Aldebaran is a giant star with a deep orange color, and it forms the eye of the constellation Taurus. It is about 63 million kilometers (39 million miles) across, about 45 times the width of the Sun. That is the width of its visible surface.

Size

Aldebaran looks larger in the sky than almost any other star, and by 2002 its width had been measured more often than any other star's. A catalog from that year lists about 60 measurements. Many of them come from lunar occultations: Aldebaran lies along the Moon's path across the sky, so the Moon often passes in front of it. The time the Moon's edge takes to cover the star shows how wide the star looks.

A 2005 study by Richichi and Roccatagliata combined new occultation and interferometer measurements. It found that Aldebaran is 20.58 thousandths of an arcsecond across, where an arcsecond is 1/3,600 of a degree. That is about the width of a human hair seen from 1 kilometer (0.6 miles) away. The Hipparcos satellite measured Aldebaran's distance at about 67 light-years, where a light-year is the distance light travels in one year, about 9.46 trillion kilometers. At that distance, the width gives a radius of 45.1 times the Sun's.

Older measurements of Aldebaran scatter more widely, from about 36 to 46 times the Sun's radius, and the reason is not known. The star's brightness changes by less than 1%, but it may have spots on its surface or change slightly in size.

Size comparisons

• Aldebaran is about 45 times as wide as the Sun and about 4,900 times as wide as Earth.
• About 92,000 Suns would fit inside it, by volume.
• If Aldebaran took the Sun's place, its surface would reach more than halfway to the orbit of Mercury.
• It is about 1.8 times as wide as Arcturus, another giant star and a smaller neighbor in the explorer.
• If the Sun were shrunk to the size of a basketball, Aldebaran would be a ball about 11 meters (36 feet) across.

A disputed planet

Since the early 1990s, astronomers have measured Aldebaran moving very slightly toward and away from Earth, in a cycle of about 629 days. A 2015 study proposed that a planet causes this wobble. It would have at least 6.5 times the mass of Jupiter and orbit about 1.46 astronomical units from Aldebaran, where one astronomical unit is the average distance from Earth to the Sun.

A 2019 study added many new measurements and found that the signal was not steady over time. Its authors concluded that their data weaken the case for a planet. They suggested that oscillations linked to the churning of gas inside the star might cause the wobble instead.

A spacecraft on its way

Pioneer 10, NASA's first mission to the outer planets, was launched in 1972 and sent its last signal to Earth in 2003. It is heading roughly toward Aldebaran and is expected to pass it in about two million years.

Sources

• Aldebaran's angular diameter: How well do we know it?. Richichi and Roccatagliata, Astronomy & Astrophysics, 2005.
• Long-lived, long-period radial velocity variations in Aldebaran: A planetary companion and stellar activity. Hatzes et al., Astronomy & Astrophysics, 2015.
• Precise radial velocities of giant stars. XII. Evidence against the proposed planet Aldebaran b. Reichert et al., Astronomy & Astrophysics, 2019.
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