How big is Altair?

Altair is a hot star in the constellation Aquila, about 17 light-years from Earth. A light-year is the distance light travels in one year. It spins so fast that it bulges at its equator. It is about 2.8 million kilometers (1.7 million miles) across at its equator, twice the width of the Sun, but only about 2.3 million kilometers from pole to pole.

Size

A star has no solid surface, so its size is the width of the glowing layer that its light comes from. In 2007, astronomers measured Altair with the CHARA Array, which combines the light of several telescopes to see detail too fine for any one of them. They used it to make an image of the star's surface, and fitted a model of a spinning star to their measurements.

The model gave a radius at the equator of 2.03 times the Sun's and a radius at the poles of 1.63 times. So the star is about a quarter wider at its equator than from pole to pole. A 2020 study, using a more detailed model of the star's interior, found an equatorial radius of 2.01 times the Sun's, within about 1% of the earlier result.

Size comparisons

Altair is about twice as wide as the Sun across its equator.
About 220 Earths side by side would stretch across its equator.
Procyon, which has the same size in the explorer, is almost exactly as wide: about 2.8 million kilometers.
Vega, another fast-spinning star, is about 3.8 million kilometers wide at its equator, so Altair is about three-quarters as wide.
If the Sun were shrunk to the size of a basketball, Altair would be a slightly squashed ball about 49 centimeters (19 inches) across at its equator and 39 centimeters (15 inches) from pole to pole.

A star that spins in under eight hours

Altair turns once every 7 hours and 46 minutes, and its equator moves at about 314 kilometers per second. The Sun, by comparison, takes about 25 days to turn once. Spinning this fast pushes Altair's equator outward, which is why the star is flattened.

The bulging equator is also cooler than the poles, an effect called gravity darkening. The 2007 study measured about 8,200 degrees Celsius at the poles and about 6,600 at the equator. Its image showed the effect directly: a bright region around the pole, and a darker band along the equator that is only 60–70% as bright as the pole.

In the night sky

Altair is one of the three bright stars of the Summer Triangle, with Vega and Deneb. The triangle is an asterism, a familiar pattern of stars that is not one of the 88 official constellations. In the Northern Hemisphere it appears high overhead on summer evenings, and it can be seen even from light-polluted cities.