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Messier object
The Messier objects are a set of astronomical objects catalogued by Charles Messier in his catalogue of Nebulae and Star Clusters first published in 1774. The original motivation behind the catalogue was that Messier was a comet hunter, and was frustrated by objects which resembled but were not comets. He therefore compiled a list of these objects.
The first edition covered 45 objects numbered M1 to M45. The total list consists of 110 objects, ranging from M1 to M110. The final catalogue was published in 1781 and printed in the Connaissance des Temps in 1784.
Many of these objects are still known by their Messier number.
Because the Messier list was compiled by astronomers in the Northern Hemisphere, it contains only objects from the north celestial pole to a celestial latitude of about –35°. Many impressive Southern objects, such as the Large and Small Magellanic Clouds are excluded from the list. Because all of the Messier objects are visible with binoculars or small telescopes (under favorable conditions), they are popular viewing objects for amateur astronomers. In early spring, astronomers sometimes gather for "Messier Marathons", when all of the objects can be viewed over a single night.
See also
- List of Messier objects
- Messier marathon
- :Category:Messier objects
See also
- Deep sky object
- New General Catalogue
External links
- http://www.seds.org/messier/
- http://www.seds.org/messier/xtra/history/CMessier.html
- http://zebu.uoregon.edu/messier.html
- http://messier45.com/messier/index.html
ja:メシエ天体
-
Astronomical object
See also
- Lists of astronomical objects
ko:천체
ja:天体
th:วัตถุท้องฟ้า
Charles Messier
Charles Messier (June 26, 1730 – (April 12, 1817) was a
French astronomer who in 1774 published a catalogue of 45 deep sky objects such as nebulae and star clusters. The purpose of the catalogue was to help comet hunters (like himself) and other astronomical observers to distinguish between permanent and transient objects in the sky.
By 1781 the catalogue had grown to 110 'Messier Objects'. The objects' designations, from M1 to M110, are mostly still in use today.
The catalog comprises some of the most important objects in the night sky - from the Crab Nebula (M1) to a small elliptical galaxy near Andromeda (M110). In Messier Martahons, many amateur astronomers compete to view all 110 of these objects in a single dusk-to-dawn session, usually in March, when conditions are most favorable.
The Messier crater on the Moon and the asteroid 7359 Messier were named in his honour.
External link
- [http://www.seds.org/messier/xtra/history/CMessier.html SEDS: Charles Messier]
- [http://www.licha.de/astro_gallery_messier.php Amateur Photos of Charles Messier Objects]
- [http://www.seds.org/messier/xtra/marathon/marathon.html Messier Marathon]
Messier, Charles
Messier, Charles
Messier, Charles
ja:シャルル・メシエ
Comet
A comet is a small body in the solar system that orbits the sun and (at least occasionally)
exhibits a coma (or atmosphere) and/or a tail — both due primarily to the effects of solar radiation
upon the comet's nucleus, which itself is a minor planet composed of rock, dust, and ices.
Due to their origins in the outer solar system and their propensity to be highly affected by
relatively close approaches to the major planets, comets' orbits are constantly evolving. Some
are moved into sungrazing orbits that destroy the comets when they near the sun, while others are thrown out of the solar system forever. But a bright comet is one of the surest celestial events to capture the
interest of the general public.
Comets are believed to originate in a cloud (the Oort cloud) at large distances from the sun consisting of debris left over from the condensation of the solar nebula; the outer edges of such nebulae are cool enough that water exists in a solid (rather than gaseous) state. Asteroids originate via a different process, but very old comets which have lost all their volatile materials may come to resemble asteroids.
Physical characteristics
Long-period comets are believed to originate in a distant cloud known as the Oort cloud, after the astronomer Jan Hendrik Oort who hypothesised its existence. They are sometimes perturbed from their distant orbits by gravitational interactions, falling into extremely elliptical orbits that bring them very close to the Sun.One theory says that when a comet approaches the inner solar system, radiation from the Sun causes its outer layers of ice to evaporate, but again there is no proof of this.
The streams of dust and gas this releases form a huge but extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the sun's radiation pressure and solar wind cause an enormous tail to form pointing away from the sun. The dust and gas each form their own distinct tail, pointed in slightly different directions — dust being left behind in the comet's orbit (so that it often forms a curved tail) and the ion tail (gas) always pointing directly away from the Sun, since the gas is more strongly affected by the solar wind than dust is, and follows magnetic field lines rather than an orbital trajectory. While the solid body of the comet (called the nucleus) is generally less than 50km across, the coma may be larger than the Sun, and the tails can extend over 150 million km (1 Astronomical unit) or more.
Both coma and tail are illuminated by the Sun, and may become visible from the Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly and the gases glowing due to ionization. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible with the naked eye. Before the invention of the telescope, comets seemed to appear out of nowhere in the sky and gradually vanish out of sight. They were usually considered bad omens of deaths of kings or noble men, or coming catastrophes. From ancient sources, such as Chinese oracle bones, it is known that their appearance have been noticed by humans for millennia. One very famous old recording of a comet is the appearance of Halley's Comet on the Bayeux Tapestry, which records the Norman conquest of England in 1066.
1066
Surprisingly, cometary nuclei are among the blackest objects known to exist in the solar system. The Giotto probe found that Comet Halley's nucleus reflects approximately 4% of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects only 2.4% to 3% of the light that falls on it; by comparison, asphalt reflects 7% of the light that falls on it. It is thought that complex organic compounds are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like tar or crude oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing.
In 1996, comets were found to emit X-rays [http://heasarc.gsfc.nasa.gov/docs/rosat/hyakutake.html]. These X-rays surprised researchers, because their emission by comets had not previously been predicted. The X-rays are thought to be generated by the interaction between comets and the solar wind: when highly charged ions fly through a cometary atmosphere, they collide with cometary atoms and molecules. In these collisions, the ions will capture one or more electrons leading to emission of X-rays and far ultraviolet photons [http://www.kvi.nl/~bodewits].
