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| Gamma Scuti |
Gamma Scuti
Gamma Scuti (γ Sct / γ Scuti) is a star in the constellation Scutum.
Gamma Scuti is a white A-type subgiant with an apparent magnitude of +4.67. It is approximately 291 light years from Earth.
Scuti, Gamma
Category:Scutum constellation
Category:White subgiants
Star:This article is about celestial bodies.
A star is a massive body of plasma in outer space that is currently producing or has produced energy through nuclear fusion. Unlike a planet, from which most light is reflected, a star emits light because of its intense heat. Scientifically, stars are defined as self-gravitating spheres of plasma in hydrostatic equilibrium, which generate their own energy through the process of nuclear fusion. Small (dwarf) stars such as the Sun generally have essentially featureless disks with only small starspots. Larger (giant) stars have much bigger, much more obvious starspots, and also exhibit strong stellar limb-darkening (the brightness decreases towards the edge of the stellar disk). Stellar astronomy is the study of stars.
Star formation and evolution
Star formation occurs in molecular clouds, large regions of high density in the interstellar medium (though still less dense than the inside of an earthly vacuum chamber). Star formation begins with gravitational instability inside those clouds, often triggered by shockwaves from supernovae or collision of two galaxies (as in a starburst galaxy). High mass stars powerfully illuminate the clouds from which they formed. One example of such a nebula is the Orion Nebula.
Stars spend about 90% of their lifetime fusing hydrogen to produce helium in high-temperature and high-pressure reactions near the core. Such stars are said to be on the main sequence.
Small stars (called red dwarfs) burn their fuel very slowly and last tens to hundreds of billions of years. At the end of their lives, they simply become dimmer and dimmer, fading into black dwarfs. However, since the lifespan of such stars is greater than the current age of the universe (13.6 billion years), no black dwarfs exist yet.
As most stars exhaust their supply of hydrogen, their outer layers expand and cool to form a red giant. In about 5 billion years, when the Sun is a red giant, it will be so large that it will consume both Mercury and Venus. Eventually the core is compressed enough to start helium fusion, and the star heats up and contracts. Larger stars will also fuse heavier elements, all the way to iron, which is the end point of the process. Since iron nuclei are more tightly bound than any heavier nuclei, they cannot be fused to release energy. Likewise, since they are more tightly bound than all lighter nuclei, energy cannot be released by fission. In old, very massive stars, a large core of inert iron will accumulate in the center of the star.
An average-size star will then shed its outer layers as a planetary nebula. The core that remains will be a tiny ball of degenerate matter not massive enough for further fusion to take place, supported only by degeneracy pressure, called a white dwarf. These too will fade into black dwarfs over very long stretches of time.
white dwarf
In larger stars, fusion continues until an iron core accumulates that is too large to be supported by electron degeneracy pressure. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons and neutrinos in a burst of inverse beta decay. The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae are so bright that they may briefly outshine the star's entire home galaxy. When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none existed before. Eventually, most of the matter in a star is blown away by the explosion (forming nebulae such as the Crab Nebula) and what remains will be a neutron star (sometimes a pulsar or X-ray burster) or, in the case of the largest stars, a black hole.
The blown-off outer layers of dying stars include heavy elements which may be recycled during new star formation. These heavy elements allow the formation of rocky planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.
Appearance and distribution of stars
All stars except the Sun appear to the human eye as shining points in the nighttime sky that twinkle because of the effect of the Earth's atmosphere. Interferometer telescopes are required in order to produce images of these objects. The Sun is also a star, but it is close enough to Earth to appear as a disk instead, and to provide daylight.
Stars are not spread uniformly across the universe, but are typically grouped into galaxies. A typical galaxy contains hundreds of billions of stars. The majority of stars are gravitationally bound to other stars, forming binary stars. Larger groups called star clusters also exist.
Astronomers estimate that there are at least 70 sextillion (7×1022) stars in the known universe [http://news.bbc.co.uk/2/hi/science/nature/3085885.stm]. That is 70 000 000 000 000 000 000 000, or 230 billion times as many as the 300 billion in our own Milky Way.
The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion kilometers, or 4.2 light years away (light from Proxima Centauri takes 4.2 years to reach Earth). Travelling at the orbit speed of the Space Shuttle (5 miles per second -- almost 30,000 kilometers per hour), it would take about 150,000 years to get there. Distances like this are typical inside galactic discs, where the Sun and Earth are located. Stars can be much closer to each other in the centres of galaxies and globular clusters, or much further apart in galactic halos.
