When Push Comes to Shove

Gaia, a galactic surveyor spacecraft, has caught our galaxy doing “the wave!” Studies of one billion stars show a wave-like motion in the stars of our galaxy’s arms. According to galactic structure theory, this shouldn’t be happening but it is. Why? A near-collision with the Sagittarius dwarf galaxy 200 million to 1 billion years ago imparted a gravitational shove that started “the wave.” When the galaxies collide again, our Milky Way will consume or incorporate the dwarf, one of the ways our galaxy got to be so big. Astronomers don’t see the arms doing the wave but they can trace the motions of the stars over the past billion years and interpret that it has been happening.

NASA celebrates its 60th anniversary on Monday. The agency was founded in 1958, after the Soviet launch of Sputnik, but was an outgrowth of the National Commission on Aeronautics, which was founded in 1915 when the United States needed to catch up to Europe in the field of aviation.