October in India heralds beginning of observing season. We start our observations from the western direction. and Face west and hold the map so that ‘West’ marked on it is in the direction of your feet and ‘East’ is up (it will be read upside down). Now look half way between the horizon and zenith you will see three fairly bright stars in almost a right angled triangle configuration. The base of this triangle is almost parallel to the horizon.
These stars belong to three different constellations. Star at the apex of the triangle is Deneb in Cygnus, the Swan [Hans हंस ]. The star to the right of the base of the triangle is Altair [Shravan श्रवण] in Aquila, the Eagle [Garuda गरूड] and the one to the left is Vega [Abhijit अभिजीत ], in Lyra, the Lyre [Swaramandal स्वरमंडल].
This group of stars was named summer triangle by Patrick Moor. All the stars are the brightest stars in their respective constellations. Vega which is the brightest star in the northern hemisphere. The second brightest of the summer triangle is Altair. Shravan is 22nd Nakshatra.
About 9 degrees north east of Debeb is a cluster of stars that can be easily picked up using a pair of binoculars. It is designated as M39which means it is 39th object in Messier catalogue. The stars of this cluster are loosely grouped together. Such a cluster is called open cluster or galactic cluster.
Right above the western horizon you can probably trace out Hercules [Shauri शौरी]. This constellation is rather too close to the horizon and will be setting soon. Try to catch it a bit early, as soon as it gets dark. The object marked M13 is another cluster but very much unlike M39. This is a compact cluster of stars comes under the category ‘globular cluster’.
Turn now to your left in the southwestern direction. Sagittarius [Dhanur धनु ] is also setting. In this direction lies center of our milky way galaxy. This area contains number of clusters and nebulae. The best one is M22. It is another globular cluster. It too can be picked up in pair of binoculars.
For the observers in the southern parts of India, Acherner [agra-nad अग्रनद] in Eridanus, the River [Yamuna यमुना] is just rising above the sought-western horizon. For the observers in the northern parts this will be too low. The name Acherner comes from ‘Al Ahir al Nahr’ or ‘Akhir an-Nahr’ meaning the End of the River. We call it Agra-nad or debaucher of river, Yamuna. The two opposite meanings for the same stars can be easily understood when we realize that from northern latitudes Acherner is seen very close to the horizon, almost at the end of the sky. Where as from the latitudes close to the equators this star is one of the first one to rise in its own constellation.
Right over head is one of the ‘land marks’ of the night sky is Great Square of Pegasus, the Winged Horse [Maha-ashwa महाअश्व]. You just cannot miss it. The sides of the square are almost aligned north-south and east-west. The stars on the western arm of the square make our 25th nakshatra Purva Bhadrapada and those on the eastern arm are of 26th nakshatra Uttra Bhadrapada. Purva and Uttara are in the sense of earlier and later respectively, in their movement in the sky and not in the sense of direction.
Halfway between Pegasus and the horizon is Cassiopeia [Sharmishata शर्मिष्ठ]. This is a constellation in ‘M’ shape with its right leg pulled too much to right. You might recall that Saptarishis सप्तर्षि are used for finding north direction in the night. This constellation can be used for finding north when Saptarishis are below the horizon. Take the first three stars of ‘M’ which make an equilateral triangle. Now take perpendicular bisector of the first and third star. This line, when extended towards horizon will go through the Polaris [Dhruva ध्रुव], Pole Star. Using a pair of binoculars if you scan on the line joining third and fourth stars of Cassiopeia and extending to east you will reach a binary open cluster of stars h &χ. These are lovely pair of star clusters.
Probably the most familiar object to the science-fiction readers (whether they see the sky or not) is the Andromeda galaxy. Easy to locate, it can be seen with the naked eyes on a clear dark night. It is marked as M31 in Andromeda [Devayani देवयानी]. Look for it between Cassiopeia and Andromeda. To the first time viewers the galaxy may not appear much of a sight, but remember, this galaxy is very much like our own galaxy, the Milky Way and with some practice you will appriciate its beauty.
Face now east. Taurus, the Bull [Vrishabha वृषभ] is rising just above the eastern horizon. This constellation has two prominent clusters – Pleiades (कृत्तिका) and Hyades both visible to the naked eyes. Hyades is a ‘V shaped cluster with a bright red star Aldebaran [Rohini रोहीणी] at the tip of one arm of the ‘V’. This stars, however, does not belong to the Hyades cluster. It just happens to be half way on the line of sight from us and the cluster. Rohini is 4th Nakshatra. Pleiades is 3rd nakshatra Krittika कृत्तिका. A person with good eye sight can count about 6 (or even 7) stars in this group.
Lastly face northeast. Right above the horizon you can see bright yellow stars Capella [Brahamha-rhuday ब्रह्मह्रदय] rising above the horizon. We will talk more about it next month. But try to identify a chain of stas connecting Pleiades and h & c in Perseus [Yayati ययाती] and then try to identify star Algol above this chain.
Algol gets its name from its Arabic name Al-Ghul, the Ghost. This star is what astronomers call a variable star. The star light goes through a periodic change in its light. Normally its light is constant but after about every 2 days and 20 hours its light stars fading. In about 5 hours it is half as its normal light and then it starts regaining its brightness and it is back to normal in next 5 hours to remain so for next 2 days and 20 hours or so. This happens because there is a cooler companion to the bright star that we see. Every time the cooler stars ‘eclipses’ the brighter one we see the drop in the light.