Pegasus, Andromeda and Perseus

On a dark autumn night a faint oval smudge can be seen with the naked eye, about 8° from the star Mirach in Andromeda. It is the Andromeda Galaxy, M31, a spiral galaxy like our own Milky Way, about 2.5 million light-years away, and it is the most distant object that most people can see without a telescope. To find it we start from the Great Square of Pegasus, four stars of the second and third magnitude that stand high in the evening sky from October to December, seen from Ankara. The Square has few bright stars inside it or near it, so it is easy to pick out even from a town. One of its corners is the first star of a chain that runs through Andromeda to Perseus, and its sides point south to Fomalhaut and Diphda and north to Cassiopeia. Seen from Sydney the Square is lower, in the north, and upside down, while Fomalhaut passes almost overhead. In this section we learn the Square first, then the chain to Perseus with the Andromeda Galaxy and Algol, then Cassiopeia and Cepheus near the pole, and then the hops south.

O1.5.1The Great Square of Pegasus

The corners of the Square are Markab (α Peg) in the south-west, Scheat (β Peg) in the north-west, Algenib (γ Peg) in the south-east and Alpheratz (α And) in the north-east. Its sides are between 13° and 16.5° long, so it is not quite a square, and it spans about one and a half fists at arm's length. Scheat looks orange, because it is a cool giant star, and the other three are white. The rest of Pegasus hangs from the western side of the Square. The horse is drawn upside down, as seen from the northern hemisphere, with its neck running west from Markab to Enif (ε Peg), magnitude 2.38, at the horse's nose. About 4° north-west of Enif lies the globular cluster M15, which binoculars show as a fuzzy star.

O1.5.2From the Square to Perseus

Andromeda is a chain of three stars of almost equal brightness, all close to magnitude 2.1, running north-east from the corner of the Square. From Alpheratz it is 14.5° to Mirach (β And), an orange giant, and then 12.5° to Almach (γ And). Continuing the line of the chain for about 16° beyond Almach brings us to Mirfak (α Per), magnitude 1.79, the brightest star of Perseus.

The Andromeda Galaxy is found from Mirach. Two fainter stars, μ And and ν And, run north-west from Mirach at right angles to the chain, 3.9° and then 2.9° apart, and the galaxy lies 1.3° beyond ν And, 7.7° from Mirach. At magnitude 3.4 it is easy to see from a dark site, where it looks like a faint elongated glow. On the opposite side of Mirach, about 7° away, lies the Triangulum Galaxy, M33, another spiral, which is fainter and spread out and needs a very dark sky and good eyes or binoculars.

Perseus, the Hero, is a curved line of stars running from Cassiopeia down towards Taurus, with Mirfak near its middle. About 9° south of Mirfak, a little to the west, lies Algol (β Per), a famous variable star. Algol is an eclipsing binary, two stars orbiting each other, and every 2.87 days the fainter star passes in front of the brighter one for about ten hours. For a few hours at mid-eclipse Algol drops from magnitude 2.1 to about 3.4, a change easy to see by comparing it with nearby stars. Chapter 8 explains how such eclipses tell us the sizes and masses of stars.

O1.5.3Cassiopeia and Cepheus

Cassiopeia is a W of five stars of the second and third magnitude, which makes it as easy to recognise as the Big Dipper. It lies on the far side of the pole from the Dipper, so when one is high the other is low, and between them they guide northern observers to Polaris at any time of night and any time of year. From west to east the W runs through Caph (β Cas), Schedar (α Cas), γ Cas, Ruchbah (δ Cas) and Segin (ε Cas). The star in the middle, γ Cas, is usually the brightest, at magnitude 2.15 in the catalogue, but it is a variable star whose brightness has changed over the last century. Seen from Ankara all five never set, while from Sydney the whole W stays below the northern horizon, or at most grazes it.

Between Cassiopeia and Perseus, about 7° from Segin, lies the Double Cluster, two open clusters side by side that the naked eye sees as a hazy knot in the Milky Way on a dark night, and binoculars show as two separate swarms of stars. West of Cassiopeia, between it and the pole, the faint figure of Cepheus, the King, looks like a child's drawing of a house, and its brightest star is Alderamin (α Cep), magnitude 2.45. In the southern part of Cepheus lies δ Cep, a star of about magnitude 4 that brightens and fades every 5.4 days. It gave its name to the Cepheid variables, whose regular pulsations let astronomers measure distances to nearby galaxies (Chapter 8).

O1.5.4South of the Square

The two sides of the Square that run north to south also point south to two bright stars. The western side, from Scheat through Markab, continued about three and a half times as far, reaches Fomalhaut (α PsA), magnitude 1.17, in Piscis Austrinus, the Southern Fish. The eastern side, from Alpheratz through Algenib, continued about two and a half times as far, reaches Diphda (β Cet), magnitude 2.04, in Cetus, the Whale or Sea Monster. Diphda is brighter than Menkar (α Cet), another case where the Bayer letters do not follow brightness. Cetus also holds Mira (ο Cet), the first star found to fade and brighten again in a repeating cycle, which rises to about the third magnitude and then fades below naked-eye visibility over a cycle of about 11 months.

South of the chain of Andromeda lies Aries, the Ram, a small bent line of three stars, of which Hamal (α Ari), magnitude 2.01, is the brightest. Hamal is 17° south-east of Mirach. Between the Square and Aries, Pisces, the Fishes, is a long V of faint stars with no star brighter than magnitude 3.6, and the March equinox point lies in it (Section O1.7). Small Triangulum, with the Triangulum Galaxy, lies between Mirach and Hamal.

O1.5.5The autumn sky from the southern hemisphere

Seen from Sydney, Santiago or Johannesburg, the constellations of this section are seen in the spring, from October to December. Fomalhaut is then the brightest star high in the evening sky, passing within 5° of the zenith of Sydney, and Diphda is also high. The Great Square stands low in the north and is upside down, with Markab 41° up and Alpheratz only 27° up when they cross the meridian. Andromeda and Perseus are lower still, Mirfak reaching only 6°, and Cassiopeia never clears the horizon. The Andromeda Galaxy can still be seen from Sydney, low in the north on spring evenings, at an altitude of about 15° when it crosses the meridian.