Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2015 January 6
See Explanation.  Clicking on the picture will download
 the highest resolution version available.

100 Million Stars in the Andromeda Galaxy
Image Credit: NASA, ESA, J. Dalcanton, B. F. Williams, L. C. Johnson (U. Washington), PHAT team, R. Gendler

Explanation: What stars compose the Andromeda galaxy? To better understand, a group of researchers studied the nearby spiral by composing the largest image ever taken with the Hubble Space Telescope. The result, called the Panchromatic Hubble Andromeda Treasury (PHAT), involved thousands of observations, hundreds of fields, spanned about a third of the galaxy, and resolved over 100 million stars. In the featured composite image, the central part of the galaxy is seen on the far left, while a blue spiral arm is prominent on the right. The brightest stars, scattered over the frame, are actually Milky Way foreground stars. The PHAT data is being analyzed to better understand where and how stars have formed in M31 in contrast to our Milky Way Galaxy, and to identify and characterize Andromeda's stellar clusters and obscuring dust.

Tomorrow's picture: Pillars of Creation

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Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)
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