Enigmatic Rare And Very Hot Stars Of Dr. Wolf And Mr. Rayet -
Still A Fascinating Subject!

11 July, 2012 - The universe is changing and stars are changing as well, even if they appear static and predictable every night.

Wolf-Rayet stars remain a fascinating subject of research for astronomers even if the first of them were discovered more than 140 years ago.

In the summer of 1867, French astronomers Charles Wolf and Georges Rayet observed for the first time three strange objects during a visual spectroscopic study of stars in the constellation Cygnus at the Observatoire de Paris, France.

These three stars known as "Wolf-Rayet Stars" were indeed very strange in the same way as the others in this group.
They are evolved, massive stars, which are losing mass rapidly due to very strong stellar wind, with speedsup to 2000 km/s.

These stars are far hotter than the sun, typically 25,000 to 50,000 degrees Celsius (45,030 to 90,030 Fahrenheit), massive (20+ solar mass) with a high rate of mass loss.

The sun has a comparatively chilly surface temperature of just 5,500 degrees Celsius (9,932 Fahrenheit).

Their spectra show a great variety of combinations of continuous spectrum and bright bands.

It is believed that the continuous spectrum in such a star comes from the more condensed central part, or core, and that the bright-line light proceeds from a hot atmosphere extending far out from the core.

The central star is actully a triplett system. Photo Credits: Josch Hambsch/

Due to their strong emission lines, they can be identified in nearby galaxies and now hundreds of Wolf-Rayet stars have been catalogued in our Milky Way Galaxy.

Thanks to deep photometric and spectroscopic surveys of the Galactic plane, the number of catalogued Wolf-Rayet Stars are growing with each year.

The enigmatic stars of Dr. Wolf and Mr. Rayet. Photo Credits: L. Drissen & R. Arsenault - (Université Laval, Québec, Canada) 2 Observatoire de Paris, Meudon, France

Many of them have been discovered in M33, a nearby Galaxy.

The Wolf-Rayet phase, which lasts less than 500 000 years, is in fact the "Swan song" of the most massive stars, shortly preceding their explosion as supernova.

Wolf-Rayet Nebulae in M33.Photo Credits: L. Drissen & R. Arsenault - (Université Laval, Québec, Canada) 2 Observatoire de Paris, Meudon, France

Most stars eject a very small part of their atmosphere in their surroundings: this phenomenon is called a "stellar wind". The wind of Wolf-Rayet stars is billions of times stronger than the Sun's.

For comparison, at this pace the Sun would be completely devoid of gas in less than 50 000 years!

Astronomers believe this variability is caused by a disc of dust which lies almost edge-on when viewed from Earth, which periodically obscures the star.

It is a Wolf–Rayet-type star — at the core of Hen 3-1333 - a late stage in the evolution of sun-sized stars. Photo Credit:NASA-Hubble/ESA

The image above (from the NASA Hubble Space Telescope) shows planetary nebula Hen 3-1333. The star at the heart of Hen 3-1333 is thought to have a mass of around 60% that of the sun, but unlike the sun, its apparent brightness varies substantially over time.

These are named after (and share many observational characteristics with) Wolf–Rayet stars, which are much larger. Why the similarity? Both Wolf–Rayet and Wolf–Rayet type stars are hot and bright because their helium cores are exposed: the former because of the strong stellar winds characteristic of these stars; the latter because the outer layers of the stars have been puffed away as the star runs low on fuel.

The Wolf–Rayet type stars such as the one at the core of Hen 3-1333 are much smaller in size but effectively mimic the appearance of their much bigger and more energetic namesakes: they are sheep in Wolf–Rayet clothing.

This visible-light image was taken by the high resolution channel of Hubble’s Advanced Camera for Surveys. The field of view is approximately 26 by 26 arcseconds.

The evolutionary status of the Wolf–Rayet stars is still not fully understood.

See also:
Spectacular Unknown Object Seen In The Skies Over New Zealand

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