Age and size of stars
galactic halo
Many stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.7 billion years old, which is the observed age of the universe. (See Big Bang theory and stellar evolution.) They range in size from the tiny neutron stars (which are actually dead stars) no bigger than a city, to supergiants like the North Star (Polaris) and Betelgeuse, in the Orion constellation, which have a diameter about 1,000 times larger than the Sun—about 1.6 billion kilometers. However, these have a much lower density than the Sun.
One of the most massive stars known is η Carinae, with 100–150 times as much mass as the Sun. Recent work by Donald Figer, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland, suggests that 150 solar masses is the upper limit of stars in the current era of the universe. He used the Hubble Space Telescope to observe about a thousand stars in the Arches cluster, a massive young star cluster near the core of the Milky Way, and found no stars over that limit despite a statistical expectation that there should be several. The reason for this limit is not precisely known, but the Eddington limit is part of the answer. The very first stars to form after the Big Bang may have been larger, up to 300 solar masses or more, due to the complete absence of elements heavier than lithium in their composition. This generation of supermassive star is long extinct, however, and currently only theoretical.
With a mass only 93 times that of Jupiter, AB Doradus C, a companion to AB Doradus A, is the smallest known star undergoing nuclear fusion in its core. Smaller bodies are brown dwarfs, which occupy a poorly-defined grey area between stars and gas giants. The minimum mass a star can have is estimated to be in the vicinity of 75 Jupiters.
Star classification
There are different classifications of stars ranging from type W, which are very large and bright, to M, which is often just large enough to start ignition of the hydrogen. Some of the more common classifications are O, B, A, F, G, K, M, and can perhaps be more easily remembered using the mnemonic "Oh, Be A Fine Girl, Kiss Me" (variant: change "girl" to "guy"), invented by Annie Jump Cannon (1863-1941). There are many other mnemonics for star classification; the most frequent addition tacks "Right Now, Sweetheart" for the red dwarf sub-types R, N and S. The new types L and T have also been recently appended to the end of the OBAFGKM sequence to classify the coldest low-mass stars and brown dwarfs, prompting such additions as "Lovingly Tonight" to the mnemonic.
Each letter has 10 subclassifications. Our Sun is a G2, which is very near the middle in terms of quantities observed. Most stars fall into the main sequence which is a description of stars based on their absolute magnitude and spectral type. The Sun is taken as the prototypical star (not because it is special in any way, but because it is the closest and most studied star we have), and most characteristics of other stars are usually given in solar units.
For example, the mass of the Sun is
:MSun = 1.9891×1030 kg
The masses of other stars can be given in terms of MSun.
Naming of stars
Most stars are identified only by catalogue numbers; only a few have names as such.
The names are either traditional names (mostly from Arabic), Flamsteed designations, or Bayer designations. The only body which has been recognized by the scientific community as having competence to name stars or other celestial bodies is the International Astronomical Union (IAU). A number of private companies (e.g. the "International Star Registry") purport to sell names to stars; however, these names are not recognized by the scientific community, nor used by them, and many in the astronomy community view these organizations as frauds preying on people ignorant of how stars are in fact named.
See star designations for more information on how stars are named. For a list of traditional names, see the list of stars by constellation.
Energy production
The energy produced by stars radiates into space as electromagnetic radiation, as a stream of neutrinos from the star's core, and as a stream of particles from the star's outer layers (its stellar wind). The peak frequency of the light depends on the temperature of the outer layers of the star. Besides the emitted visible light, the ultraviolet and infrared components are typically significant. The apparent brightness of a star is measured by its apparent magnitude.
Nuclear fusion reaction pathways
A variety of different nuclear fusion reactions take place inside the cores of stars, depending upon their mass and composition (see Stellar nucleosynthesis).
Stars begin as a cloud of mostly hydrogen with about 25% helium and heavier elements in smaller quantities. In the Sun, with a 107 K core, hydrogen fuses to form helium in the proton-proton chain:
:41H → 22H + 2e+ + 2νe (4.0 MeV + 1.0 MeV)
:21H + 22H → 23He + 2γ (5.5 MeV)
:23He → 4He + 21H (12.9 MeV)
These reactions result in the overall reaction:
:41H → 4He + 2e+ + 2γ + 2νe (26.7 MeV)
In more massive stars, helium is produced in a cycle of reactions catalyzed by carbon, the carbon-nitrogen-oxygen cycle.
