Shapley-Ames Catalog

Lore & Background

Shapley and Ames began their study of all nearby galaxies in 1926. An important finding from this research was that the galaxies were not evenly distributed (they violated the isotropy assumption) in that the northern hemisphere contained more galaxies than the southern hemisphere. It also found that the Virgo cloud extended further than previously believed. From this data, Shapley and Ames therefore created a new hierarchy of clusters called a supercluster, which is a cluster of galaxy clusters, and called this Virgo cloud in the northern hemisphere a 'Local Supercluster'.

With the help of new photographic recordings, which also contained comparison stars of known brightness, the brightness of many galaxies were measured and recorded only up to a magnitude of 13.2. They identified 1189 objects based on the New General Catalogue and 48 based on the Index Catalogue.

In 1981, Allan Sandage and Gustav Tammann published an updated version known as the Revised Shapley-Ames Catalog (RSA). The original list of galaxies was maintained, with the exception of three objects which were no longer considered galaxies. The information on the 1246 individual galaxies have been updated and substantially extended.

Reader's Guide

For the next 60 years after its publication, astronomers referred to this catalog as a primary source for information about redshifts and galaxy types. The catalog's identification of the uneven distribution of galaxies in the northern and southern hemispheres, and its discovery that the Virgo cloud extended further than previously believed, led to the creation of the concept of a supercluster—a cluster of galaxy clusters—which Shapley and Ames called the 'Local Supercluster'. This work fundamentally changed the understanding of large-scale structure in the universe. The 1981 Revised Shapley-Ames Catalog (RSA) by Allan Sandage and Gustav Tammann updated and substantially extended the information on the 1246 individual galaxies, maintaining the original list except for three objects no longer considered galaxies. The catalog remained a primary reference for astronomers for decades, providing foundational data on galaxy positions, brightness, sizes, and Hubble classifications.

Did You Know?

• The catalog includes 1249 objects brighter than 13.2 magnitude.
• It was the first compilation of bright galaxies covering both the northern and southern sky.
• Shapley and Ames identified a 'Local Supercluster' from the uneven distribution of galaxies.
• The 1981 Revised Shapley-Ames Catalog removed three objects no longer considered galaxies.

Defining the Catalogue Tradition

An astronomical catalogue, at its core, is a structured listing or tabulation of celestial objects. Rather than being a random collection, these entries are deliberately grouped together because the objects share a unifying characteristic—whether that be a common type, a particular morphology, a shared origin, a specific means of detection, or a common method of discovery. This organizing principle is what transforms a mere list into a scientific instrument. The catalogue becomes a reference framework, allowing astronomers to locate, compare, and study objects that belong to the same family. In the broader landscape of astronomical literature, catalogues occupy a foundational role: they are the index cards of the universe, the way we impose order on the overwhelming diversity of what the sky contains. Every entry carries with it the implication that the object fits within a defined category, making the catalogue not just a record but a classification system in its own right.

Survey as the Foundation

The creation of an astronomical catalogue is rarely an isolated act of observation. In most cases, these tabulations are the direct product of a broader astronomical survey—a systematic, often large-scale effort to scan portions of the sky and record what is found. The survey provides the raw material; the catalogue is the distilled, organized output. This relationship between survey and catalogue means that the scope, sensitivity, and methodology of the original observation are baked into the final list. A catalogue of radio sources, for instance, reflects the capabilities and pointing strategy of the radio telescope used, while a catalogue of X-ray sources depends on the orbital path and detector sensitivity of the satellite that captured them. The catalogue thus inherits the strengths and limitations of its parent survey. Understanding a catalogue's provenance—what survey produced it, under what conditions, with what instruments—remains essential for interpreting its contents and recognizing what it may have missed.

A Spectrum of Celestial Objects

The objects recorded across astronomical catalogues span an extraordinary range of celestial phenomena. Some catalogues focus on double stars, compiling pairs that share a common gravitational bond. Others target open star clusters or globular clusters, grouping stars that formed together. Still others catalogue planetary nebulae, dark nebulae, peculiar galaxies, or emission-line objects. The detection methods vary just as widely: radio telescopes identify sources at specific frequencies, satellites capture X-ray and gamma-ray emissions, and ground-based optical or infrared surveys map visible-light and thermal objects across the sky. Even within a single type of object, catalogues can differ in scope—one might list all stars within eight parsecs, while another catalogs ultracool dwarfs identified through a broader infrared survey. This diversity ensures that no single catalogue captures the full picture of the sky.

The Architecture of Designations

The identification system for astronomical catalogues is a layered architecture of letters, numbers, and personal names. Some entries are purely numerical, such as the sequential Cambridge Catalogues of Radio Sources running from 1C through 10C. Others combine a number with a letter to denote a specific survey version or instrument, as seen in designations like 1RXH for ROSAT pointed observations or 2MASS for the Two Micron All Sky Survey. A large portion of catalogues bear the name of their creator or principal contributor—Abell, Abt, Alden, Alessi, Arp, Barnard, Baade, and many others—linking the list directly to the individual who compiled it. Cross-referencing is common: a single open star cluster might appear under multiple names in different catalogues, connected by equivalences. This web of aliases and designations reflects the collaborative, cumulative nature of astronomical cataloguing, where each new survey adds another thread to an ever-expanding tapestry of celestial identification.