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14 November 2003

The November Sky

by Paul Curtin

November continues the autumnal trend of faint and rather indistinct star patterns. Southern observers get their first glimpse of the Large Magellanic Cloud late in the evening and Northerners spot the harbinger of bright winter constellations as the Pleiades (M45) rise in the east. Perseus likewise presages the rich winter sky-look for its double cluster NGC 884 and 869- a fine sight in binoculars.

Readers in Antarctica are due for a total solar eclipse 23-24 November. This will be partial for Australia and Southern New Zealand.

The planets this month

Venus remains in twilight. Finding it can be an interesting challenge if you have an unobstructed western horizon.

Keep an eye on Mars, still bright though receding. Larger telescopes are now needed to see detail.

Saturn is in Gemini and thus can be observed throughout the night.

The unsettled firmament: Demons, wonders and other variable stars.

I have often wondered what the ancients really thought of the wanderings and inconstant brightness of the planets. Judging from the names bestowed on these objects, those of gods, goddesses and mighty supernatural beings, one may assume they considered the planets to be worthy of worship. The response to stars that refused to shine faithfully with a fixed brightness was more complex. Two November stars illustrate this. O Ceti and b Persei were well known to early watchers of the skies. The first is a giant called Mira "the wonderful" with a period of 332 days ranging from magnitude 3.4 to 10, well below the naked eye limit. The second is an eclipsing binary that varies from magnitude 2.1 to 3.3 and back again in 2.9 days. This has been called Algol, the demon star. Variable stars, demonic or otherwise, can be studied through observing programs of wide ranging sophistication and can illustrate a broad array of phenomena in stellar composition and dynamics.

The variability of stars such as Algol results from periodic eclipses of b Persei by a companion star. Many multiple star systems present similar changes. Others display effects that suggest the presence of unusual companion stars. Kaler describes the strange case of e Aurigae, a supergiant apparently eclipsed for two years by an enormous non-luminous object every 27 years. Unlike eclipsing systems, a large subset of stars are variable because of internal conditions sometimes resulting in mass loss. Pulsating variables include the completely regular Cepheids, short period giants, supergiants with alternating deep and shallow minima, as well as cooler giants and supergiants displaying occasional variations or lacking a regular period altogether. Giants of the o Ceti type, having long periods and large magnitude swings, often suffer mass loss leading to the formation of planetary nebulae. Eruptive variables are generally quiescent for extended spans of time only to exhibit abrupt changes in brightness. Of these the most spectacular are the supernovae transforming themselves, in one massive explosion, into a neutron star. Novae result from the transfer of mass from a main sequence star to its white dwarf companion. Dwarf novae show sudden fluctuations after years of constant luminosity. Some dwarves form carbon soot cloaks that result in dramatic drops in magnitude. Rotating variables have large spots that lead to reduced brightness although eclipsing companions may also sometimes play a role. Flare stars exhibit erratic, brief bright episodes followed by a slow return to original magnitude. Equally unpredictable are the luminous blue variables. The most famous of these is h Carinae which brightened to magnitude -1 in 1840, dimmed radically and subsequently became enveloped in nebular matter.

A full list of variable types is beyond such a forum. Suffice it to say that a complete study of such objects will bring one in contact with practically all stellar types and combinations.

Anyone interested in studying variable stars can begin the next clear evening. The basic method is to make regular comparisons of the target using nearby non-variable stars to estimate the current magnitude. As in all such endeavors careful observation and consistent record keeping are of prime importance. Naked eye magnitude estimates can be carried out even in urban settings. See Gupta for a good first project in observing and plotting the periodic changes of a Orionis. Burnham, Webb and Kepple list variable stars by constellation and give descriptions of the more prominent and/or interesting. Hynes notes a number of variables associated with planetary nebulae although these may require large telescopes for detailed study. Observers wishing to develop more precise techniques should see Moore for various visual tips as well as suggestions for the use of artificial comparison star methods. Advanced amateurs, or those wishing to start with a truly ambitious project should consult Hall and Genet. Photoelectric photometry of variable stars, especially in conjunction with automated observing programs, is a field in which amateur astronomers are still able to make a significant contribution.

Bibliography

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

Gupta, R. (ed.) Observers Handbook 2003. Toronto: University of Toronto Press, 2002.

Hall, D. S., Genet, R. M. (ed.) Photoelectric Photometry of Variable Stars. Richmond, VA: Willmann-Bell, 1988.

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

Kaler, J. B. Extreme Stars. Cambridge: Cambridge University Press, 2001.

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

Lozinskaya, T. A. Supernovae and Stellar Wind in the Interstellar Medium. New York: American Institute of Physics, 1992.

Moore, P. (ed.) Practical Amateur Astronomy. London: Lutterworth Press, 1975.

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