An asteroid passed in front of one of the brightest stars in the sky yesterday – but there is little chance you would have noticed it. The event, called an occultation, happened when the space rock crossed the face of Betelgeuse, the red supergiant star marking the armpit in the constellation of Orion.
This line-up with asteroid 2005 UW381 was theoretically “visible” along a track from south-east Asia to the south-western tips of England and Ireland. But it caused Betelgeuse to fade by only a hundredth of a magnitude – an imperceptible amount – for around three and a half seconds.
We are used to thinking of the stars as pinpoints of light because of their vast distances. But astronomers are able to put a figure to the diameter of Betelgeuse at a tiny 31 milliarcseconds (mas), which is the largest observed for a star. It turns out that the asteroid, estimated to be about 3km (2 miles) wide and 283 million km (176 million miles) away from the Earth, had a far smaller apparent size of 3mas, around a tenth that of Betelgeuse. The result was that it had an almost insignificant effect as it passed across the face of the star, which is also known as alpha Orionis.
However, the event was sufficiently interesting to be the subject of a scientific paper prepared by Italian astrophysicist Costantino Sigismondi of the Galileo Ferraris Institute and International Center for Relativistic Astrophysics, in Rome.
He remarked that attempts to measure the occultation would be of interest to help astronomers learn more about how to study transits of extrasolar planets across the face of their host stars. The drop in brightness was far greater than NASA’s Kepler space telescope is detecting from such transits.
Betelgeuse lies at a relatively close distance of 640 light-years from Earth but is one of the biggest and most luminous stars known, being nearly 1,000 times larger than the Sun and shining more than 100,000 times more brightly. It changes in brilliance anyway over time in an irregular manner as one of the sky’s brightest variable stars. Violent behaviour on its unstable surface lead astronomers to believe that it is a prime candidate to become a supernova at any moment, though this is unlikely to happen in the near future.
A paper about the occultation, which appeared in the pre-print archive arXiv.org, will be published in the January-March issue of the Journal of Occultation Astronomy.
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