May 31, 2020

Astro-Event: An Asteroid Occults a Bright Star.

One of the best occultations of a bright star occurs this week for observers along a line from western Canada down the U.S. west coast. At around 10:00 UT, on the morning of April 6th, 14th magnitude asteroid 824 Anastasia will occult, or pass in front of, the bright +2.5 magnitude star Zeta Ophiuchi for up to 8.6 seconds. This is a rare event in that the occulted star will be visible with the naked eye! Stellar occultations give us the rare opportunity to profile the shape of an asteroid; if enough folks are lined up along the graze line and make and submit accurate observations, a chord map of the “shadow” of the asteroid can be plotted. Binary asteroids have even been discovered by amateur astronomers using this method! Anyway, if you’re located anywhere along the predicted path and the sky is clear, don’t miss this rare event!

This week’s astro-term of the week is runaway star. A very few select stars show a large annual proper motion; Zeta Ophiuchi is one such star. At 520 light years, Zeta Oph shows an annual proper motion of 0.02” arc seconds, and a radial velocity of about 11.5 miles per second in approach to us. Runaway stars have relative stellar velocities of up toward 60 miles per second, compared with about 12 miles per second for our own Sun. Like many runaway stars, Zeta Oph can be traced back to its origins, in this case the Scorpius-Centaurus Association. It’s likely that Zeta Oph was accelerated by a close stellar encounter somewhere in its history. At 4,000x the luminosity of our Sun, Zeta Oph would be one of the brightest stars in our sky in intergalactic dust didn’t intervene!


  1. AK says:

    Unfortunately, it shifted to the west. Nobody on the predicted path saw it.

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