Orbital characteristics
ions, illustrating the high eccentricity of the orbit and more rapid motion when closer to the Sun]]
Comets are classified according to their orbital periods. Short period comets have orbits of less than 200 years, while Long period comets have longer orbits but remain gravitationally bound to the Sun. Single-apparition comets have parabolic or hyperbolic orbits which will cause them to permanently exit the solar system after one pass by the Sun.
Modern observations have revealed a few genuinely hyperbolic orbits, but no more than could be accounted for by perturbations from Jupiter. If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of kilometres per second). If such objects entered the solar system, they would have positive total energies, and would be observed to have genuinely hyperbolic orbits. A rough calculation shows that there might be 4 hyperbolic comets per century, within Jupiter's orbit, give or take one and perhaps two orders of magnitude [http://www.astro.lsa.umich.edu/users/cowley/lecture34/ †].
On the other extreme, the short period Comet Encke has an orbit which never places it farther from the Sun than Jupiter. Short-period comets are thought to originate in the Kuiper belt, whereas the source of long-period comets is thought to be the Oort cloud. A variety of mechanisms have been proposed to explain why comets get perturbed into highly elliptical orbits, including close approaches to other stars as the Sun follows its orbit through the Milky Way Galaxy; the Sun's hypothetical companion star Nemesis; or an unknown Planet X.
Because of their low masses, and their elliptical orbits which frequently take them close to the giant planets, cometary orbits are often perturbed. Short period comets display a strong tendency for their aphelia to coincide with a giant planet's orbital radius, with the Jupiter family of comets being the largest, as the histogram shows. It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined, in addition to being the swiftest of the giant planets.
histogram
Also because of gravitational interactions, a number of periodic comets discovered in earlier decades or previous centuries are now lost, since their orbits were never known well enough to know where to look for their future appearances. However, occasionally a "new" comet will be discovered and upon calculation of its orbit it turns out to be an old "lost" comet. An example is Comet 11P/Tempel-Swift-LINEAR, which was discovered in 1869 but became unobservable after 1908 due to perturbations by Jupiter, and was not found again until accidentally rediscovered by LINEAR in 2001.
Comet nomenclature
The names given to comets have followed several different conventions over the past two centuries. Before any systematic naming convention was adopted, comets were named in a variety of ways. Prior to the early 20th century, most comets were simply referred to by the year in which they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "Great Comet of 1680" (Kirch's Comet), the "Great September Comet of 1882," and the "Daylight Comet of 1910" ("Great January Comet of 1910"). After Edmund Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759, that comet became known as Comet Halley. Similarly, the second and third known periodic comets, Comet Encke and Comet Biela , were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred by the year of their apparition.
In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet is named after up to three independent discoverers. In recent years, many comets have been discovered by instruments operated by large teams of astronomers, and in this case, comets may be named for the instrument. For example, Comet IRAS-Araki-Alcock was discovered independently by the IRAS satellite and amateur astronomers Genichi Araki and George Alcock. In the past, when multiple comets were discovered by the same individual, group of individuals, or team, the comets' names were distinguished by adding a numeral to the discoverers' names; thus Comets Shoemaker-Levy 1–9. Today, the large numbers of comets discovered by some instruments (in August 2005, SOHO discovered its 1000th comet) has rendered this system impractical, and no attempt is made to ensure that each comet has a unique name. Instead, the comets' systematic designations are used to avoid confusion.
Until 1994, comets were first given a provisional designation consisting of the year of their discovery followed by a lowercase letter indicating its order of discovery in that year (for example, Comet Bennett 1969i was the 9th comet discovered in 1969). Once the comet had been observed through perihelion and its orbit had been established, the comet was given a permanent designation of the year of its perihelion, followed by a Roman numeral indicating its order of perihelion passage in that year, so that Comet Bennett 1969i became Comet Bennett 1970 II (it was the second comet to pass perihelion in 1970) .
Increasing numbers of comet discoveries made this procedure awkward, and in 1994 the International Astronomical Union approved a new naming system. Comets are now designated by the year of their discovery followed by a letter indicating the half-month of the discovery and a number indicating the order of discovery (a system similar to that already used for asteroids), so that the fourth comet discovered in the second half of February 2006 would be designated 2006 D4. Prefixes are also added to indicate the nature of the comet, with P/ indicating a periodic comet, C/ indicating a non-periodic comet, X/ indicating a comet for which no reliable orbit could be calculated, D/ indicating a comet which has broken up or been lost, and A/ indicating an object that was mistakenly identified as a comet, but is actually a minor planet. After their second observed perihelion passage, periodic comets are also assigned a number indicating the order of their discovery. So Halley's Comet, the first comet to be identified as periodic, has the systematic designation 1P/1682 Q1. Comet Hale-Bopp's designation is C/1995 O1.
History of comet study
Early observations and thought
Historically, comets were thought to be unlucky, or even interpreted as attacks by heavenly beings against terrestrial inhabitants. Some authorities interpret references to "falling stars" in Gilgamesh, Revelation and the Book of Enoch as references to comets, or possibly bolides.
In the first book of his Meteorology, Aristotle propounded the view of comets that would hold sway in Western thought for nearly two thousand years. He rejected the ideas of several earlier philosophers that comets were planets, or at least a phenomenon related to the planets, on the grounds that while the planets confined their motion to the circle of the Zodiac, comets could appear in any part of the sky. Instead, he described comets as a phenomenon of the upper atmosphere, where hot, dry exhalations gathered and occasionally burst into flame. Aristotle held this mechanism responsible for not only comets, but also meteors, the aurora borealis, and even the Milky Way.
A few later classical philosophers did dispute this view of comets. Seneca the Younger, in his Natural Questions, observed that comets moved regularly through the sky and were undisturbed by the wind, behavior more typical of celestial than atmospheric phenomena. While he conceded that the other planets do not appear outside the Zodiac, he saw no reason that a planet-like object could not move through any part of the sky, humanity's knowledge of celestial things being very limited. However, the Aristotelean viewpoint proved more influential, and it was not until the 16th century that it was demonstrated that comets must exist outside the earth's atmosphere.
