Tuesday, April 15, 2008
Red dwarf + funny monkey
According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type. They constitute the vast majority of stars and have a mass of less than one-half that of the Sun (down to about 0.075 solar masses, which are brown dwarfs) and a surface temperature of less than 3,500 K.
Brown Dwarf-History-.-
Brown dwarfs, a term coined by Jill Tarter in 1975, were originally called black dwarfs, a classification for dark substellar objects floating freely in space which were too low in mass to sustain stable hydrogen fusion (the term black dwarf currently refers to a white dwarf that has cooled down so that it no longer emits heat or light). Alternative names have been proposed, including Planetar and Substar.
Early theories concerning the nature of the lowest mass stars and the hydrogen burning limit suggested that objects with a mass less than 0.07 solar masses for Population I objects or objects with a mass less than 0.09 solar masses for Population II objects would never go through normal stellar evolution and would become a completely degenerate star (Kumar 1963). The role of deuterium-burning down to 0.012 solar masses and the impact of dust formation in the cool outer atmospheres of brown dwarfs was understood by the late eighties. They would however be hard to find in the sky, as they would emit almost no light. Their strongest emissions would be in the infrared (IR) spectrum, and ground-based IR detectors were too imprecise at that time to readily identify any brown dwarfs.
Since those earlier times, numerous searches involving various methods have been conducted to find these objects. Some of those methods included multi-color imaging surveys around field stars, imaging surveys for faint companions to main sequence dwarfs and white dwarfs, surveys of young star clusters and radial velocity monitoring for close companions.
For many years, efforts to discover brown dwarfs were frustrating and searches to find them seemed fruitless. In 1988, however, University of California at Los Angeles professors Eric Becklin and Ben Zuckerman identified a faint companion to GD 165 in an infrared search of white dwarfs. The spectrum of GD 165B was very red and enigmatic, showing none of the features expected of a low-mass red dwarf star. It became clear that GD 165B would need to be classified as a much cooler object than the latest M dwarfs then known. GD 165B remained unique for almost a decade until the advent of the Two Micron All Sky Survey (2MASS) when Davy Kirkpatrick, out of the California Institute of Technology, and others discovered many objects with similar colors and spectral features.
Today, GD 165B is recognized as the prototype of a class of objects now called "L dwarfs". While the discovery of the coolest dwarf was highly significant at the time, it was debated whether GD 165B would be classified as a brown dwarf or simply a very low mass star, since observationally, it is very difficult to distinguish between the two.
Interestingly, soon after the discovery of GD 165B other brown dwarf candidates were reported. Most failed to live up to their candidacy however, and with further checks for substellar nature, such as the lithium test, many turned out to be stellar objects and not true brown dwarfs. When young (up to a gigayear old), brown dwarfs can have temperatures and luminosities similar to some stars, so other distinguishing characteristics are necessary, such as the presence of lithium. Stars will burn lithium in a little over 100 Myr, at most, while most brown dwarfs will never acquire high enough core temperatures to do so. Thus, the detection of lithium in the atmosphere of a candidate object ensures its status as a brown dwarf.
In 1995 the study of brown dwarfs changed dramatically with the discovery of three incontrovertible substellar objects, some of which were identified by the presence of the 6708 Li line. The most notable of these objects was Gliese 229B which was found to have a temperature and luminosity well below the stellar range. Remarkably, its near-infrared spectrum clearly exhibited a methane absorption band at 2 micrometres, a feature that had previously only been observed in gas giant atmospheres and the atmosphere of Saturn's moon, Titan. Methane absorption is not expected at the temperatures of main-sequence stars. This discovery helped to establish yet another spectral class even cooler than L dwarfs known as "T dwarfs" for which Gl 229B is the prototype.
Since 1995, when the first brown dwarf was confirmed, hundreds have been identified. Brown dwarfs close to Earth include Epsilon Indi Ba and Bb, a pair of dwarfs gravitationally bound to a sunlike star, around 12 light-years from the Sun.
Brown Dwarf !!!
Brown dwarfs are sub-stellar objects with a mass below that necessary to maintain hydrogen-burning nuclear fusion reactions in their cores, as do stars on the main sequence, but which have fully convective surfaces and interiors, with no chemical differentiation by depth. Brown dwarfs occupy the mass range between that of large gas giant planets and the lowest mass stars; this upper limit is between 75and 80 Jupiter masses (MJ).Currently there is some debate as to what criterion to use to define the separation between a brown dwarf from a giant planet at very low brown dwarf masses (~13 MJ ),and whether brown dwarfs are required to have experienced fusion at some point in their history. In any event, brown dwarfs heavier than 13 MJ do fuse deuterium and those above ~65 MJ also fuse lithium. The only planet known to orbit a brown dwarf is 2M1207b.
