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42-496: The Messier objects are a set of 110 astronomical objects catalogued by the French astronomer Charles Messier in his Catalogue des Nébuleuses et des Amas d'Étoiles ( Catalogue of Nebulae and Star Clusters ). Because Messier was interested only in finding comets , he created a list of those non-comet objects that frustrated his hunt for them. This list, which Messier created in collaboration with his assistant Pierre Méchain ,

84-427: A supermassive black hole , which may result in an active galactic nucleus . Galaxies can also 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 typically assembled in clusters from

126-458: 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 . The evolving star may eject some portion of its atmosphere to form a nebula, either steadily to form a planetary nebula or in a supernova explosion that leaves a remnant . Depending on the initial mass of the star and the presence or absence of

168-542: A companion, a star may spend the last part of its life as a compact object ; either a white dwarf , neutron star , or black hole . The IAU definitions of planet and dwarf planet require that a Sun-orbiting astronomical body has undergone the rounding process to reach a roughly spherical shape, an achievement known as hydrostatic equilibrium . The same spheroidal shape can be seen on smaller rocky planets like Mars to gas giants like Jupiter . Any natural Sun-orbiting body that has not reached hydrostatic equilibrium

210-499: A line, divide this line in half, then at right angles draw another imaginary line through the sky until it meets the line from the Southern Cross. This point is 5 or 6 degrees from the south celestial pole. Very few bright stars of importance lie between Crux and the pole itself, although the constellation Musca is fairly easily recognised immediately beneath Crux. The second method uses Canopus (the second-brightest star in

252-426: A web that spans the observable universe. Galaxies have a variety of morphologies , with irregular , elliptical 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

294-657: Is a supernova remnant , known as the Crab Nebula , and the great spiral Andromeda Galaxy is M31. Further inclusions followed; 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, and M110 by Kenneth Glyn Jones in 1967. The first edition of 1774 covered 45 objects ( M1 to M45 ). The total list published by Messier in 1781 contained 103 objects, but

336-587: Is classified by the IAU as a small Solar System body (SSSB). These come in many non-spherical shapes which are lumpy masses accreted haphazardly by in-falling dust and rock; not enough mass falls in to generate the heat needed to complete the rounding. Some SSSBs are just collections of relatively small rocks that are weakly held next to each other by gravity but are not actually fused into a single big bedrock . Some larger SSSBs are nearly round but have not reached hydrostatic equilibrium. The small Solar System body 4 Vesta

378-543: Is large enough to have undergone at least partial planetary differentiation. Stars like the Sun are also spheroidal due to gravity's effects on their plasma , which is a free-flowing fluid . Ongoing stellar fusion is a much greater source of heat for stars compared to the initial heat released during their formation. The table below lists the general categories of bodies and objects by their location or structure. Celestial pole The north and south celestial poles are

420-474: Is now known as the Messier catalogue . The Messier catalogue is one of the most famous lists of astronomical objects, and many objects on the list are still referenced by their Messier numbers. The catalogue includes most of the astronomical deep-sky objects that can be easily observed from Earth's Northern Hemisphere ; many Messier objects are popular targets for amateur astronomers. A preliminary version of

462-687: Is very far away from the celestial pole. The third method is best for moonless and clear nights, as it uses two faint "clouds" in the Southern Sky . These are marked in astronomy books as the Large and Small Magellanic Clouds (the LMC and the SMC). These "clouds" are actually dwarf galaxies near the Milky Way . Make an equilateral triangle, the third point of which is the south celestial pole. Like before,

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504-556: Is within one degree of the bright star Polaris (named from the Latin stella polaris , meaning " pole star "). This makes Polaris, colloquially known as the "North Star", useful for navigation in the Northern Hemisphere : not only is it always above the north point of the horizon, but its altitude angle is always (nearly) equal to the observer's geographic latitude (though it can, of course, only be seen from locations in

546-517: The Andromeda nebula as a different galaxy, along with many others far from the Milky Way. 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 , often within larger superclusters , that are strung along great filaments between nearly empty voids , forming

588-614: The Moon , and sunspots on the Sun. Astronomer Edmond Halley was able to successfully predict the return of Halley's Comet , which now bears his name, in 1758. In 1781, Sir William Herschel discovered the new planet Uranus , being the first discovered planet not visible by the naked eye. In the 19th and 20th century, new technologies and scientific innovations allowed scientists to greatly expand their understanding of astronomy and astronomical objects. Larger telescopes and observatories began to be built and scientists began to print images of

