Photographs and charts

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Palomar, SRC, and ESO sky surveys The true comprehensive charts of the faint stars in the sky at optical wavelengths are actually deep (i.e., sensitive) photographs of the sky. The first and most famous of these is the Palomar Observatory Sky Survey (POSS-I) carried out in the early 1950s. It consists of 936 pairs of large glass plates (350 mm x 350 mm) taken with the large Schmidt telescope at Palomar Mountain in California. The plates cover declinations +90 to -30. At each position, a red-sensitive and a blue-sensitive plate were exposed. The glass plate provides stability against bending and thermal expansion. Each plate covers a large portion of the sky, about 6.5° x 6.5°, and shows stars as faint as ~21st magnitude. (The brightest stars have magnitude m ~ 0, and the faintest stars visible to the naked eye on a dark night have magnitude m ~ 6. See Section 8.3 for more.)

The red and blue plates were taken with specific types of emulsion and filters. The developed plate is a black and white photograph, but the relative "brightnesses" of the stellar images on a given plate give the relative star-to-star intensities in the color band of the plate. Comparison of the intensities of a given star on two different plates, i.e., in two different color bands, provides a good indication of the temperature of the object. A hot object will be relatively more intense in the blue than in the red, as compared to a cool object which will be more intense in the red than in the blue. The shape of images on such photographs also conveys a great deal of information. Point-like objects (stars) and diffuse nebulosities (supernova remnants, globular clusters, galaxies, etc.) are seen in abundance on such plates.

In the recent decades, similar surveys with somewhat better sensitivity have been carried out in the southern hemisphere, at observatories in Chile and Australia. (ESO-R and SRC-J surveys). A second, more sensitive Palomar survey (POSS-II) of the northern sky is now being completed at Palomar and digitized at the Space Telescope Institute.

Astronomical libraries have collections of paper prints of these plates, and digitized versions are now available on compact disks. These are produced with machines that either scan or photograph digitally. A reproduction of the original photograph can then be produced in a computer from these data. Computer analysis of the digitized reproduction can yield celestial positions (a,8) and star colors of each object. From this information one can generate a catalog. The POSS-I/SRC/ESO survey has yielded a catalog of 526 million stars, the US Naval Observatory (USNO) catalog. The USNO also provides the digitized images.

Often a star or other object will reside anonymously on one of these sky photographs or charts for many years until it suddenly becomes of interest to astronomers. This interest arises because it is found to have new or unusual characteristics. This new-found fame can be due to careful surveys of the optical sky with different or more sensitive instrumentation. Often, observations in other wavelength bands, e.g., radio, infrared, ultraviolet or x-ray, will call attention to a previously anonymous optical object.

Quasars, for example, are point-like objects that appear to be ordinary stars on all but the highest quality images. They first attracted attention because of their radio emission. Quasars are now also discovered in other frequency bands such as the optical and x-ray and are known to be the very active nuclei at the centers of galaxies. Neutron-star binary systems were first discovered in the x-ray band. In many such cases, when the celestial position of the radio, infrared, or x-ray object is known to sufficient precision, an examination of the optical survey plates will reveal the optical counterpart to be a star, a galaxy, or other interesting object that can then be studied at many wavelengths.

The Schmidt plates are very sensitive to nebulosities because the telescopes have a short focal length and large aperture (see Section 5.3 for definitions). A large solid angle of sky is compressed onto a single plate; hence a single grain of film receives a lot of energy from a diffuse object. The prints are quite beautiful and fascinating when examined by eye and with a small magnifier. They reveal dramatically the complexity and richness of the sky, e.g., the Pleiades in Fig. 1.7a.

Once an interesting astronomical object is identified on a survey plate, astronomers usually wish to study it in detail with powerful astronomical instruments mounted on a telescope, such as a spectrometer that disperses the light to reveal its spectrum. To do this, they need the equatorial coordinates of the object so the telescope can be properly pointed. The celestial coordinates of a faint object may be uncataloged. One can measure its physical (x, y) position on a survey photograph along with the x, y positions of nearby reference stars that have precise cataloged celestial coordinates. One can then solve to obtain the celestial position (a ,8) of the unknown object.