White Dwarf >>> Black Dwarf
A black dwarf is a hypothetical star,created when a white dwarf becomes sufficiently cool to no longer emit significant heat or light.Since the time required for a white dwarf to reach this state is calculated to be longer than the current age of the universe,13.7 billion years, no black dwarfs are expected to exist in the Universe yet, and the temperature of the coolest white dwarfs is one observational limit on the age of the universe.
A white dwarf is what remains of a main sequence star of low or medium mass (below approximately 9 to 10 solar masses),after it has either expelled or fused all the elements which it has sufficient temperature to fuse.What is left is then a dense piece of electron-degenerate matter which cools slowly by thermal radiation, eventually becoming a black dwarf.If black dwarfs were to exist, they would be extremely difficult to detect, since, by definition, they would emit very little,if any, radiation. One theory is that they might be detectable through their gravitational influence.
Since the far-future evolution of white dwarfs depends on physical questions,such as the nature of dark matter and the possibility and rate of proton decay, which are poorly understood, it is not known precisely how long it will take white dwarfs to cool to blackness.IIIE,IVA.Barrow and Tipler estimate that it would take 1015 years for a white dwarf to cool to 5 K;however, if weakly interacting massive particles exist, it is possible that interactions with these particles will keep some white dwarfs much warmer than this for approximately 1025 years.IIIE.If the proton is not stable, white dwarfs will also be kept warm by energy released from proton decay. For a hypothetical proton lifetime of 1037 years, Adams and Laughlin calculate that proton decay will raise the effective surface temperature of an old one-solar mass white dwarf to approximately 0.06 K. Although cold, this is thought to be hotter than the temperature that the cosmic background radiation will have 1037 years in the future.IVB.
The name black dwarf has also been applied to sub-stellar objects which do not have sufficient mass, approximately 0.08 solar masses, to maintain hydrogen-burning nuclear fusion.These objects are now generally called brown dwarfs, a term coined in the 1970s.Also, black dwarfs should not be confused with black holes or neutron stars.
Friday, April 11, 2008
Formation of the White Dwarf
Formation
White dwarfs are thought to represent the end point of stellar evolution for main-sequence stars with masses from about 0.07 to 10 solar masses.The composition of the white dwarf produced will differ depending on the initial mass of the star.
Stars with very low mass
If the mass of a main-sequence star is lower than approximately half a solar mass, it will never become hot enough to fuse helium at its core. It is thought that, over a lifespan exceeding the age (~13.7 billion years)of the Universe, such a star will eventually burn all its hydrogen and end its evolution as a helium white dwarf composed chiefly of helium-4 nuclei. Owing to the time this process takes, it is not thought to be the origin of observed helium white dwarfs. Rather, they are thought to be the product of mass loss in binary systems or mass loss due to a large planetary companion.
Stars with low to medium mass
If the mass of a main-sequence star is between approximately 0.5 and 8 solar masses, its core will become sufficiently hot to fuse helium into carbon and oxygen via the triple-alpha process, but it will never become sufficiently hot to fuse carbon into neon. Near the end of the period in which it undergoes fusion reactions, such a star will have a carbon-oxygen core which does not undergo fusion reactions, surrounded by an inner helium-burning shell and an outer hydrogen-burning shell. On the Hertzsprung-Russell diagram, it will be found on the asymptotic giant branch. It will then expel most of its outer material, creating a planetary nebula, until only the carbon-oxygen core is left. This process is responsible for the carbon-oxygen white dwarfs which form the vast majority of observed white dwarfs.
Stars with medium to high mass
If a star is sufficiently massive, its core will eventually become sufficiently hot to fuse carbon to neon, and then to fuse neon to iron. Such a star will not become a white dwarf as the mass of its central, non-fusing, core, supported by electron degeneracy pressure, will eventually exceed the largest possible mass supportable by degeneracy pressure. At this point the core of the star will collapse and it will explode in a core-collapse supernova which will leave behind a remnant neutron star, black hole, or possibly a more exotic form of compact star.Some main-sequence stars, of perhaps 8 to 10 solar masses, although sufficiently massive to fuse carbon to neon and magnesium, may be insufficiently massive to fuse neon. Such a star may leave a remnant white dwarf composed chiefly of oxygen, neon, and magnesium, provided that its core does not collapse, and provided that fusion does not proceed so violently as to blow apart the star in a supernova.[97][98] Although some isolated white dwarfs have been identified which may be of this type, most evidence for the existence of such stars comes from the novae called ONeMg or neon novae. The spectra of these novae exhibit abundances of neon, magnesium, and other intermediate-mass elements which appear to be only explicable by the accretion of material onto an oxygen-neon-magnesium white dwarf.
