Explanation:
1. Radio waves from matter at the edge of the black hole were used to create the first image of a black hole. Radio waves emitting from the gas around the black hole were used.
2. Galaxies can be grouped by the age of stars they tend to contain, with spiral galaxies having the youngest stars and elliptical galaxies having the oldest Stars. Spiral galaxies are rich in gas and has a mixture of young and old stars, whereas elliptical galaxies have little gas and only old stars.
3. The model of Galaxy formation in which sections of gas clouds collapse into small pieces which repeatedly combine into larger pieces under the influence of gravity is called the inside-out model or the top-down model.
4. Most stars not destroyed in Galaxy Collision since galaxies are mostly empty space and have no internal stars to collide. The gravity also will distort the other galaxies and then make them to merge with each other without collision.