A less precise, but simple, measurement technique is to make use of the plastic transparent overlays that are provided (by Ohio State Univ.) for the Palomar Schmidt plates. The overlay for a given plate contains a coordinate grid (black lines) and also crosses at the locations of the brightest stars. If one places the overlay precisely over a paper print of the plate, using the bright-star positions for alignment, the celestial position of a previously uncataloged object may be obtained to about 20". Today, this can be done on the computer with the scanned (digitized) images.

Finding charts

Astronomers traditionally went to telescopes with less precise, ~1', celestial positions for a couple of reasons. It was quite an onerous task to obtain a very precise 1" position of a previously unmeasured star. Also, many telescopes do not point to a position on the sky more accurately than about 1'. In these cases, astronomers point the telescope in the approximate direction of the star and then use a finding chart to locate the exact star they wish to study.

A finding chart is simply an enlarged photographic print of the sky (e.g., a plate scale of 4''/mm) with the star of interest indicated by a pen mark. When the telescope is pointed in the approximate direction of the desired star, the astronomer looks through the eyepiece (or nowadays, at the TV monitor) to examine the stars in the field of view of the telescope. The pattern of stars should look similar to those on the finding chart, and the star of interest can then be identified. The telescope is then pointed more precisely so the light from this star falls precisely on the correct position for measurement, for example, the entrance slit of a spectroscope. Data accumulation can then begin.

The astronomical literature (and now the internet) abounds with finding charts; each is simply a photo of a piece of the sky with the star or galaxy of interest indicated. With such a chart, other astronomers can tell which star the author has studied. For example, the photo of the Crab nebula with the Crab pulsar marked (Fig. 1.3) would serve as a finding chart for the pulsar. These photographs made the astronomical literature more attractive and fun to peruse. One can see the beauty of the sky while sitting in the library. Nowadays, of course one does all this on the internet, and in color too!

Printed charts

Charts, or maps, of the sky historically complement photographs of the sky. These are printed maps that are produced from the measured positions and brightnesses of the objects of interest with the brighter stellar objects shown as larger dark circles. In times past, it could be a large project to make such plots. Now with computers and a file listing of star coordinates and intensities, one can quickly generate charts of any desired portion of the sky or of the whole sky. A chart of the POSS/SRC/ESO

survey could in principle be created, but it would be no more valuable than the reproduced digitized images one can now call up at will.

Historically, charts of the sky have been a staple of astronomy; perusal of old charts is intriguing indeed. A modern useful set of charts for the amateur astronomer is Norton's 2000.0 Star Atlas. It includes stars down to 6.5 magnitude on 14 charts, a total of ~8700 objects. This convenient atlas contains all the stars visible to the naked eye and includes much useful information. It serves as finding charts for the brighter stars also.

Charts can also be made from catalogs of other types of astronomical objects, e.g.,radio sources, infrared sources, x-ray sources, etc. An example is the x-ray sky shown as the large background illustration on the front cover of this text.

Catalogs of celestial objects

The brightest of objects or those that exhibit characteristics of particular interest are often cataloged in a book, journal, or in computer-readable form. There are numerous catalogs or lists of different kinds of stars (e.g., white dwarfs, emission-line stars, variable stars, etc.) and of interesting extragalactic objects (Seyfert galaxies, BL Lacertae objects, quasars, etc.). Other catalogs contain a mix of different types of objects that were detected in a survey of the sky by a single instrument. Catalogs typically list the intensity (brightness), the equatorial coordinates, the type of object (if known), special characteristics (e.g., the proper motion, the spectral type and/or broad band colors, or period of a pulsar), and references to other work on the object.

The goal of understanding the cosmos requires the classification of the types of objects in it. Catalogs of objects of a given type allow astronomers to study the class of objects as a whole to ferret out the extent of their characteristics and behavior.

