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.

2002 November 11
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The Outer Shells of Centaurus A
Credit: E. Peng and H. Ford (JHU), K. Freeman (ANU), R. White (STScI), CTIO, NOAO, NSF

Explanation: What causes the surrounding shells in peculiar galaxy Cen A? Last month a fascinating image of peculiar galaxy Centaurus A was released, processed to highlight a faint blue arc indicating an ongoing collision with a smaller galaxy. Another interesting feature of Cen A, however, is the surrounding system of shells, better visible here in this recently released wider pan from the four meter Blanco telescope at Cerro Tololo Inter-American Observatory. Faint shells around galaxies are not unusual and considered by themselves as evidence of a previous galaxy merger, analogous to water ripples on a pond. An unexpected attribute of these shells is the abundance of gas, which should become separated from existing stars during the collision.

Tomorrow's picture: Terkezi Oasis

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