Fate
A white dwarf is stable once formed and will continue to cool almost indefinitely; eventually, it will become a black white dwarf, also called a black dwarf. Assuming that the Universe continues to expand, it is thought that in 1019 to 1020 years, the galaxies will evaporate as their stars escape into intergalactic space.§IIIA. White dwarfs should generally survive this, although an occasional collision between white dwarfs may produce a new fusing star or a super-Chandrasekhar mass white dwarf which will explode in a type Ia supernova.§IIIC, IV. The subsequent lifetime of white dwarfs is thought to be on the order of the lifetime of the proton, known to be at least 1032 years. Some simple grand unified theories predict a proton lifetime of no more than 1049 years. If these theories are not valid, the proton may decay by more complicated nuclear processes, or by quantum gravitational processes involving a virtual black hole; in these cases, the lifetime is estimated to be no more than 10200 years. If protons do decay, the mass of a white dwarf will decrease very slowly with time as its nuclei decay, until it loses so much mass as to become a nondegenerate lump of matter, and finally disappears completely.§IV.
Stellar system
A white dwarf's stellar and planetary system is inherited from its progenitor star and may interact with the white dwarf in various ways. Infrared spectroscopic observations made by NASA's Spitzer Space Telescope of the central star of the Helix Nebula suggest the presence of a dust cloud, which may be caused by cometary collisions. It is possible that infalling material from this may cause X-ray emission from the central star.[102][103] Similarly, observations made in 2004 indicated the presence of a dust cloud around the young white dwarf star G29-38 (estimated to have formed from its AGB progenitor about 500 million years ago), which may have been created by tidal disruption of a comet passing close to the white dwarf.[104] If a white dwarf is in a binary system with a stellar companion, a variety of phenomena may occur, including novae and Type Ia supernovae.
The Combined XSC and PSC Allsky
2MASS Local Universe: As seen in the 2MASS Near-Infrared bands: J (1.2 microns), H (1.6 microns) and Ks (2.2 microns). The All Sky image is composed of sources with integrated fluxes brighter than Ks=14th mag, comprising the 2MASS Extended Source Catalog (XSC) -- more than 1.6 million galaxies, and the Point Source Catalog (PSC) -- nearly 0.5 billion Milky Way stars (here tinted in blue to show contrast with the background galaxies.) The map is projected with an equal area aitoff in the Geo-equatorial system (centered at 6 hr Right Ascension). The plane of the Milky Way runs diagonally across the image, with the Galactic anti-center facing you.
Note: Known Milky Way sources (e.g., HII regions) have been removed from the XSC ALLSKY image, and thus the remaining XSC image is nearly 100% extragalactic in nature. Galactic (Milky Way) sources are primarily confined to the Plane of the Galaxy, as is easily seen in the XSC vs. PSC movie shown below.
SuperGalactic View of the Local Universe
2MASS Integrated Flux Allsky Movie
Aitoff equal-area projection of the integrated PSC and XSC flux for cummulative mag bins. See the LSS chart for image orientation and Large Scale Structure map. The PSC FITS images are courtesy of John Carpenter.
Note: Known Milky Way sources (e.g., HII regions) have been removed from the XSC ALLSKY image, and thus the remaining XSC image is nearly 100% extragalactic in nature. Galactic (Milky Way) sources are primarily confined to the Plane of the Galaxy, as is easily seen in the XSC vs. PSC movie shown below.
SuperGalactic View of the Local Universe
2MASS Integrated Flux Allsky Movie
Aitoff equal-area projection of the integrated PSC and XSC flux for cummulative mag bins. See the LSS chart for image orientation and Large Scale Structure map. The PSC FITS images are courtesy of John Carpenter.
Radiation and cooling
The visible radiation emitted by white dwarfs varies over a wide color range, from the blue-white color of an O-type main sequence star to the red of a M-type red dwarf.White dwarf effective surface temperatures extend from over 150,000 K to under 4,000 K.In accordance with the Stefan-Boltzmann law, luminosity increases with increasing surface temperature; this surface temperature range corresponds to a luminosity from over 100 times the Sun's to under 1/10,000th that of the Sun's.Hot white dwarfs, with surface temperatures in excess of 30,000 K, have been observed to be sources of soft (i.e., lower-energy) X-rays. This enables the composition and structure of their atmospheres to be studied by soft X-ray and extreme ultraviolet observations.
