What do the different ways of identifying stars with numbers mean?

How do we catalogue stars in the night sky?


Asked by Martin Bromberg

You have probably heard of stars that have names such as Vega, Alcor, Sirius and the like. However, since there are so many stars in the sky, not all of them can be assigned names. Another naming system needs to be put in place.

An early attempt to systematically catalogue stars came in 1603 when Johann Bayer began to name the brightest stars in each constellation using greek letters. His system worked on the principle that the brightest star in the constellation was designated the alpha star, the second brightest was the beta star and so on. But seeing as his catalogue only named 1,564 naked eye stars, many stars were left without form of identification.

So in 1712 John Flamsteed, who was then an astronomer at the Royal Observatory Greenwich produced a star catalogue that numbered some of the undesignated stars. An example of a Flamsteed Number is 51 Pegasi, the first Sun-like star found to have a planet orbiting it, where 51 is the Flamsteed number and Pegasus its constellation. But even he only managed to catalogue 2,554 stars, leaving many thousands of stars unnamed or unnumbered.

Subsequent star catalogues have sought to fill in the missing numbers, for example the Henry Draper catalogue which initially listed 225,300 stars to a low magnitude of 10 and was later extended to 359,083 stars.

Variable stars, which are stars whose brightness is seen to fluctuate from Earth, are also numbered in a similar way. In general, astronomers use a variation on the Bayer designation combined with the Latin name of the constellation in which the star lies. If a star does not have an existing Greek letter from the Bayer designation, then astronomers start with a letter. Examples are R Coronae Borealis and YZ Ceti. After using a combination of 334 letters, variable stars start to take on numbers such as V335, V336 and so on.

Image courtesy of ESO/B. Tafreshi/TWAN

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