In stars with cores at 108 K and masses between 0.5 and 10 solar masses, helium can be transformed into carbon in the triple-alpha process:
:4He + 4He + 92 keV → 8
- Be
:4He + 8
- Be + 67 keV → 12
- C
:12
- C → 12C + γ + 7.4 MeV
For an overall reaction of:
:34He → 12C + γ + 7.2 MeV
Star mythology
As well as certain constellations and the Sun itself, stars as a whole have their own mythology. They were thought to be the souls of the dead, or gods/goddesses.
References
- Cliff Pickover (2001) "The Stars of Heaven", Oxford University Press
- John Gribbin, Mary Gribbin (2001) "Stardust: Supernovae and Life — The Cosmic Connection", Yale University Press.
See also
- Black hole
- Blue straggler
- Overview of star constellations
- Nursery rhyme Twinkle twinkle little star
- sidereal clock
- Star count
- Star clocks
- Stars with articles in Wikipedia
- Stellar navigation
- Stellar evolution
- Timeline of stellar astronomy
- Variable star
Related lists
- List of brightest stars (apparent & absolute magnitude)
- List of heaviest stars (by solar mass)
- List of largest stars (by diameter)
- List of mnemonics for star classification
- List of nearest bright stars
- List of nearest stars
- List of the most important stars
- List of stars by constellation
- List of stars with confirmed extrasolar planets
External links
- [http://www.mrao.cam.ac.uk/telescopes/coast/betel.html Images of starspots on the surface of Betelgeuse]
- [http://simbad.u-strasbg.fr/sim-fid.pl Find out what is known about any given star by entering its name or position]
- [http://www.enchantedlearning.com/subjects/astronomy/stars/startypes.shtml Star Classification]
Category:Astronomical objects
ko:항성
ms:Bintang
ja:恒星
simple:Star
th:ดาวฤกษ์
Scutum
:Other uses:
:
- For the Roman shield, see scutum (shield);
:
- For the zootomical term, see scute.
Scutum (Latin for shield) is a small constellation. It is one of the 88 modern constellations. It was introduced in 1690 by Hevelius. It was originally known as Scutum Sobiescii (Sobieski's Shield) after the Polish king and hero Jan III Sobieski. It is the only constellation that is associated with a modern historical figure.
Notable features
With an area of 109 square degrees, Scutum is the fifth smallest of the 88 modern constellations. It has few bright stars; the brightest star, α Scuti, has a magnitude of 3.85. The Milky Way runs through the constellation, and the Scutum star cloud can be found in the northeastern corner of the constellation.
Notable deep sky objects
Scutum contains several open clusters, as well as a
globular cluster and a planetary nebula.
The two best known deep sky objects in Scutum are
M11 (NGC 6705), the Wild Duck Cluster, a dense open cluster, and
M26, another open cluster also known as NGC 6694.
The globular cluster NGC 6712 and the planetary nebula IC 1295
can be found in the eastern part of the constellation, only 24 arcminutes
apart.
Mythology
Being a modern constellation, Scutum has no mythology associated with it.
It was deigned to represent the shield of Jan III Sobieski.
Notable and named stars
Source: The Bright Star Catalogue, 5th Revised Ed., The Hipparcos Catalogue, ESA SP-1200
See also
External links
- [http://www.allthesky.com/constellations/scutum/ The Deep Photographic Guide to the Constellations: Scutum]
ko:방패자리
ja:たて座
th:กลุ่มดาวโล่
Stellar classificationIn astronomy, stellar classification is a classification of stars based initially on photospheric temperature and its associated spectral characteristics, and subsequently refined in terms of other characteristics. Stellar temperatures can be classified by using Wien's displacement law; but this poses difficulties for distant stars. Stellar spectroscopy offers a way to classify stars according to their absorption lines; particular absorption lines can be observed only for a certain range of temperatures because only in that range are the involved atomic energy levels populated. An early schema (from the 19th century) ranked stars from A to P, which is the origin of the currently used spectral classes.
Morgan-Keenan spectral classification
This stellar classification is the most commonly used. The common classes are normally listed from hottest to coldest, and are:
- 1 = Sun. Values are averages.
