Andromedagalaxen
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| Andromedagalaxen | |
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| Observationsdata | |
|---|---|
| Stjärnbild | Andromeda |
| Rektascension | 00t 42m 44.3s[1][2] |
| Deklination | +41° 16′ 9″[1][2] |
| Rödförskjutning | -301 ± 1[3] km/s |
| Avstånd | 2,53 ± 0,07 miljoner[2][6] ljusår |
| Typ | Spiralgalax av typ Sb[2] |
| Skenbar storlek | 3.167° × 1°[1] |
| Skenbar magnitud | 4,36[1] |
| Fysiska egenskaper | |
| Radie | cirka 150 000 ljusår. |
| Absolut magnitud | -20,0[5] |
| Andra beteckningar | |
| NGC 224, UGC 454, PGC 2557, MCG +07-02-016, CGCG 535-17, IRAS 00400+4059, 2MASX J00424433+4116074, GC 116, h 50, Bode 3, Flamsteed 58, Hevelius 32, Ha 3.3, IRC +40013 | |
| Se även: Galaxer, Lista över galaxer | |
Andromedagalaxen, NGC 224 eller M31, är en spiralgalax av typ Sb i Vintergatans närområde som är svagt synlig med blotta ögat. Den återfinns i stjärnbilden Andromeda som givit den dess namn.
Andromedagalaxen ligger på ett avstånd av 2,5–2,9 miljoner ljusår[7][5] från Vintergatan och är den närmaste granngalaxen om man inte räknar Vintergatans små satellitgalaxer. Mätningar visar att den är på kollisionskurs med Vintergatan med en radiell hastighet på cirka 120 km/s och galaxerna kommer sannolikt att krocka om cirka 3 miljarder år, beroende på hur stor den okända transversella hastigheten är. Resultatet av denna kollision blir förmodligen att de smälter samman till en stor elliptisk galax.[8]
Andromedagalaxen är med en synlig diameter på 165 000–200 000 ljusår 1,5–2 gånger så stor som Vintergatan och är den största galaxen i den lokala galaxhopen. Dess utsträckning på stjärnhimlen är cirka 3×1 grader vilket är mycket större än månens (cirka 0,5 grad). För blotta ögat är dock storleken mindre eftersom endast den centrala ljusa kärnan är synlig utan teleskop.
Andromedagalaxen åtföljs av ett 10-tal satellitgalaxer, varav ett par (M32 och M110, se bilden) observerades redan av Messier. M32 tycks ha haft "närkontakt" med huvudgalaxen och därvid både stört spiralstrukturen på denna och förlorat en viss del av sitt eget stjärnbestånd.
Endast en gång har en supernova observerats i Andromedagalaxen. Det skedde 1885 och var den första supernovan utanför Vintergatan som någonsin observerats.
Observationshistoria
Trots att den är synlig för blotta ögat, om än svagt, tycks M31 inte ha varit känd i antiken; den nämns inte i till exempel Almagest. Likväl säger sig några engelska forskare som gått igenom gamla astronomiska observationer från århundradena kring vår tideräknings början, ha funnit att kinesiska astronomer observerat ett stillastående sken på himlen – en supernova – i den stjärnbild som motsvarar Andromeda mellan 10 mars och 7 april år 5 f.Kr. Samma observation tycks ha gjorts av koreanska astronomer.[9] Trots att supernovor har observerats i Andromedagalaxen, är det extremt svårt att senare detektera en supernovarest i en annan galax, än mindre i dag skaffa sig en tillförlitlig datering för dess inträffande.
M31 omnämns i övrigt först av den persiske astronomen Abd al-Rahman Al-Sufi[10] år 964 i dennes Fixstjärnornas (stjärnbildernas?) bok (Kitab al-Kawakib och kallas där det "lilla molnet"[10], men den var antagligen känd av persiska astronomer redan tidigare.[7]
Den återupptäcktes på 1600-talet av flera europeiska astronomer oberoende av varandra, och Charles Messier anger, när han 1764 registrerar galaxen som det 31:a objektet i sin katalog, Simon Marius räknas som den förste som gjort teleskopiska observationer av galaxen år 1612.
