An image with 25 times the resolution the Hubble Space Telescope can produce has shown unprecedented levels of detail on the star Altair. Located 15 light years away in the constellation Aquila (The Eagle), Altair is a young, hot star about twice the size of the Sun, known to rotate at 300 kilometers per second at its equator. That’s about sixty times Sol’s rotation rate, fast enough to flatten Altair into an oval, its radius larger at the equator than the poles. In fact, it’s 22 percent wider than it is tall.
[kml_flashembed movie=”https://centauri-dreams.org/wp-content/uploads/2007/05/altair_small.swf” height=”300″ width=”450″ /]
Animation credit: Ming Zhao (University of Michigan)
How do you get a surface image of a star a million times farther away than our own? The technique is optical interferometry, combining the light from multiple telescopes to simulate a much larger instrument. In this case, the four telescopes used (at Georgia State University’s Center for High Angular Resolution Astronomy on Mt. Wilson, California) fed infrared light into a device called the Michigan Infrared Combiner. The result: A simulated instrument 265 meters by 195 meters in size.
And the imagery delivered a surprise. Rapidly rotating stars have been assumed to feature a dark band along their equators, the result of so-called ‘gravity darkening.’ The latter occurs because the star’s swollen equator is farther from its nuclear core, making that region cooler than the poles. The new work does confirm gravity darkening but shows a larger effect than earlier theories had predicted. The physics behind the darkening process may require an alternative model.
Hot, rapidly rotating stars are intriguing because of their effects on the stars around them. From the paper:
A signiﬁcant fraction of hot stars are rapid rotators with surface rotational velocities of more than 100 km/s. These rapid rotators are expected to traverse very different evolutionary paths than their slowly rotating kin and rotation-induced mixing alters stellar abundances. While hot stars are relatively rare by number in the Milky Way Galaxy, they have a disproportional effect on galactic evolution due to their high luminosities, strong winds, and their ﬁnal end as supernovae (for the most massive stars). Recently, rapid rotation in single stars has been invoked to explain at least one major type of gamma ray bursts and binary coalescence of massive stars/remnants for another.
Previous stellar imaging, apart from our own Sun, has focused on enormous red giant stars, but this is the first time a main-sequence star has been seen in such detail. The researchers hope to move on to other rapid rotators like Vega, but I like what Michigan graduate student Ming Zhao, who performed the stellar modeling on Altair, has to say about future work: “Imaging stars is just the start. We are going to next apply this technology to imaging extrasolar planets around nearby stars.”
The paper is Monnier et al., “Imaging the Surface of Altair,” published by Science online on May 31, 2007 (abstract available).