30 July 2004

The August Sky: Nebulae and the Lives of Stars

Paul Curtin

This month we revisit the constellations introduced in June and July. Once we have become familiar with the general star patterns a more detailed investigation is called for. Scattered throughout the rich star fields of the Milky Way we find a large number of deep sky objects composed of thin gas/dust structures glowing either by excitation or reflection. Such objects displaying a compact, regular shape are known as "planetary" nebulae. Gas clouds or dust structures of irregular shape are known simply as "gaseous" nebulae. This morphology captures more than a visual distinction as the history, composition and energy profiles of these two groups are profoundly different.

Planetary nebulae form when certain mid-sized main sequence stars use up most of their hydrogen and become red giants. Such objects have cool outer regions surrounding a heavy contracting core. Energy released in this process pushes gas outwards forming an expanding gas shell. The hot core excites this shell causing the gas to glow. Often the expanding shell is easily visible while the energy source is too faint for observation. Locating planetary nebulae can be difficult due to their relatively small apparent size. Some can only be distinguished from stars by using a prism held between the eyepiece and the eye to disperse the stars into tiny spectra. Planetary nebulae, resulting from the excitation of gas, emit light at discrete wavelengths and will thus appear unchanged through the prism. Very bright objects may display one or more identical copies of the original object at discrete intervals. Some planetaries benefit from the use of a nebula filter.

Gaseous nebulae are regions of space occupied by gas and dust. In the absence of energy sources they are dark areas that block out more distant star fields. These are the dark lanes observed in the Milky Way and other galaxies. When stars are present the gas and dust glow either as a result of ultraviolet induced radiation or by reflecting star light. Many gaseous nebulae are regions of star birth and contain a mixture of gas, dust, proto-stars and very young hot stars. Many gaseous nebulae can be observed with binoculars or wide field telescopes. Nebula filters often help improve contrast but remember to draw the star field first, then use the filter as stars are considerably fainter in nebula filters.

The two most famous northern planetary nebulae are M57 and M27. The Ring Nebula (M57) is located between the southernmost stars of Lyra. It looks like a tiny smoke ring in small telescopes. The Dumbbell Nebula (M27) is located due north of the tip of Sagitta, the Arrow. M27 is larger than M57 and displays a pronounced greenish tinge.

Sagittarius and environs host an impressive array of gaseous nebulae. Beginning north of the teapot"s spout we find the bright Lagoon Nebula (M8) with the star cluster NGC 6530 embedded in its center. North and slightly west of M8 is the Trifid Nebula (M20) which has a reflection nebula component. North of the teapot's lid is the Horseshoe or Omega Nebula (M17) and due north of this, in Serpens Cauda, is M16, a complex object of dark lanes and glowing gas enveloping an open star cluster.

All these objects are visible in binoculars and small telescopes. Repeated observing will bring out details not brought out by sporadic sightings. As with all observations drawing is an excellent way to train ones eyes. Detailed finder charts for the above and other challenging objects can be found in the Webb Society guide.

The planets this month

Mercury is an early morning object for southern observers.

Venus is the brightest object in the predawn sky.

Mars is out of sight most of the summer.

Jupiter is now quite low in the west after sunset. A fine view to the naked eye but atmospheric interference precludes telescopic work. By the end of the month it will pass into the sun's glare.

Saturn is in the morning sky but won't be in dark skies until next month.

Uranus and Neptune, in Aquarius and Capricornus respectively, make their first evening appearances of 2004. Gupta has finder charts for these planets.

Bibliography

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

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

Hynes, S. J. Planetary Nebulae. Richmond, VA: Willman-Bell, 1991.

Jones, K. G. Webb Society Deep-Sky Observer's Handbook. Volume II Planetary and Gaseous Nebulae. Hillside, NJ: Enslow Publishers, 1979.

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.

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