The heart of one of the Milky Way
galaxy's most massive star clusters harbors as many as five pinwheels,
a strange and relatively newly discovered type of stellar object,
astronomers say.
Initially, scientists using the Hubble Space Telescope
took a close look at the Quintuplet Cluster, naming it after the five
red massive and enigmatic stars found at its center. It was unclear if
the stars, called "cocoon stars" for the dust surrounding them, were
young or old.
Now, the Keck Observatory's 10-meter Keck I telescope
on Mauna Kea in Hawaii, the world's biggest telescope, has shown that
all five of the stars are approaching the ends of their lives and that
at least two of the stars look like pinwheels, rotating around one
another. The other stars also might be pinwheels, the scientists say,
spewing out dust in a spiral arc in the same way a rotating lawn
sprinkler creates a spiral of water. [Image]
"With five times greater resolution on the Keck
Telescope, we could really focus in on the core of the star and drill
deep down into the physics of these massive stars, finally answering
the mystery of what these enigmatic cocoon stars are," said
astrophysicist Peter Tuthill of the University of Sydney, Australia.
Tuthill led the group that published the pinwheel images in the journal
Science.
Pinwheels, or spirals, are quite rare and exotic in
our galaxy, he said. "To find a whole little garden of them in this
remote cluster was startling and beautiful," Tuthill told SPACE.com.
The dust plumes around the two pinwheel stars are
typical of objects called colliding-wind binaries, he said. These
stars, between 10 and 20 times the mass of our Sun and 10,000 to 100,000 times brighter than it, are exotic rarities found only among old massive binaries.
The pinwheel discovery suggests that many of the very luminous stars in our galaxy, most of which are surrounded by dust, are actually massive binaries, not single stars, Tuthill said.
The five brightest Quintuplet stars lived fast and
are dying young. They have burned off all their hydrogen and now are
fueled by helium and fuse it into heavier elements, he said. In a
binary system, such stars generate strong stellar winds that collide to
create a lot of dust. The dust at the collision front between the
stellar winds is carried around as the stars orbit, trailing dust that
turns to create the pinwheel effect. For one of the pinwheels, the
spiral measures 300 times the radius of Earth's orbit.
The binary in the pinwheels cannot be directly
observed because it is shrouded in dust, but Tuthill and his colleagues
William C. Danchi at NASA Goddard Spaceflight Center and John Monnier
at the University of Michigan, recognized by the spiral tails that they
were looking at a colliding wind binary because they had reported the
first such spiral nebula in 1999 associated with the binary star system
WR 104.
The Quintuplet Cluster is located near the center of our galaxy, 25,000 light years from Earth in the constellation Sagittarius, and is one of the most massive young star clusters known.
Massive binary star systems like those in the
Quintuplet Cluster explode three times in their lives. There are two
explosions when each of the pair separately undergoes a core-collapse supernova.
Then a third explosion occurs as the two stars spiral into each other
and merge. The Quintuplet stars imaged by the Keck are at the end of
their normal, stable lives, just before the final supernova explosion,
Tuthill said. A gamma-ray burst will likely follow.
Supernovae are a phenomenon of high-mass stars, and
the study of pinwheels and other massive stars will help
astrophysicists better understand the subsequent supernovae explosions,
he said. "To understand the supernovae we see, often at immense
distances in the universe, it is very important to understand the
precursor stars—these are what make supernovae and gamma-ray bursts
happen," Tuthill said.
The finding also has implications for the number of
stars and supernovae in the Milky Way. "The finding of these 'buried'
binary stars, which are immediate supernova precursors, tells us that
sometimes there are two stars where we thought there to be only one,"
Tuthill said. "This has some influence on our census of the statistics
of future supernovae in the galaxy."
This article is part of SPACE.com's weekly Mystery Monday series.
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