Brian Jones: Autumn stars
The autumn night sky plays host to Cassiopeia, which can be found at or near the overhead point during the months of autumn. A little way to the south of Cassiopeia is the line of stars forming Andromeda. Sirrah, the westernmost star in Andromeda, forms the upper left corner of the adjoining and very conspicuous Square of Pegasus. This huge quadrilateral of stars, located roughly a third of the way to the south from the overhead point during autumn, is a striking feature of the late-summer, autumn and early-winter night sky. If the sky is really dark and moonless you might like to try your hand at counting the number of stars visible to the naked eye within the Square of Pegasus. If viewing conditions are good, you have really keen eyesight, you might spot twenty or more.
Located just to the south east of Andromeda are the two tiny but prominent constellations Triangulum and Aries and extending in a long and meandering line from Andromeda to below the Square of Pegasus is the large but generally faint constellation Pisces, the Fishes.
Staying with the watery theme, bordering the south eastern edge of Pisces is another faint constellation, this being Cetus, the Whale, its tail marked by the fairly prominent star Deneb Kaitos, its name derived from the Arabic for ‘Tail of the Whale’. Deneb Kaitos can be located by following a line from Sirrah, through Algenib (in the Square of Pegasus) a little way towards the south.
By far the most interesting object in Cetus is Mira, a long-period variable star, whose brightness varies from around second or third magnitude (roughly that of Deneb Kaitos) down to around tenth magnitude, which is below the light grasp of all but the most powerful binoculars (for more on magnitudes, see below). The period of variability is around 331 days, during which time the star undergoes a complete cycle of variability. Mira was the first variable star to be discovered and, as is the case with other long-period variables, the period of Mira is by no means constant. Periods ranging from as little as 304 days to as much as 355 days have been recorded.
During late-August and early-September, Mira will be around maximum brightness and so this will be a good time to identify the star. It lies roughly on a line from Eta, Omicron and Alrisha in the neighbouring constellation Pisces, as shown on the chart. Get to know the stars of Cetus and, once you’ve picked Mira out, keep an eye on it over the following few months and watch it as it slowly disappears from view. The name of this star means ‘Wonderful’ which indeed it must have been to the stargazers of long ago who must have often noticed the star, only to watch it disappear and reappear over time.
Our final port of call this time is the bright star Fomalhaut in the constellation Piscis Austrinus, the Southern Fish. Fomalhaut, whose name means ‘Mouth of the Fish’ lies just above the southern horizon during late-evenings in September. It can be found by following a line from Scheat, through Markab, both in the Square of Pegasus and down towards the south as shown here. If the sky close to your horizon is very clear you should spot Fomalhaut and, using binoculars, you may be able to pick out some of the fainter stars in Piscis Austrinus and the neighbouring constellation, the Sculptor, although exceptional seeing conditions would be needed…
…but don’t let that stop you having a go! Until the next blog, happy stargazing
MAGNITUDES
In around 150BC the Greek astronomer Hipparchus divided the stars up into six classes of apparent brightness, the brightest stars being ranked as first class and the faintest as sixth. This system is known as apparent magnitude and classifies the stars and other celestial objects according to how bright they actually appear to the observer. In 1856 the English astronomer Norman Robert Pogson (1829 – 1891) refined Hipparchus’s system by classing a first magnitude star as being 100 times as bright as one of sixth magnitude, giving a difference between successive magnitudes of 5√100 or 2.512. A star of magnitude 1.00 is 2.512 times as bright as one of magnitude 2.00 and 6.31 (2.512 x 2.512) times as bright as a star of magnitude 3.00 and so on. The same basic system is used today, although modern telescopes enable us to determine values to within 0.01 of a magnitude. Negative values are used for the brightest objects including the Sun (-26.8), Venus (-4.4 at its brightest) and Sirius (-1.42). Generally speaking, the faintest objects that can be seen with the naked eye under good viewing conditions are around sixth magnitude, whilst binoculars will allow you to see stars and other objects down to around ninth magnitude or so.
