Sky & Telescope - February 2023 - 13

T. A. RECTOR (UNIV. OF ALASKA, ANCHORAGE) AND H. SCHWEIKER
(WIYN AND NOIRLAB / NSF / AURA)
olf-Rayet Stars
L
These rare jewels in the galaxy create powerful
winds and mesmerizing dust spirals.
ive fast and die young - it's a well-used phrase in
many facets of our world. In astronomy it surely sums
up the life story of Wolf-Rayet stars, one of the rarest
of stellar types in our galaxy, yet increasingly recognized as
wielding great influence on both the interstellar medium and
star formation.
Estimated to shine for only 5 million years,
and with exuberant stellar winds second to
none, Wolf-Rayets enrich the surrounding
interstellar medium with gas and dust as they
dispel up to ten-millionths of a solar mass
each year. They each end their lives in a supernova,
perhaps even as a long gamma-ray burst
- the most energetic form of supernova. Their
luminosity is hundreds of thousands of times
that of the Sun, thanks to their high temperatures
and masses ranging from around 10 to
more than 80 solar masses.
The first catalog of Wolf-Rayets, published
During their survey, Wolf and Rayet's attention was
drawn to three 8th-magnitude stars - HD 191765, HD
192103, and HD 192641. The spectrum of each of these stars
displayed broad emission bands, rather than the thin absorption
lines that had been seen in other stars.
This find puzzled astronomers for more than half a
This find
puzzled
century. As late as 1929, the American astronomer Donald
Menzel described Wolf-Rayet stars and their spectral characteristics
as " . . . a door yet unopened and with a key so curious
that we are not even sure how to insert it
in the lock. "
That same year, however, Canadian
astronomers
for more
than half a
century.
in 1894, numbered just 54 members, and even by 1981 the
count had only risen to 159. Since then, dedicated optical and
infrared surveys have increased the Milky Way's tally to 667,
according to the online Galactic Wolf-Rayet Catalogue maintained
by Paul Crowther (University of Sheffield, UK). Some
studies have suggested that only a thousand or so Wolf-Rayets
should currently exist in the Milky Way, out of a galactic
population of perhaps several hundred billion stars overall. It
certainly makes them a treasured rarity.
The Nature of Wolf-Rayets
Our knowledge of Wolf-Rayet stars dates back to 1867. That
year, French astronomers Charles Wolf and Georges Rayet
were conducting a survey of the constellation Cygnus, using
the 15.7-inch (40-cm) Foucault telescope at Paris Observatory
to record the spectra of individual stars. Stellar spectroscopy,
particularly for stars other than our Sun, was very
much in its infancy at this time.
LOSING ITS COOL Wolf-Rayet stars form from massive stars that
shed their outer layers shortly before death. Here, WR 134 (second star
from top in the central four-star string) appears with some of its lost gas,
visible where it collided with surrounding gas to create the prominent
blue arc. The star lies about 6,200 light-years away in Cygnus, the Swan.
astronomer Carlyle Beals confirmed that the
broad bands in the spectrum appeared to be
Doppler-shifted emission, smeared out by the
motion of a continuous outflow of material
from the star. In a paper published in Monthly
Notices of the Royal Astronomical Society, Beals
presented spectra taken of known, northernsky
Wolf-Rayet stars brighter than 11th
magnitude. He showed that several aspects of
the emission bands' appearance matched what
should arise if the emission came from atoms hurtling away
from the stars, pushed out by the stars' own light.
An Evolving Picture
It would be many more decades before astronomers appreciated
how Wolf-Rayet stars evolved to their current state. By
the 1980s, though, astronomers generally accepted that these
stars were once O-type supergiants, the hottest, brightest
stars in the stellar menagerie. Commencing on the main
sequence with masses equivalent to at least 25 Suns, the
progenitors of Wolf-Rayets burn hydrogen for a few million
years, forming a helium core.
Wolf-Rayet Subtypes
There are multiple kinds of Wolf-Rayet stars,
split according to the star's spectral lines.
Nitrogen-rich (WN) ones are dominated by
nitrogen; WC ones are dominated by carbon
and oxygen. In a rarer third class, WO, oxygen
appears more prevalent than carbon.
sk yand tele scope .o r g * FEBRUARY 2023 13
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