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Squished stars have researchers in a spin

Zoomed-in view of the star Regulus

A "zoomed in" view of the star Regulus. Measurements of the temperature of the star's poles and equator have shown up flaws in a century-old astronomical theory about hot, fast-spinning stars.

  • Hot stars spin so fast that they become slightly flattened
  • Theory predicts temperature differences between poles and equator
  • New measurements show the theory has major flaws

THE HOTTEST STARS IN THE UNIVERSE spin so quickly that they become a bit squished at their poles and dimmer around their middle.

But the 90-year-old theory that predicts the extent of this “gravity darkening” phenomenon has major flaws, according to a new study led by University of Michigan (U-M) astronomers.

The von Zeipel law, named for its creator Swedish astronomer Edvard Hugo von Zeipel, has been used for the better part of a century to predict the difference in surface gravity, brightness and temperature between a rapidly rotating star’s poles and its equator.

Using a technique called interferometry, the U-M researchers essentially ‘zoomed in’ to take close-up pictures and measurements of the giant star Regulus.

If Regulus were spinning just a few percent faster, it would fly apart.

The astronomers found that the actual difference in temperature between its equator and poles is much less than the old theory predicts.

Measurements don’t match the theory

“It is surprising to me that von Zeipel’s law has been adopted in astronomy for such a long time with so little solid observational evidence,” said Xiao Che, a doctoral student in the Department of Astronomy who is first author of a paper on the findings to be published in Astrophysical Journal on April 20.

It’s important to get the numbers right, says John Monnier, an associate professor in the U-M Department of Astronomy.

“In some cases, we found a 5,000-degree Fahrenheit [2,750 degrees Celsius] difference between what the theory predicts and what our actual measurements show,” Monnier said.

CHARA telescope array

Combining the light from the six CHARA telescopes (arrowed) with the Michigan Infra-Red Combiner, gives astronomers a virtual telescope 100 times as big as the Hubble Space Telescope.

“That has a big effect on total luminosity. If we don’t take this into account, we get the star’s mass and age and total energy output wrong.”

Zooming in with virtual telescope

Monnier led the creation of the Michigan Infra-Red Combiner (MIRC) instrument that was used to take the measurements. MIRC combines the light from four telescopes at the CHARA array at Georgia State University, producing a virtual telescope 100 times larger than the Hubble Space Telescope.

This interferometry technique enables astronomers see the shape and surface characteristics of stars. Without this technique, stars look like mere points of light even through the largest telescopes.

In this case, zooming in on Regulus let the researchers measure its poles and equator temperatures separately.

“Normally, you would just be able to get an average temperature,” Monnier said.

So where did von Zeipel go wrong? Monnier believes his Swedish predecessor didn’t take into account circulation patterns on stars that are not unlike wind patterns on Earth.

“The Earth has a hot equator and cold poles and that causes air circulation,” Monnier said.

“The hot air wants to flow toward the poles and equilibrate, bringing the temperatures closer together. This is a source of some weather patterns on Earth.”

Adapted from information issued by the University of Michigan. Regular image courtesy Xiao Che.

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