How big is Spica?

Spica is a pair of hot stars in the constellation Virgo, about 250 light-years from Earth, where a light-year is the distance light travels in one year. The larger of the two is about 10.4 million kilometers (6.5 million miles) across, about 7.5 times the width of the Sun, and that is the star the explorer shows. Its partner is about half as wide. The two stars orbit so closely that, seen from Earth, they look like a single star.

Size

A star has no solid surface, so its size is the width of the glowing layer that its light comes from. Spica's sizes come from watching the pair as it orbits.

A 2016 study combined measurements of the stars' speeds with precise readings of their brightness from the MOST satellite, a small space telescope. It found that the larger star has a radius 7.47 times the Sun's, with an uncertainty of about 7%. The smaller star's radius is 3.74 times the Sun's, so it is about 5.2 million kilometers across.

An earlier measurement, made in 1971 with an interferometer, which combines the light from separate telescopes, gave the larger star a radius 8.1 times the Sun's, or about 11.3 million kilometers across. Neither star is quite round. Each star's gravity stretches the other into a slightly egg-like shape, so the pair's brightness rises and falls as they turn.

Size comparisons

Spica's larger star is about 7.5 times as wide as the Sun, and its smaller star about 3.7 times.
About 820 Earths side by side would stretch across the larger star.
Regulus, the next smaller star in the explorer, is about 5.8 million kilometers wide at its equator, so Spica's larger star is about 1.8 times as wide.
The centers of the two stars are, on average, about 19.6 million kilometers apart, about one-third of the distance from the Sun to Mercury.
If the Sun were shrunk to the size of a basketball, Spica's larger star would be a ball about 1.8 meters (5.9 feet) across, a little taller than a person 1.7 meters tall. Its partner would be a ball about 90 centimeters (3 feet) across, and the two would be about 3.4 meters (11 feet) apart.

Two stars in a four-day orbit

The two stars circle each other once every 4.0145 days. Seen from Earth, they are less than 2 thousandths of an arcsecond apart, where an arcsecond is 1/3,600 of a degree. Astronomers first learned that Spica is double in 1890, from the way lines in its light shift back and forth as the stars move. The tides the stars raise on each other also change those lines over each orbit.

Both stars are heavy and young. The larger has about 11.4 times the Sun's mass and the smaller about 7.2 times. Their surfaces are about 25,000 and 20,600 degrees Celsius, and the pair is about 12.5 million years old.

A pulsing star

The larger star also pulsates, in three rhythms at once. In one of them the whole star swells and shrinks, and in the other two its shape changes. One of those rhythms is tied to the orbit: its frequency is an exact multiple of how often the stars go around each other, which suggests that the partner's tides help drive it.