Question

Describe the difference in how astronomers believe spiral and elliptical galaxies are formed. What sort of collisions take place in the formation of each type?

When we look at the blackbody spectrum of an entire regular galaxy at once, we see a spectrum that looks like a brighter version of the spectrum of a star. But the spectrum of an active galaxy looks different. How is it different and what is the cause of this extra radiation?

Answer 1

Spiral galaxies are formed by the evolution of the stars in the galactic halo. On the other way the elliptical galaxies are formed by merging of two spiral galaxies.

For a normal galaxy the optical observation mainly captures the features of the stars. Hence the similarity is there for a normal galaxy and a star.

In case of a active nucleus galaxy the optical spectrum is different from a star mainly due to the presence of optical emission from the accretion disk about the central massive black hole.