Book of Fixed Stars
The Book of Fixed Stars is an astronomical text written by Abd al-Rahman al-Sufi around 964. The book was written although the author himself was Persian, it was an attempt to create a synthesis of the comprehensive star catalogue in Ptolemy’s Almagest with the indigenous Arabic astronomical traditions on the constellations. The book was illustrated along with observations and descriptions of the stars, their positions, their magnitudes and their color, his results, as in Ptolemy's Almagest, were set out constellation by constellation. For each constellation, he provided two drawings, one from the outside of a celestial globe, the other from the inside; the work was influential and survives in numerous manuscripts and translations. The oldest manuscript, kept in the Bodleian Library, dates to 1009 and is the work of the author's son. There is a thirteenth-century copy in the British Library, he has the earliest known descriptions and illustrations of what he called "a little cloud", the Andromeda Galaxy.
He mentions it as lying before the mouth of an Arabic constellation. This "cloud" was commonly known to the Isfahan astronomers probably before 905; the first recorded mention of the Large Magellanic Cloud was given in the Book of Fixed Stars. These were the first galaxies other than the Milky Way to be observed from Earth; the Great Andromeda Nebula he observed was the first true nebula to be observed, as distinct from a star cluster. He also cataloged the Omicron Velorum star cluster as a "nebulous star", an additional "nebulous object" in Vulpecula, a cluster now variously known as Al-Sufi's Cluster, the "Coathanger asterism", Brocchi's Cluster or Collinder 399. Moreover, he mentions the two Magellanic Clouds, that they are not visible from Iraq nor Najd, but visible from Tihama, that they are called al-Baqar. There has not been a published English translation of the book, though it was translated into French by Hans Schjellerup in 1874; as of March 2012, one is in preparation by Ihsan Hafez of Townsville.
Text and French translation of Ṣūfī's introduction by J. J. A. Caussin de Perceval in Notices et extraits des manuscrits XII, Paris, 1831, pp. 236f. H. C. F. C. Schjellerup, Description des étoiles fixes par Abd-al-Rahman al-Sûfi, St. Petersburg, 1874. Complete French translation from two late mss. with selected portions in Arabic. Ketāb ṣowar al-kawākeb al-ṯābeta, edited from five mss. and accompanied by the Orǰūza of Ebn al-Ṣūfī, India, 1954. Facsimile edition of the Persian translation by Naṣīr-al-dīn Ṭūsī, Tehran, 1348 Š./1969. Critical edition of Ṭūsī's translation by Sayyed Moʿezz-al-dīn Mahdavī, Tehran, 1351 Š./1972. The star nomenclature of the Castilian version, of an Italian translation made from Castilian, was critically edited by O. J. Tallgren, "Los nombres árabes de las estrelas y la transcripción alfonsina", in Homenaje a R. Menéndez Pidal II, Madrid, 1925, with'Correcciones y adiciones' in Revista de filología española 12, 1925, pp. 52f. The Italian translation was edited by P. Knecht, I libri astronomici di Alfonso X in una versione fiorentina del trecento, Saragossa, 1965.
Paul Kunitzsch, The Arabs and the Stars: Texts and Traditions on the Fixed Stars, Their Influence in Medieval Europe Paul Kunitzsch, Arabische Sternnamen in Europa, Wiesbaden, 1959, pp. 230f. Paul Kunitzsch, "Ṣūfī Latinus", Zeitschrift der Deutschen Morgenländische Gesellschaft, 115, 1965, pp. 65–74. Paul Kunitzsch, "Al-Ṣūfī" in: Dictionary of Scientific Biography, XIII, New York, 1976, pp. 149–50. J. Upton, "A Manuscript of “The Book of the Fixed Stars” by ʿAbd ar-Raḥmān aṣ-Ṣūfī", Metropolitan Museum Studies, 4, 1933, pp. 179–97. E. Wellesz, An Islamic Book of Constellations, Oxford, 1965. H. J. J. Winter, "Notes on al-Kitab Suwar Al-Kawakib", Archives Internationales d’Histoire des Sciences, 8, 1955, pp. 126–33. Bodleian copy of Suwar al-Kawakib al-Thabitah Biography of Al Sufi Copy of al-Sufi's Book of the Fixed Stars Ulugh Beg in www.atlascoelestis.com Liber locis stellarum fixarum, 964, manoscritto del 1417 riprodotto il 1730 in www.atlascoelestis.com Pergamenthandschrift M II 141 in www.atlascoelestis.com A page about Muslim Astronomers Al-Sufi's constellations Al-Ṣūfī’s Book of the Constellations of the Fixed Stars and its Influence on Islamic and Western Celestial Cartography - includes a detailed bibliography and a list of all known manuscripts of al-Ṣūfī's Book of the Fixed Stars.