A comparison between the white dwarf IK Pegasi B (center), its A-class companion IK Pegasi A (left) and the Sun (right). This white dwarf has a surface temperature of 35,500 K.Unless the white dwarf accretes matter from a companion star or other source, this radiation comes from its stored heat, which is not replenished. White dwarfs have an extremely small surface area to radiate this heat from, so they remain hot for a long time.As a white dwarf cools, its surface temperature decreases, the radiation which it emits reddens, and its luminosity decreases. Since the white dwarf has no energy sink other than radiation, it follows that its cooling slows with time. Bergeron, Ruiz, and Leggett, for example, estimate that after a carbon white dwarf of 0.59 solar mass with a hydrogen atmosphere has cooled to a surface temperature of 7,140 K, taking approximately 1.5 billion years, cooling approximately 500 more kelvins to 6,590 K takes around 0.3 billion years, but the next two steps of around 500 kelvins (to 6,030 K and 5,550 K) take first 0.4 and then 1.1 billion years.Although white dwarf material is initially plasma—a fluid composed of nuclei and electrons—it was theoretically predicted in the 1960s that at a late stage of cooling, it should crystallize, starting at the center of the star.The crystal structure is thought to be a body-centered cubic lattice.In 1995 it was pointed out that asteroseismological observations of pulsating white dwarfs yielded a potential test of the crystallization theory and in 2004, Travis Metcalfe and a team of researchers at the Harvard-Smithsonian Center for Astrophysics estimated, on the basis of such observations, that approximately 90% of the mass of BPM 37093 had crystallized.Other work gives a crystallized mass fraction of between 32% and 82%.
Most observed white dwarfs have relatively high surface temperatures, between 8,000 K and 40,000 K. A white dwarf, though, spends more of its lifetime at cooler temperatures than at hotter temperatures, so we should expect that there are more cool white dwarfs than hot white dwarfs. Once we adjust for the selection effect that hotter, more luminous white dwarfs are easier to observe, we do find that decreasing the temperature range examined results in finding more white dwarfs. This trend stops when we reach extremely cool white dwarfs; few white dwarfs are observed with surface temperatures below 4,000 K,and one of the coolest so far observed, WD 0346+246, has a surface temperature of approximately 3,900 K.The reason for this is that, as the Universe's age is finite,there has not been time for white dwarfs to cool down below this temperature. The white dwarf luminosity function can therefore be used to find the time when stars started to form in a region; an estimate for the age of the Galactic disk found in this way is 8 billion years.
A white dwarf will eventually cool and become a non-radiating black dwarf in approximate thermal equilibrium with its surroundings and with the cosmic background radiation. However, no black dwarfs are thought to exist yet.
Outer space-White dwarf
White dwarf
Image of Sirius A and Sirius B taken by the Hubble Space Telescope. Sirius B, which is a white dwarf, can be seen as a faint dot to the lower left of the much brighter Sirius A.A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. As white dwarfs have mass comparable to the Sun's and their volume is comparable to the Earth's, they are very dense. Their faint luminosity comes from the emission of stored heat.[1] They comprise roughly 6% of all known stars in the solar neighborhood.[2] The unusual faintness of white dwarfs was first recognized in 1910 by Henry Norris Russell, Edward Charles Pickering and Williamina Fleming;[3], p. 1 the name white dwarf was coined by Willem Luyten in 1922.[4]
White dwarfs are thought to be the final evolutionary state of all stars whose mass is not too high—over 97% of the stars in our Galaxy.[5], §1. After the hydrogen-fusing lifetime of a main-sequence star of low or medium mass ends, it will expand to a red giant which fuses helium to carbon and oxygen in its core by the triple-alpha process. If a red giant has insufficient mass to generate the core temperatures required to fuse carbon, an inert mass of carbon and oxygen will build up at its center. After shedding its outer layers to form a planetary nebula, it will leave behind this core, which forms the remnant white dwarf.[6] Usually, therefore, white dwarfs are composed of carbon and oxygen. It is also possible that core temperatures suffice to fuse carbon but not neon, in which case an oxygen-neon-magnesium white dwarf may be formed.[7] Also, some helium[8][9] white dwarfs appear to have been formed by mass loss in binary systems.
