Anne’s Picture of the Day: The Smile Nebula

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January 12, 2013

The Smile Nebula, an emission nebula in Carina

Image Credit & Copyright: Ken Crawford, Rancho Del Sol Observatory (http://www.imagingdeepsky.com)

The Smile Nebula (NGC 3199) is a bright emission nebula of about 75 light-years across, located some 11,736 light-years away in the southern constellation of Carina. It is created by the Wolf-Rayet star 18 (also known as HD 89358), blowing a strong stellar wind into the surrounding interstellar medium.

Wolf-Rayet stars (named for their discoverers) are extremely rare and short-lived super-hot stars, nearly at the end of their stellar lives. These stars start their lives with dozens of times the mass of our Sun, and when they age, material which the stars have cooked up in their central nuclear furnaces (like carbon and oxygen) gradually reach the surface of the star. When enough material reaches the surface, it absorbs so much of the intense light from the star that an enormously strong wind starts to blow from the star’s surface. This wind becomes so thick that it totally obscures the star – so when we look at a Wolf-Rayet star, we’re really just seeing this thick wind. The amount of material which the wind carries away from the star each year, is equivalent to that of the entire Earth.

Wolf-Rayet stars are highly luminous, from tens of thousands to several million times the luminosity of the Sun, although not exceptionally bright visually since most of their output is in far ultraviolet and even soft X-rays. WR stars are probably progenitors of long-duration Gamma Ray Bursts. Often occurring in binary systems, very massive stars likely become Wolf-Rayet stars within a few million years before they explode in supernovae (though no one has yet seen such a star explode).

Though the Smile nebula has a more or less complete ring shape, it looks very lopsided with a much brighter edge at the lower right, which was thought to indicate a bow shock produced as the Wolf-Rayet star (near the center of the nebula) plowed through a uniform medium, like a boat through water. But measurements have shown that the star is moving at about 60 kilometers per second, but not really directly toward the bright edge. So a more likely explanation is that the material surrounding the star is not uniform, but clumped and denser near the bright edge of the windblown nebula.

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