The age of the Milky Way galaxy has been estimated to be at least 13 billion years based ona. observations of globular clusters.b. observations of open clusters.c. 21-cm radiation from H I regions.d. the rotation curve of the galaxy.e. the energy produced by Sagittarius A*.

Nucleosynthesisa. is the process by which energy is produced at the center of the galaxy.b. is the process by which hydrogen and helium are converted into heavier elements.c. describes the structure of a globular cluster.d. describes how the magnetic field of the galaxy traps cosmic rays.e. describes the method by which neutral hydrogen produces 21 cm radiation.

The traditional theory states that the galaxy formeda. as a large spherical cloud of gas that was rotating very slowly.b. from a large cloud of material that broke off a larger galaxy.c. from material that had been ejected in the violent explosion of a dying galaxy.d. as a result of mergers between several smaller groups of gas, dust, and stars.e. as two massive galaxies collided.

Sgr A* is believed to be the center of the Milky Way galaxy because

I. It lies in the general direction of the center of the galaxy based on observations of globular clusters in the plane of the Milky Way.II. Long wavelength dust penetrating images show it to be a region crowded with starsIII. Orbits of stars around it indicate a mass several million times that of the Sun within a volume a few light hours in sizeIV. Radio observations show it is a concentrated source at the center of swirling clouds of gas

a. I & II onlyb. II & III onlyc. I & IV onlyd. II & IV onlye. All of I, II, III, & IV

The galactic halo or corona is believed to contain mostlya. O and B stars.b. G, K, and M stars.c. open clustersd. globular clusterse. dark matter.