Spica...the rest of the story

Spica, also known as Alpha Virginis, is the brightest star in the constellation Virgo. Because it lies on the ecliptic, the path of the Sun, Moon and planets across the sky, it is sometimes occulted (eclipsed) by the Moon.

Other than on those occasions, to the unaided eye Spica shines constantly at magnitude o.98, making it the 16th brightest star in the sky. But like so many other stars you wouldn't suspect are actually variable stars, Spica is much more than meets the eye.

As a side note, five of the top sixteen brightest stars are variable stars. They are Betelgeuse, Hadar, Aldebaran, Antares and Spica.

Spica is an interesting case, because not only does it pulsate, causing it to fluctuate in brightness, but it is part of an exotic pair of stars doing a cosmic dance in very close proximity to each other.

Spica is an ellipsoidal variable. Ellipsoidal variables are binary systems where the two components are close enough to distort their shapes into elongated, egg-shaped stars. As they rotate around each other they show us varying amounts of combined surface area. When we see them both from the side we see the maximum amount of surface area. Since brightness is directly related to the amount of surface area throwing light our way we see the pair at its brightest when we see them both from the side. When they rotate around to the point we are looking at them from the end of one or the other star, we see the least amount of surface area shining at us, so the star appears slightly dimmer.

The time it takes to go from bright to faint and then back to maximum light can be measured, and this in turn tells us the period of rotation of the system. In Spica's case, that's about four days. Keep in mind though, these variations are not really detectable without using photoelectric photometry or a CCD. Spica varies from 0.92 - 1.04 magnitudes, too small a change to record visually.

This might seem like a small, insignificant thing to note or study, but consider these facts. The primary component of the pair is 11 times more massive and 8 times the radius of our Sun. Its smaller partner is 7 times as massive as our sun and 4 times as big. These two massive stars spin around a common center of mass so quickly that it only takes 96 hours to complete a cycle. Their combined light output is on the order of 15,000 times the luminosity of our Sun. Its a good thing Spica is a comfortable 260 light years away, or there would be no night time on earth.

Not only does Spica have all this craziness going on, but due to instabilities in its interior, the primary has reached a point in its evolution where it is now pulsating. Yes, its atmosphere is actually expanding and contracting periodically. Beta Cepheids (not to be confused with the better known Delta Cepheids) have periods of 2 -12 hours or so. Maximum light occurs when the star is contracted down to its smallest hottest state, and minimum corresponds to its expanded cooler phase. This variation is also quite small, a few hundredths of a magnitude.

So to the unaided eye Spica shines fairly constantly and only winks at us when the Moon passes in front of it once in a while during an occultation. But to those in the know, Spica is an active constantly changing system, containing two massive stars whipping around each other in space, distorting their shapes into eggs while one of them throws its whole atmosphere up and down in a matter of hours.

As Paul Harvey would have said, "and that my friends, is the rest of the story."

1 comment:

Stephen said...

Spica at 41 AU - about the distance of Pluto, would be as bright as the Sun. We'd have night half of the year. Of course, we could say that the Sun, us would all be orbiting Spica.

Spica at 4 light years, about the distance to Proxima Centauri, would be magnitude -8. That's not as bright as the Moon, but would still be visible in the day, and punch through clouds and such.

I would not want a supernova going off 4 light years away, as these big stars are want to do.