Hubble

Peering upward on late September evenings, the sky is dominated by the Milky Way, which arches from the southwest to the northeast.  High overhead, we can easily view the Great Rift, an area within the Milky Way but, seemingly, almost completely devoid of stars.  In fact, this absence of stars is due to the gas and dust, common in spiral galaxies such as our own, which obscures the stars beyond.

NASA

Most of us know the old nursery rhyme that begins “Star light, star bright, first star I see tonight.” Have you ever asked yourself “How do we measure star bright?” The history of measuring brightness goes back to the Greek astronomer, Hipparchus, who lived in the second century B.C.