Skylight: The Big Dipper Through Time

by Irene Pease on Jul 28, 2017 6:32 pm

Stars aren’t still—they move through space. Our Sun and the seven stars that form the Big Dipper in the constellation Ursa Major all orbit the center of the Milky Way at different speeds. So why do today’s constellations closely resemble those depicted by ancient astronomers?

Find out why they, like us, saw just a snapshot of cosmic time.

The Big Dipper Through Time - Transcript [View of night sky as stars move through the night. Music fades up.] The constellation Ursa Major swings high overhead each summer night. Seven of its brightest stars form the Big Dipper. [The Big Dipper is in view. A red line connects the seven stars forming the shape of a dipper, and the shape moves to center of view.] Five of these stars are part of a group that moves around the galaxy together. [Green circles appear around the five stars: the inner two of the handle, and the three in the cup closest to the handle.] The other two are farther from Earth, and separate from the group. [Gold circles appear around the stars at the end of the handle and the far end of the Dipper's cup. Distances from Earth are shown for all seven stars. The green-circled stars range 80-83 light years. The gold-circles stars are 104 and 124 light years.] [Distance labels and gold circles disappear. View pans out and around the stars of the Dipper in 3D, it's dipper-shape distorting as angle of view and distance change.] Stars in the Milky Way orbit the galactic center. Some travel in groups, some in pairs, others by themselves. [As view moves away and pans around the Dipper, the green circled stars appears very close from a different angle; the other two stars are clearly not part of the group. The central region of the galaxy passes through the background. The green circles disappear.] Typically, stars closer to the center move slower, and stars farther from the center move faster. [View continues to move away from the Dipper stars, the red line shrinking into the distance, no longer visible. The whole Milky Way galaxy comes into view, rotating through an edge-on view.] Given that the stars are moving at different speeds, why haven't the constellations changed since antiquity? [Galaxy stops rotating, viewed face-on. A yellow circle appears marking the Sun's orbit around the galaxy.] Because the Sun takes 225,000,000 years to orbit the Milky Way, stellar motion is insignificant over a few millennia. [View of galaxy dissolves to view of outlined Big Dipper.] Over several thousand years, the Sun's motion relative to the stars contributes more to the changes in our night sky. [View of Big Dipper is shown for tonight, in 2,000 years, 4,000, 6,000, 8,000, 10,000, showing very small changes in stars' positions. View continues in 5,000-year increments, showing small but noticeable changes in star positions. Dates end at 50,000 years from now. View shifts between 50,000 and tonight, showing small drift of Dipper stars.] [Dissolve to face-on view of Milky Way. Circle indicating orbit of the sun appears again. View is zooming in towards left side of the orbit.] Stars take millions of years to travel around the galaxy. Over the entire history of our species, the Sun has moved only this far. [Tiny green dot appears along the orbit, and a green arrow pointing to the dot. Zoom stops, duplicate of the dot is elongated to a line with a label of 200,000 years.] We each witness only a snapshot of cosmic time. Credits roll. [Fade to black, credits begin to roll. Music fades.]