The material in a white dwarf no longer undergoes fusion reactions, so the star has no source of energy, nor is it supported against gravitational collapse by the heat generated by fusion. It is supported only by electron degeneracy pressure, which enables it to be extremely dense. The physics of degeneracy yields a maximum mass for a nonrotating white dwarf, the Chandrasekhar limit—approximately 1.4 solar masses—beyond which it cannot be supported by degeneracy pressure. A carbon-oxygen white dwarf that approaches this mass limit, typically by mass transfer from a companion star, may explode as a Type Ia supernova via a process known as carbon detonation.[6][1]
A white dwarf is very hot when it is formed, but since it has no source of energy, it will gradually radiate away its energy and cool down. This means that its radiation, which initially has a high color temperature, will lessen and redden with time. Over a very long time, a white dwarf will cool to temperatures at which it is no longer visible and become a cold black dwarf.[6] However, since no white dwarf can be older than the age of the Universe (approximately 13.7 billion years),[10] even the oldest white dwarfs still radiate at temperatures of a few thousand kelvins, and no black dwarfs are thought to exist yet.[5][1]
Wednesday, April 9, 2008
Monday, April 7, 2008
Connie Talbot: Wonderful World
Connie you are wonderful! really!! connie in the world u were wonderful angel i've ever seen. i love you dear.
CONNIE TALBOT - I HAVE A DREAM
Beautiful video and song! Everyday & everynight i hear your song at least twice then i feel my life will become bright it is like a energy booster.... Thanks for your song...little cute girl.I also have a dream and my dream is wish u create more sweety song for everyone.Everyone support u........
Connie Talbot - Walking in the Air - The Snowman
A nice pretty girl,lovely voice,incredible talents,no matter young or old also love your song.Hope you can bring out more best voice for us. Cute........................
CAn you imagine thAT??! i think you can't......
If it happened right infront of your eyes,what should you do????!
苏打绿 Latest Album Cover
蘇打綠（sodagreen）是一個台灣的樂團，於2001年由國立政治大學的學生組成，2003年時確立了了現在的六人陣容：主唱吳青峰（青峰）、貝斯手謝馨儀（馨儀）、電結他手劉家凱（家凱）、鍵盤與中提琴手龔鈺祺（阿龔）以及木結他手何景揚（阿福）；其中除了阿龔為台北藝術大學音樂研究所的學生外，其餘五名成員都曾就讀政治大學。
2004年8月，蘇打綠獲得海洋音樂祭《評審團大賞》，當場被林暐哲音樂社正式簽下，成為正式樂團，並於2005年9月發行第一張同名專輯《蘇打綠》。
現在的蘇打綠雖然是一支簽約樂隊，但林暐哲並沒有限制蘇打綠的創作，使蘇打綠保持著獨立樂團的風格。這一特點也成為蘇打綠備受歡迎的原因之一。
让我取暖
歌手：彭羚 歌曲：让我取暖
彭:看起来朋友很多知心的没几个而最关心的就是你
尤其在这些年后分开的那么远感情就更难说出口
王:回程的机票在手也许明天就走
其实都可以更改的只要你开口留我只要一个理由就能让我停留
彭:别太晚别太乱别太烦告诉我有没有人让你取暖
谈情感谈孤单谈平凡虽然所有相聚都可能面对离散
王:下一晚下一站下一段告诉我有没有人让你取暖
如果能再回到你身边那些走在大街的日子多简单多自然
合:一直到天黑了人散了谁也都不要离开
一直到我们都相信了还有爱
彭:看起来朋友很多知心的没几个而最开心的还是你
王:回程的机票在手也许明天就走除非是你留我
Titanic - My Heart Will Go On
Every night in my dreams
I see you, I feel you,
That is how I know you go on
Far across the distance
And spaces between us
You have come to show you go on
Near, far, wherever you are
I believe that the heart does go on
Once more you open the door
And you're here in my heart
And my heart will go on and on
Love can touch us one time
And last for a lifetime
And never let go till we're one
Love was when I loved you
One true time I hold to
In my life we'll always go on
Near, far, wherever you are
I believe that the heart does go on
Once more you open the door
And you're here in my heart
And my heart will go on and on
There is some love that will not go away
You're here, there's nothing I fear,
And I know that my heart will go on
We'll stay forever this way
You are safe in my heart
And my heart will go on and on
Track Time: 4:40
Written By: James Horner and Will Jennings
Published By: Famous Music Corp. (ASCAP), Blue Sky Rider Songs (BMI)
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