In 1577, a bright comet was visible for several months. The Danish astronomer Tycho Brahe used measurements of the comet's position taken by himself and other, geographically separated observers to determine that the comet had no measureable parallax. Within the precision of the measurements, this implied the comet must be at least four times more distant from the earth than the moon.
Orbital studies
parallax's Principia.]]
Although comets had now been demonstrated to be in the heavens, the question of how they moved through the heavens would be debated for most of the next century. Even after Johannes Kepler had determined in 1609 that the planets moved about the sun in elliptical orbits, he was reluctant to believe that the laws that governed the motions of the planets should also influence the motion of other bodies—he believed that comets travel among the planets along straight lines. Galileo Galilei, although a staunch Copernicanist, rejected Tycho's parallax measurements and held to the Aristotelean notion of comets moving on straight lines through the upper atmosphere.
The first suggestion that Kepler's laws of planetary notion should also apply to the comets was made by William Lower in 1610. In the following decades, other astronomers, including Pierre Petit, Giovanni Borelli, Adrien Auzout, Robert Hooke, and Jean-Dominique Cassini, all argued for comets curving about the sun on elliptical or parabolic paths, while others, such as Christian Huygens and Johannes Hevelius, supported comets' linear motion.
The matter was resolved by the bright comet that was discovered by Gottfried Kirch on November 14, 1680. Astronomers throughout Europe tracked its position for several months. In his Principia Mathematica of 1687, Isaac Newton proved that an object moving under the influence of his inverse square law of universal gravitation must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.
In 1705, Edmond Halley applied Newton's method to twenty-four cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758-9. (Earlier, Robert Hooke had identified the comet of 1664 with that of 1618, while Jean-Dominique Cassini had suspected the identity of the comets of 1577, 1665, and 1680. Both were incorrect.) Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy. When the comet returned as predicted, it became known as Comet Halley or Halley's Comet (its official designation is 1P/Halley). Its next appearance is due in 2061.
Among the comets with short enough periods to have been observed several times in the historical record, Comet Halley is unique in consistently being bright enough to be visible to the naked eye. Since the confirmation of Comet Halley's periodicity, many other periodic comets have been discovered through the telescope. The second comet to be discovered to have a periodic orbit was Comet Encke (official designation 2P/Encke). Over the period 1819-1821 the German mathematician and physicist Johann Franz Encke computed orbits for a series of cometary apparitions observed in 1786, 1795, 1805, and 1818, concluded they were same comet, and successfully predicted its return in 1822. By 1900, seventeen comets had been observed at more than one perihelion passage and recognized as periodic comets. As of November 2005, 173 comets have achieved this distinction, though several have since been destroyed or lost.
Studies of physical characteristics
:Hast thou ne'er seen the Comet's flaming flight?
Isaac Newton described comets as compact, solid, fixed, and durable bodies: in one word, a kind of planets, which move in very oblique orbits, every way, with the greatest freedom, persevering in their motions even against the course and direction of the planets; and their tail as a very thin, slender vapour, emitted by the head, or nucleus of the comet, ignited or heated by the sun. Comets also seemed to Newton absolutely requisite for the conservation of the water and moisture of the planets; from their condensed vapours and exhalations all that moisture which is spent on vegetations and putrefactions, and turned into dry earth, might be resupplied and recruited; for all vegetables were thought to increase wholly from fluids, and turn by putrefaction into earth. Hence the quantity of dry earth must continually increase, and the moisture of the globe decrease, and at last be quite evaporated, if it have not a continual supply. Newton suspected that the spirit which makes the finest, subtilest, and best part of our air, and which is absolutely requisite for the life and being of all things, came principally from the comets.
Another use which he conjectured comets might be designed to serve, is that of recruiting the sun with fresh fuel, and repairing the consumption of his light by the streams continually sent forth in every direction from that luminary —
:"From his huge vapouring train perhaps to shake
:Reviving moisture on the numerous orbs,
:Thro' which his long ellipsis winds; perhaps
:To lend new fuel to declining suns,
:To light up worlds, and feed th' ethereal fire."
As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion. In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor in the 1835 apparition of Comet Halley, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit and argued that the non-gravitational movements of Comet Encke resulted from this mechanism.
However, another comet-related discovery overshadowed these ideas for nearly a century. Over the period 1864–1866 the Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteors, and based on orbital similarities, correctly hypothesized that the Perseids were fragments of Comet Swift-Tuttle. The link between comets and meteor showers was dramatically underscored when in 1872, a major meteor shower occurred from the orbit of Comet Biela, which had been observed to split into two pieces during its 1846 apparition, and never seen again after 1852. A "gravel bank" model of comet structure arose, according to which comets consist of loose piles of small rocky objects, coated with an icy layer.
By the middle of the twentieth century, this model suffered from a number of shortcomings: in particular, it failed to explain how a body that contained only a little ice could continue to put on a brilliant display of evaporating vapor after several perihelion passages. In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock. This "dirty snowball" model soon became accepted. It was confirmed when an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) flew through the coma of Halley's comet in 1986 to photograph the nucleus and observed the jets of evaporating material. The American probe Deep Space 1 flew past the nucleus of Comet Borrelly on September 21 2001 and confirmed that the characteristics of Comet Halley are common on other comets as well.
2001
The Stardust spacecraft, launched in February 1999, has already collected particles from the coma of Comet Wild 2 in January 2004, and will return the samples to Earth in a capsule in 2006. Claudia Alexander, a program scientist for Rosetta from NASA's Jet Propulsion Laboratory who has has modeled comets for years, reported to space.com about her astonishment at the number of jets, their appearance on the dark side of the comet as well as the light side, their ability to lift large chunks of rock from the surface of the comet and the fact that comet Wild 2 is not a loosely-cemented rubble pile.[http://www.space.com/scienceastronomy/stardust_results_040617.html]
Forthcoming space missions will add greater detail to our understanding of what comets are made of. In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. And in 2014, the European Rosetta probe will orbit comet Comet Churyumov-Gerasimenko and place a small lander on its surface.