Sun]
A popular mnemonic for remembering the order is "Oh Be A Fine Girl, Kiss Me" (there are many variants of this mnemonic). This scheme was developed in the 1900s, by Annie J. Cannon and the Harvard College Observatory. The Hertzsprung-Russell diagram relates stellar classification with absolute magnitude, luminosity, and surface temperature. It should be noted that while these descriptions of stellar colors are traditional in astronomy, they really describe the light after it has been scattered by the atmosphere. The Sun is not in fact a yellow star, but has essentially the color temperature of a black body of 5780 K; this is a white with no trace of yellow which is sometimes used as a definition for standard white.
The reason for the odd arrangement of letters is historical. When people first started taking spectra of stars, they noticed that stars had very different hydrogen spectral lines strengths, and so they classified stars based on the strength of the hydrogen balmer series lines from A (strongest) to Q (weakest). Other lines of neutral and ionized species then came into play (H&K lines of calcium, sodium D lines etc). Later it was found that some of the classes were actually duplicates and those classes were removed. It was only much later that it was discovered that the strength of the hydrogen line was connected with the surface temperature of the star. The basic work was done by the "girls" of Harvard College Observatory, primarily Cannon and Antonia Maury, based on the work of Williamina Fleming. These classes are further subdivided by arabic numbers (0-9). A0 denotes the hottest stars in the A class and A9 denotes the coolest ones. The sun is classified as G2.
Spectral types
- Class O stars are very hot and very luminous, being strongly blue in colour. O-stars shines with a power close to a million times solar. These stars have prominent ionized and neutral helium lines and only weak hydrogen lines. Class O stars emit most of their radiation in ultra-violet.
:Examples: Zeta Puppis, Epsilon Orionis
- Class B stars are again extremely luminous. Their spectra have neutral helium and moderate hydrogen lines. As O and B stars are so powerful, they live for a very short time. They do not stray far from the area in which they were formed as they don't have the time. They therefore tend to cluster together in what we call OB1 associations, which are associated with giant molecular clouds. The Orion OB1 association is an entire spiral arm of our Galaxy (brighter stars make the spiral arms look brighter, there aren't more stars there) and contains all of the constellation of Orion.
:Examples: Rigel, Spica
- Class A stars are amongst the more common naked eye stars. As with all class A stars, they are white. Many white dwarfs are also A. They have strong hydrogen lines and also ionized metals.
:Examples: Vega, Sirius
- Class F stars are still quite powerful but they tend to be main sequence stars. Their spectra is characterized by the weaker hydrogen lines and ionized metals, their colour is white with a slight tinge of yellow.
:Examples: Canopus, Procyon
- Class G stars are probably the most well known if only for the reason that our Sun is of this class. They have even weaker hydrogen lines than F but along with the ionized metals, they have neutral metals. G is host to the "Yellow Evolutionary Void". Supergiant stars often swing between O or B (blue) and K or M (red). While they do this, they do not stay for long in the G classification as this is an extremely unstable place for a supergiant to be.
:Examples: Sun, Capella
- Class K are orangish stars which are slightly cooler than our Sun. Some K stars are giants and supergiants, such as Arcturus while others like Alpha Centauri B are main sequence stars. They have extremely weak hydrogen lines, if they are present at all, and mostly neutral metals.
:Examples: Arcturus, Aldebaran
- Class M is by far the most common class if we go by the number of stars. All our red dwarfs go in here and they are plentiful; more than 90% of stars are red dwarfs, such as Proxima Centauri. M is also host to most giants and some supergiants such as Antares and Betelgeuse, as well as Mira variables. The spectrum of an M star shows lines belonging to molecules and neutral metals but hydrogen is usually absent. Titanium oxide can be strong in M stars. The red color is deceptive; it is because of the dimness of the star. When an equally hot object, a halogen lamp (3000 K) which is white hot is put at a few kilometers distance, it appears like a red star.
:Examples: Betelgeuse, Barnard's star
Spectral types for rare stars
A number of new spectral types have been taken into use for rare types of stars, as they have been discovered:
- W: Up to 70,000 K - Wolf-Rayet stars.
- L: 1,500 - 2,000 K - Stars with masses insufficient to run the regular hydrogen fusion process (brown dwarfs). Class L stars contain lithium which is rapidly destroyed in hotter stars.
- T: 1,000 K - Cooler brown dwarfs with methane in the spectrum.
- C: Carbon stars.
