Two very faint and distant galaxies are detected on photographs but are too dist
ID: 232486 • Letter: T
Question
Two very faint and distant galaxies are detected on photographs but are too distant to allow identification of spiral arms and other morphological features. Using a spectrograph, you find that one is reddish with a spectrum of K-type stars, while the other is bluish with a spectrum of B- and A-type stars. What type of galaxy would you expect each of those galaxies to be? Irregular galaxies are dominated by stellar associations, open clusters, and gas and dust clouds. All of these features Indicate stellar youthfulness. Does this prove that the galaxies themselves formed recently? Why does the prominent light from star-forming regions in galaxies come from massive, hot, blue stars? Why are these stars not seen in regions with no star formation?Explanation / Answer
(a) If spectra showed that a faint distant galaxy was reddish with a spectrum of K-type stars, you would expect it to be an elliptical galaxy. If spectra showed that the galaxy was bluish with B- and A-type stars, you would expect it to be an irregular galaxy.
(b) The dominance of young stellar features in irregular galaxies does not prove that the galaxies themselves formed recently. It simply means that so much star formation is occurring in these galaxies at the present time that the light from the young stars being created dominates over the light of whatever population of older stars might also exist within the galaxy.
(c) The prominent light from star-forming regions comes from massive, hot, blue stars, because these stars are far brighter that the less massive stars which are forming in these regions at the same time.
(d) Massive, hot, blue stars are not see outside the star-forming regions, because their lifetimes are so short that they die before they have moved very far from their birthplaces.
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