Till en början trodde man att Andromedagalaxen var en nebulosa som låg relativt nära oss. Först på 1920-talet började man inse att den låg utanför Vintergatan och Edwin Hubble lyckades genom att studera cepheider i dess utkanter uppskatta dess avstånd och storlek (publicerat i Monthly Notices of the Royal Astronomical Society, 1929). Ofullständiga kunskaper om cepheidernas natur gav dock siffror som bara är hälften av dagens.
Kärnan
M31 är känd för att hysa en tät och kompakt stjärnhop mitt i sitt centrum. I ett stort teleskop skapar den ett synligt intryck av en stjärninbäddad kärna i den diffusa omgivande bulben. Denna stjärnbildning med unga blå stjärnor är mycket nära galaxens centrum[11], är hittills oförklarad, men har av företrädare för alternativa kosmologiska modeller spekulerats kunna vara en typ av recirkulation av väte. Kärnans luminositet överträffar de mest ljusstarka klotformiga stjärnhoparna.
Nyheter
De senaste åren har man med satellitbaserade teleskop hittat flera källor till röntgenstrålning i galaxens centrala del. I dess centrum finns en ovanligt "kall" källa som man tror är ett massivt svart hål.
De senaste åren har man också med rymdteleskopet Hubble upptäckt att dess centrum verkar ha två kärnor som ligger på cirka 5 ljusårs avstånd från varandra. Man tror att den ena är den verkliga kärnan och den andra formas av materia som roterar runt och är på väg in mot kärnan.
Se även
Källor
- ^ [a b c d e] ”NASA/IPAC Extragalactic Database – Results for Messier 31”. http://nedwww.ipac.caltech.edu/cgi-bin/objsearch?objname=M+31&extend=no. Läst 1 november 2006.
- ^ [a b c d] ”SIMBAD Astronomical Database”. http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=M+31&submit=SIMBAD+search. Läst 29 oktober 2016.
- ^ [a b] Karachentsev, I. D.; Kashibadze, O. G. (2006). ”Masses of the local group and of the M81 group estimated from distortions in the local velocity field”. Astrophysics 49: sid. 3-18. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2006Ap.....49....3K.
- ^ I. D. Karachentsev, V. E. Karachentseva, W. K. Hutchmeier, D. I. Makarov (2004). ”A Catalog of Neighboring Galaxies”. Astronomical Journal 127: sid. 2031–2068. http://adsabs.harvard.edu/abs/2004AJ....127.2031K.
- ^ [a b c] I. Ribas, C. Jordi, F. Vilardell, E.L. Fitzpatrick, R.W. Hilditch, F. Edward (2005). ”First Determination of the Distance and Fundamental Properties of an Eclipsing Binary in the Andromeda Galaxy”. Astrophysical Journal 635: sid. L37-L40. http://adsabs.harvard.edu/abs/2005ApJ...635L..37R.
- ^ McConnachie, A. W.; Irwin, M. J.; Ferguson, A. M. N.; Ibata, R. A.; Lewis, G. F.; Tanvir, N. (2005). ”Distances and metallicities for 17 Local Group galaxies”. Monthly Notices of the Royal Astronomical Society 356: sid. 979-997. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2005MNRAS.356..979M.
- ^ [a b] ”Om 3 miljarder år kolliderar vi med Andromedagalaxen”. Allt om Vetenskap (5): sid. 70-76. 2006. http://www.alltomvetenskap.se/nyheter/om-3-miljarder-ar-kolliderar-vi-med-andromedagalaxen.
- ^ The collision between the Milky Way and Andromeda http://adsabs.harvard.edu/abs/2008MNRAS.tmp..333C
- ^ Bethlehemsstjärnan: En Typ Ia/Ic Supernova i Andromedagalaxen. F.J. Tipler; Dept. of Mathematics and Dept. of Physics, Tulane University; New Orleans, LA 70118 (Mars 2005)
- ^ [a b] ”The Night Sky Observer's Guide, Volume 1”. Willmann-Bell, Inc. 1998. sid. 16. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2005MNRAS.356..979M.
- ^ Unga blåa stjärnor förbryllar F&F (nr 8/2005)
Externa länkar
Wikimedia Commons har media som rör Andromedagalaxen.- SEDS sidor om Andromedagalaxen
- Bilder, solstation.com
- Hubbleteleskopets zoombara bild med över Andromedagalaxen med mer än 100 miljoner stjärnor.