:
- R: Formerly a class on its own representing the carbon star equivalent of Class K stars, e.g. S Camelopardalis.
:
- N: Formerly a class on its own representing the carbon star equivalent of Class M stars, e.g. R Leporis.
- S: Similar to Class M stars, but with zirconium oxide replacing the regular titanium oxide.
- D: White dwarfs, e.g. Sirius B.
Class W represents the superluminous Wolf-Rayet stars, being notably different since they have mostly helium instead of hydrogen. They are thought to be dying supergiants with their hydrogen layer blown away by hot stellar winds caused by their high temperatures, thereby directly exposing their hot helium shell. Class W is subdivided into subclasses WN and WC according to the dominance of nitrogen or carbon in their spectra (and outer layers).
Class L stars get their designation from the lithium present in their core. Any lithium would be destroyed in ongoing nuclear reactions in regular stars, which indicates that these objects have no ongoing fusion processes. They are a very dark red in colour and brightest in infrared. Their gas is cool enough to allow metal hydrides and alkali metals to be prominent in the spectrum.
Class T stars are very young and low density stars often found in the interstellar clouds they were born in. These are stars barely big enough to be stars and others that are substellar, being of the brown dwarf variety. They are black, emitting little or no visible light but being strongest in infrared. Their surface temperature is a stark contrast to the fifty thousand degrees or more for Class O stars, being merely up to 1,000 K. Complex molecules can form, evidenced by the strong methane lines in their spectra.
Class T and L could be more common than all the other classes combined, if recent research is accurate. From studying the number of proplyds (protoplanetary discs, clumps of gas in nebulae from which stars and solar systems are formed) then the number of stars in the galaxy should be several orders of magnitude higher than what we know about. It’s theorised that these proplyds are in a race with each other. The first one to form will become a proto-star, which are very violent objects and will disrupt other propylids in the vicinity, stripping them of their gas. The victim propylids will then probably go on to become main sequence stars or brown dwarf stars of the L and T classes, but quite invisible to us. Since they live so long (no star below 0.8 solar masses has ever died in the history of the galaxy) then these smaller stars will accumulate over time.
Class R and N stars are carbon stars (red giants thought to reach the end of their life) which run parallel to the normal classification system from roughly mid G to late M. These have more recently been remapped into a unified carbon classifier C, with N0 starting at roughly C6.
Class S stars have ZrO lines rather than TiO, and are in between the Class M stars and the carbon stars. Class S stars have their carbon and oxygen abundances almost exactly equal, and both elements are locked up almost entirely in CO molecules. For stars cool enough for CO to form that molecule tends to "eat up" all of whichever element is less abundant, resulting in "leftover oxygen" on the normal main sequence, "leftover carbon" on the C sequence, and "leftover nothing" on the S sequence.
In reality the relation between these stars and the traditional main sequence suggest a rather large continuum of carbon abundance and if fully explored would add another dimension to the stellar classification system.
Finally, the classes P and Q are occasionally used for certain non-stellar objects. Type P objects are planetary nebulae and type Q objects are novae.
White dwarf classifications
The class D is sometimes used for white dwarfs, the state most stars end their life in. Class D is further divided into classes DA, DB, DC, DO, DZ, and DQ. Note the letters are not related to the letters used in the classification of true stars, but instead indicate the composition of the white dwarf's outer layer or "atmosphere".
The white dwarf classes are as follows:
- DA: a hydrogen-rich "atmosphere" or outer layer, indicated by strong Balmer hydrogen spectral lines.
- DB: a helium-rich "atmosphere" or outer layer, indicated by neutral helium spectral lines.
- DQ: a carbon-rich "atmosphere" or outer layer, indicated by atomic or molecular carbon lines.
- DZ: a 'metal'-rich "atmosphere" or outer layer, indicated by calcium II lines.
- DC: no strong spectral lines indicating one of the above categories.
- DX: spectral lines are insufficiently clear to classify into one of the above categories.
All class D stars use the same sequence from 1 to 9, with 1 indicating a temperature above 37,500 K and 9 indicating a temperature below 5,500 K. [http://www.physics.uq.edu.au/people/ross/ph3080/whitey.htm]
Yerkes spectral classification
The Yerkes spectral classification, also called the MKK system from the authors' initials, is a system of stellar spectral classification introduced in 1943 by William Wilson Morgan, Phillip C. Keenan and Edith Kellman of Yerkes Observatory.
