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.

2023 January 17
A deep image of M31, the Andromeda galaxy, shows unexpected
oxygen-glowing arcs to its left. 
Please see the explanation for more detailed information.

Unexpected Clouds Toward the Andromeda Galaxy
Image Credit & Copyright: Yann Sainty & Marcel Drechsler

Explanation: Why are there oxygen-emitting arcs near the direction of the Andromeda galaxy? No one is sure. The gas arcs, shown in blue, were discovered and confirmed by amateur astronomers just last year. The two main origin hypotheses for the arcs are that they really are close to Andromeda (M31), or that they are just coincidentally placed gas filaments in our Milky Way galaxy. Adding to the mystery is that arcs were not seen in previous deep images of M31 taken primarily in light emitted by hydrogen, and that other, more distant galaxies have not been generally noted as showing similar oxygen-emitting structures. Dedicated amateurs using commercial telescopes made this discovery because, in part, professional telescopes usually investigate angularly small patches of the night sky, whereas these arcs span several times the angular size of the full moon. Future observations -- both in light emitted by oxygen and by other elements -- are sure to follow.

Tomorrow's picture: JWST lensing


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