Note: This is an article I wrote for kids (6 to 12 years) for a newsletter. Now that the news of ‘Ophiuchus’ being the 13th constellation of the Zodiac is public, I thought there might be some interest in stars and constellations among adults too!
Have you ever tried going to a dark spot away from city lights looking at the sky from there? You’ll be amazed at the range of colors and brightness variations of stars in the night sky. Can you guess the number of stars you can see in a very dark sky? Would you think you can see millions of stars? No! At the most you can see about six thousand stars in the sky. That means you probably can’t see more than three thousand stars at any given time. Strange, but true!
Ursa Major constellation, or “Big Dipper”. In India it is called Saptarshi Mandala. The stars Merak & Dubhe (Indian names: Pulaha and Krtu are called pointers, because it you extend an imaginary line through them, you’ll reach the Pole star (Dhruva)
When you look up in a dark sky you will see stars of many different hues – bluish white, bright white, bright red, orange, yellow and several other colors in between. Some stars visible to naked eye are extremely bright, while a large number of the stars are faint. But have you wondered if these stars had any names?
Many of the bright stars in the sky that you might be able to see even from a light-polluted city sky have proper names. In India, many of these stars were named thousands of years ago and the same names are in vogue today. The names of twenty seven asterisms (stars, or groups of stars) starting with Ashwini, ending with Revathi which are part of the twelve constellations in the zodiac have existed since then. By the way, many of the stars from this list of Indian asterisms are not very bright but they were named because they helped ancient Indians to formulate their calendar based on the movement of Sun and the Moon in the background of these stars.
Apart from these, names in Indian languages are available to few other bright stars outside the zodiac as well. The pole star, called ‘Dhruva’ is probably the most well known of such stars. The word ‘Dhruva’ in Samskrta means ‘constant’. This is a very apt name because the position of this star in the sky never changes and stays constant. The Pole star is a not an exceptionally bright star, but is a notable star because of its position it occupies in the sky. All stars in the sky appear as though they rotate around it. The Pole star never rises or sets, nor does it show any kind of movement in the sky. If you were at the North Pole, you would see the Pole star directly overhead, and all other stars go around, never rising or setting.
Many of the star names in English are taken from their Roman or Greek names. A large number of star names in English come from Arabic as well. For those stars for which there is no native Indian name, Indian stargazers use their international (English) names.
For those stars that don’t have proper names, there is a very interesting way of nomenclature. The sky is divided into 88 constellations. Constellations are imaginary star patterns in the sky. Some constellations actually resemble what they are supposed to resemble, and for some constellations, you must have an extremely eccentric imagination to relate a constellation to the figure it is supposed to mean! But that is beside the point. Any star you that you can see, belongs to one constellation or the other. The brightest star in a constellation is normally denoted by the Greek letter alpha, the second brightest beta, the third brightest gamma and so on. Thus, the brightest star in the constellation of Centaur would be called Alpha Centauri; the second brightest star in the constellation Leo would be called Beta Leonis. In this system, the Pole star would be called Alpha Ursa Minoris, because it is the brightest star in the constellation of Ursa Minor. Thus almost every star that has a proper name also has a name based on the constellation and the brightness of the star within the constellation it belongs to.
This method of naming although very useful has given some incorrect names too. For example, the bright red star Betelgeuse is called Alpha Orionis, meaning it is the brightest star in the constellation Orion. But if you look up the sky now to look at Orion, you will notice there is one more star that is brighter than Betelgeuse! That star is Rigel (or Beta Orionis, as you might have guessed). Betelgeuse is a variable star, meaning its brightness varies over time. There was a time when Betelgeuse might have actually looked brighter to bare eyes than Rigel, and that’s when this name must have come from and it has stayed on.
Picture source: Astronomy picture of the day website (http://apod.nasa.gov/apod/ap081015.html)
Using Greek letters as prefixes to stars would only work for a handful of stars in any constellation. When you look at the sky with a telescope, you’d see thousands of new stars, invisible to naked eye. A new problem of naming these arises. Astronomers have a very interesting way of dealing with this problem.
Just like the Earth is divided by imaginary lines called longitudes from North Pole to South Pole, the sky is also divided by imaginary lines going from the north pole of the sky to the South Pole. Just as there is a prime meridian on Earth (0 degree longitude), there is a 0 hours right ascension (RA) line in the sky. The line that goes through the First point of Aries (The point in sky where the Sun would be seen on the spring equinox) is called 0 hours RA. Any star in sky can be located by its co-ordinates – how many hours (and/or minutes) away from the first point of Aries and how far is it from the equator of the sky. This is very similar to locating a place on the Earth knowing the longitude and latitude.
Now you must be guessing how this helps in naming stars! Every star in a constellation is given a number by the order of right ascension. As an example, the star within the area marked for the constellation Virgo and with the least right ascension will be labeled 1 Virginis. The star with next higher right ascension will be 2 Virginis and so on. Here there is no correlation between the number and the brightness of the star.
The first few months of the year are a treat to star gazers. There are a bunch of bright stars and constellations in the eastern sky. So what are you waiting for? There are many resources on the Internet to help you identify the stars and constellations. Get out and check out constellations Orion, Taurus, Gemini, Auriga and Perseus shining in the winter sky with bright stars like Sirius, Betelgeuse, Rigel, Aldebaran and Procyon. The three stars from the belt of Orion (see figure) and Pleiades star cluster are unmistakable from any corner on the Northern hemisphere. I bet you will have lots of fun star hunting!