How Astronomical Objects Are Named

by Dave Snyder
Written: November, 2001
Revised: February, 2002

Constellations

The sky is divided into 88 regions known as constellations.  Every point in the sky belongs to exactly one of these constellations.  Observers living on the equator have the opportunity of seeing all of the 88 constellations over the course of a year; Other observers can see some of the constellations but not others.

The following chart lists these 88 constellations.  For each constellation you will see the name, the genitive form (this is used to form star names as explained below) and a three letter abbreviation (this is used in tables and on star charts as explained below).

Note

1. Camelopardalis is sometimes referred to by the name Camelopardus.  Camelopardus has the genitive form Camelopardi.
2. Scorpius is sometimes referred to by the name Scorpio.
3. Serpens has two parts which are not contiguous.  Sometimes the parts are given the names Serpens Caput and Serpens Cauda, however the genetive form Serpentis is always used.

Star Names

Before telescopes, stars were given names usually derived from Latin, Greek or Arabic words.  Examples include Vega, Regulus and Polaris.

Assigning names to thousands of stars would get cumbersome and only the brightest stars were given names.  The first systematic attempt at naming stars is the Beyer system.  In the Beyer system, the brightest stars in a constellation are each given lower case greek letters.  The proper name for such stars is the greek letter followed by the genitive form of the constellation name (for example, Alpha Canis Majoris is a bright star in Canis Major, Beta Geminorus is a bright star in Gemini and so on).  The greek alphabet is as follows:

The Beyer designation has some odd features

1. While the “alpha” star is often the brightest star in a constellation, this is not always the case.
2. The other letters are often assigned in order which does not match the order of brightness.
3. There are 24 greek letters, so you might think that each constellation has 24 stars with greek letters, this is true for some constellations.  However others have less than 24 (for example Lynx has only one, Alpha).
4. Others have more than 24; numbers are used to distinguish between stars that use the same letter.  For example the six stars that make up the shield in the constellation Orion have the names Pi One, Pi Two, Pi Three, Pi Four, Pi Five and Pi Six.

An alternative to the Beyer system are Flamsteed numbers.  The brightest stars in each constellation (anywhere from ten or so to over a hundred) are each assigned numbers starting in the west going east.  Stars with Flamsteed numbers may also have Beyer designations.  The proper name for such stars is the number followed by the genitive form of the constellation name.  For example the star 66 Geminorum is also known as Alpha Geminorum.

A few stars that are assigned roman letters (such as “e”, “s” or “N”).  The letter must be followed by the genitive form of the constellation name.  (If you see what looks like the letter “o”, it is probably the greek letter omicron; a few other greek letters look similar to roman letters, if in doubt it is probably a greek letter).

Variable stars are often assigned names with a single upper case letter from R to Z, two upper case letters (such as RR) or the letter V followed by a three or four digit number (such as V335).  In each case these designations must be followed by the genitive form of the constellation name (for example RR Lyrae is a variable star in the constellation Lyra).

To cover stars that were not assigned names by Beyer, Flamsteed, roman letters or variable star designations; a variety of catalogs have been developed.  The most commonly seen are the Smithsonian Astrophysical Observatory catalog, the Henry Draper catalog and Aiken’s catalog of double stars, however a number of others are use.  Typically such stars are indicated by an abbreviation of the catalog name (for example SAO, HD, ADS) followed by some numbers.

Deep Sky Objects

Deep sky objects (that is objects that are not stars and not within our solar system) may have common names, but most are found in catalogs.  The earliest catalog that is still in wide use is the Messier catalog which includes 110 objects, they are usually referred by a letter M followed by a number (for example M31 is the andromeda galaxy).  Two more extensive catalogs are known as NGC and IC.  Objects in these catalogs are indicated by the letters NGC or IC followed by a number (for example NGC 7000 is the North American Nebula).

Note, a few deep sky objects have names that look like star names.  In particular, Omega Centarus and 47 Tucanae are both globular clusters, not stars.

Star Charts

Star charts typically show constellation boundaries, so it is clear which objects belong to which constellation.  The constellation name is printed only once and stars are usually labeled with an abbreviated form of the name.  If the star has two different names, they are typically both given, usually separated by a dash or comma.  Within the boundary of Orion, you may see the labels:

This indicates the star 7 Orionis also known as Pi One Orionis, the star 2 Orionis also known as Pi Two Orionis and so on.  Elsewhere in Orion, you may find the labels:

This indicates four stars, 58 Orionis also known as Alpha Orionis, 19 Orionis also known as Beta Orionis and the two variable stars W Orionis and BL Orionis.

Single letters will sometimes been seen.  For example in the constellation of Eridanus you will find the following letters: e, f, g, h, p, s, and y.  Each of these represents a different star.

Occasionally you will find a star labeled with a letter or number followed by a three letter abbreviation.  In the constellation Aries, you may find a star labeled 85Cet, Cet is the abbreviation for Cetus and the proper name for this star is 85 Ceti.  While uncommon, constellation boundaries have changed, stars moved and errors have been made and corrected; thus there are stars with names that imply they are in one constellation while they actually are in a neighboring constellation.

Deep sky objects are usually marked with their abbreviated names, however a few charts mark NGC objects with the number only, so NGC 7000 is labeled as 7000.  This can create an ambiguity as stars are also marked with numbers.  Generally you can tell whether a label refers to an NGC object or a star by looking at the symbol, a dot generally indicates a star, some other symbol indicates a deep sky object (there should be a legend somewhere that shows what the symbols mean).

References

For a list of the reference materials used to produce this guide, see the University Lowbrow Astronomer’s Book List.

Links

• University Lowbrow Astronomers Observer’s Guide.
• The Home Page for the University Lowbrow Astronomers.
• The University of Michigan Astronomy Department.
• The University of Michigan Gateway.
• A List of Other Astronomy Clubs in SE Michigan.
• Other Astronomy Related Pages.

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