Forgotten History: Al-Sufi's Book Of Fixed Stars Slides and audio recording from a presentation on the book, with images and quotations from many different manuscripts. Illustrations from Book of Fixed Stars by ‛Abd al-Rahman ibn ‛Umar al-Ṣūfī. Moya Carey, Painting the Stars in a Century of Change: A thirteenth-century copy of al-Sufi's "Treatise on the Fixed Stars" - British Library Or.5323
The Caldwell catalogue is an astronomical catalogue of 109 star clusters and galaxies for observation by amateur astronomers. The list was compiled by Patrick Moore as a complement to the Messier catalogue. While the Messier catalogue is used by amateur astronomers as a list of deep-sky objects for observation, Moore noted that Messier's list was not compiled for that purpose and excluded many of the sky's brightest deep-sky objects, such as the Hyades, the Double Cluster, the Sculptor Galaxy; the Messier catalogue was compiled as a list of known objects that might be confused with comets. Moore observed that since Messier compiled his list from observations in Paris, it did not include bright deep-sky objects visible in the Southern Hemisphere, such as Omega Centauri, Centaurus A, the Jewel Box, 47 Tucanae. Moore compiled a list of 109 objects to match the accepted number of Messier objects, the list was published in Sky & Telescope in December 1995. Moore used his other surname – Caldwell – to name the list, since the initial of "Moore" is used for the Messier catalogue.
Entries in the catalogue are designated with the catalogue number. Unlike objects in the Messier catalogue, which are listed in the order of discovery by Messier and his colleagues, the Caldwell catalogue is ordered by declination, with C1 being the most northerly and C109 being the most southerly, although two objects are listed out of sequence. Other errors in the original list have since been corrected: it incorrectly identified the S Norma Cluster as NGC 6067 and incorrectly labelled the Lambda Centauri Cluster as the Gamma Centauri Cluster. Star cluster Nebula Galaxy Messier Catalogue Herschel 400 Catalogue New General Catalogue Index Catalogue Revised New General Catalogue Revised Index Catalogue The Caldwell Catalogue at SEDS The Caldwell Club Caldwell Star Charts and more Searchable Caldwell Catalogue list Clickable Caldwell Object table
The Messier objects are a set of 110 astronomical objects cataloged by the French astronomer Charles Messier in his Catalogue des Nébuleuses et des Amas d'Étoiles. Because Messier was interested in finding only comets, he created a list of non-comet objects that frustrated his hunt for them; the compilation of this list, in collaboration with his assistant Pierre Méchain, is known as the Messier catalogue. This catalogue of objects is one of the most famous lists of astronomical objects, many Messier objects are still referenced by their Messier number; the catalogue includes some astronomical objects that can be observed from Earth's Northern Hemisphere such as deep-sky objects, a characteristic which makes the Messier objects popular targets for amateur astronomers. A preliminary version first appeared in the Memoirs of the French Academy of Sciences in 1771, the last item was added in 1966 by Kenneth Glyn Jones, based on Messier's observations; the first version of Messier's catalogue contained 45 objects and was published in 1774 in the journal of the French Academy of Sciences in Paris.
In addition to his own discoveries, this version included objects observed by other astronomers, with only 17 of the 45 objects being Messier's. By 1780 the catalogue had increased to 80 objects; the final version of the catalogue containing 103 objects was published in 1781 in the Connaissance des Temps for the year 1784. However, due to what was thought for a long time to be the incorrect addition of Messier 102, the total number remained 102. Other astronomers, using side notes in Messier's texts filled out the list up to 110 objects; the catalogue consists of a diverse range of astronomical objects, ranging from star clusters and nebulae to galaxies. For example, Messier 1 is a supernova remnant, known as the Crab Nebula, the great spiral Andromeda Galaxy is M31. Many further inclusions followed in the next century when the first addition came from Nicolas Camille Flammarion in 1921, who added Messier 104 after finding Messier's side note in his 1781 edition exemplar of the catalogue. M105 to M107 were added by Helen Sawyer Hogg in 1947, M108 and M109 by Owen Gingerich in 1960, M110 by Kenneth Glyn Jones in 1967.