This classification is based on spectral lines sensitive to stellar surface gravity which is related to luminosity, as opposed to the Harvard classification which is based on surface temperature.
Since the radius of a giant star is much larger than a dwarf star while their masses are roughly comparable, the gravity and thus the gas density and pressure on the surface of a giant star are much lower than for a dwarf.
These differences manifest themselves in the form of luminosity effects which affect both the width and the intensity of spectral lines which can then be measured.
A number of different luminosity classes are distinguished:
- 0 hypergiants (later addition);
- Ia most luminous supergiants;
- Ib less luminous supergiants;
- II bright giants;
- III normal giants;
- IV subgiants;
- V main sequence stars (dwarfs);
- VI subdwarfs (rarely used);
- VII white dwarfs (rarely used)
Marginal cases are allowed; for instance a star classified as Ia-0 would be a very luminous supergiant, verging on hypergiant.
UBV system
The UBV system, also called the Johnson system, is a photometric system for classifying stars according to their magnitude. The letters U, B, and V stand for ultraviolet, blue, and visual magnitudes, which are measured for a star in order to classify it in the UBV system. The choice of colors on the blue end of the spectrum is because of the bias that photographic film has for those colors. It was introduced in the 1950s by American astronomers Harold Lester Johnson and William Wilson Morgan.
External links
- [http://www.twcac.org/Tutorials/spectral_classes.htm www.twcac.org/Tutorials/spectral_classes.htm]
- [http://www.ucm.es/info/Astrof/invest/actividad/spectra.html Libraries of stellar spectra, D. Montes, UCM]
Classification
ja:スペクトル分類
SubgiantSubgiant star is a class of stars that are brighter than normal main sequence (dwarf) stars, but not as bright as true giant stars. They are believed to be stars that are ceasing or already ceased fusing hydrogen in their cores. In stars of roughly a solar mass, this causes the core to contract, which increases the star's central temperature enough to move hydrogen fusion into a shell surrounding the core. This swells the star on the way to becoming a true giant; however, luminosity typically does not increase while the star is on the subgiant branch. Subgiant stars have larger diameters and lower temperature than stars of similar mass in the main sequence. In Yerkes spectral classification their luminosity class is IV.
Category:Star types
-
Apparent magnitudeThe apparent magnitude (m) of a star, planet or other heavenly body is a measure of its apparent brightness. The brighter the object appears the lower its value.
The scale upon which magnitude is measured has its origin in the Hellenistic practice of dividing those stars visible to the naked eye into six magnitudes. The brightest stars were said to be of first magnitude (m = 1), while the faintest were of sixth magnitude (m = 6), the limit of human visual perception (without the aid of a telescope). Each grade of magnitude was considered to be twice the brightness of the following grade. This somewhat crude method of indicating the brightness of stars was popularized by Ptolemy in his Almagest, and is generally believed to have originated with Hipparchus. This original system did not measure the magnitude of the Sun. Because the response of the eye to
light is logarithmic, the resulting scale is also logarithmic.
In 1856, Pogson formalized the system by defining a typical first magnitude star as a star which is 100 times brighter than a typical sixth magnitude star; thus, a first magnitude star is about 2.512 times brighter than a second magnitude star. The fifth root of 100, an irrational number about (2.512) is known as Pogson's Ratio. Pogson's scale was originally fixed by assigning Polaris a magnitude of 2. Astronomers later discovered that Polaris is slightly variable, so they first switched to Vega as the standard reference star, and then switched to using tabulated zero points for the measured fluxes (see second Reference below). The magnitude depends on the wavelength band (see below).
The modern system is no longer limited to 6 magnitudes or only to visible light. Really bright objects have negative magnitudes. For example, Sirius, the brightest star of the celestial sphere, has an apparent magnitude of −1.44 to −1.46. The modern scale includes the Moon and the Sun; the Moon has an apparent magnitude of −12.6 and the Sun has an apparent magnitude of −26.8. Hubble has located stars with magnitudes of 30 at visible wavelengths and the Keck telescopes have located similarly faint stars in the infrared.
These are only approximate values at visible wavelengths (in reality the values depend on the precise bandpass used) — see airglow for more details of telescope sensitivity.
