How big is La Superba?

La Superba, also called Y Canum Venaticorum, is a carbon star: a red giant, late in its life, with more carbon than oxygen in the gas around it. It lies about 1,000 light-years from Earth in the constellation Canes Venatici, the Hunting Dogs. La Superba is about 660 million kilometers across, roughly 470 times the width of the Sun. That estimate is for its visible surface and is uncertain by about an eighth.

Size

Astronomers work out the star's width from how wide it looks in the sky and how far away it is. In 2003 a team used the IOTA interferometer, which combined the light of three telescopes, to measure its apparent width in infrared light. They found about 14 thousandths of an arcsecond, where an arcsecond is 1/3,600 of a degree.

The Gaia space telescope later measured its distance at about 310 parsecs, or 1,010 light-years, where a parsec is about 3.26 light-years. Combining the two, a 2021 study gave a radius of 330 million kilometers, give or take 40 million. That makes La Superba about 660 million kilometers across. Older estimates of its distance were as close as about 710 light-years, which made the star seem smaller.

La Superba is a variable star. Its brightness rises and falls in an irregular rhythm. Studies of the star have used a cycle of about 157 days, but the main catalog of variable stars now lists one of about 3,000 days.

Size comparisons

La Superba is about 470 times as wide as the Sun.
If La Superba took the Sun's place, its surface would reach past the orbit of Mars, about 40% of the way to the orbit of Jupiter.
About 100 million Suns could fit inside it, by volume.
Antares, a red supergiant and the next larger star in the explorer, is about 1.4 times as wide as La Superba. S Doradus, at its largest, is about four-fifths as wide.
If the Sun were shrunk to the size of a basketball, La Superba would be a ball about 110 meters (360 feet) across, about the length of an American football field.

A carbon star

Cool giant stars are sorted by the balance of carbon and oxygen in the gas around them. In carbon stars there is more carbon than oxygen, and in oxygen-rich stars the reverse. La Superba's surface is about 2,760 kelvins (degrees measured from absolute zero), less than half as hot as the Sun's.

La Superba belongs to a smaller group, the J-type carbon stars, which make up about 10–15% of carbon stars. They are unusually rich in carbon-13, a heavier form of carbon. In the gas around La Superba there are only about 2.5 atoms of ordinary carbon-12 for every atom of carbon-13, while on Earth there are about 92. How J-type stars became so rich in carbon is not yet known.

A shell of lost gas

La Superba is shedding its outer layers in a slow wind that leaves at about 8 kilometers per second. It loses roughly one Sun's worth of material every 7–10 million years. Far from the star, the wind runs into the thin gas between the stars and slows down, piling up into a hollow shell of gas and dust. The shell is about 3 light-years across, and it has been imaged in infrared light and detected in radio waves.