30 April 2004

The May Sky: Between galaxies and globular clusters

Paul Curtin

A Reminder: If you are interested in observing the transit of Venus this June, you should start a regular observing plan now in order to avoid technical surprises on the big day. Also remember to check local times for this event, as much of North America will have a tiny window at sunrise. Choose your observing site carefully!

May brings a major transition from the galaxy rich fields of Virgo to the leading edge of the summer Milky Way. The western sky still teems with deep sky objects far outside our own galaxy, while the globular clusters, the ‘halo' of our own galaxy, rise in the east. These first sightings provide a hint of what to expect in the coming (hopefully warmer) months of the northern summer as we slowly turn our gaze back to the galactic center. Like the Autumn sky, the meridian this month is sparse in terms of deep sky objects, but, unlike the Autumn darkness, May boasts striking patterns and some dazzling stars.

Last month we mentioned a technique for identifying some important spring stars. Remember that you can follow the curve of the Big Dipper's handle to Arcturus and continue on to Spica in Virgo. Arcturus, the third brightest star in sky (fourth if one counts the sun), is situated at the base of a kite-shaped collection of stars known as Bootes, the herdsman. Bootes and its eastern neighbor, the bowl shaped Corona Borealis, or Northern Crown, contain few deep sky objects, but both warrant a sweep with binoculars or a small telescope. Hunt for double stars and interesting star patterns.

East of Corona Borealis is a distinct, though not extremely bright, keystone-shaped group of four stars that forms the body of Hercules. Hercules has two important globular clusters that warrant close inspection. M13 is found due south of the northwestern star in the keystone directly on a line to the southwestern star. M92 is slightly harder to find but is well worth the effort. Look for it due north of the northeastern star in the keystone. To estimate how far north to go, bear in mind that M92 forms an equilateral triangle with the two northern stars of the keystone. For these and other globular clusters, observe with successively higher powers until you are able to resolve individual stars. Note any outlying star patterns and compare different globulars for luminosity distribution, compactness and overall size. The practice of regularly drawing what you observe often sharpens the observer's sensitivity to subtle differences among such objects.

Our final globular cluster for this month is M5 in Serpens. Serpens is a rather unusual constellation, divided as it is into two widely separated parts. Serpen Caput, the head of the snake, is a spring constellation, whereas Serpens Cauda, the tail of the snake, is in the summer sky. Ophiuchus, which divides the two, will be featured next month.

M5 is more of a challenge than its northern cousins. Though it is a very impressive globular cluster, it is located in a particularly sparse region of the sky where Virgo, Bootes, Libra and Serpens Caput meet. If you are familiar with Libra, start at Beta Librae and sweep due north. You can also begin with the easternmost star of the “kite” of Bootes and make a long sweep due south.

The southern sky in May is dominated by Centaurus, the Centaur, and Crux, the Southern Cross.  Both constellations contain bright stretches of the Milky Way divided by dark lanes. Look for the dark nebula known as the “coalsack” in the southeastern quadrant of Crux. Both constellations contain impressive objects for all telescope apertures. In addition to globular clusters, this region abounds in open clusters, emission nebulae and some beautiful planetary nebulae.

The planets this month

Mercury is a bright object late in the month for observers in the Southern Hemisphere. Northerners will have to wait for a better view.

Venus reaches maximum magnitude early in May. See if it casts shadows after dusk. This is a good opportunity to observe Venus in its crescent phase. A good pair of binoculars suffices to show this.

Mars passes into Gemini and can be seen in conjunction with Saturn on May 24.

Jupiter, in Leo, is now the best planet for telescopic study. On May 17 there is a double shadow transit. See Gupta for details.

Saturn is in Gemini and is much less prominent.

Bibliography

Burnham, Robert Jr. Burnham's Celestial Handbook. New York: Dover, 1978.

Gupta, R. (ed.) Observers Handbook 2004. Toronto: University of Toronto Press, 2003.

Jones, K. G. (ed.) Webb Society Deep-Sky Observers Handbook. Enslow Publishers, 1982.

Kepple, G. R., Sanner, G. W. The Night Sky Observers Guide. Richmond, VA: Willman-Bell, 1999.

Webb, T. W. Celestial Objects for Common Telescopes. New York: Dover, 1962.