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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