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Media som används på denna webbplats
Författare/Upphovsman: Tkgd2007, Licens: CC BY-SA 3.0
A new incarnation of Image:Question_book-3.svg, which was uploaded by user AzaToth. This file is available on the English version of Wikipedia under the filename en:Image:Question book-new.svg
Författare/Upphovsman: Adam Evans, Licens: CC BY 2.0
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years away in the constellation Andromeda. The image also shows Messier Objects 32 and 110, as well as NGC 206 (a bright star cloud in the Andromeda Galaxy) and the star Nu Andromedae. This image was taken using a hydrogen-alpha filter.
This artist's concept shows a view across a mysterious disk of young, blue stars encircling a supermassive black hole at the core of the neighboring Andromeda Galaxy (M31). The region around the black hole is barely visible at the center of the disk. The background stars are the typical older, redder population of stars that inhabit the cores of most galaxies. Spectroscopic observations by the Hubble Space Telescope reveal that the blue light consists of more than 400 stars that formed in a burst of activity about 200 million years ago. The stars are tightly packed in a disk that is only a light-year across. Under the black hole's gravitational grip, the stars are traveling very fast: 2.2 million miles an hour (3.6 million kilometers an hour, or 1,000 kilometers a second) Object Names: M31, Andromeda Galaxy, NGC 224.
Författare/Upphovsman:
- Andromeda_constellation_map.png: Torsten Bronger
- derivative work: Kxx (talk)
Andromeda constellation map
Hot stars burn brightly in this image from NASA's Galaxy Evolution Explorer, showing the ultraviolet side of a familiar face. At approximately 2.5 million light-years away, the Andromeda galaxy, or M31, is our Milky Way's largest galactic neighbor. The entire galaxy spans 260,000 light-years across -- a distance so large, it took 11 different image segments stitched together to produce this view of the galaxy next door.
The bands of blue-white making up the galaxy's striking rings are neighborhoods that harbor hot, young, massive stars. Dark blue-grey lanes of cooler dust show up starkly against these bright rings, tracing the regions where star formation is currently taking place in dense cloudy cocoons. Eventually, these dusty lanes will be blown away by strong stellar winds, as the forming stars ignite nuclear fusion in their cores. Meanwhile, the central orange-white ball reveals a congregation of cooler, old stars that formed long ago.
When observed in visible light, Andromeda’s rings look more like spiral arms. The ultraviolet view shows that these arms more closely resemble the ring-like structure previously observed in infrared wavelengths with NASA’s Spitzer Space Telescope. Astronomers using Spitzer interpreted these rings as evidence that the galaxy was involved in a direct collision with its neighbor, M32, more than 200 million years ago.
Andromeda is so bright and close to us that it is one of only ten galaxies that can be spotted from Earth with the naked eye. This view is two-color composite, where blue represents far-ultraviolet light, and orange is near-ultraviolet light.
A team of astronomers using NASA's Hubble Space Telescope has discovered a "double nucleus" in the center of the neighboring spiral galaxy M31, located in the constellation Andromeda.
"Hubble shows that the M31 nucleus is much more complex than previously thought," says Dr. Tod R. Lauer of the National Optical Astronomy Observatories, Tuscon, Arizona.
A nucleus is a dense clustering of stars at the very center of a galaxy.
The astronomers report that the brighter member of the double nucleus might be the remnants of another galaxy cannibalized by M31. They say that an alternative possibility is that dust might dim the core to create the illusion of a pair of separate star clusters.
"The Hubble images intensify the mystery of what's happening in the center of this galaxy," says Lauer. "Neither interpretation offers a complete explanation of the M31 nuclear structure."
The double nucleus discovery is based on image analysis conducted by Lauer, Dr. Sandra M. Faber of the University of California, Santa Cruz, and other members of the HST Wide Field/Planetary Camera Imaging Team.
The HST pictures show two bright spots at the heart of the M31 galaxy. The dimmer of the two "light-peaks" appears to mark the exact center of the galaxy. The brighter peak is at least five light years away from the true center, but corresponds to what astronomers had previously thought was the nucleus of M31, based on ground-based observations.