Rosetta observed the Deep Impact event, and with its set of very sensitive instruments for cometary investigations, it used its capabilities to observe Tempel 1 before, during and after the impact. At a distance of about 80 million kilometres from the comet, Rosetta was in the most privileged position to observe the event. Rosetta measured the water vapour content and the cross-section of the dust created by the impact. European scientists could then work out the corresponding dust/ice mass ratio, which is larger than one, suggesting that comets are composed more of dust held together by ice, rather than made of ice comtaminated with dust. Hence, they are now 'icy dirtballs' rather than 'dirty snowballs' as previously believed.
Debate over comet composition
Comet Churyumov-Gerasimenko
As late as 2002, there is conflict on how much ice is in a comet. NASA's Deep Space 1 team, working at NASA's Jet Propulsion Lab, obtained high-resolution images of the surface of comet Borrelly. They announced that comet Borrelly exhibits distinct jets, yet has a hot, dry surface. The assumption that comets contain water and other ices led Dr. Laurence Soderblom of the U.S. Geological Survey to say, "The spectrum suggests that the surface is hot and dry. It is surprising that we saw no traces of water ice." However, he goes on to suggest that the ice is proabably hidden below the crust as "either the surface has been dried out by solar heating and maturation or perhaps the very dark soot-like material that covers Borrelly's surface masks any trace of surface ice".[http://www.jpl.nasa.gov/releases/2002/release_2002_80.html]
The recent Deep Impact probe has also yielded preliminary results suggesting there is less ice in comets then originally predicted.
Great comets
While hundreds of tiny comets pass through the inner solar system every year, only a very few comets make any impact on the general public. About every decade or so, a comet will become bright enough to be noticed by a casual observer — such comets are often designated Great Comets. In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, the Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions, while the appearance of Comet Hale-Bopp in 1997 triggered the mass suicide of the Heaven's Gate cult. To most people, however, a great comet is simply a beautiful spectacle.
Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions. Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it will have a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular, but failed to do so. Comet West, which appeared three years later, had much lower expectations (perhaps because scientists were much warier of glowing predictions after the Kohoutek fiasco), but became an extremely impressive comet.
The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick succession — Comet Hyakutake in 1996, followed by Hale-Bopp, which reached maximum brightness in 1997 having been discovered two years earlier. As yet, the 21st century has not seen the arrival of any great comets.
Peculiar comets
Of the thousands of known comets, some are very unusual. Comet Encke orbits from inside the orbit of Jupiter to inside the orbit of Mercury while Comet 29P/Schwassmann-Wachmann orbits in a nearly circular orbit entirely between Jupiter and Saturn. 2060 Chiron, whose unstable orbit keeps it between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed. Similarly, Comet Shoemaker-Levy 2 was originally designated asteroid 1990 UL3. Some near-earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.
Some comets have been observed to break up. Comet Biela was one significant example, breaking into two during its 1846 perihelion passage. The two comets were seen separately in 1852, but never again after that. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A lesser meteor shower, the Andromedids, occurs annually in November, and is caused by the Earth crossing Biela's orbit [http://comets.amsmeteors.org/meteors/showers/andromedids.html].
Several other comets have been seen to break up during their perihelion passage, including great comets West and Comet Ikeya-Seki. Some comets, such as the Kreutz Sungrazers, orbit in groups and are thought to be pieces of a single object that has previously broken apart.
Another very significant cometary disruption was that of Comet Shoemaker-Levy 9, which was discovered in 1993. At the time of its discovery, the comet was in orbit around Jupiter, having been captured by the planet during a very close approach in 1992. This close approach had already broken the comet into hundreds of pieces, and over a period of 6 days in July 1994, these pieces slammed into Jupiter's atmosphere — the first time astronomers had observed a collision between two objects in the solar system. However, it has been suggested that the object responsible for the Tunguska event in 1908 was a fragment of Comet Encke.
Comets in fiction
Comets are popular subjects for science fiction authors and filmmakers although they are often misrepresented as fiery objects, rather than icy.
- Jules Verne's Hector Servadac, Voyages et aventures à travers le Monde Solaire (Off on a Comet, 1877) is a deeply implausible Victorian vision of touring the solar system via a handy comet.
- H. G. Wells' In the Days of the Comet (1905) is a account of how the vapours of a comet's tail cause an instantaneous worldwide utopian society.
- Tove Jansson's Comet in Moominland (1946) depicts the world of the Moomins threatened by a fiery comet.
- The Day of the Triffids (1951) is a novel by John Wyndham in which a meteor shower causes permanent and irreversible blindness in the population and renders them easy prey to giant mobile vegetables.
- Lucifer's Hammer (1977), a novel by Larry Niven and Jerry Pournelle, is an apocalyptic survival story featuring a comet impact on Earth.
- In Heart of the Comet (1987), a novel by Gregory Benford and David Brin, a multinational team colonizes Halley's Comet, building a habitat within the ice.
- Arthur C. Clarke's novel 2061: Odyssey Three (1988) includes a detailed description of a manned mission to Halley's Comet.
- The Hammer of God (1993) is a novel by Arthur C. Clarke in which an object (Kali) which threatens to strike earth appears to be an almost dead comet.
- The Paramount/DreamWorks motion picture Deep Impact (1998) tells the story of a comet on a collision course with Earth, and focuses primarily on the emotional reactions of those who are affected by the impending disaster.
See also
- List of periodic comets
- List of non-periodic comets
- Torino Scale for categorizing the impact hazard
References
# Aristotle (ca. 350 B.C.) Meteorologia. An English translation by E.W. Webster is [http://classics.mit.edu/Aristotle/meteorology.1.i.html available online].
# Bill Arnett. (2000). "Astronomical Names." [http://www.nineplanets.org/names.html Available online].
# Committee on Small Body Nomenclature (1994). "Cometary Designation System." [http://cfa-www.harvard.edu/cfa/ps/lists/CometResolution.html Available online].
# European Southern Observatory. (2003). "A Brief History of Comets." Available online: [http://www.eso.org/outreach/info-events/hale-bopp/comet-history-1.html Part I], [http://www.eso.org/outreach/info-events/hale-bopp/comet-history-2.html Part II].
