Galakse

En galakse fotograferet af Hubble-rumteleskopet.

En galakse, (af græsk:γαλαξίας som betyder mælkeagtig), er et komplekst system af stjerner, mørkt stof og interstellart stof, bundet sammen af tyngdekraften. Solen er blot én af mange stjerner i Mælkevejsgalaksen. Solsystemet indeholder Jorden og alle de andre himmellegemer, der kredser om Solen. Mælkevejen er en større galakse med 3×1011-4×1011 (300-400 milliarder) stjerner, hvoraf ca. 73-78% er ret små, lyssvage røde dværge.

Galakser kan indeholde mange solsystemer, stjernehobe, og forskellige interstellare skyer. Galakser indeholder som minimum mere end 1 million stjerner og har sandsynligvis alle et sort hul i deres center, som visse galakser tilmed roterer om. Galakser er så store, at deres størrelser måles i lysår eller kiloparsec. Mælkevejen har en diameter på 100.000 lysår og en tykkelse på 1.000 lysår.

Den fjerneste kendte galakse UDFy-38135539 i et udsnit af Hubble Ultra Deep Field.

En galakses relative bevægelsesretning og radialhastighed kan bestemmes ud fra forskydningen af dens lysfrekvens på grund af dopplereffekten. Det er ændringen af lysets bølgelængde (farve), set fra observationspunktet, der benyttes til at bestemme, om objektet bevæger sig væk fra eller mod observatøren. Hvis den observerede galakse udsender lys, der er rødforskudt, betyder det at afstanden øges, og hvis dens lys er blåforskudt formindskes afstanden til observatøren. Rød og blå refererer til det synlige lys, men dopplereffekten gælder alle elektromagnetiske bølgelængder. Til afstandsbestemmelse af galakser benyttes Cepheidemetoden ved observationer af lysstyrken af specielle lysstærke, variable Cepheidestjerner i galakserne. Den fjerneste galakse, der er observeret (oktober 2010) er UDFy-38135539 13,1 milliarder lysår borte[1].

Galaksehobe er samlinger af galakser, Mælkevejen ligger i en galaksehob der hedder Den lokale galaksegruppe. Galaksehobe er samlet i superhobe og Den lokale galaksegruppe tilhører Virgo-superhoben.

Galaksetyper

Galakseformer og -typer.

Historisk set er galakser blevet kategoriseret efter deres udseende. Edwin Hubble var den første der i 1926 lavede et system til klassifikation af galakser efter deres udseende (som han antog var deres egentlige form). Systemet bruges i vid udstrækning den dag i dag og kendes som Hubble sekvensen (eller Hubbles gaffeldiagram[2]). Galaksetyperne i den oprindelige Hubble sekvens var som følger:

  • Spiralgalakser (Sa, Sb og Sc)
    • Bjælkegalakser (SBa, SBb og SBc) – Mælkevejen menes at være en bjælkespiralgalakse.[3]
  • Linseformede galakser (S0)
  • Elliptiske galakser (E0→E7)
  • Irregulære galakser (Ir I og Ir II)

En spiralgalakse har form som en flad skive med spiralarme, en elliptisk galakse er ellipseformet og en linseformet galakse er en blanding af spiral- og ellipseform. Irregulære galakser er galakser der ikke har en åbenlys regelmæssig struktur, hverken spiral- eller ellipseform. Bjælkegalaksen er en underkategori af spiralgalaksen. Armene i en spiralgalakse kan forstås med tæthedsbølgeteori.[4][5]

Senere udvidelser til Hubbles klassifikationsmodel, kom til at indeholde bl.a. ringgalakser, dværggalakser og aktive galakser, men klassificeringen er ikke længere kun baseret på galaksens form og udseende.[8] Udseendet af galakser afhænger også af afstanden, da fjerne galakser ses, som de så ud da de var meget unge, kortere tid efter Big Bang.

Referencer

  1. ^ Hidtil fjerneste galakse Hentet d. 26. oktober 2010
  2. ^ Galaksestudie Arkiveret 1. juni 2015 hos Wayback Machine Astronomibladet.dk
  3. ^ Sommerens Stjernetræf Arkiveret 9. marts 2012 hos Wayback Machine Anja C. Andersen, Dark Cosmology Center Arkiveret 9. marts 2012 hos Wayback Machine
  4. ^ Lin, C.C.; Shu, F.H. (1964). "On the spiral structure of disk galaxies". Astrophysical Journal. 140: 646-655. Bibcode:1964ApJ...140..646L. doi:10.1086/147955.
  5. ^ Shu, Frank H. (2016-09-19). "Six Decades of Spiral Density Wave Theory". Annual Review of Astronomy and Astrophysics. 54 (1): 667-724. Bibcode:2016ARA&A..54..667S. doi:10.1146/annurev-astro-081915-023426. ISSN 0066-4146.
  6. ^ "Billede fra STScI/NASA: "Hubble Unveils a Galaxy in Living Color"". Arkiveret fra originalen 10. august 2003. Hentet 14. september 2003.
  7. ^ "Billede fra STScI/NASA". Arkiveret fra originalen 1. oktober 2003. Hentet 14. september 2003.
  8. ^ Tiden læger deforme galakser Arkiveret 9. februar 2010 hos Wayback Machine Ingeniøren

