Altair is the brightest star in the constellation known as the eagle, and one of the 12 brightest stars in the night sky. But a fainter star that appears just above it as night falls is actually a whole lot brighter. It only looks fainter because it’s a whole lot farther away.
Tarazed looks only about one-sixth as bright as Altair. But it’s 35 times farther than Altair. When you take that into account, Tarazed shines a couple of hundred times brighter.
Over the past few years, astronomers have refined the distance to the star using observations from Gaia, a European space telescope. It’s measuring the distances to more than a billion stars, and compiling detailed profiles of many of them.
Gaia determines a star’s distance by measuring its parallax. It observes a star at six-month intervals, when Gaia is on opposite sides of the Sun. The star appears to move back and forth a tiny bit against the background of more-distant objects. The angle of that shift reveals the star’s distance.
Gaia found that Tarazed is a couple of hundred light-years farther than astronomers had thought. That means it’s also bigger and brighter than thought — and much more impressive than nearby Altair.
Watch Tarazed as it leads Altair across the sky on summer nights. The stars are low in the east as darkness falls, due south in the wee hours of the morning, and high in the southwest at dawn.
Tomorrow: planets in the dawn sky.
Script by Damond Benningfield