#
# Gary W. Kronk. (2001–2005). Cometography. [http://cometography.com Available online].
# I.S. Newton (1687). Philosophiæ Naturalis Principia Mathematica. Londoni: Josephi Streater.
# Samuel Pepys (1893). The Diary of Samuel Pepys, M.A., F.R.S.. London: George Bell & Sons.
# Vigyan Prasar (2001). "Development of Cometary Thought." Available online: [http://www.vigyanprasar.com/dream/mar2001/comets.htm Part I], [http://www.vigyanprasar.com/dream/apr2001/comets.htm Part II].
# Reading Museum Service (2000-2004). Britain's Bayeux Tapestry. [http://www.bayeuxtapestry.org.uk/ Available online]. Accessed 22 April 2005.
#
# Solar and Heliospheric Observatory. (2005). "The SOHO 1000th Comet Contest." [http://soho.nascom.nasa.gov/comet1000/ Available online].
#
External links
- [http://www.cometography.com/ Cometography.com]
- [http://www.ifa.hawaii.edu/faculty/jewitt/comet.html David Jewitt overview of the comets]
- [http://cfa-www.harvard.edu/iau/Headlines.html Listing of newly discovered comets]
- [http://cfa-www.harvard.edu/icq/icq.html Source of useful comet-related material on the Web]
- Open Directory Project: [http://www.dmoz.org/Science/Astronomy/Solar_System/Asteroids,_Comets_and_Meteors/Comets/ Comets]
- [http://fax.libs.uga.edu/QB721xM635/ ESSAY ON COMETS], which gained the first of Dr. Fellowes's prizes, proposed to those who had attended the University of Edinburgh within the last twelve years. By David Milne. Publisher: Edinburgh, Printed for A. Black; 1828. (a searchable facsimile at the University of Georgia Libraries; DjVu & [http://fax.libs.uga.edu/QB721xM635/1f/essay_on_comets.pdf layered PDF] format)
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ko:혜성
ms:Komet
ja:彗星
simple:Comet
th:ดาวหาง
Pleiades (star cluster)s surrounded by reflection nebulosity]]
The Pleiades (also known as M45 or the Seven Sisters) is an open cluster in the constellation of Taurus. It is among the nearest to the Earth of all open clusters, probably the best known and certainly the most striking to the naked eye.
The distance of the cluster is very important as it is a crucial step in determining the distance scale of the whole universe. The Hipparcos satellite measured a distance for the cluster which was 10% smaller than most previous measurements, but was later found to have suffered from a systematic error when observing the Pleiades. The cluster is now known to lie at a distance of about 135 parsecs (440 light years).
The cluster is dominated by hot blue stars, which have formed within the last 100 million years. Dust that was erroneously thought at first to be remaining from the formation of the cluster forms faint reflection nebulosity around the brightest stars. The cluster will in time disperse due to gravitational interactions with the spiral arms of the galaxy and giant molecular clouds. The cluster's lifetime will probably be about 250 million years.
History
giant molecular cloud]]
The Pleiades are a prominent sight in the northern hemisphere in winter, and have been known since antiquity to cultures all around the world, including the Maori and Australian Aborigines, the Japanese and the Sioux of North America. Some Greek astronomers considered them to be their own constellation, and they are mentioned in Homer's Iliad and Odyssey, and also by Hesiod. They are also mentioned three times in the Bible (Job 9:9, 38:31; Amos 5:8).
They have long been known to be a physically related group of stars rather than any chance alignment. The Reverend John Michell calculated in 1767 that the probability of a chance alignment of so many bright stars was only 1 in 500,000, and so correctly surmised that the Pleiades and many other clusters of stars must be physically related . When studies were first made of the stars' proper motions, it was found that they are all moving in the same direction across the sky, at the same rate, further demonstrating that they were related.
Charles Messier measured the position of the cluster and included it as M45 in his catalogue of comet-like objects, published in 1771. Along with the Orion Nebula and the Praesepe cluster, Messier's inclusion of the Pleiades has been noted as curious, as most of Messier's objects were much fainter and more easily confused with comets—something which seems scarcely possible for the Pleiades. One possibility is that Messier simply wanted to have a larger catalogue than his scientific rival Lacaille, whose 1755 catalogue contained 42 objects, and so he added some bright, well-known objects to boost his list.
Distance
The distance to the Pleiades has been estimated by many methods, as it is an important step in calibrating distance scales for the whole universe. Accurate knowledge of the distance to the Pleiades allows astronomers to plot a Hertzsprung-Russell Diagram for the cluster, which, when compared to those plotted for clusters whose distance is not known, allows their distances to be estimated. Other methods can then extend the distance scale from open clusters to galaxies and clusters of galaxies, and a cosmic distance ladder can be constructed.
Results prior to the launch of the Hipparcos satellite generally found that the Pleiades were about 135 parsecs away from Earth. Hipparcos caused consternation among astronomers by finding a distance of only 118 parsecs by measuring the parallax of stars in the cluster—a technique which should yield the most direct and accurate results. Later work has consistently found that the Hipparcos distance measurement for the Pleiades was in error, but it is not known why the error occurred . The distance to the Pleiades is currently thought to be the higher value of about 135 parsecs , .
Composition
parallax
The cluster is about 12 light years in diameter and contains approximately 500 stars in total. It is dominated by young, hot blue stars, up to 14 of which can be seen with the naked eye depending on local observing conditions. The arrangement of the brightest stars is somewhat similar to Ursa Major and Ursa Minor. The total mass contained in the cluster is estimated to be about 800 solar masses .
The cluster contains many brown dwarfs—objects with less than about 8% of the Sun's mass, which are not heavy enough to become proper stars. They may constitute up to 25% of the total population of the cluster, although they contribute less than 2% of the total mass . Astronomers have made great efforts to find and analyse brown dwarfs in the Pleiades and other young clusters—because they are still relatively bright and observable—while brown dwarfs in older clusters have faded and are much more difficult to study.