Well-known as the Andromeda Galaxy, M31 (the 31st object in a catalog of non-stellar objects compiled by French astronomer Charles Messier in 1774) is located only 2.3 million light years away, making it the nearest major galaxy to our own Milky Way. M31 dominates the small group of galaxies (of which our own Milky Way is a member), and can be seen with the naked eye as a spindle-shaped "cloud" the width of the full moon.
Like the Milky Way, M31 is a giant spiral-shaped disk of stars, with a bulbous central hub of older stars. M31 has long been known to have a bright and extremely dense grouping of a few million stars clustered at the very center of its spherical hub. As seen from large ground-based telescopes, the starlight blends to resemble a single, bright, almost point-like source. Previous ground-based observations gave little hint of the true structure of the core, which is now revealed by Hubble.
In the 1960's the first high resolution photographs of M31's core were obtained by Stratoscope II, a balloon-borne observatory. The images were not as sensitive as Hubble's, and so only showed a single bright cluster of stars.
An important clue came with observations obtained in 1986 by the late Jean-Luc Nieto, then at the Pic du Midi Observatory in France. He found that the bright nucleus was offset by several light-years from the exact center of the galaxy's central bulge. The new HST images show that the dimmer peak instead is the true nucleus, and that the bright point of light evident from ground-based telescopes corresponds to the brighter of two peaks.
One possible explanation for the second cluster being offset from the exact center is that it is the remnant of a smaller galaxy that fell into M31 perhaps a billion or so years ago. The smaller galaxy's core is the only surviving fossil relic of the galactic collision.
A problem with the collision scenario is that the remnant core should be torn apart by the massive black hole hypothesized to dwell at the exact center of M31. The suspected black hole would be located in the middle of the dimmer peak uncovered by HST.
In 1988, the first evidence for a black hole at the exact center of M31 came from ground-based observations by Dr. John Kormendy (now at the University of Hawaii), and independently by Dr. Alan Dressler(Observatories of the Carnegie Institution of Washington), and Dr. Douglas O.Richstone (University of Michigan). Their data indicated an abrupt increase in the orbital velocities of stars in the center of the M31 nucleus.
This led the astronomers to conclude that M31 must have a strong but unseen concentration of mass at its center. A black hole at least ten million times the mass of the Sun is the most likely type of object matching these characteristics. A black hole is a theoretical object that is so dense that even light cannot escape its intense gravitational pull.
If such a black hole really exists, than the remnant core from the cannibalized galaxy would be torn apart in just a few hundred thousand years. "This is very short in cosmic time," says Lauer. "We would have to be looking at the galaxy at a very special time to see it now."
One way for the remnant to survive for a much longer time is if it has its own massive black hole. Gravity from a black hole in the remnant would hold it together against destruction from the other black hole at the M31 center.
"In retrospect, there may be evidence for this possibility in the spectra obtained by Kormendy, Dressler, and Richstone, says Lauer. "One problem with this picture, however, is if the black hole in the remnant were too big, it would distort even the true nucleus of M31."
Another interpretation of the "twin peaks" is that the bright spot is just the outer portion of a large nuclear star cluster, and that the central portions have been obscured by dust. A thick ring of dust might even cut across the nucleus, creating the illusion of two separate objects rather than one elongated structure.
Lauer explains that the problem with this idea is that normal galactic dust would scatter the light such that it would appear reddened. "But this is not the case, there are no color effects at all," he emphasizes. "This means that the dust grains would have to be much larger than average." In our own galaxy, however, the interstellar dust grains are roughly the same size. "We can only guess that earlier nuclear activity in M31 would have destroyed all the fine dust grains that would cause color effects," says Lauer.
M31 is an ideal target for Hubble once the telescopes's optics are improved during a space shuttle servicing mission in December. Spectrographs aboard Hubble will dissect the light from the two peaks of the double nucleus and determine if they are truly separate clusters. Astronomers will be able to measure the velocity of stars to pin down whether there is a black hole in either or both.
This is the latest in a series of Hubble observations that have uncovered unusual structures in the cores of galaxies. Some of them might be fossil evidence of galactic collisions. Hubble has also resolved very dense concentrations of stars in the cores of other galaxies that are circumstantial evidence that massive black holes are common among galaxies.