Introductory Astronomy: Galaxy Classifications

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1. Thank you!  I have just a question about the elliptical galaxies; I notice they have a large luminous circle in the very center that looks like a large star and the rest of it is mostly vacant with just a few stars here and there.  What is the large luminous bulge in the center?  Is that just a large cluster of stars held by gravity just like we have in the center of the Milky Way?  Or could it possibly be one large star? It's just something I've always wondered about elliptical galaxies. 
2. Very good explained, thanks.
   It is Introductory, (so you did not explained how to read the spectrum
   of a galaxy and what are scientific highlights can find there and what are the
   meaning of them.) Waaa, I think I found it Introductory Astronomy: The H-R Diagram, Thanks. But I still do not know what it means when a Galaxy got in the spectrum high H alpha NII(N2) peak, hmmm.  
   
   There are some Questions about it?
   Are there indications for the age of a Galaxy, I mean
   there are also young elliptical so this is not an indication or ?
   when it got a lot of O and B Type Stars is it then a new, young, modern one ?
   a yellow would be mid age ? 
   and a red would be a very old one in the early universe  ?
   Or do we just say more complex the galaxy is the newer and younger it is in time ?  
   
   And when I found a Galaxy where the center is very bright much more
   then of 100 of Galaxies I did classify before, is there a reason for that or is it just the Galaxy has too less gas so I the center is giving more light.
   Most Galaxies are looking in the center very balanced and the center is merging with the halo, but some got a very bright lighted point in the center, you can really see a point there - with a very clear edge and not only light going out to the rings or whatever.
   S0 galaxies got a very dominant center right ? The arms or the disc is very small ?
   Do elliptical Galaxies have always a dot in the middle ?
   So without they are irregular Galaxies or could they sometimes also classified as
   an elliptical, maybe the irregular galaxies will be in the evolution diagram too, after S0, why got irregular galaxies got mostly O&B Type Stars, dustlanes and a lot of white grayish look (dwarfs), I never saw an irregular galaxies which is red, yellow, orange in color only blue white gray     ?
   Are there Galaxies, which got not a black hole center ,
   some got an neutron star ?
   And how are Galaxies classified when they look very like a disc one without arms but on the edge they got little structures of arms, spirals without arms or with or both  ?