Also present in the cluster are several white dwarfs. Given the young age of the cluster normal stars are not expected to have had time to evolve into white dwarfs, a process which normally takes several billion years. It is believed that, rather than being individual low- to intermediate-mass stars, the progenitors of the white dwarfs must have been high-mass stars in binary systems. Transfer of mass from the higher-mass star to its companion during its rapid evolution would result in a much quicker route to the formation of a white dwarf.
Age and future evolution
Ages for star clusters can be estimated by comparing the H-R diagram for the cluster with theoretical models of stellar evolution, and using this technique, ages for the Pleiades of between 75 and 150 million years have been estimated. The spread in estimated ages is a result of uncertainties in stellar evolution models. In particular, models including a phenomenon known as convective overshoot, in which a convective zone within a star penetrates an otherwise non-convective zone, result in higher apparent ages.
Another way of estimating the age of the cluster is by looking at the lowest-mass objects. In normal main sequence stars, lithium is rapidly destroyed in nuclear fusion reactions, but brown dwarfs can retain their lithium. Due to its very low ignition temperature of 2.5 million Kelvin, the highest-mass brown dwarfs will burn lithium eventually, and so determining the highest mass of brown dwarfs still containing lithium in the cluster can give an idea of its age. Applying this technique to the Pleiades gives an age of about 115 million years .
Like most open clusters, the Pleiades will not stay gravitationally bound forever, as the component stars are moving faster than the escape velocity of the cluster. Calculations suggest that the cluster will take about 250 million years to disperse.
Reflection nebulosity
escape velocity
Under ideal observing conditions, some hint of nebulosity may be seen around the cluster, and this shows up in long-exposure photographs. It is a reflection nebula, caused by dust reflecting the blue light of the hot, young stars.
It is often thought that the dust was left over from the formation of the cluster, but at the age of about 100 million years generally accepted for the cluster, almost all the dust originally present would have been dispersed by radiation pressure. Instead, it seems that the cluster is simply passing through a particularly dusty region of the interstellar medium.
Studies show that the dust responsible for the nebulosity is not uniformly distributed, but is concentrated mainly in two layers along the line of sight to the cluster. These layers may have been formed by deceleration due to radiation pressure as the dust has moved towards the stars .
Names and technical information
The nine brightest stars of the Pleiades are named for the Seven Sisters of Greek mythology: Asterope, Merope, Electra, Maia, Taygete, Celaeno and Alcyone, along with their parents Atlas and Pleione. As daughters of Atlas, the Hyades were sisters of the Pleiades. The name of the cluster itself is of Greek origin, though of uncertain etymology. Suggested derivations include: from πλει̂ν plein, to sail, making the Pleiades the "sailing ones"; from pleos, full or many; from peleiades, flock of doves; or from the ancient Persian equivalent name of Parvin. The following table gives details of the brightest stars in the cluster:
Pronunciation guide: a as in cat (when stressed), or in sofa (when not); ay as in day; ai as in air; e as in pet; ee as in feet; i as in bit; eye as in bite; oh as in bone; s as in hiss. Stress is indicated by an apostrophe after the stressed syllable (af'-ter).
The Pleiades in folklore
Persian
The Pleiades' high visibility in the night sky has guaranteed it a special place in many cultures, both ancient and modern. To the Vikings, they were Freya's hens, and their name in many old European languages compares them to a hen with chicks. In Sanskrit Pleiades is known as Krittika nakshatra. According to Indian astrology, those born at a time when the Sun is in Krittika are "stubborn, harsh in speech with uncontrollable fiery temper, this makes them illogical in thoughts, speech and actions. They have strained relations with relatives and friends. However, these persons are glutton, fond of spicy foods, well-versed academically, fond of opposite sex, bright in appearance , miser, worried nature and of widespread fame." In Greek mythology, they represented the Seven Sisters.
The Maori of New Zealand call the Pleiades Mataariki, and their heliacal rising signifies the beginning of the new year (around June). The Australian Aborigines believed they were a woman who had been nearly raped by Kidili, the man in the moon. Alternatively, they were seven sisters called the Makara.
The Sioux of North America had a legend that linked the origin of the Pleiades to Devils Tower. It was common among the indigenous peoples of the Americas to measure keenness of vision by the number of stars the viewer could see in the Pleiades, a practice which was also used in historical Europe, especially in Greece.
In Japan, the Pleiades are known as Subaru, and have given their name to the car manufacturer. In Chinese constellations, they are 昴 mao, the hairy head of the white tiger of the West, while the name of the Hindu God Kartikeya means him of the Pleiades.
In Western astrology they represent coping with sorrow and were considered a single one of the medieval fixed stars. As such, they are associated with quartz and fennel, and the kabbalistic sign Image:Agrippa1531_Pleiades.png.
The word has acquired a meaning of "multitude", inspiring the name of the French literary movement La Pléiade and an earlier group of Alexandrian poets.
References
- Adams, Joseph D.; Stauffer, John R.; Monet, David G.; Skrutskie, Michael F.; Beichman, Charles A. (2001), The Mass and Structure of the Pleiades Star Cluster from 2MASS, The Astronomical Journal, v.121, p.2053
- Basri G., Marcy G. W., Graham J. R. (1996), Lithium in Brown Dwarf Candidates: The Mass and Age of the Faintest Pleiades Stars, Astrophysical Journal v.458, p.600
- Frommert, Hartmut (1998). [http://www.seds.org/messier/m-q&a.html#why_M42-45 "Messier Questions & Answers"]. Retrieved March 1, 2005.
- Gibson, Steven J.; Nordsieck, Kenneth H. (2003), The Pleiades Reflection Nebula. II. Simple Model Constraints on Dust Properties and Scattering Geometry, The Astrophysical Journal, v.589, p. 362
- Michell J. (1767), An Inquiry into the probable Parallax, and Magnitude, of the Fixed Stars, from the Quantity of Light which they afford us, and the particular Circumstances of their Situation, Philosophical Transactions, v. 57, p. 234-264
- Moraux, E.; Bouvier, J.; Stauffer, J. R.; Cuillandre, J.-C. (2003), Brown dwarfs in the Pleiades cluster: Clues to the substellar mass function, Astronomy and Astrophysics, v.400, p.891
- Morse, Eric (1988). The Living Stars. London: Amethyst Books.
