Lecture on Galaxies

Galaxies are large assembly of stars and nebulae. Most galaxies are billions of years old. The youngest known galaxy is 1 Zwicky 18. At an estimated age of a mere 500 million years, it is a babe in diapers compared to the Milky Way at 10-14 billion years, which is the average age of the known galaxies. Our galaxy, the Milky Way, is typical: it has hundreds of billions of stars, enough gas and dust to make billions more stars, and at least ten times as much dark matter as all the stars and gas put together.