A standard catalog of bright stars is the Yale Bright Star Catalog and its Supplement, now in its 5th Edition. Together, they contain about 11 700 stars down to 7.1 magnitude. It is the basis of the Norton star charts described above. The Smithsonian Star Catalog 2000 contains 260000 stars down to about 9th magnitude. The contemporary Hipparcos Catalog lists coordinates to ~1 mas and proper motions for 118 000 stars mostly brighter than ~10 mag. The US Naval Observatory-A2 Megastar Catalog was obtained from the digitizing scans of the POSS/SRC/ESO Schmidt plates. It reaches down to 20th magnitude and lists the positions of 526 million stars with ~0.2" precision. In the infrared, a ground-based survey at wavelength 2 (xm has yielded the Two-Micron All Sky Survey (2MASS) catalog of 20 million objects. Stars known to have variable brightness are listed in the General Catalog of Variable Stars with 31918 entries in 1998 (Edition 4.1).

In the past, catalogs were created from hand measurements of individual objects, and this limited the number of objects one could practically list. The huge numbers of objects in current catalogs has become possible only recently with automated computer data acquisition, measurements, and listing. Rather than books, catalogs are now distributed on compact disks, or simply made available on the internet.

Objects that appear to have finite extent on a photographic plate are called nebulae. Some nebulae cataloged in early times are now known to be galaxies external to the Galaxy. Nebulae were first listed by Messier (1781). His list has 110 items and includes both galactic and extragalactic objects according to our present knowledge. (See Norton's 2000 Star Atlas, p. 156.) Messier made his list to keep track of objects that could be confused with comets and thereby would slow him in his competitive searches for new comets. Today the Andromeda nebula (our sister galaxy) is known as Messier 31, the 31st entry on Messier's list. It is also known as NGC 224 because it is listed as #224 in a much more complete New General Catalogue of Nebulae and Clusters of Stars published by J. Dreyer in 1888 or its two supplements, Index Catalogue (IC; 1895 and 1908). Currently one can refer to the Third Reference Catalog of Bright Galaxies (RC3) with 23 022 galaxies and the APM Bright Galaxy Catalog of the Southern Sky which lists 14 681 galaxies. The First Byurakan Survey lists 1469 galaxies with strong ultraviolet continuum which is a characteristic of active galactic nuclei (AGN).

Catalogs of the radio sky include the Fourth Cambridge Radio Survey (4C) which lists 4844 northern sources at 178 MHz, two California Institute of Technology lists at 960 MHz (CTA and CTB), and the Australian Parkes Observatory survey of the southern sky with 8264 sources at several frequencies between 178 MHz and 22 GHz. A survey of the x-ray sky by the HEAO-1 satellite in 1977-8 yielded a catalog of the brightest x-ray sources at 1-20 keV; it contains 842 sources and is the origin of the x-ray cover chart. A soft (<2 keV) x-ray survey of the sky has been carried out by the German ROSAT satellite; it yielded >50 000 sources.

Many catalogs are maintained on the internet for use by astronomers together with tools for using them. The Strasbourg Astronomical Data Center (CDS) is a repository for about 4000 of them. There are so many that there are actually catalogs of catalogs! Try for the VizieR catalog service and the Simbad service which provides coordinates and references for your favorite celestial object.

Names of astronomical objects

The catalogs must give a name or number to each entry. In many cases this number becomes the name of the object. If the object was evident and conspicuous to early civilizations it probably has a historical name given in classical times. Subsequent catalogs can result in new names for the old objects, e.g., M31 = NGC 224 as noted above. Thus the names of astronomical objects are rich and diverse; they derive from many researchers working in a variety of frequency domains (radio, optical, etc.). Many names derive from the celestial constellation in which the object resides.


The constellations are 88 regions on the sky noted for particular arrangements of the stars. The origins of the names of the constellations in large part are lost in antiquity. Over the centuries, new constellations have been defined and other large ones broken up into smaller ones. The most recent activity of this sort applied mostly to the southern sky which until recently had been less familiar to northern cultures. The boundaries of the constellations were not specified with care until 1930 when the International Astronomical Union (IAU) defined constellation boundaries that are fixed among the stars and are in accord with the traditional constellations. The boundaries were chosen to follow the lines of constant right ascension and declination for epoch 1875. Due to the earth's precession, these now deviate somewhat from the current lines of constant right ascension and declination, but they do remain fixed relative to the stars and continue to specify the constellation boundaries.

