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.

April 16, 1996

Cometary Knots in the Helix Nebula
Credit: R. O'Dell and K. Handron (Rice University), NASA

Explanation: Four hundred fifty light-years from Earth, the wind from a dying, sun-like star produced a planetary nebula popularly known as the Helix. While exploring the Helix's gaseous envelope with the Hubble Space Telescope (HST), astronomers discovered indications of 1,000s of striking "cometary knots" like those shown above. So called because of their resemblence to comets, they are actually much larger - their heads are several billion miles across (roughly twice the size of the our solar system itself) while their tails, pointing radially away from the central star, stretch over 100 billion miles. Previously known from ground based observations, the sheer number of cometary knots found in this single nebula is astonishing. What caused them to form? Hot, fast moving shells of nebular gas overrunning cooler, denser, slower shells ejected by the star during an earlier expansion may produce these droplet-like condensations as the two shells intermix and fragment. An intriguing possibility is that instead of dissipating over time, these objects, could collapse and form pluto-like bodies. If so, these icy worlds created near the end of a star's life, would be numerous in our galaxy.

Tomorrow's picture: NGC 7293: The Helix Nebula

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