Astronomy

Capturing a Butterfly

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Science  11 Dec 2009:
Vol. 326, Issue 5959, pp. 1460
DOI: 10.1126/science.326.5959.1460-b
CREDIT: NASA/ESA/HUBBLE SM4 ERO TEAM

Stars radiate because thermonuclear reactions in their cores transform hydrogen into helium. For most of their lives, radiation pressure balances gravity, preventing expansion or collapse. Once hydrogen starts to run out, however, the core contracts rapidly, leading to a temperature increase and an expansion of the star's outer layers: The star becomes a red giant. Helium fusion then starts, leading to instabilities that provoke ejection of most of the star's atmosphere. The ultraviolet radiation from the now exposed, hot surface of the star ionizes its surroundings and drives a fast outflow into the circumstellar material, heating it up and making it glow as a planetary nebula. Sometimes the central dying star may be hidden by the nebula itself and by a circumstellar disk of gas and dust. Such is the case with NGC6302, also known as the Butterfly Nebula because of its bipolar morphology. Its long-elusive central star has now been detected by Szyszka et al., who acquired optical images with the Wide Field Camera 3 recently installed on the Hubble Space Telescope. The star is situated in the center of the large-scale outflows, as expected, lying on the eastern edge of the thick dust lane. The data suggest that it has a high mass as compared to other planetary nebulae.

Astrophys. J. 707, L32 (2009).

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