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The evidence to date for the existence of black holes relies on
GRAVITATIONAL EFFECTS ON SURROUNDING OBJECTS.
We believe the Milky Way to be a spiral galaxy, based on
THE OVERALL FLAT SHAPE AND ROTATION OF THE MILKY WAY.
Early attempts to estimate the size of the Milky Way were defeated by
INTERSTELLAR ABSORPTION BY DUST.
Very detailed observations of distant galaxies show that
THERE WERE ONCE MANY MORE SPIRALS IN CLUSTERS THAN WE NOW SEE
Type 2 Stars
Brightest stars are red, older, globular clusters, bulge plus halo. When population 2 stars formed, the galaxy was round.
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