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Spring's Water Snake

Map 18: South Equatorial 3
Canis Major to Centaurus
ngc_3242_noao_250
Planetary nebula NGC 3242 is located in the constellation Hydra.
Adam Block/NOAO/AURA/NSF

The southern portion of Hydra — the sky's largest constellation — dominates the top part of the next star map. Because it is so large, Hydra boasts a variety of celestial gems.

Take a break from observing traditional deep-sky objects to spot V Hydrae, a single star many observers rate as the reddest in the sky. V Hydrae is a variable star with a period (from one peak brightness to the next) of 531 days. If it's at its maximum magnitude of 6.6, you may be able to spot it with unaided eyes from a dark site. At minimum, the star's brightness drops to magnitude 9.0. Its color, however, gives it away. Use a telescope, and slightly defocus the image to see V Hydrae at its reddest.

A bright galaxy in Hydra, NGC 3621, shines at magnitude 8.9. Twice as long as it is wide, NGC 3621 has a bright nucleus that covers only 15". NGC 3621 is a spiral galaxy, but you'll need a 14-inch or larger scope to see any of its structure.

Not many planetary nebulae shine as brightly as magnitude 7.8, so when you have a chance to observe one, don't miss it. Such an object, the Ghost of Jupiter (NGC 3242), lies 2° south of Mu Hydrae. Objects like the Ghost of Jupiter inspired English astronomer John Herschel to coin the term "planetary nebulae." NGC 3242 measures 13" across, which is about the same as Mars at a distant opposition (the point in its orbit when it lies opposite the Sun in our sky). At low magnification, the Ghost's color — blue-green — resembles Uranus'. Once you've verified this, crank up the power in steps. You'll see a faint shell 40" across surrounding a brighter, football-shaped interior. An even closer look reveals an empty space 10" across that contains the planetary's central star.

The constellation due south of the Ghost of Jupiter is Antlia the Air Pump. Small and sparse, Antlia contains only four stars brighter than 5th magnitude. Another slightly fainter star you'll spot easily from a dark site is U Antliae. This star is similar in color to V Hydrae, described above, only brighter. Such objects are known as carbon stars, and they're all intensely red.

Magnitude 9.3 NGC 2997 is one of only two galaxies in Antlia that show any detail in medium-size telescopes. Through a 10-inch scope, you'll see an indistinct glow 5' by 3' aligned east-west. The other galaxy is NGC 3175, a nearly edge-on spiral 3 times as long as it is wide. With a length of 3' and a magnitude of only 11.3, NGC 3175 appears faint.

It's worth stopping in Pyxis to view two open star clusters. The first is magnitude 8.4 NGC 2627. If you use an 8-inch scope at 50x, you'll see 40 stars in a region 6' across and many more hinted at in the background.

Now move to NGC 2818. At magnitude 8.2, it's a bit brighter than NGC 2627, but it's special for a different reason: NGC 2818 contains a planetary nebula. The nebula looks like a small version of the Dumbbell Nebula (M27) in Vulpecula. You'll see 30 stars in an area about 9' across.

Vela is a large constellation with few bright stars. In the 18th century, the constellation Vela did not exist. Its stars were part of a larger constellation named Argo Navis, which, according to Greek mythology, was the ship Jason and the Argonauts used to find the Golden Fleece. The constellation Argo Navis was so unwieldy that later astronomers subdivided it into Carina the Keel, Puppis the Deck, and Vela the Sails.

The Vela Supernova Remnant is the sky's largest supernova remnant, covering 5°. The best way to observe this object is to use a 12-inch or larger telescope with a low-power eyepiece and a nebula filter, such as an OIII. Either disengage the drive motor, or set your slewing speed at "medium," and scan the area. Only one portion of this region has an NGC number assigned to it. The east-southeast region carries the designation NGC 2736 and is often called the Pencil Nebula.

Look 2° south of Gamma Velorum for NGC 2547. At magnitude 4.7, it's an easy naked-eye object. NGC 2547 measures 1° across, but in a 6-inch or larger scope, you'll see nearly all of its stars in a ½° area. And, yes, the magnitude 6.5 star is part of the cluster, not a foreground star.

Get out your binoculars (and Map 23) and look 2° north-northwest of Delta Velorum for IC 2391, a magnitude 2.5 open cluster almost 1° across. Binoculars highlight this object's brightest stars, but a telescope with a wide-angle eyepiece will let you go a bit deeper. In all, expect to see about 30 stars brighter than 12th magnitude.

The Eight-Burst Nebula (NGC 3132) lies in Vela's northeast corner. This is a magnitude 9.7 planetary nebula comprising a 10th-magnitude central star surrounded by a shell measuring 60" by 45". The shell's outer regions are brightest, but a 10-inch or larger telescope will allow you to see this object's splotchy interior.

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