When a star tracker on NASA’s Galileo spacecraft
temporarily lost a star being used as a reference point for
monitoring the spacecraft’s attitude, engineers suspected an
aberration in the equipment, not in the star.

After all, this particular star is one of the 50
brightest in the sky, brighter than the North Star. And it
wasn’t listed — at least not yet, when Galileo stopped
recognizing it for about eight hours last June — among the
thousands of stars known to vary in brightness. It is Delta
Velorum, in the southern-hemisphere constellation Vela, the
sail.

“I spent about a week working on it, and concluded the
star scanner wasn’t broken, but perhaps the star was,” said
Paul Fieseler, an engineer for the Galileo project at NASA’s
Jet Propulsion Laboratory, Pasadena, Calif.

Galileo was never intended to make discoveries about
distant stars while it orbits Jupiter, studying that planet
and its surroundings. But combining information from the
spacecraft’s star tracker with careful observations by amateur
astronomers in South America and Africa has led to a
scientific report, published by the International Astronomical
Union, that Delta Velorum does indeed vary in brightness.

“Variable stars are common, but it’s been a surprise that
a star so bright could be variable without anybody reporting
it before,” said Fieseler. He co-authored the report with
amateur astronomer Sebastian Otero of Buenos Aires, Argentina,
and astronomer Christopher Lloyd of the Rutherford Appleton
Laboratory in England.

Fieseler wanted to figure out why the star tracker acted
as if Delta Velorum had disappeared. The star is one of about
150 bright ones the tracker is programmed to recognize by
their brightness and their position in relation to other
bright stars. He quickly checked that Delta Velorum wasn’t
listed as a variable star, then tried to determine why the
star scanner wasn’t working right.

“We were at a loss to understand what happened, but the
problem didn’t happen again, so we just went on to other
things,” said Fieseler. He did, however, send an e-mail to
the American Association of Variable Star Observers, letting
that network of amateur and professional astronomers know
about the incident. He also found data from a few hours in
November 1989 when the instrument measured a brief dip in the
same star’s brightness.

Through various forwardings, Fieseler’s message reached
Otero, who likes to check the sky for errors and omissions in
catalogues of stars’ apparent brightness. Otero had noticed a
one-night dimming of Delta Velorum in 1997, and had checked it
frequently since then, observing it dim three additional
times. He contacted Fieseler in October about his
observations.

“I had almost forgotten about the whole thing when I got
this e-mail from Argentina,” Fieseler said.

Otero and Lloyd used the information from Galileo, as
well as Otero’s observations, to calculate the rhythm of the
dimming events. They predicted the next two, at 45-day
intervals. Several amateur astronomers in South America,
Africa and Australia, watched Delta Velorum and confirmed the
accuracy of the predictions.

Delta Velorum was previously known to be a multiple star,
a tight grouping of at least five stars. The pattern of
dimming indicates that what had been seen as the brightest
individual member of the group is actually two stars of
similar brightness orbiting each other and periodically coming
in front of each other, according to Otero, Fieseler and
Lloyd. When either one eclipses the other, the total
brightness declines by about 30 percent. Such mutually
eclipsing binary stars are one of the common types of variable
stars.

Galileo’s star tracker knew how to recognize Delta
Velorum at its usual brightness, but when the brightness
dipped, that dot of light no longer fit the programmed
criteria, Fieseler said. If the instrument had not lost the
star, Fieseler wouldn’t have been looking for an explanation
and Galileo would not have added a star’s variability to its
list of discoveries.

Two factors may explain why nobody appears to have
noticed this star’s variability before Otero. The amount of
change in brightness is small enough that it is difficult to
gauge by eye, and the change happens only during a few hours
every 45 days.

More information about Galileo, which has been orbiting
Jupiter since 1995, is available at
http://galileo.jpl.nasa.gov . JPL, a division of the
California Institute of Technology in Pasadena, manages the
mission for NASA’s Office of Space Science, Washington, D.C.