The first edition of 1771 covered 45 objects numbered M1 to M45. The total list published by Messier in 1781 contained 103 objects, but the list was expanded through successive additions by other astronomers, motivated by notes in Messier's and Méchain's texts indicating that at least one of them knew of the additional objects; the first such addition came from Nicolas Camille Flammarion in 1921, who added Messier 104 after finding a note Messier made in a copy of the 1781 edition of the catalog. M105 to M107 were added by Helen Sawyer Hogg in 1947, M108 and M109 by Owen Gingerich in 1960, M110 by Kenneth Glyn Jones in 1967. M102 was observed by Méchain. Méchain concluded that this object was a re-observation of M101, though some sources suggest that the object Méchain observed was the galaxy NGC 5866 and identify that as M102. Messier's final catalogue was included in the Connaissance des Temps for 1784, the French official yearly publication of astronomical ephemerides; these objects are still known by their "Messier number" from this list.
Messier did his astronomical work at the Hôtel de Cluny, in Paris, France. The list he compiled contains only objects found in the sky area he could observe: from the north celestial pole to a celestial latitude of about −35.7°. He did not observe or list objects visible only from farther south, such as the Large and Small Magellanic Clouds; the Messier catalogue comprises nearly all the most spectacular examples of the five types of deep-sky object – diffuse nebulae, planetary nebulae, open clusters, globular clusters, galaxies – visible from European latitudes. Furthermore all of the Messier objects are among the closest to Earth in their respective classes, which makes them studied with professional class instruments that today can resolve small and visually spectacular details in them. A summary of the astrophysics of each Messier object can be found in the Concise Catalog of Deep-sky Objects. Since these objects could be observed visually with the small-aperture refracting telescope used by Messier to study the sky, they are among the brightest and thus most attractive astronomical objects observable from Earth, are popular targets for visual study and astrophotography available to modern amateur astronomers using larger aperture equipment.
In early spring, astronomers sometimes gather for "Messier marathons", when all of the objects can be viewed over a single night. Lists of astronomical objects List of Messier objects Caldwell catalogue Deep-sky object Herschel 400 Catalogue New General Catalogue SEDS Messier Database Charles Messier Charles Messier's Catalog of Nebulae and Star Clusters History of the Messier Catalog Interactive Messier Catalog Greenhawk Observatory Listing of Copyright-free Images of all Messier Objects CCD Images of Messier Objects 12 Dimensional String Messier Gallery The Messier Catalogue Merrifield, Mike. "Messier Objects". Deep Sky Videos. Brady Haran. Messier Objects at Constellation Guide
An astrograph is a telescope designed for the sole purpose of astrophotography. Astrographs are used in wide field astronomical surveys of the sky and for detection of objects such as asteroids and comets. Most research telescopes in this class are refractors, although there are many reflecting designs such as the Ritchey-Chrétien and catadioptrics such as the Schmidt camera; the main parameters of an Astrograph are the diameter and f-ratio of the objective, which determine the field of view and image scale on the photographic plate or CCD detector. The objective of an astrograph is not large, on the order of 20 to 50 cm; the shape of the focal plane is designed to work in conjunction with a specific shaped photographic plate or CCD detector. The objective is designed to produce a large and distortion-less image at the focal plane, they may be designed to focus certain wavelengths of light to match the type of film they are designed to use. Wide-angle astrographs with short f-ratios are used for photographing a huge area of sky.
Astrographs with higher f-ratios are used in more precise measurements. Many observatories of the world are equipped with the so-called normal astrographs with an aperture of around 13 inches and a focal length of 11 feet; the purpose of a "normal astrograph" is to create images where the scale of the image at the focal plane is a standard of 60 arcsecs/mm. Astrographs used in astrometry record images that are used to "map" the positions of objects over a large area of the sky; these maps are published in catalogs to be used in further study or to serve as reference points for deep-space imaging. Astrographs used for stellar classification sometimes consist of two identical telescopes on the same mount; each sky field can be photographed in two colors. Each telescope may have individually designed non-achromatic objectives to focus the desired wavelength of light, paired with the respective color-sensitive photographic plate. In other cases a single telescope is used to make two exposures of the same part of the sky with different filters and color sensitive film used on each exposure.
