Starship Asterisk*

http://en.wikipedia.org/wiki/CHARA_array wrote:
<<The Center for High Angular Resolution Astronomy (CHARA) Array is an optical astronomical interferometer operated by Georgia State University. CHARA is the World's highest angular resolution telescope at near-infrared wavelengths. It is located at the Mount Wilson Observatory, near Los Angeles, California. The CHARA Array is an interferometer formed from six 1 meter (40-inch) telescopes arranged along three axes with a maximum separation length of 330 m. The light beams travel through vacuum tubes and are combined optically, requiring a building 100 meters long with movable mirrors to keep the light in phase as the earth rotates. CHARA began scientific use in 2002 and began "routine operations" in early 2004. In the infrared, the array has an interferometric imaging resolution of 0.5 mas. All six telescopes are in regular use for scientific observations and as of late 2005 imaging results are routinely acquired. The array captured the first image of the surface of a main sequence star other than the sun published in early 2007.>>

• Altair (Alpha Aquilae) IV is the main setting of the film Forbidden Planet.
  
  The Altairian dollar is the principal medium of exchange throughout
  the Milky Way galaxy in The Hitchhiker's Guide to the Galaxy series.

http://en.wikipedia.org/wiki/Altair wrote:
<<Altair ("the flying eagle") rotates rapidly (i.e., period = 8.9 hours), with a velocity at the equator of around 286 km/s. The angular diameter of Altair was measured interferometrically by R. Hanbury Brown and his coworkers at Narrabri Observatory in the 1960s. They found a diameter of 3 mas. Although Hanbury Brown et al. realized that Altair would be rotationally flattened, they had insufficient data to experimentally observe its oblateness. Altair was later observed to be flattened by infrared interferometric measurements made by the Palomar Testbed Interferometer in 1999 and 2000.

Theory predicts that, owing to Altair's rapid rotation, its surface gravity and effective temperature should be lower at the equator, making the equator less luminous than the poles. This phenomenon, known as gravity darkening or the von Zeipel effect, was confirmed for Altair by measurements made by the Navy Prototype Optical Interferometer in 2001, and analyzed by Ohishi et al. and Peterson et al. (2006).

Altair is one of the few stars for which a direct image has been obtained. In 2006 and 2007, J. D. Monnier and his coworkers produced an image of Altair's surface from 2006 infrared observations made with the MIRC instrument on the CHARA array interferometer; this was the first time the surface of any main-sequence star, apart from the Sun, had been imaged.



In this image, North (the direction towards the North Celestial Pole) is up and East is left, and the white line is the rotational axis of Altair. The black grid shows lines of latitude and longitude in an Altair-centric coordinate system. The image is false-color, with brighter regions shown in white and darker regions in blue. The von Zeipel effect can be observed in the image, which shows a white spot near the pole and a darker equator. The equatorial radius of the star was estimated to be 2.03 solar radii, and the polar radius 1.63 solar radii—a 20% increase of the stellar radius from pole to equator.>>

• In Carl Sagan's science fiction novel Contact , Earth receives a message
  from an extraterrestrial transmitter array orbiting Vega (Alpha Lyrae).

http://en.wikipedia.org/wiki/Vega wrote:
<<When the radius of Vega ("the stone eagle") was measured to high accuracy with an interferometer, it resulted in an unexpectedly large estimated value of 2.73 ± 0.01 times the radius of the Sun. This is 60% larger than the radius of the star Sirius, while stellar models indicated it should only be about 12% larger. However, this discrepancy can be explained if Vega is a rapidly rotating star that is being viewed from the direction of its pole of rotation. Observations by the CHARA array in 2005–06 confirmed this deduction.

Size comparison of Vega (left) to the Sun (right).


The pole of Vega—its axis of rotation—is inclined no more than five degrees from the line-of-sight to the Earth. The equator of Vega has a rotation velocity of 274 km/s (for a rotation period of about 12.5 hours), which is 93% of the speed that would cause the star to start breaking up from centrifugal effects. This rapid rotation of Vega produces a pronounced equatorial bulge, so the radius of the equator is 23% larger than the polar radius. (The estimated polar radius of this star is 2.26 ± 0.02 solar radii, while the equatorial radius is 2.78 ± 0.02 solar radii.) From the Earth, this bulge is being viewed from the direction of its pole, producing the overly large radius estimate.

The local gravitational acceleration at the poles is greater than at the equator, so, by the Von Zeipel theorem, the local luminosity is also higher at the poles. This is seen as a variation in effective temperature over the star: the polar temperature is near 10,000 K, while the equatorial temperature is 7,600 K. As a result, if Vega were viewed along the plane of its equator, then the luminosity would be about half the apparent luminosity as viewed from the pole. This large temperature difference between the poles and the equator produces a strong 'gravity darkening' effect. As viewed from the poles, this results in a darker (lower intensity) limb than would normally be expected for a spherically symmetric star. The temperature gradient may also mean Vega has a convection zone around the equator, while the remainder of the atmosphere is likely to be in almost pure radiative equilibrium.

If Vega was actually a slowly rotating, spherically symmetric star and it was radiating the same energy as viewed from the Earth, then the luminosity of Vega would be 57 times the luminosity of the Sun. This value is much larger than the luminosity of a typical slowly rotating star with the same mass as Vega. Thus the discovery of fast rotation of Vega resolved this discrepancy. The true full luminosity of Vega is about 37 times the luminosity of the Sun.

As Vega had long been used as a standard star for calibrating telescopes, the discovery that it is rapidly rotating may challenge some of the underlying assumptions that were based on it being spherically symmetric. With the viewing angle and rotation rate of Vega now better known, this will allow for improved instrument calibrations.>>

Orca wrote:That size-comparison image looks like it was produced using Celestia. You guys ever played with that program? It's really fun (and free...a sort of Google Earth for the universe). If you have it, compare our sun at 15 AU with say, Betelgeuse at 12 AU. Quite an eye-opener!