- Percival, S. M.; Salaris, M.; Groenewegen, M. A. T. (2005), The distance to the Pleiades. Main sequence fitting in the near infrared, Astronomy and Astrophysics, v.429, p.887
- Soderblom D.R., Nelan E., Benedict G.F., McArthur B., Ramirez I., Spiesman W., Jones B.F. (2005), Confirmation of Errors in Hipparcos Parallaxes from Hubble Space Telescope Fine Guidance Sensor Astrometry of the Pleiades, The Astronomical Journal, v. 129, pp. 1616-1624.
- Zwahlen, N.; North, P.; Debernardi, Y.; Eyer, L.; Galland, F.; Groenewegen, M. A. T.; Hummel, C. A. (2004), A purely geometric distance to the binary star Atlas, a member of the Pleiades, Astronomy and Astrophysics, v.425, p.L45
External links
- [http://www.ras.ucalgary.ca/~gibson/pleiades/ Photos and information on the Pleiades from the University of Calgary]
- [http://www.seds.org/messier/m/m045.html Information on the Pleiades from SEDS]
- [http://www.aao.gov.au/images.html/captions/uks018.html Information and images from the Anglo-Australian Observatory]
Category:Messier objects
Category:Pleiades Open Cluster
Category:Taurus constellation
ko:플라이아데스 성단
ja:プレアデス星団
1781
1781 was a common year starting on Monday (see link for calendar).
Events
- January 5 - American Revolutionary War: Richmond, Virginia is burned by British naval forces led by Benedict Arnold.
- January 30 - Articles of Confederation ratified by 13th state, Maryland.
- January - William Pitt the Younger, later Prime Minister, enters Parliament.
- March 1 - American Continental Congress implements the Articles of Confederation.
- March 13 - Sir William Herschel discovers the planet Uranus. Originally he calls it Georgium Sidus (George's Star) in honour of King George III of England.
- March 15 - American Revolutionary War: American General Nathanael Greene loses Battle of Guilford Court House to British.
- July 27 - French spy Francis Henry de la Motte executed in Tyburn prison in England for high treason
- August 30 - American Revolutionary War: French fleet under Comte de Grasse enters Chesapeake Bay, cutting British General Charles Cornwallis off from escape by sea.
- September 4 - Los Angeles is founded as El Pueblo de Nuestra Señora La Reina de Los Ángeles de Porciuncula (City of Our Lady the Queen of the Angels of Porciuncula) by a group of 44 Spanish settlers.
- September 5 - British fleet under Thomas Graves arrives and fights de Grasse, but to no effect.
- September 6 - The British army attacks a fort in Groton, Connecticut which became known as the Battle of Groton Heights.
- September 10 - Graves gives up trying to break through the now-reinforced French fleet and returns to New York, leaving Cornwallis to his fate.
- October 19 - General Charles Cornwallis surrenders at Yorktown, Virginia, ending the armed struggle of the American Revolutionary War.
- November 5 - John Hanson is elected President of the Continental Congress.
- November 29 - The slave ship Zong dumps its living cargo into the sea in order to claim insurance.
- December 12 - French and British fleets fight in the Second Battle of Ushant.
- Bank of North America is chartered by the Continental Congress.
- Charles Messier publishes final catalog of Messier objects.
- Carl Wilhelm Scheele discovers tungsten.
- Immanuel Kant publishes Critique of Pure Reason.
- Jeremy Bentham formulates utilitarian ethics.
- Reverend Samuel Peters publishes General History of Connecticut, using the term blue law for the first time.
- Antonio Salieri selected as music teacher of Princess of Württemberg over Mozart.
Births
- January 26 - Achim von Arnim, German writer (d. 1831)
- January 30 - Adelbert von Chamisso, German writer (d. 1838)
- February 17 - Rene Theophile Hyacinthe Laennec, French physician and inventor (d. 1826)
- March 4 - Rebecca Gratz, American educator and philanthropist (d. 1869)
- March 13 - Karl Friedrich Schinkel, German architect and painter (d. 1841)
- June 9- George Stephenson, English engineer (d. 1848)
- June 21 - Siméon-Denis Poisson, French mathematician and physicist (d. 1840)
- July 6 - Thomas Stamford Raffles, English founder of Singapore (d. 1826)
- July 6 - John D. Sloat, American naval officer (d. 1867)
- July 27 - Mauro Giuliani, Italian composer (d. 1828)
- September 3 - Eugène de Beauharnais, French nobleman, son of Napoleon's wife Josephine (d. 1824)
- September 6 - Anton Diabelli, Austrian music publisher, editor, and composer (d. 1858)
- October 1 - James Lawrence, U.S. Navy officer (d. 1813)
- November 6 - Lucy Aikin, English writer (d. 1864)
- November 20 - Karl Friedrich Eichhorn, German jurist (d. 1854)
- November 29 - Andrés Bello, Venezuelan poet, lawmaked, teacher, philosopher and sociologist (d. 1865)
- November 30 - Alexander Berry, Scottish adventurer and Australian pioneer (d. 1873)
- December 11 - Sir David Brewster, Scottish physicist (d. 1868)
- William Williams of Wern, minister (d. 1840)
Deaths
- January 12 - Richard Challoner, English Catholic prelate (b. 1691)
- January 15 - Marianne Victoria of Borbón, queen regent of Portugal (b. 1718)
- February 15 - Gotthold Ephraim Lessing, German author and philosopher (b. 1729)
- February 23 - George Taylor, American signer of the Declaration of Independence
- February 24 - Edward Capell, English critic (b. 1713)
- March 18 - Anne Robert Jacques Turgot, Baron de Laune, French statesman and economist (b. 1727)
- April 23 - James Abercrombie, British general (b. 1706)
- April 28 - Cornelius Harnett, American delegate to the Continental Congress (b. 1723)
- May 8 - Richard Jago, English poet (b. 1715)
- May 27 - Giovanni Battista Beccaria, Italian physicist (b. 1716)
- July 18 - Padre Francisco Garcés, Spanish missionary (killed) (b. 1738)
- July 23 - John Joachim Zubly, Swiss-born Continental Congressman (b. 1724)
- September 28 - William Henry Nassau de Zuylestein, 4th Earl of Rochford, British diplomat and statesman (b. 1717)
- October 16 - Edward Hawke, 1st Baron Hawke, British naval officer (b. 1705)
- November 4 - Johann Nikolaus Götz, German poet (b. 1721)
- Peter Scheemakers, Flemish sculptor (b. 1691)
Category:1781
ko:1781년
ms:1781
1784
1784 was a leap year starting on Thursday (see link for calendar).
