December 10, 2014
The night sky on your birthday is never quite the same twice — the planets shift, the constellations drift, the Moon wears a different phase.
The Reddening of M71
Birthday · December 10, 2014
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APOD published December 10, 2014. Credit: Bob Franke. A publication-date match, not a photo taken on your date. Independent of NASA.
Now known to be a globular star cluster at the tender age of 10 billion years, M71 is a mere 13,000 light-years away within the narrow boundaries of the faint constellation Sagitta. Close to the plane of the Milky Way galaxy in planet Earth's sky, its 10,000 or so member stars are gathered into a region about 27 light-years across near the center of this color composite view. In fact, the line-of-sight to M71 passes along the galactic plane through much intervening diffuse interstellar dust. The dust dims starlight and scatters blue light more efficiently, masking the brightness of M71's stars and shifting true star colors toward the red. How much are the star colors shifted? Slide your cursor over the image (or follow this link) to use an estimate of the dust reddening or galactic extinction to correct the star colors in M71. Corrections to the brightness and colors of M71 member stars are needed to measure the cluster's distance and age using a Color-Magnitude diagram.
Image credit: Bob Franke