630-639: The Scientific Revolution , in 1543, Nicolaus Copernicus's heliocentric model was published. This model described the Earth , along with all of the other planets as being astronomical bodies which orbited the Sun located in the center of the Solar System . Johannes Kepler discovered Kepler's laws of planetary motion , which are properties of the orbits that the astronomical bodies shared; this

672-466: The Southern Cross (Crux) and its two "pointer" stars α Centauri and β Centauri . Draw an imaginary line from γ Crucis to α Crucis —the two stars at the extreme ends of the long axis of the cross—and follow this line through the sky. Either go four-and-a-half times the distance of the long axis in the direction the narrow end of the cross points, or join the two pointer stars with

714-491: The protoplanetary disks that surround newly formed stars. The various distinctive types of stars are shown by the Hertzsprung–Russell diagram (H–R 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, then its physical properties can cause it to become

756-592: The "North Star", though it will be about six degrees from the true north celestial pole. To find Polaris, from a point in the Northern Hemisphere, face north and locate the Big Dipper (Plough) and Little Dipper asterisms. Looking at the "cup" part of the Big Dipper, imagine that the two stars at the outside edge of the cup form a line pointing upward out of the cup. This line points directly at

798-563: The Hôtel de Cluny (now the Musée national du Moyen Âge ), 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 of

840-483: The Moon and other celestial bodies on photographic plates. New wavelengths of light unseen by the human eye were discovered, and new telescopes were made that made it possible to see astronomical objects in other wavelengths of light. Joseph von Fraunhofer and Angelo Secchi pioneered the field of spectroscopy , which allowed them to observe the composition of stars and nebulae, and many astronomers were able to determine

882-493: The Northern Hemisphere). Polaris is near the north celestial pole for only a small fraction of the 25,700-year precession cycle. It will remain a good approximation for about 1,000 years, by which time the pole will have moved closer to Alrai ( Gamma Cephei ). In about 5,500 years, the pole will have moved near the position of the star Alderamin (Alpha Cephei), and in 12,000 years, Vega (Alpha Lyrae) will become

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924-454: The SMC, LMC, and the pole will all be points on an equilateral triangle on an imaginary circle. The pole should be placed clockwise from the SMC and anticlockwise from the LMC. Going in the wrong direction will land you in the constellation of Horologium instead. A line from Sirius , the brightest star in the sky, through Canopus, the second-brightest, continued for the same distance lands within

966-731: 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 relatively small-aperture refracting telescope (approximately 100 mm ≈ 4 inches) used by Messier to study the sky from downtown Paris , they are among the brightest and thus most attractive astronomical objects (popularly called deep-sky objects ) observable from Earth, and 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

1008-549: The catalogue first appeared in 1774 in the Memoirs of the French Academy of Sciences for the year 1771. The first version of Messier's catalogue contained 45 objects, which were not numbered. Eighteen of the objects were discovered by Messier; the rest had been previously observed by other astronomers. By 1780 the catalogue had increased to 70 objects. The final version of the catalogue containing 103 objects

1050-455: The date of the rotation axis; J2000.0 is the current standard. An analogous concept applies to other planets: a planet's celestial poles are the points in the sky where the projection of the planet's axis of rotation intersects the celestial sphere. These points vary because different planets' axes are oriented differently (the apparent positions of the stars also change slightly because of parallax effects). The north celestial pole currently

1092-402: The entire comet with its diffuse coma and tail . Astronomical objects such as stars , planets , nebulae , asteroids and comets have been observed for thousands of years, although early cultures thought of these bodies as gods or deities. These early cultures found the movements of the bodies very important as they used these objects to help navigate over long distances, tell between

1134-581: 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 catalogue. M105 to M107 were added by Helen Sawyer Hogg in 1947, M108 and M109 by Owen Gingerich in 1960, and M110 by Kenneth Glyn Jones in 1967. M102

1176-679: The masses of binary stars based on their orbital elements . Computers began to be used to observe and study massive amounts of astronomical data on stars, and new technologies such as the photoelectric photometer allowed astronomers to accurately measure the color and luminosity of stars, which allowed them to predict their temperature and mass. In 1913, the Hertzsprung-Russell diagram was developed by astronomers Ejnar Hertzsprung and Henry Norris Russell independently of each other, which plotted stars based on their luminosity and color and allowed astronomers to easily examine stars. It

1218-463: The most spectacular examples of the five types of deep-sky object – diffuse nebulae , planetary nebulae , open clusters , globular clusters , and galaxies – visible from European latitudes. Furthermore, almost all of the Messier objects are among the closest to Earth in their respective classes, which makes them heavily studied with professional class instruments that today can resolve very small and visually significant details in them. A summary of