As the amount of light received actually depends on the thickness of the atmosphere in the line of sight to the object, the apparent magnitudes are normalized to the value it would have outside the atmosphere. The dimmer an object appears, the higher its apparent magnitude. Note that apparent brightness is not equal to actual brightness — an extremely bright object may appear quite dim, if it is far away. The rate at which apparent brightness changes, as the distance from an object increases, is calculated by the inverse-square law (at cosmological distance scales, this is no longer quite true because of the curvature of space). The absolute magnitude, M, of a star or galaxy is the apparent magnitude it would have if it were 10 parsecs away; that of a planet (or other solar system body) is the apparent magnitude it would have if it were 1 astronomical unit away from both the Sun and Earth. The absolute magnitude of the Sun is 4.83 in the V band (yellow) and 5.48 in the B band (blue).
The apparent magnitude in the band x can be defined as
:
where is the observed flux in the band x,
and is a constant that depends on the units of the flux and the band. The constant is defined in Aller et al 1982 for the most commonly used system.
The second thing to notice is that the scale is logarithmic: the relative brightness of two objects is determined by the difference of their magnitudes. For example, a difference of 3.2 means that one object is about 19 times as bright as the other, because Pogson's ratio raised to the power 3.2 is 19.054607...
The logarithmic nature of the scale is due to the fact of the human eye itself having a logarithmic response, see Weber-Fechner law.
Magnitude is complicated by the fact that light is not monochromatic. The sensitivity of a light detector varies according to the wavelength of the light, and the way in which it varies depends on the type of light detector. For this reason, it is necessary to specify how the magnitude is measured in order for the value to be meaningful. For this purpose the UBV system is widely used, in which the magnitude is measured in three different wavelength bands: U (centred at about 350 nm, in the near ultraviolet), B (about 435 nm, in the blue region) and V (about 555 nm, in the middle of the human visual range in daylight). The V band was chosen for spectral purposes and gives magnitudes closely corresponding to those seen by the light-adapted human eye, and when an apparent magnitude is given without any further qualification, it is usually the V magnitude that is meant, more or less the same as visual magnitude.
Since cooler stars, such as red giants and red dwarfs, emit little energy in the blue and UV regions of the spectrum their power is often under-represented by the UBV scale. Indeed, some L and T class stars have an estimated magnitude of well over 100, since they emit extremely little visible light, but are strongest in infrared.
Measures of magnitude need cautious treatment and it is extremely important to measure like with like. On early 20th-century and older orthochromatic (blue-sensitive) photographic film, the relative brightnesses of the blue supergiant Rigel and the red supergiant Betelgeuse irregular variable star (at maximum) are reversed compared to what our eyes see since this archaic film is more sensitive to blue light than it is to red light. Magnitudes obtained from this method are known as photographic magnitudes, and are now considered obsolete.
For a objects within our Galaxy with a given absolute magnitude, 5 is added to the apparent magnitude for every tenfold increase in the distance to the object. This relationship does not apply for objects at very great distances (far beyond our galaxy) as a correction for General Relativity must be taken into account due to the non-Euclidean nature of space.
See also
- Absolute magnitude
- List of Brightest stars
- List of nearest bright stars
- List of nearest stars
References
- [http://articles.adsabs.harvard.edu//full/1857MNRAS..18...47P Magnitudes of Thirty-six of the Minor Planets for the first day of each month of the year 1857], N. Pogson, MNRAS 17 pp 12 1856 -- in which Pogson first introduced his magnitude system
- [http://ukads.nottingham.ac.uk/cgi-bin/nph-bib_query?bibcode=1982lbor.book.....A&db_key=AST Landolt-Börnstein: Numerical Data and Functional Relationships in Science and Technology - New Series " Gruppe/Group 6 Astronomy and Astrophysics " Volume 2 Schaifers/Voigt: Astronomy and Astrophysics / Astronomie und Astrophysik " Stars and Star Clusters / Sterne und Sternhaufen] Aller, L. H. et al, ISBN # 3-540-10976-5; 0-387-10976-5 1982 -- modern definition of the zero point for the most common magnitude system
External links
- [http://www.astronomynotes.com/starprop/s4.htm The Magnitude system]
- [http://csep10.phys.utk.edu/astr162/lect/stars/magnitudes.html About stellar magnitudes]
- [http://simbad.u-strasbg.fr/sim-fid.pl Obtain the magnitude of any star from SIMBAD]
Category:Observational astronomy
als:Scheinbare Helligkeit
ko:겉보기 등급
ja:等級 (天文)
th:โชติมาตรปรากฏ
Light-years:There was also a 1988 animated science fiction film named "Light Years".
