Astronomers have caught a peek at a rare moment in the final stages of astar's life: a ballooning shroud of gas cast off by a dying starflicking on its stellar light bulb. NASA's Hubble Space Telescope hascaptured the unveiling of the Stingray nebula (Hen-1357), the youngestknown planetary nebula. Twenty years ago, the nebulous gas entombing thedying star wasn't hot enough to glow.
The Stingray nebula (Hen-1357) is so named because its shape resembles astingray fish. Images of a planetary nebula in its formative years can yield new insights into the last gasps of ordinary stars like our sun.
A planetary nebula forms after an aging, low-mass star swells to becomea "red giant" and blows off some of its outer layers of material. As thenebula expands away from the star, the star's remaining core gets hotterand heats the gas until it glows. A fast wind - material propelledoutward from the hot central star -- compresses the gas and pushes thegas bubble outward.
The central star in the Stingray nebula has heated up quite fast. "Sucha fast evolution of the object actually came as a surprise," saysKailash Sahu of the Space Telescope Science Institute in Baltimore, Md."The current theoretical models do not predict such fast evolution forlow-mass stars like that of the Stingray nebula."
Adds Matt Bobrowsky of Orbital Sciences Corp. in Greenbelt, Md: "TheStingray nebula is, in human terms, just an infant because only withinthe past 20 years did its central star rapidly heat up enough to makethe nebula start to glow. It is extraordinary to catch a star in thisexceedingly brief phase of its evolution. While stars typically last forbillions of years, the transition to a visible planetary nebula takesonly about 100 years -- the blink of an eye compared to a star'slifetime. It is therefore not surprising that no younger planetarynebula has ever been identified."
The nebula is one-tenth the size of most planetary nebulae and is 18,000light-years away in the direction of the southern constellation Ara (theAltar). Because of its small size, no details of the Stingray nebulawere visible before Hubble observations were carried out.
The creation of twin bubbles of gas, which shape so many planetarynebulae, has always been a mystery to astronomers. The jets of gasrevealed in the Hubble images are of great interest to astronomersbecause many types of astronomical objects - from young stars to activegalaxies - produce similar, opposing flows of gas. Many theories havebeen proposed to explain these jets, but the details of their formationare not yet fully understood. The Hubble images actually reveal how thejets in the Stingray are produced.
"Both theory and observations have indicated that a ring or disk ofmatter plays a role in forming the opposing outflows," Bobrowsky says."But these images are significant in showing that, at least in somecases, the situation is somewhat more complex."
The images that Bobrowsky and collaborators acquired show a ring of gassurrounding the central star, with bubbles of gas above and below thering. The wind emanating from the central star has created enoughpressure to blow open holes in the ends of the bubbles, allowing gas toescape. The bubbles act like nozzles that direct the escaping gas intotwo opposing streams. The images also show bright gas that is heated bya "shock" caused when the central star's wind hits the walls of thebubbles.
A further discovery is a second star within the nebula, indicating thatthe Stingray's central star is part of a binary star system. This secondstar is important because astronomers have theorized that a companionmight be necessary for the formation of the ring, bubbles, and columnsof gas.
There is also evidence that some of the gas in the nebula may bedistorted due to the gravity from the companion star - anotherphenomenon never before seen in a planetary nebula. This appears in theimages as a spur of gas forming a bridge to the companion star.
Bobrowsky first observed the Stingray nebula with the Hubble telescopein 1993. Those images were the first to show the structure of thenebula. The new observations were taken in 1997 by Bobrowsky and hiscollaborators: Sahu of the Space Telescope Science Institute, M.Parthasarathy of the Indian Institute of Astrophysics in Bangalore,India, and Pedro Garcia-Lario of the ISO Science Operations Center inMadrid, Spain. The results are described in the April 2 issue of thejournal Nature.
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The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).
Materials provided by Space Telescope Science Institute. Note: Content may be edited for style and length.
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