1260-422: The objects can be viewed over a single night. Astronomical objects Examples of astronomical objects include planetary systems , star clusters , nebulae , and galaxies , while asteroids , moons , planets , and stars are astronomical bodies. A comet may be identified as both a body and an object: It is a body when referring to the frozen nucleus of ice and dust, and an object when describing

1302-410: The poles of the celestial equatorial coordinate system , meaning they have declinations of +90 degrees and −90 degrees (for the north and south celestial poles, respectively). Despite their apparently fixed positions, the celestial poles in the long term do not actually remain permanently fixed against the background of the stars. Because of a phenomenon known as the precession of the equinoxes ,

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1344-492: The poles trace out circles on the celestial sphere, with a period of about 25,700 years. The Earth's axis is also subject to other complex motions which cause the celestial poles to shift slightly over cycles of varying lengths (see nutation , polar motion and axial tilt ). Finally, over very long periods the positions of the stars themselves change, because of the stars' proper motions . To take into account such movement, celestial pole definitions come with an epoch to specify

1386-635: The seasons, and to determine when to plant crops. During the Middle-Ages , cultures began to study the movements of these bodies more closely. Several astronomers of the Middle-East began to make detailed descriptions of stars and nebulae, and would make more accurate calendars based on the movements of these stars and planets. In Europe , astronomers focused more on devices to help study the celestial objects and creating textbooks, guides, and universities to teach people more about astronomy. During

1428-562: The sky) and Achernar . Make a large equilateral triangle using these stars for two of the corners. But where should the third corner go? It could be on either side of the line connecting Achernar and Canopus, and the wrong side will not lead to the pole. To find the correct side, imagine that Archernar and Canopus are both points on the circumference of a circle. The third corner of the equilateral triangle will also be on this circle. The corner should be placed clockwise from Achernar and anticlockwise from Canopus. The third imaginary corner will be

1470-450: The south celestial pole. If the opposite is done, the point will land in the middle of Eridanus , which isn't at the pole. If Canopus has not yet risen, the second-magnitude Alpha Pavonis can also be used to form the triangle with Achernar and the pole. In this case, go anticlockwise from Achernar instead of clockwise, form the triangle with Canopus, and the third point, the pole, will reveal itself. The wrong way will lead to Aquarius, which

1512-676: The star at the tip of the Little Dipper's handle. That star is Polaris, the North Star. The south celestial pole is visible only from the Southern Hemisphere . It lies in the dim constellation Octans , the Octant. Sigma Octantis is identified as the south pole star, more than one degree away from the pole, but with a magnitude of 5.5 it is barely visible on a clear night. The south celestial pole can be located from

1554-493: The two points in the sky where Earth 's axis of rotation , indefinitely extended, intersects the celestial sphere . The north and south celestial poles appear permanently directly overhead to observers at Earth's North Pole and South Pole , respectively. As Earth spins on its axis, the two celestial poles remain fixed in the sky, and all other celestial points appear to rotate around them, completing one circuit per day (strictly, per sidereal day ). The celestial poles are also

1596-399: The various condensing nebulae. The great variety of stellar forms are determined almost entirely by the mass, composition and evolutionary state of these stars. Stars may be found in multi-star systems that orbit about each other in a hierarchical organization. A planetary system and various minor objects such as asteroids, comets and debris, can form in a hierarchical process of accretion from

1638-536: Was found that stars commonly fell on a band of stars called the main-sequence stars on the diagram. A refined scheme for stellar classification was published in 1943 by William Wilson Morgan and Philip Childs Keenan based on the Hertzsprung-Russel Diagram. Astronomers also began debating whether other galaxies existed beyond the Milky Way , these debates ended when Edwin Hubble identified

1680-676: Was observed by Méchain, who communicated his notes to Messier. Méchain later concluded that this object was simply 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 pour l'Année 1784 [ Knowledge of the Times for the Year 1784 ], the French official yearly publication of astronomical ephemerides . Messier lived and did his astronomical work at

1722-513: 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, eventually filled out the list up to 110 objects. The catalogue consists of a diverse range of astronomical objects, from star clusters and nebulae to galaxies . For example, Messier 1

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1764-473: Was used to improve the heliocentric model. In 1584, Giordano Bruno proposed that all distant stars are their own suns, being the first in centuries to suggest this idea. Galileo Galilei was one of the first astronomers to use telescopes to observe the sky, in 1610 he observed the four largest moons of Jupiter , now named the Galilean moons . Galileo also made observations of the phases of Venus , craters on

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