away from the crown.

Gamma (γ) Corona Australis is a tight double star. The separation of the two components is a close 1.6” but the stars are both 5th magnitude and it’s easier to separate stars of equal magnitude than ones of unequal magnitude. You’ll need high magnification for this one so get out your short focal length eyepieces. For comparison, the two stars that make up Epsilon Lyr’s e1 (the northern star pair in the famous double-double) star are 2.6 arc seconds apart, so this one is tighter than that.

An easier double is Kappa (κ) Cra at 21.4” separation. This is a visual magnitude 6.3 star so it’ll be a bit more challenging to find.

Variables— ε CrA — an eclipsing binary 4.7-5.0m, Appx 14 hour period
R CrA — Pre-main sequence Herbig-Haro star 9.7-13.9m
S CrA— T Tauri star—a low mass pre-main sequence star
TY CrA — 8.7-12.4m — surrounding nebulosity brightens and dims with star

Finder Charts for Objects in Corona Australis. From TheSkyX

Detail from chart above (boxed area). From TheSkyX

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