As one of the first steps towards a coherent theory of galaxy evolution, the American astronomer Edwin Hubble, developed a classification scheme of galaxies in 1926. The image centered above represents this early coherent theory of galaxy evolution.
Galaxies form on the right side of the diagram into a variety of diffuse complex forms, then begin accreting smaller forms and converge toward elliptical galaxies on the left.
Instead of large gas clouds collapsing to form a galaxy in which the gas breaks up into smaller clouds, it is proposed that matter started out in these “smaller” clumps (mass on the order of globular clusters), and then many of these clumps merged to form galaxies, which then were drawn by gravitation to form galaxy clusters.
A distinguishing feature of elliptical galaxies is that the velocity of the stars does not necessarily contribute to flattening of the galaxy, such as in spiral galaxies.
Elliptical galaxies have two main stages of evolution: a supermassive black hole grows by accreting cooling gas and the black hole is stabilized by suppressing gas cooling, leaving the elliptical galaxy in a stable state. The mass of the black hole is also correlated to a property called velocity dispersion (sigma) which is the dispersion of the velocities of stars in their orbits, known as the M-sigma relation.
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