Two-color photography lets astronomers measure the color, as well as the brightness, of each star imaged. Colors tell the star's "temperature”. Knowing the color type and magnitudes lets astronomers determine the distance of a star. Sky fields that are photographed twice, decades apart in time, will reveal a nearby star's proper motion when measured against the background of distant stars or galaxies. By taking two exposures of the same section of the sky days or weeks apart, it is possible to find objects such as asteroids, comets, variable stars and unknown planets. By comparing the pair of images, using a device such as a blink comparator, astronomers are able to find objects that moved or changed brightness between the two exposures or appear in one image only, as in the case of a nova or meteor. Sometimes objects can be found in one exposure since a fast moving object will appear as a "line" in a long exposure. One well-known case of an astrograph used in a discovery is Clyde Tombaugh’s discovery of the dwarf planet Pluto in 1930.
Tombaugh was given the job of hunting for a suspected "9th planet" to be achieved by systematically photographing the area of the sky around the ecliptic. Tombaugh used Lowell Observatory's 13-inch, f/5.3 refractor astrograph, which recorded images on 14-by-17-inch glass plates. In the amateur astronomy field many types of commercial and amateur built telescopes are designed for astrophotography and labeled "astrographs". Optical designs of amateur astrographs vary but include apochromatic refractors, variations of Cassegrain reflectors, Newtonian reflectors. Most optical designs do not produce large and well-corrected imaging fields and therefore require some type of optical correction by way of field flatteners or coma correctors. Amateur astrographs have purpose-built focusers, are constructed of thermally stable materials like carbon fiber, are put on heavy duty mounts to facilitate accurate tracking of deep sky objects for long periods of time. BOOTES List of telescope types The Double Astrograph of the Yale Southern Observatory The Carnegie Double Astrograph Pluto Imaging Challenge: Images Construction of the Tycho Reference Catalogue - 2 Source Catalogues
Hipparcos was a scientific satellite of the European Space Agency, launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky; this permitted the accurate determination of proper motions and parallaxes of stars, allowing a determination of their distance and tangential velocity. When combined with radial velocity measurements from spectroscopy, this pinpointed all six quantities needed to determine the motion of stars; the resulting Hipparcos Catalogue, a high-precision catalogue of more than 118,200 stars, was published in 1997. The lower-precision Tycho Catalogue of more than a million stars was published at the same time, while the enhanced Tycho-2 Catalogue of 2.5 million stars was published in 2000. Hipparcos' follow-up mission, was launched in 2013; the word "Hipparcos" is an acronym for HIgh Precision PARallax COllecting Satellite and a reference to the ancient Greek astronomer Hipparchus of Nicaea, noted for applications of trigonometry to astronomy and his discovery of the precession of the equinoxes.
By the second half of the 20th century, the accurate measurement of star positions from the ground was running into insurmountable barriers to improvements in accuracy for large-angle measurements and systematic terms. Problems were dominated by the effects of the Earth's atmosphere, but were compounded by complex optical terms and gravitational instrument flexures, the absence of all-sky visibility. A formal proposal to make these exacting observations from space was first put forward in 1967. Although proposed to the French space agency CNES, it was considered too complex and expensive for a single national programme, its acceptance within the European Space Agency's scientific programme, in 1980, was the result of a lengthy process of study and lobbying. The underlying scientific motivation was to determine the physical properties of the stars through the measurement of their distances and space motions, thus to place theoretical studies of stellar structure and evolution, studies of galactic structure and kinematics, on a more secure empirical basis.
Observationally, the objective was to provide the positions and annual proper motions for some 100,000 stars with an unprecedented accuracy of 0.002 arcseconds, a target in practice surpassed by a factor of two. The name of the space telescope, "Hipparcos" was an acronym for High Precision Parallax Collecting Satellite, it reflected the name of the ancient Greek astronomer Hipparchus, considered the founder of trigonometry and the discoverer of the precession of the equinoxes; the spacecraft carried a single all-reflective, eccentric Schmidt telescope, with an aperture of 29 cm. A special beam-combining mirror superimposed two fields of view, 58 degrees apart, into the common focal plane; this complex mirror consisted of two mirrors tilted in opposite directions, each occupying half of the rectangular entrance pupil, providing an unvignetted field of view of about 1°×1°. The telescope used a system of grids, at the focal surface, composed of 2688 alternate opaque and transparent bands, with a period of 1.208 arc-sec.