A light year (or light-year), abbreviated ly, is the distance light travels in one year: about 9.461 × 1015 metres (9.461 petametres), or about 5.879 × 1012 (nearly six trillion) miles. More specifically, a light year is defined as the distance that a photon would travel, in free space and infinitely distant from any gravitational or magnetic fields, in one Julian year (365.25 days of 86,400 seconds each). Since the speed of light in a vacuum is exactly 299,792,458 m/s by the definition of metre, one light year is exactly equal to 9,460,730,472,580,800 m.
The light year is often used to measure distances to stars: a light year is not a unit of time. In astronomy, the preferred unit of measurement for such distances is the parsec which is defined as the distance at which an object will generate one arcsecond of parallax when the observing object moved one astronomical unit perpendicular to the line of sight to the observer. This is equal to approximately 3.26 light years. The parsec is preferred because it can be more easily derived from, and inter-compared with, observational data. However, outside scientific circles, the term light year is more widely used by the general public.
A light year is also equal to about 63,241.077 astronomical units (AU). For a list of lengths on the order of one light year, see the article 1 E15 m.
Units related to the light year are the light minute and light second, the distance light travels in a vacuum in one minute and one second, respectively. A light minute is equal to 17,987,547,480 m. Since light travels 299,792,458 m in one second, a light second is 299,792,458 m in length.
Miscellaneous facts
- It takes 8.3 minutes for light to travel from the Sun to the Earth (a distance of light years).
- The most distant space probe, Voyager 1, was 13 light hours (only light years) away from Earth in September 2004. It took Voyager 27 years to cover that distance.
- The nearest known star, Proxima Centauri is 4.22 light years away.
- The center of our galaxy, the Milky Way, is about 28,000 light years away. The Galaxy is about 100,000 light years across.
- The nearest large galaxy cluster, the Virgo Cluster, is about 60 million light years away.
- The particle horizon (observable part) of the universe has a radius of about 46 billion light years, but light from the edge of the observable universe was emitted only 13.7 billion years ago (the age of the universe). The figures differ because distant objects have continued to recede from us due to cosmological expansion (see Hubble's law).
- One gigaparsec is equal to approximately 3.2 billion light years.
See also
- Conversion of units
- Orders of magnitude (length)
External link
- [http://www.ex.ac.uk/trol/scol/ccleng.htm Conversion Calculator for Units of LENGTH]
Category:Astronomical units of length
ko:광년
ms:Tahun cahaya
ja:光年
simple:Light year
th:ปีแสง
Category:Bayer objectsJohann Bayer originated a method of systematically naming stars, now called the Bayer designation, in his star atlas Uranometria.
:Help keep this category in order, modify Category:Bayer objects to genitive, designator on pages that categorize here.
Category:Astronomical catalogues
Category:White subgiantsWhite subgiants are subgiant stars of spectral type A.
Category:Subgiant stars
Category:Type-A stars
Matthias LejaMatthias Leja (
- 1962 in Lüneburg) ist ein deutscher Schauspieler.
Leja studierte anfangs Philosophie und Theaterwissenschaft in München, wechselte ab 1983 für 2 Jahre an der Hochschule für Musik und Darstellende Kunst in Hamburg und studierte dort Schauspiel. Währenddessen war er als Gast am Schauspiel Frankfurt (Fortinbras in Hamlet) engagiert.
1995 wird er von Leander Haußmann für das Schauspielhaus Bochum engagiert. Mit Dimiter Gotscheff entstehen Amphitrion, Die Zimmerschlacht, Der zerbrochne Krug und Sechs Personen suchen einen Autor. Bei Haußmann spielt er in Germania 3, unter Karin Henkels Regie in "Eines langen Tages Reise in die Nacht".
Leja hat auch in zahlreichen TV-Produktionen gespielt, unteranderem spielt er ab 1996 den Hubschrauberpiloten eines Rettungshubschraubers in der ZDF-Serie "Die Rettungsflieger".
Weblinks
-
Leja, Matthias
Leja, Matthias
Leja, Matthias
Leja, Matthias
Leja, Matthias
Randki cytaty Skrty angielskie praca w anglii Architekci wntrz
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Thermochromism is the ability of a substance to change colour due to a change in temperature. A mood ring is an excellent example of this, but it has many other uses. Thermochromism is one of several types of chromism.
__NOTOC__
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