ScienceWeek

ASTROPHYSICS: VEGA IS A WHIRLING STAR

The following points are made by Richard Gray (Nature 2006 440:873):

1) New Work [1] presents measurements that convincingly show that Vega -- the second brightest star in the northern night sky -- is not turning unhurriedly as had been assumed, but rotating at a speed near to that at which it would fly apart. Confirming this behavior required cutting-edge technology, as Vega's rotational axis is pointing almost directly at Earth. The discovery would seem to demand a rethink of some established ideas -- among them, our notions of the distribution of energy in the electromagnetic spectrum of astronomical objects, and perhaps even of the time required for planets to form around stars.

2) For astronomers, Vega is arguably the most important star in the sky. In the standard Morgan-Keenan system, which classifies stars into types O, B, A, F, G, K and M in order of decreasing temperature, Vega is the archetypal A-type star. These stars, with temperatures around 10,000 kelvin and with a bluish-white tinge, are among the most common stars visible to the naked eye. Vega is also the fundamental "spectrophotometric" star, serving as the standard calibrating source of light for telescopes working in regions of the electromagnetic spectrum from the far ultraviolet through the visible to the infrared. In addition, astronomers have historically used Vega -- which is an apparently simple, slowly rotating star -- to test and develop their theories of stellar atmospheres and interiors. Finally, the star is the archetype of a class of stars with disks of dust and debris surrounding them that are possible sites of planetary formation.

3) For some decades, however, there has been an uneasy feeling that all is not right with Vega. In 1964, the star failed to conform to R. M. Petrie's model [2] that allowed the intrinsic luminosities of most hot stars to be calculated from the strength of the hydrogen lines in their spectra. Vega was found to be too luminous by a factor of nearly two, leading Petrie to suggest that it was actually two stars, too close to one another to be resolved by a telescope. Because no Doppler shifts -- changes in the observed frequency of radiation from moving objects -- were observed in Vega's spectrum, Petrie speculated that our line of sight must be exactly perpendicular to the orbital plane of the double, so that, as they orbit one another, neither star is ever moving towards or away from Earth.

4) Three years later, however, intensity interferometry revealed that Vega is a single star [3]. In the ensuing decade, it was demonstrated theoretically [4] that a rapidly rotating star viewed in its equatorial plane would seem fainter and redder than a slow rotator, whereas a rapid rotator seen pole-on would be unusually bright. This phenomenon occurs because a rapidly rotating star is flattened by the greater centrifugal force at its equator, which has the effect of making the surface temperature there significantly cooler than at its polar regions. In rapidly rotating stars seen in their equatorial plane, Doppler shifts from the high rotational velocities both towards and away from us -- on opposite sides of the star as we look at it --combine to smear out absorption frequencies and produce broadened spectral absorption lines. Slowly rotating stars and rapid rotators seen pole-on, in contrast, have narrow spectral lines, as the variation of rotational velocity along our line of sight is small. In all respects except its anomalously high luminosity, therefore, a rapid rotator seen pole-on mimics a slow rotator.[5]

References (abridged):

1. Peterson, D. M. et al. Nature 440, 896-899 (2006)

2. Petrie, R. M. Publ. Dominion Astrophys. Obs. Victoria 12, 317-338 (1964)

3. Hanbury Brown, R. , Davis, J. , Allen, L. R. & Rome, J. M. Mon. Not. R. Astron. Soc. 137, 393 (1967)

4. Collins, G. W. III & Sonneborn, G. H. Astrophys. J. Suppl. Ser. 34, 41-94 (1977)

5. Gray, R. O. J. R. Astron. Soc. Can. 82, 336-348 (1988)

Nature http://www.nature.com/nature

ScienceWeek http://scienceweek.com