12. A Larger, Expanding Universe

    • The Sun is not at the center of our stellar system, the Milky Way. Its remote center, known as the galactic center, is inferred from observations of globular star clusters.

    • The distances of globular star clusters can be determined from observations of the periods of their Cepheid variable stars.

    • All of the stars in the Milky Way are rotating about the galactic center with speeds that decrease with increasing distance from the center.

    • The Milky Way has a spiral shape.

    • The center of the Milky Way contains a super-massive black hole, that coincides with a strong radio source Sagittarius A*. It contains an invisible mass equivalent to about 4 million times the mass of the Sun.

    • The outer parts of the Milky Way are moving so rapidly that vast amounts of dark matter are needed to hold them together. The mass of this invisible dark halo is about ten times the mass of all the visible stars in the Milky Way.

    • Bigger telescopes enable the detection of fainter cosmic objects, and these telescopes have been used to discover billions of galaxies that cannot be seen without a telescope.

    • The first galaxies were known as spiral nebulae, due to their spiral shape, and as extragalactic nebulae, since they were found to be located outside the Milky Way, our Galaxy.

    • The distance of the closest galaxy, Andromeda or M 31, was inferred from Edwin Hubble’s observations of Cepheid variable stars in it.

    • The galaxies are all moving away from us and from each other with speeds that increase with their distance, in a relationship known as the Hubble law. It was first inferred by combining Vesto Slipher’s observations of the radial velocities of spiral nebulae with Edwin Hubble’s determinations of their distances.

    • Clusters of galaxies must be held together by dark matter whose total mass is about ten times that of the visible galaxies.

    • Galaxies move together in streams and are interconnected into filaments that extend as far as telescopes can see.

Copyright 2010, Professor Kenneth R. Lang, Tufts University