The most well known constellation names are the 12 signs of the zodiac, well known to horoscope readers. These are the constellations that lie along the ecliptic through which the sun passes on its annual cycle, but not at the times suggested by horoscopes (See Section 4.3).


The brightest stars have classical or medieval names like Sirius, Castor, Diphda, etc. A system by Mayer in 1603 named stars according to the constellation within which they reside, the brightest being given the first letter of the Greek alphabet, a, the second p, etc., more or less in order of brightness. The stars Sirius, Castor, and Diphda are thus also known as a CMa (alpha Canis Majoris), a Gem (alpha Geminorum), and p Cet (p Ceti). These names are the genitive forms for the constellations Canis Major (big dog), Gemini (twins), and Cetus (whale).

When the Greek alphabet was exhausted in a given constellation, small roman letters were used (b Cygni) and then capital letters, (A Cygni) up through the letter Q. A general star catalog by Flamsteed published in 1725 named faint stars that had no Greek-Roman name with numbers, e.g., 61 Cyg. For the most part, these were ordered by increasing right ascension within each constellation. Precession of the earth since 1725 causes this ordering to not be strictly preserved in today's coordinates.

Variable stars have been cataloged by constellation with a similar naming convention, but with the unused capital letters R, S, T, etc. (R Cyg). When the single capital letters were exhausted, double capital letters were invoked. For example, the object AM Her (Herculis) is the first-discovered (prototype) example of a highly magnetic binary stellar system. It consists of a very compact star (a white dwarf) that is accreting gas from a companion star. Subsequent star systems of the same type are given their own names, but, as a class called AM Her-type objects. Variable star objects are added to the General Catalog of Variable Stars as they are discovered and reported in the literature.

Modern names ("telephone numbers")

The earliest radio detections of the brightest discrete sources in a given constellation were named after the constellation with the suffix A, B, etc. in order of descending intensity. Thus Taurus A, and Sagittarius A, Cyg A are, respectively, the radio sources associated with the Crab nebula, the center of the Galaxy, and a distant active galaxy with luminous radio jets emerging from it.

The sources in modern catalogs are often listed in order of right ascension (with the epoch specified) with no heed paid to constellations. The name given the source is very likely to be something exciting like 1956 + 350, another name for Cygnus X-1, a bright x-ray source that was the first to provide evidence for the existence of black holes. The digits often refer to the epoch B1950 celestial position, a (B1950) = 19 h 56 m, 5 (B1950) = +35.0°. Many important objects are known by their seven-digit name, jokingly called a "telephone number".

The original Twin quasar, an example of gravitational lensing, is often called QSO 0957 + 561, where the label stands for Quasi Stellar Object, an early name for quasars. The famous binary radio pulsar that is losing energy to gravitational radiation is known as PSR 1913 + 16 where the label stands for "PulSaR". In other cases, the number of the source in a catalog will become the commonly used name. The first discovered quasar is always called 3C273 because it is the 273rd source listed in the Third Cambridge Catalogue of Radio Sources. The 85th cluster of galaxies listed in the catalog prepared by Abell is commonly called Abell 85.

The telephone numbering system was initiated when the epoch B1950 was in use. Unfortunately, in epoch J2000 coordinates, the familiar telephone numbers of our favorite sources would change. It would be like changing the street number in your house address just because someone changed the coordinate system in your town.

The solution is that the sources that were labeled in the former epoch keep their old names, possibly with a "B" precursor, e.g., QSO B0957 + 561, to indicate epoch B1950.0 while sources newly discovered or recently found to be interesting are given names based on their epoch J2000 coordinates. An example of this is the x-ray and radio source XTE J1748-288, a binary system probably containing a black hole. X-ray astronomers still assign prefixes that indicate the satellite that made the discovery of the object, e.g., "XTE" for the Rossi X-ray Timing Explorer.

It helps x-ray astronomers remember the object, but prefixes that identify the type of object would probably be more helpful to other astrophysicists.

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