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Introduction

Lying under a clear, dark night sky, listening to the nocturnal chorus of the insects, gazing up and pointing at the brightest star, Sirius, with a green laser, my younger brother beamed the red flashlight on my face and asked, “Why do these stars have names?”, “Why are they named so?”, “Can a star have two names- a formal name and a pet name?”

Just like I had encountered these questions, you might also have had. Or if not, then these must be popping up in your head right now. So here is an acceptable explanation.

Why are they named?

We all have names so that we can be distinctly identified. The stars are named for the same purpose. Astronomy is one of the first natural sciences, and so, the stars have been studied by astronomers for thousands of years. It is conclusive that astronomers and stars are the best friends. So, to distinctly identify their best friends, ancient astronomers named these stars. Now, if you wonder about when the first star was named and which one, then the answer to the first question seems obvious- as soon as we had enough language to name anything around us. To answer the second question, not quite obvious but it was our star- the Sun, which was the first star to be named.

The Ancient Naming System

As we all might be familiar with, the constellations are patterns among stars. There are 88 constellations recognized by the International Astronomical Union (IAU) that cover the entire northern and southern sky. Few of them, about which we were taught since grade three- Orion, Ursa Major, Cassiopeia, Leo, Taurus, and Scorpius are easily recognizable in the night sky. Let us choose Orion. Now, as the tale goes, the Ancient Greeks saw the star pattern as the mighty hunter Orion. With his dogs Canis Major and Canis Minor, he faced the charging bull Taurus as shown below.

Credit: Stellarium Web
Credit: Orion Constellation|Astronoo

The stars which constituted this pattern were named for what they signified in the figure. As shown in the image above, the brightest star in the constellation of Orion, Rigel, marks the left foot of the hunter. Literally, the word ‘Rigel’ is derived from an Arabic term meaning “the left leg of the giant,” referring to the figure of Orion. Betelgeuse, that yellowish star, which is a red supergiant star marks the shoulder of the hunter. The word ‘Betelgeuse’ is derived from the Arabic word “bat al-jawzā”, which means “the giant’s shoulder.”

Similarly, most of the prominent stars of the constellations were named this way by Ancient Astronomers. Though, over the time, certain ancient names have changed but many of them are still used. But what about the other stars? Out of tens of thousands of stars visible to the unaided eye, only a few hundreds of them have been given proper names. It is analogous to the case that if there are only 18 students in a class, the teacher can distinctly identify them by their names. But if the number goes up to 118 students, then it is no longer possible for the teacher to distinctly identify students by their names. That is when the role of roll numbers becomes significant and the students are designated using numbers from 1 to 118. Likewise, rest of the stars, whether visible to unaided eye or not, but which are observed with ground or space based telescopes have been designated by the Modern Astronomers.

Designating the Stars

In the past, different cultures gave different names to the same star. Astronomy, being one of the first natural sciences, has advanced a lot over centuries. So, there was a need of a universal cataloging system in which a particular star is given a particular label irrespective of different cultures. This was necessary to bring a uniformity throughout the field.

Johann Bayer was the first one to produce such a catalog in 1603 using a set of rules for designating the stars. This catalog was included in his star atlas called Uranometria (meaning ‘Measuring the Heavens’). The stars were labelled on the basis of their brightness. He used lowercase Greek letters (𝛂,𝛃,𝛄,𝛅, etc.) to designate the stars in approximate order of their apparent brightness. It is analogous to the roll numbers being used to designate the students on the basis of their names in increasing alphabetical order. He labelled the brightest star in a constellation as alpha (𝛂), the second brightest star as beta (𝛃), and so on, followed by the genitive of their Latin constellation names. For example, the brightest star in the constellation Ursa Major (the Great Bear) was designated as 𝛂-Ursae Majoris and the brightest star is the constellation Leo (the Lion) was designated as 𝛂-Leonis. Now, to answer the question whether stars have both formal and informal names, the answer is a yes. The already named stars were given Stellar Designations too, though, not all stars were named. This Stellar Designation is popularly referred to as Bayer Designation of Stars.

As mentioned above, the stars were designated based on their apparent brightness and this was usually but not always correct. So, what significance does the term ‘apparent’ have and is there any scale to measure brightness, like the one we use to measure length?

