How Stars Are Named: Why This Menagerie of Names & Catalog Numbers For Extrasolar Planets?

How Stars Are Named: Why This Menagerie of Names & Catalog Numbers For Extrasolar Planets?
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How Stars Are Named

Let’s start with how stars are named. Most of the brightest stars have proper names that date back centuries to a time when ancient civilizations believed their fate lay in the heavens. Many of the names have their origins in the Greek and Arabic languages. Hence, the brightest star in the sky is called Sirius, which translates into “sparkling”. A star in the constellation Perseus, which varies so much in brightness that it regularly disappears has the proper name Algol, a corruption of the Arabic “al ghul” meaning “ghoul” or “demon,” and we can understand why the ancients thought it was demonic.

When Johannes Bayer (1572-1625) created his Uranometria celestial atlas he moved way from proper names and systematically identified the stars using Greek characters. The brightest star in a constellation was called α (alpha), the second brightest β (beta) and so on. Well, more or less. He sometimes made mistakes and, for some strange reason, the ordering of the stars in Pisces went haywire, but generally it worked.

As astronomy developed and telescopes became more advanced, astronomers could see fainter stars and required better catalogs. John Flamsteed (1646-1720), Britain’s first Astronomer Royal, cataloged the stars using numbers instead of letters. A century later Friedrich Argelander (1799-1875) compiled his 300,000 star Bonner Durchmusterung catalog (BD) and identified those stars that varied in brightness by using the double letters RR-RZ, SR-SZ and so on all the way up to ZZ. Another important catalog was compiled by the American astronomer Henry Draper (HD) and there have been many others, most notably the Harvard Revised Catalog (HR), the Smithsonian Astrophysical Observatory catalog (SAO) and specialist publications such as the Washington Double Star (WDS) catalog.

The advent of the Space Age saw telescopes lifted above Earth’s blinding atmosphere and millions of new stars were recorded, most recently by Hipparcos and Tycho. In addition to these general catalogs there are many specialist lists on, for example, X-ray sources, supernova remnants and pulsars to name but a few. A team at the University of Strasbourg has undertaken the herculean task of bringing most of these catalogs into a single massive database, which includes cross-referencing between the catalogs. So we can easily find out, for example, that α Persei (alpha) is also listed in the Flamsteed catalog as 33 Persei, in Draper’s as HD 20902, in Harvard as HR 1017, in Hipparcos as HIP 15863 and in Tycho as TYC 3320-2808-1. We also discover that it’s proper name is Mirfak, it is variable and is listed as a Newly Suspected Variable NSV 1125 and that it has shown up in the Infra-Red Astronomy Satellite data as IRAS 03207+4941. In all α Persei has no fewer than 43 names or, to be more precise, catalog numbers.

Multiple Star Systems

Many of the stars in our galaxy are not alone. The often have two or more companions all orbiting a common center of gravity. In order to identify the various components the stars are usually given a superscript character.

Suppose the brightest star in “Constellation”, a hypothetical star group, is a binary. Astronomers usually identify the two components as:

αA Con and αB Con.

Sometimes numbers are used instead of Roman letters:

α1 Con and α2 Con.

And, in more recent decades, the superscript is converted to normal type and placed at the end of the Con:

α Con A and α Con B.

This appears to have come about to simplify inputting into computerized databases.

Now, if it turns out that α Con A actually has another companion – in other words, it is a triple star and not a binary star system – then the notation is extended:

α Con Aa and α Con Ab and α Con B

The Discovery of Planets Outside of the Solar System

So far, so good. But then along came the planet hunters and things started to go awry.

A couple of the early exoplanet discoveries, in 1996, were around the stars τ Bootis (tau) and υ Andromedae (upsilon). These planets were cataloged as τ Boo b and υ And b. The letter “a” is, in effect, not used as it signifies the star, a bit of a strange decision as the star is already identified by it Bayer/Flamsteed/HD etc catalog number. A further three planets were discovered in Andromeda, which came to be listed as υ And c, υ And d and υ And e. Now it should have been obvious at this early stage that there was room for confusion: “α Con B” could easily be misread for “α Con b” and vice versa, and if a planet is discovered around “α Con A” then it becomes “α Con Ab,” which does rather sound like a binary star system!

If you think that is confusing then read on.

The planet hunters obviously decided to use Bayer notation for planets discovered around the brighter stars. Hence, when a planet was found to be orbiting the brightest star in Piscis Australis, it was obviously going to be called a PsA b. Except it wasn’t. They called it after the star’s proper name, Fomalhaut b (pronounced foe-ma-low). Oh, and just to muddy the celestial waters further, Piscis Australis is also known as Piscis Austrinus.

With ρ1 Cancri (rho) the planet hunters were on safer ground as the star does not have a proper name. The five planets in orbit around the star could logically be cataloged as ρ1 Cnc b, c, d, e and f. But that would have been too easy! Instead the Flamsteed number was used and so they became 55 Cnc b, c, d, e and f.

Perhaps we are on safer ground with μ Arae (mu), a star so far south that Flamsteed could not catalog it, and nor does it have a proper name. But no, the four planets around μ Arae have been listed as HD 160691 b,c,d and e using the Henry Draper catalog.

Okay, the Henry Draper Catalog covers far more stars than Bayer or Flamsteed so perhaps we should use that instead? It makes some sort of sense, doesn’t it? Hence, the single detected planet around HD 204961 in Grus will be called HD 204961 b. No chance! It is listed as GJ 832 b from yet another catalog, Gliese-Jahreiss. Then there is ι Horologii b (iota) which could have been HD 17051 b or even GJ 108 b but is, instead, HR 810 b, one of just two entries from the Harvard Revised Catalog.

With around 1,400 confirmed or suspected planets the list of confusing names seems to go on forever. Wouldn’t you think that someone could co-ordinate the naming and cataloguing?


Strangely enough, it is the International Astronomical Union’s role to do just that. They may not have any legal authority to do so, operating simply on a nod and a wink from the rest of the astronomical community, but it is the only body geared up to the task. So why has it all gone so horribly wrong?

The impression I get is that the IAU was a bit slow off the mark. That’s unfortunate because it leaves astronomers to their own devices and they couldn’t catalog their own corn flake collection even if their lives depended on it. Over the last century we have seen just as much confusion in other areas: with comets, asteroids, double and multiple star systems, open clusters and meteorites (though I’ll accept that those who collect and catalog meteorites are not always astronomers). Letting astronomers loose on the galaxy is just asking for trouble. They need coordinating. The thing is, when the IAU does get its act together, they are really good. They’ve done a brilliant job of setting out the rules for naming features on other Solar System planets and moons. The geologic formations found on the Saturnian satellite Tethys, for example, are named after people and places mentioned in Homer’s Odyssey, while those on Venus are named after famous women and goddesses.

In 2009 one researcher, W. Lyra of the Max Planck Institute for Astronomy, suggested naming extrasolar planets after the mythology of the constellation in which they were discovered. The IAU rejected this suggestion on the basis that there were likely to be so many discoveries that the names would quickly run out. Undeterred, Dr Lyra published a list of names for all 409 then known extrasolar planets. The most obvious problem with this list, however, is that many of the names are already assigned to asteroids and planetary features. So a good decision by the IAU, but perhaps for the wrong reason. And if you are wondering where you have heard the name “Lyra” before then, yes, it is the name of a constellation. But, that’s just coincidence.

The Future

The IAU did finally address the issue of naming extrasolar planets last year, but it could well be a case of too little too late. No one knows what new objects await discovery in our Universe, but early intervention by the IAU could avoid the confusion we have seen over the past 20 years of extrasolar planetary research.