Behind this grid system, an image dissector tube with a sensitive field of view of about 38-arc-sec diameter converted the modulated light into a sequence of photon counts from which the phase of the entire pulse train from a star could be derived. The apparent angle between two stars in the combined fields of view, modulo the grid period, was obtained from the phase difference of the two star pulse trains. Targeting the observation of some 100,000 stars, with an astrometric accuracy of about 0.002 arc-sec, the final Hipparcos Catalogue comprised nearly 120,000 stars with a median accuracy of better than 0.001 arc-sec. An additional photomultiplier system viewed a beam splitter in the optical path and was used as a star mapper, its purpose was to monitor and determine the satellite attitude, in the process, to gather photometric and astrometric data of all stars down to about 11th magnitude. These measurements were made in two broad bands corresponding to B and V in the UBV photometric system.
The positions of these latter stars were to be determined to a precision of 0.03 arc-sec, a factor of 25 less than the main mission stars. Targeting the observation of around 400,000 stars, the resulting Tycho Catalogue comprised just over 1 million stars, with a subsequent analysis extending this to the Tycho-2 Catalogue of about 2.5 million stars. The attitude of the spacecraft about its center of gravity was controlled to scan the celestial sphere in a regular precessional motion maintaining a constant inclination between the spin axis and the direction to the Sun; the spacecraft spun around its Z-axis at the rate of 11.25 revolutions/day at an angle of 43° to the Sun. The Z-axis rotated about the sun-satellite line at 6.4 revolutions/year. The spacecraft consisted of two platforms and six vertical panels, all made of aluminum honeycomb; the solar array consisted of three deployable sections. Two S-band antennas were located on the top and bottom of the spacecraft, providing an omni-directional downlink data rate of 24 kbit/s.
An attitude and orbit-control subsystem ensured correct dynamic attitude control and determination during the operational lifetim
Atlas of Peculiar Galaxies
The Atlas of Peculiar Galaxies is a catalog of peculiar galaxies produced by Halton Arp in 1966. A total of 338 galaxies are presented in the atlas, published in 1966 by the California Institute of Technology; the primary goal of the catalog was to present photographs of examples of the different kinds of peculiar structures found among galaxies. Arp realized that the reason why galaxies formed into spiral or elliptical shapes was not well understood, he perceived peculiar galaxies as small "experiments" that astronomers could use to understand the physical processes that distort spiral or elliptical galaxies. With this atlas, astronomers had a sample of peculiar galaxies; the atlas does not present a complete overview of every peculiar galaxy in the sky but instead provides examples of the different phenomena as observed in nearby galaxies. Because little was known at the time of publication about the physical processes that caused the different shapes, the galaxies in the atlas are sorted based on their appearance.
Objects 1–101 are individual peculiar spiral galaxies or spiral galaxies that have small companions. Objects 102 -- 145 are elliptical-like galaxies. Individual or groups of galaxies with neither elliptical nor spiral shapes are listed as objects 146–268. Objects 269–327 are double galaxies. Objects that do not fit into any of the above categories are listed as objects 332–338. Most objects are best known by their other designations, but a few galaxies are best known by their Arp numbers. Today, the physical processes that lead to the peculiarities seen in the Arp atlas are thought to be well understood. A large number of the objects have been interpreted as interacting galaxies, including M51, Arp 220, the Antennae Galaxies. A few of the galaxies are dwarf galaxies that do not have enough mass to produce enough gravity to allow the galaxies to form any cohesive structure. NGC 1569 is an example of one of the dwarf galaxies in the atlas. A few other galaxies are radio galaxies; these objects contain active galactic nuclei.
The atlas includes the nearby radio galaxies M87 and Centaurus A. Many of the peculiar associations present in the catalogue have been interpreted as galaxy mergers, though Arp refuted the idea, rather, that apparent associations were prime examples of ejections, he writes in "Seeing Red": For me, the whole lesson of the Atlas of Peculiar Galaxies was that galaxies are ejected material. The merger mania seems to be a first guess based on a cursory look at galaxies; the Atlas of Peculiar Galaxies contained a interesting class of galaxies called spirals with companions on the ends of arms. How had they gotten there? Not by accidental collisions or by the beginning of a merger process, fashionably used to "explain" everything in the extragalactic realm; these are dwarf galaxies or poorly defined spiral galaxies that have low surface brightnesses. Low surface brightness galaxies are quite common; the exception is NGC 2857, an Sc spiral galaxy. This category contains spiral galaxies with arms; this category contains spiral galaxies with arms.