Let’s Measure- The Brightness Scale

This is an easy thought experiment which can help in understanding what apparent brightness actually is. Suppose, you light a candle and hold it at a distance of about 30 cm from your eyes. Or if imagining holding a candle feels tiring, then imagine a table too and place the candle on it such that it is 30 cm away from your eyes. Further, imagine that you, the table, and the candle are on one side of the street and let us imagine a street light on the opposite end of the street. Which one, out of the candle and the street light, seems brighter in your imagination? Now, notice that I have used the word ‘seems’ and not ‘is’. Reason being the candle seems to be brighter, although it is not, than the street light because it is much closer to your eyes than the street light is. Had they been observed from the same distance, the street light, of course, turns out to be brighter because it ‘is’ brighter than the candle.

The case for the stars stands the same. A star ‘A’ which is, suppose, bigger and brighter than another star ‘B’ when observed from the same distance will appear dimmer to us on the Earth if it is much much farther away than the star ‘B’. However, if there is a star ‘C’ which is much much brighter than a star ‘D’ (when observed from the same distance) but a bit farther away, it will still seem to be brighter than the star ‘D’ even if ‘D’ is closer but dimmer. The goal of this thought experiment was to give a clear and easy visualization of the term ‘apparent’. If you will search Google for the meaning of the word ‘apparent’, it comes out to be ‘seeming real or true, but not necessarily so’. However, measuring the brightness from the same distance gives the absolute magnitude or actual brightness of the stars.

Ancient Greek Astronomer Hipparchus was the first to classify the stars based on their apparent brightness. Basically, he looked at the stars in the sky and put them on a magnitude or brightness scale ranging from 1 to 6. He labelled the brightest star he saw as magnitude 1 star, the next brightest star was classified as a magnitude 2 star and the faintest star that he could see was given a magnitude of 6.

Credit: Self-Made

With our modern telescopes, we can now see the stars which have an apparent magnitude beyond 6 (dimmer stars). Below here shown is a modern apparent magnitude (brightness) scale. The more negative the number (magnitude) is, the more brighter that object seems to be in the sky. The apparent brightness goes on decreasing as we move towards the right end of the scale.

Credit: What is Stellar Magnitude?|EarthSky

So, that is what apparent brightness means and now you know how Bayer designated stars based on their apparent brightness.

The alpha, beta Discrepancies

The Bayer designations are in use even till date, however, this scheme has some discrepancies. For example, the brightest star in the constellation Orion, Rigel is designated as 𝛃- Orionis and the second brightest star in the Orion, Betelgeuse is designated as 𝛂- Orionis. The reason behind this is that Betelgeuse is a variable star and its magnitude (apparent) varies between 0.2 and 1.2. At its brightest, the 0.2 magnitude of Betelgeuse is very close to Rigel’s 0.18 magnitude (apparent). So, telling which one is brighter becomes difficult with an unaided eye.

However, there is another story to reason for this. It happened sometimes, that the brightest stars of a constellation belonged to the same magnitude class, when observed with an unaided eye. So, Bayer used a different criteria to designate these stars. It seems, that he observed Betelgeuse and Rigel to be of same magnitude class. And so, he designated them as alpha and beta Orionis respectively because the shoulder of the hunter which is represented by Betelgeuse is above the left foot which is represented by Rigel.

Why did Bayer’s Scheme run into Difficulties?

One limitation, as mentioned above, were the mis-estimates and other irregularities which lead to discrepancies in Stellar Designations. For a constellation which had more than 24 stars, designating them posed a problem because the Greek alphabet only has 24 letters. To fix this problem, Bayer firstly used the lower-case (a-z) and then the upper-case (A-Z) letters from the modern Latin alphabet.

Despite all this, Bayer Designations are used till date. However, there are many more Star Catalogs, based on different schemes, which are presently being used to designate and distinctly identify bright, faint, and variable stars.

A Homework Exercise

So, here I leave my readers with an interesting homework exercise. Next time when you gaze up and look at those thousands of twinkling dots, observe them with a different perspective. Try designating them according to your own schemes. Who knows, one day you might become as popular as Johann Bayer.

References

[1] Howell, Elizabeth. “Brightest Stars: Luminosity & Magnitude.” Space.com, Space, 11 Oct. 2017, www.space.com/21640-star-luminosity-and-magnitude.html.

[2] “International Astronomical Union.” IAU, www.iau.org/public/themes/naming_stars/.

[3] Yamani, Avivah, and Name *. “Jan 15th: Why Did Betelgeuse Become Alpha Orionis Instead of Rigel?” Jan 15th: Why Did Betelgeuse Become Alpha Orionis Instead of Rigel? | 365 Days of Astronomy, cosmoquest.org/x/365daysofastronomy/2015/01/15/jan-15th-why-did-betelgeuse-become-alpha-orionis-instead-of-rigel/.

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