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Southern Cross - August 2000

• Constellation of the Month - Apus
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Constellation of the Month - Apus

Albert Brakel

Apus, the Bird of Paradise, is positioned between Triangulum Australe and Octans, and must feel a bit uncomfortable at being put so close to the pole by Bayer in 1603. In fact, only three constellations extend further south. Despite its name it is not a paradise for observers, but it does contain some objects worth pursuing.

First, have a look at a target best suited to binoculars, the double star Delta Apodis (RA 16h 20.3m, Dec -78 42'). Both components are orange (like the other bright stars in Apus), and are so bright (mags. 4.7 and 5.1) and so far apart (103") that they are designated Delta-1 and Delta-2. They still look good in a low power telescope field because of their color and brightness, but are more spread out of course.

IC 4499 (15h 00.3m, -82 13'), a faint mag. 10.1 globular cluster in the far south, does not exactly hit you in the eye. That it was missed by John Herschel tells you something, and with a C8 it's a bit of a struggle. Wait for a dark night and look for a small circular smudge 3' in diameter.

Less difficult but still no showpiece is NGC 6101 (16h 25.8m, -72 12'), a 3'-4' mag. 9.2 globular on the northern side of Apus 2 degrees south of Zeta Trianguli Australe. Like globulars generally, it is roundish with a dense centre. On a steady night, stars can be resolved at the fringes with a 20 cm aperture at high power.

Innes 236 (14h 53.2m, -73 11') is the best double star after Delta Aps. With a separation of 2" it is not difficult on a steady night, but if there's turbulence in the air that's another matter. The yellow m.5.8 primary has a paler yellow m. 8.0 attendant.

Needing better seeing is the 1.6" pair CapO 15 (15h 06.4m, -72 10'). The m. 7.5 white component has a m.9.0 white companion on its NE side.

Holden 284 (18h 12.6m, -73 40') is a 2.5" pair with a 6.0/9.0 magnitude difference that makes it more difficult to resolve than you might expect. The stars are yellow and ashen respectively.

There are a number of faint galaxies in the constellation, for which your typical suburban sky and a small 'scope just won't do. They require minimal light pollution and an instrument like the 14" Bobroff reflector in the CAS Dome. I didn't have the opportunity to hunt them down in a dark sky, but you could have a go at the following four.

NGC 5799 is a faint spiral lying 25' south of CapO 15.

NGC 5833 (15h 11.8m, -72 52'), another half degree or so to the SE, is a mag. 12 spiral 3' long with bright inner arms. It is interacting with an even fainter galaxy to the NE that is difficult because of a nearby star.

NGC 5612 (14h 34.1m, -78 24') lies just over a degree NW of Alpha Aps, and is an oblique mag. 12.1 spiral 2' long.

NGC 5967 (15h 48.1m, -75 40') is another oblique mag. 12 spiral 3' long. On its NW side look for the mag. 12.5 barred spiral NGC 5967A, 2' across and with a relatively bright nucleus and faint arms.

If you want to finish off with another challenge, try the small, 10"-diameter, bluish mag. 11.6 planetary nebula He 2-131, a.k.a. PK 315-13.1 (15h 37.2m, -71 55'). For once an OIII filter doesn't help much, as the nebula has feeble OIII emission. Malin & Frew (Hartung 2nd edition) say it blinks well with a H-beta filter, if you're one of the few observers to own such a luxury item.

So go to it now, and may your skies be clear and steady.