Some spiral arm segments may appear detached because dust lanes in the spiral arms obscure the arms' starlight. Other spiral arms may appear segmented because of the presence of bright star clusters in the spiral arms. Most spiral galaxies contain two defined spiral arms, or they contain only fuzzy filamentary spiral structures. Galaxies with three well-defined spiral arms are rare. One-armed spiral galaxies are rare. In this case, the single spiral arm may be formed by a gravitational interaction with another galaxy; the spiral arms in these galaxies have an asymmetric appearance. One spiral arm may appear to be brighter than the other. In the photographic plates produced by Arp, the bright arm would look dark or "heavy". While most of these galaxies are asymmetric spiral galaxies, NGC 6365 is an interacting pair of galaxies where one of the two galaxies is viewed edge-on and just happens to lie where the spiral arm for the other face-on galaxy would be visible; these are galaxies. Some objects, such as IC 167, are ordinary spiral galaxies viewed from an unusual angle.
Other objects, such as UGC 10770, are interacting pairs of galaxies with tidal tails that look similar to spiral arms. Many of these spiral galaxies are interacting with the low surface brightness galaxies in the field of view. In some cases, however, it may be difficult to determine whether the companion is physically near the spiral galaxy or whether the companion is a foreground/background source or a source on the edge of the spiral galaxy. Again, many of these spiral galaxies are interacting with companion galaxies, although some of the identified companion galaxies may be foreground/background sources or bright star clusters within the individual galaxies. Galaxies in this category are always interacting sources; the most famous of these objects is the Whirlpool galaxy, composed of a spiral galaxy NGC 5194, interacting with a smaller elliptical galaxy NGC 5195. The inter
An astronomical object or celestial object is a occurring physical entity, association, or structures that exists in the observable universe. In astronomy, the terms object and body are used interchangeably. However, an astronomical body or celestial body is a single bound, contiguous entity, while an astronomical or celestial object is a complex, less cohesively bound structure, which may consist of multiple bodies or other objects with substructures. Examples of astronomical objects include planetary systems, star clusters and galaxies, while asteroids, moons and stars are astronomical bodies. A comet may be identified as both body and object: It is a body when referring to the frozen nucleus of ice and dust, an object when describing the entire comet with its diffuse coma and tail; the universe can be viewed as having a hierarchical structure. At the largest scales, the fundamental component of assembly is the galaxy. Galaxies are organized into groups and clusters within larger superclusters, that are strung along great filaments between nearly empty voids, forming a web that spans the observable universe.
The universe has a variety of morphologies, with irregular and disk-like shapes, depending on their formation and evolutionary histories, including interaction with other galaxies, which may lead to a merger. Disc galaxies encompass lenticular and spiral galaxies with features, such as spiral arms and a distinct halo. At the core, most galaxies have a supermassive black hole, which may result in an active galactic nucleus. Galaxies can have satellites in the form of dwarf galaxies and globular clusters; the constituents of a galaxy are formed out of gaseous matter that assembles through gravitational self-attraction in a hierarchical manner. At this level, the resulting fundamental components are the stars, which are assembled in clusters from the various condensing nebulae; the great variety of stellar forms are determined entirely by the mass and evolutionary state of these stars. Stars may be found in multi-star systems. A planetary system and various minor objects such as asteroids and debris, can form in a hierarchical process of accretion from the protoplanetary disks that surrounds newly formed stars.
The various distinctive types of stars are shown by the Hertzsprung–Russell diagram —a plot of absolute stellar luminosity versus surface temperature. Each star follows an evolutionary track across this diagram. If this track takes the star through a region containing an intrinsic variable type its physical properties can cause it to become a variable star. An example of this is the instability strip, a region of the H-R diagram that includes Delta Scuti, RR Lyrae and Cepheid variables. Depending on the initial mass of the star and the presence or absence of a companion, a star may spend the last part of its life as a compact object; the table below lists the general categories of bodies and objects by their structure. List of light sources List of Solar System objects List of Solar System objects by size Lists of astronomical objects SkyChart, Sky & Telescope at the Library of Congress Web Archives Monthly skymaps for every location on Earth