08/18/2020 – Ephemeris – What is the Milky Way really like?

This is Ephemeris for Tuesday, August 18th. Today the Sun will be up for 13 hours and 53 minutes, setting at 8:42, and it will rise tomorrow at 6:50. The Moon is new today, and won’t be visible.

Last Friday and yesterday we looked at what the Milky Way looked like, but what is the Milky Way really like. It’s hard to tell, being inside it. In the 1920s astronomers gradually learned that the Milky Way was similar to the great number of spiral nebulae seen outside of the milky band. It was a flattened pinwheel of stars, gas and dust. At first they and the Milky Way were called island universes because until then it was thought that the Milky Way was the entire universe. The word for the Milky Way and those other structures is galaxy. The word galaxy is related to the Greek word for milk. Astronomers in the latter half of the 20th century using radio telescopes were able to trace out the clouds of gas that give our galaxy its spiral structure.

The event times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.

Addendum

The shape of the universe (Milky Way) as measured by William Herschel by counting stars in the eyepiece fields of his telescope pointed in various directions. The large indent on the right is caused by the Great Rift, clouds of gas and dust that block the light of the stars behind it, not the lack of stars in that direction that he thought. The Great Rift is easily seen in the summer sky running through the Milky Way.

1950’s radio map of the Milky Way Galaxy. The original source is for the map is The galactic system as a spiral nebula Oort, J. H.; Kerr, F. J.; Westerhout, G. MNRAS 118, (1958) p. 379.

Our place in the Milky Way. Note that we appear to be in a barred spiral galaxy. The arms are numbered and named. 3kpc is the 3 kiloparsec arm. 3kpc = 9,780 light years. The Sun is about 27,000 light years from the center. Credit NASA and Wikimedia Commons, via EarthSky.org.

One of the many spiral nebulae visible, namely the Pinwheel Galaxy, M101 near the handle of the Big Dipper. Credit Scott Anttila.