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NGC 4589 Hubble WikiSky.jpg
NGC 4589 galaxy by Hubble space telescope
Caldwell.png
Forfatter/Opretter: Roberto Mura, Licens: CC-BY-SA-3.0
Collage of photos from Wikimedia Commons. Some Caldwell Catalogue's photos.
M81 Galaxy from the Mount Lemmon SkyCenter Schulman Telescope courtesy Adam Block.jpg
Forfatter/Opretter: Ngc1535, Licens: CC BY-SA 4.0
Deep exposures of Galaxies

using the 0.8m Schulman Telescope at the Mount Lemmon SkyCenter

Credit Line & Copyright Adam Block/Mount Lemmon SkyCenter/University of Arizona
Milky Way Galaxy.jpg
Artist's conception of the Milky Way galaxy.
IC 335 - Potw1451a.tif
(c) ESA/Hubble, CC BY 4.0
This new NASA/ESA Hubble Space Telescope image shows the galaxy IC 335 in front of a backdrop of distant galaxies. IC 335 is part of a galaxy group containing three other galaxies, and located in the Fornax Galaxy Cluster 60 million light-years away. As seen in this image, the disc of IC 335 appears edge-on from the vantage point of Earth. This makes it harder for astronomers to classify it, as most of the characteristics of a galaxy’s morphology — the arms of a spiral or the bar across the centre — are only visible on its face. Still, the 45 000 light-year-long galaxy could be classified as an S0 type. These lenticular galaxies are an intermediate state in galaxy morphological classification schemes between true spiral and elliptical galaxies. They have a thin stellar disc and a bulge, like spiral galaxies, but in contrast to typical spiral galaxies they have used up most of the interstellar medium. Only a few new stars can be created out of the material that is left and the star formation rate is very low. Hence, the population of stars in S0 galaxies consists mainly of aging stars, very similar to the star population in elliptical galaxies. As S0 galaxies have only ill-defined spiral arms they are easily mistaken for elliptical galaxies if they are seen inclined face-on or edge-on as IC 335 here. And indeed, despite the morphological differences between S0 and elliptical class galaxies, they share a some common characteristics, like typical sizes and spectral features. Both classes are also early-type galaxies, as they are evolving passively. However, elliptical galaxies may be passively evolving when we observe them, but they had violent interactions with other galaxies in their past. Whereas S0 galaxies are either aging and fading spiral galaxies, which never had any interactions with other galaxies, or they are the aging result of a single merger between two spiral galaxies in the past. The exact nature of these galaxies is still a matter of debate. Links Structure and Formation of S0 and Spheroidal Galaxies Can Early Type Galaxies Evolve from Fading the Disks of Late Types?
NGC 4414 (NASA-med).jpg
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ID: GPN-2000-000933

Other ID: PR99-25
Hubble2005-01-barred-spiral-galaxy-NGC1300.jpg
Barred spiral galaxy NGC 1300 photographed by Hubble telescope.

In the core of the larger spiral structure of NGC 1300, the nucleus shows its own extraordinary and distinct "grand-design" spiral structure that is about 3,300 light-years (1 kiloparsec) long. Only galaxies with large-scale bars appear to have these grand-design inner disks — a spiral within a spiral. Models suggest that the gas in a bar can be funneled inwards, and then spiral into the center through the grand-design disk, where it can potentially fuel a central black hole. NGC 1300 is not known to have an active nucleus, however, indicating either that there is no black hole, or that it is not accreting matter.

The image was constructed from exposures taken in September 2004 by the Advanced Camera for Surveys onboard Hubble in four filters. Starlight and dust are seen in blue, visible, and infrared light. Bright star clusters are highlighted in red by their associated emission from glowing hydrogen gas. Due to the galaxy's large size, two adjacent pointings of the telescope were necessary to cover the extent of the spiral arms. The galaxy lies roughly 69 million light-years away (21 megaparsecs) in the direction of the constellation Eridanus.
Saturn template.svg
Forfatter/Opretter: Urutseg, Licens: CC BY-SA 3.0
Astronomy stub
NGC 4013HSTPart.jpg
NGC 4013 closeup view
Irregular galaxy NGC 1427A (captured by the Hubble Space Telescope).jpg
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