02 July 2004

The July Sky: Star clouds and nebulae

Paul Curtin

After months of cold weather observing, summer astronomy is a delightful change. Waiting longer for a dark sky poses no problem now that frostbite isn't a concern. In the North the Milky Way, with its attendant deep sky objects, provides observers with a huge variety of targets for all telescope apertures. This month we discuss the highlights of the summer sky. In August we will concentrate on observing planetary and emission nebulae.

The star fields and dark lanes of the northern summer Milky Way stretch from Cygnus in the north to Corona Australis at the southern horizon. Sweeping this area with binoculars will reveal some of the most rewarding and beautiful objects in the night sky. Begin due east of Hercules at Vega, a very bright blue-white star. Vega, together with the small parallelogram to the east and south, form Lyra, the Lyre. Epsilon Lyrae, just east of Vega and forming a line with the western side of the parallelogram, is a famous "double-double" star. See if you can split the two bright components into pairs with a small telescope. With larger scopes one can find M57, the Ring Nebula, roughly between the two southernmost stars of Lyra. Using binoculars or a spotting telescope, follow the two southern stars of the parallelogram east to find M56, a compact globular cluster.

East of Lyra is a large cross shaped constellation, Cygnus (the Swan). At its northern tip is Deneb, at the southern end in Albireo. Albireo is a double star that displays a stunning contrast in stellar colors. The brighter star is orange while the fainter one is blue. Cygnus lies in a very bright section of the Milky Way and abounds in dark lanes and dense star fields. The North American Nebula, due east of Deneb, and the Veil Nebula, located south east of the easternmost star of the cross, are very difficult to observe but you should look for them should you find yourself in exceptionally dark skies.

Having located Deneb and Vega, look to the south for the bright star Altair, flanked by two fainter stars. Ranging to the south and west are a string of stars that make up Aquila, the Eagle. Aquila, like Cygnus, is roughly shaped like a cross. Altair, Vega and Deneb comprise the "Summer Triangle". This asterism is especially notable at twilight, when only the brightest stars are visible.

At the heart of the Milky Way, east of Scorpius and south of Aquila, we find the "teapot" of Sagittarius. Some see the handle of the teapot as an inverted dipper. Scan northward from the spout to find the Lagoon Nebula, an object that rivals M42 in Orion. Like the Orion Nebula this is a region of star formation and very young, still unstable T-Tauri stars. The juxtaposition of open cluster and emission nebula makes this a truly impressive object in a wide field telescope. South of Sagittarius is the bowl shaped Corona Australis, the Southern Crown. Though lacking in bright stars this constellation contains interesting star clouds and some worthwhile deep sky objects.

While July an August bring the Milky Way back to the north, in the south we turn from the bright star fields of Ara and the Southern Triangle to the globular clusters and galaxies of Pavo and Apus. Pavo, the Peacock, is due east of the Southern Triangle. The northernmost star in Pavo is a beautiful orange and green double. South of Pavo is Octans, the Octant, an indistinct collection of stars. The Southern Celestial Pole is found here, the nearest star to it being sigma Octanis, at magnitude 5.5.

We will return to these constellations next month in search of deep sky objects.

The planets this month

Mercury is an early evening object low in the western sky. Southern observers have a better chance of finding it at dusk.

Venus, having stolen the show in June, now moves into the early morning sky. We can now begin the cycle of observing her phases from almost full to crescent.

Mars has slipped into the sun's glare and will be out of sight most of the summer.

Jupiter is still well placed for observing throughout the month. Look for it in the western sky after dark.

Saturn is now lost in twilight glare and will not emerge until August.

Bibliography

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

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

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

Mallas, J., Kreimer, E. The Messier Album. Cambridge, MA: Sky Publishing Corp., 1978.

Moore, P. Amateur Astronomy. New York: Norton & Co., 1968.

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