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Spica: The Close Binary

Arc to Arcturus
From midnorthern latitudes on spring evenings, the Big Dipper, Boötes, and Virgo can be found high in the sky from the northeast to southeast. Follow the red arrows from the Dipper's handle to "arc to Arcturus" and the kite-shape of Boötes, and then proceed to "speed to Spica" in Virgo.

Spica is a bright binary star, the 16th brightest in Earth's night sky, visible in the northern constellation Virgo. It is visible not only because of its size, but also because of its relatively close distance: it is about 260 light-years away from Earth.

While the star appears as a single point of light to the naked eye, Spica is a binary system. One star, however, is much brighter than the other one.

The star system is also a massive source of X-rays, which opened a new understanding of Spica when X-ray astronomy became prominent in the 1960s.

Locating Spica

Spica is a little hard to spot, so astronomers sometimes use other stars to find it. Starting from the handle of the Big Dipper, one common phrase used among amateurs goes: "follow the arc to Arcturus and speed on to Spica." Spica's location is:

  • Right ascension: 13 hours 25 minutes 11.6 seconds
  • Declination: -11 degrees 09 minute 41 seconds

Spica in history and culture

The name "Spica" derives from a Latin phrase saying that Virgo is holding an "ear of grain" or "ear of wheat." It also was noted and named in other ancient cultures, such as Arabic, Chinese and Hindu. Legend commonly connects Virgo to Dike, the Greek goddess of justice, and Persephone, the daughter of Demeter, the harvest goddess. 

Spica is featured on Brazil's national flag, above the Portuguese inscription "Ordem e Progresso" (Order and Progress). It is meant to represent the state of Para. 

The star is also mentioned prominently in a 2000s-era Japanese manga called "Twin Spica," a series that follows high schoolers attending a fictional astronaut training school. 

Spica in more modern times

It wasn't until the invention of the telescope that it became clear that Spica is a binary star. The two stars orbit each other at just 12 percent the equivalent distance between Earth and the sun.

 

The two stars, according to Jim Kaler, a professor emeritus of astronomy at the University of Illinois, are both class B stars, with the brighter one "nearing the end of its stable lifetime." According to NASA, the primary star is roughly double the sun's size and almost 2,000 times as bright

It was once thought that the binaries' variability in brightness was because one star was passing in front of the other and dimming its light. 

More recent measurements show that it's likely because the stars tidally distort each other with their close proximity. They whirl so close to each other than an orbit only takes four days to complete.

Alternate views of Spica

More recently, Spica's system has been considered an interesting source of X-ray emission. One 2007 paper explored the link between the stars' X-ray emissions and their photospheres, or the physical area within the stars where their energy is released as light.

Another 2001 paper noted that the system is "one of a handful to display the Struve-Sahade effect", referring to changes in the spectral lines in the smaller star. The lines become stronger when the smaller star moves towards the observer, and then weaken when it shifts away.

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