Moon to Hide Supergiant

Moon to Hide Supergiant
by Dave Adalian

A fat full Moon is usually a problem for astronomers, its bright, white light washing fainter objects from the sky. Other times, it’s the star of the show, as it will be the night of May 23-24 when the Moon occults the red supergiant star Antares.

Occultation is a body blocking the view of another more distant body, and a few minutes before midnight on May 23 the Moon will do just that to the ruddy star at the heart of Scorpius. The show, however, starts at sunset.

In Arabic, Antares is called Kalb al Akrab, heart of the scorpion, and staying true to this metaphor, early Muslims named two bright nearby stars Al Niyat, the arteries. The brighter of the two artery stars, Al Niyat (Sigma), will be behind the Moon when it rises. Stargazers with access to a very clear horizon will see this third-magnitude star reemerge from behind the lunar disk at about 8:48.

About then Antares will become visible above the horizon, one and a half degrees or so below the Moon. As the Moon travels west across the sky, it moves east through against the starry background at about half a degree or one full-Moon width an hour, edging ever closer to Antares as midnight approaches.

The Moon appears much brighter and larger than Antares, but this is a mere deception of distance. The Moon is just a quarter million or so miles away and is roughly 2,000 miles wide at its equator. Antares, on the other hand, lies 600 light years away, is 12,000 times brighter than the Sun and measures 745 million miles across. Keep this in mind when the Moon makes supergiant Antares disappear from the sky at four minutes from midnight.

Another thing to keep in mind is Antares advanced age. While it is younger than Sol, our star, it is much larger and brighter, meaning a far shorter lifespan. When the end comes for Antares, it will be with a bang--because of its size, Antares will end life as a supernova.

That explosion could come at anytime, a million years from now or while the Moon is hiding it from view. Likely, Antares will emerge from behind the Moon at 1:12 a.m. no different than when it disappeared. Or, it may reappear as the aftermath of a titanic stellar explosion, outshining all the other stars in our galaxy combined.

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This column appeared originally in the Visalia (Calif.) Times-Delta in April 2005.