Events
- January 6 - the Turks agree to Russia's annexation of the Crimea in the Treaty of Constantinople
- January 14 - The US Congress ratifies the Treaty of Paris with England to end the American Revolutionary War
- February 27 – Count of St Germain dies of pneumonia in Schleswig-Holstein
- February 28 - John Wesley charters the Methodist Church
- August 10 - Jeanne de la Motte fools Cardinal de Rohan - Queen's Necklace Affair begins
- December 25 - Methodist Episcopal Church in USA officially formed at so-called "Christmas Conference", led by Thomas Coke and Francis Asbury
- The Japanese famine continues as 300,000 die of starvation
- Benjamin Franklin tries in vain to persuade the French to alter their clocks in winter to take advantage of the daylight
- Benjamin Franklin invents bifocal spectacles
- Antoine Lavoisier pioneers quantitative chemistry
- Britain receives its first bales of imported American cotton
- Britain creates the colony of New Brunswick
- Emperor Josef II suspends the Hungarian Constitution because of a Revolution in Transylvania
- Huge locust swarm in South Africa
Births
- January 28 - George Hamilton Gordon, 4th Earl of Aberdeen, Prime Minister of the United Kingdom (d. 1860)
- March 13 - Jean Moufot, French philosopher and mathematician (d. 1842)
- April 5 - Louis Spohr, German violinist and composer (d. 1859)
- April 13 - Friedrich Graf von Wrangel, Prussian field marshal (d. 1877)
- July 22 - Friedrich Bessel, German mathematician and astronomer (d. 1846)
- October 13 - King Ferdinand VII of Spain (d. 1833)
- October 19 - John McLoughlin, Canadian fur trader (d. 1857)
- October - Sarah Biffen, English painter (d. 1850)
- November 24 - Zachary Taylor, 12th President of the United States (d. 1850)
Deaths
- April 26 - Nano Nagle, Irish convent founder (b. 1718)
- May 12 - Abraham Trembley, Swiss naturalist (b. 1710)
- June 13 - Henry Middleton, American president of the Continental Congress (b. 1717)
- June 26 - Caesar Rodney, American lawyer and signer of the Declaration of Independence (d. 1728)
- July 1 - Wilhelm Friedemann Bach, German composer (b. 1710)
- July 31 - Denis Diderot, French philosopher and encyclopedist (b. 1713)
- August 4 - Giovanni Battista Martini, Italian musician (b. 1706)
- August 10 - Allan Ramsay, Scottish painter (b. 1713)
- August 14 - Nathaniel Hone, Irish-born painter (b. 1718)
- August 28 - Junípero Serra, Spanish Franciscan missionary (b. 1713)
- September 4 - César-François Cassini de Thury, French astronomer (b. 1714)
- September 8 - Ann Lee, American religious leader (b. 1736)
- December 4 - Wiseman Claget, English classical scholar (b. 1721)
- December 13 - Samuel Johnson, English writer and lexicographer (b. 1709)
- December 25 - Yosa Buson, Japanese poet and painter (b. 1716)
- December 26 - Seth Warner, American revolutionary leader (b. 1743)
Category:1784
ko:1784년
ms:1784
List of Messier objects
See also
- Messier object
- Wikipedia Project: Astronomical Objects
Category:Messier objects
ja:メシエ天体
Messier marathonMessier marathon is an attempt undertaken by amateur astronomers to find as many Messier objects as possible during one night. The Messier catalogue consists of 110 relatively bright deep space objects (galaxies, nebulae, and star clusters). The number of Messier objects visible in one night varies depending on the season and the location of the observer. At a latitude close to 25 degrees North it is possible to observe all Messier objects in one night. The only time of year that this is possible is during late March or early April. The reason for this is because at other times of year, some of the objects are in the sky only during daylight.
Typically an observer attempting a Messier Marathon will need to begin observing directly at sundown and work until sunrise to view all 110 objects. The objects must be viewed in more or less a prescribed order. This corresponds to the order in which the objects set below the horizon. An observer will start observing objects low in the western sky at sunset, hoping to view them before they disappear below the horizon. Then he or she will work eastward across the sky, viewing objects along the way. By sunrise, the successful observer will be observing the last few objects low on the eastern horizon, hoping to see them before the sky becomes too bright due to the rising sun.
Many astronomy organizations organize a star party to attempt a Messier Marathon every year. Observers are typically given a certificate for their hard work the morning after certifiying how many objects they found. For more information, please see the links below.
External links
- http://www.seds.org/messier/xtra/marathon/marathon.html
Category:Messier objects
Category:Astronomy
Deep sky objectDeep sky object (DSO) is a term used often in amateur astronomy to denote objects in the night sky other than solar system objects (such as planets, comets and asteroids), single stars and multiple star systems. With a few exceptions such as the Andromeda galaxy, these objects are not visible with the naked eye. The brighter ones can be seen with a small telescope or with a good pair of binoculars, and many DSOs can be photographed through small telescopes with extended exposure times. For visual observation in good clarity a larger telescope is required.
binoculars
Types of DSO's:
- Star clusters
- Open clusters
- Globular clusters
- Nebulae
- Bright nebulae
- Emission nebulae
- Reflection nebulae
- Dark nebulae
- Planetary nebulae
- Galaxies
- Quasars
These are classified by the Messier catalogue of 110 objects and the much more comprehensive | | |
