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.

2025 September 17
A starfield surrounds a several large nebulae that appear mostly red but also white and blue. Dark dust and blue filaments also populate the frame. Please see the explanation for more detailed information.

Nebulae and Clusters in Sagittarius
Image Credit & Copyright: J. De Winter, C. Humbert, C. Robert & V. Sabet; Text: Ogetay Kayali (MTU)

Explanation: Can you spot famous celestial objects in this image? 18th-century astronomer Charles Messier catalogued only two of them: the bright Lagoon Nebula (M8) at the bottom, and the colourful Trifid Nebula (M20) at the upper right. The one on the left that resembles a cat's paw is NGC 6559, and it is much fainter than the other two. Even harder to spot are the thin blue filaments on the left, from supernova remnant (SNR G007.5-01.7). Their glow comes from small amounts of glowing oxygen atoms that are so faint that it took over 17 hours of exposure with just one blue colour to bring up. Framing this scene of stellar birth and death are two star clusters: the open cluster M21 just above Trifid, and the globular cluster NGC 6544 at lower left.

Tomorrow's picture: NGC 6914

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