🌌 Milky Way

The Milky Way is our home galaxy and also the pale band of stars across the night sky. The name is about milk: in a dark place the band looks like spilled milk. The Greek word galaxias, which gave us "galaxy", means "milky".

A Greek myth explains it. Milk from the goddess Hera spilled across the sky while she was nursing the infant Heracles. Artists loved the story. In the National Gallery in London hangs Tintoretto's The Origin of the Milky Way, painted around 1575. In it Jupiter puts the infant Hercules to the breast of the sleeping Juno. Milk that spurts upward becomes the Milky Way, and milk that falls down becomes white lilies. The lilies are missing from the canvas now: about a third was cut off the bottom before 1727. We know what was there from a copy made in the 17th century.

The reality is different. The Milky Way is a barred spiral with two major arms. The Sun lies in the small Orion Arm, some 25,000 to 26,000 light years from the center. One orbit takes it about 250 million years. The disk is about 100,000 light years across. Astronomers put the star count at 100 to 400 billion; counting from the inside is hard, which is why estimates differ.

You can see the band with the naked eye under dark skies. From the northern hemisphere you look toward an outer arm, while from the southern hemisphere the galactic core stands much higher.

The theme of mothers and feeding continues on the pages mother and breastfeeding. For milk in culture overall, see milk in culture and language and the page on milk.