HST News & Updates Archive
( October 29, 1997, continued)
Hubble Catches Up with a Blue Straggler Star
Astronomers studied a blue straggler in the tumultuous heart of the nearby globular
cluster 47 Tucanae (47 Tuc), located 15,000 light-years away in the southern constellation
Tucana. The observation was made October 27, 1995 by astronomers Michael M. Shara of the
Space Telescope Science Institute in Baltimore, Md.; Rex A. Saffer of Villanova University
in Villanova, Pa.; and Mario Livio, also of the Institute. Their analysis will appear in
the Nov. 1 issue of the Astrophysical Journal Letters.
"This is an extremely exciting result," Saffer said, "because it may
help distinguish between competing theories of blue straggler star formation and
"Allan Sandage (an astronomer with Carnegie Observatories in Pasadena, Calif.)
discovered blue stragglers in the globular cluster M3 nearly 45 years ago. Since that
time, they have been assumed to be stars much like the Sun, although their bluer color and
larger brightness imply that they are more massive and much younger than normal globular
cluster stars. Our analysis confirms that, but without having to make any assumptions
about the state of blue straggler star evolution."
Using the Hubble telescope's Faint Object Spectrograph, which was removed during the
Second Servicing Mission in February, the astronomers analyzed the spectrum of one blue
straggler, measuring its temperature, radius, and rotation rate. The team then combined
these measurements with the blue straggler's apparent brightness, taken from a Hubble
telescope Wide Field and Planetary Camera archival image, to obtain the star's mass. The
derived temperature and mass are consistent with the characteristics of a normal star with
a mass about 1.7 times that of the Sun. However, the star is spinning at least two to
three times faster than stars of its kind.
"Masses are among the most fundamental properties of stars," Saffer
explained. "Yet, except for some stars in binary systems, they are difficult to
measure directly. Measurements of the masses and rotation rates of blue stragglers in
globular clusters are extremely important, since they allow us to trace the history of
cluster formation and evolution. This is possible because different formation theories
make different predictions about these characteristics."
"A direct mass measurement of a blue straggler is a kind of acid test of theory
and observations," Shara added. "In this case, the mass theory has been tested
and stands up well."
By analyzing a blue straggler's mass, temperature, and rotation velocity, astronomers
can investigate how it formed. Astronomers now believe that blue stragglers are created by
the merger of two low-mass stars. But they have two different views of how these stars
interact to create blue stragglers. One merger theory proposes that a violent collision of
two unrelated stars creates a blue straggler. Another hypothesizes that a slow coalescence
of a gravitationally bound pair creates the straggler star.
Based on their analysis of the blue straggler in 47 Tuc, the team favors the slower,
gentler merger scenario between binary stars. In double-star systems where the stars are
close enough to touch each other, the more massive star can cannibalize its partner,
producing a single, even more massive star. This process, astronomers believe, more likely
results in a rapidly spinning merger product where the fast orbital motions of the binary
star produces the rapid spin of the consolidated pair.
"Our measurement of a rapid rotation rate for the blue straggler star in 47 Tuc
tends to strongly favor this second mechanism," Saffer said, "at least for this
The second merger scenario involves a collision between two unrelated stars, which run
into each other by chance in the dense star cluster core.
"It's a bit like a head-on wreck between two tractor trailers," Saffer
explained, "where the enormous energy carried by the fast-moving stars is deposited
in the debris from the collision."
The merged star is tremendously heated and swells into a red giant star, where it can
easily lose its spin through magnetic activity. The swollen star's distended magnetic
field throws off mass at high speed, like mud flung off the rim of a rapidly rotating
wheel. Eventually, when the star shrinks to normal size after radiating the heat generated
by the collision, it spins itself down and becomes a slow rotator, Shara said.
Saffer credits the Hubble telescope's superior spatial resolution with being able to
peer into a swirling mix of stars to capture a blue straggler in the cluster core.
"While some blue stragglers are found in globular cluster outskirts, in 47 Tuc the
blue stragglers are only found in the cluster core," Saffer said. "The crowding
of the stars there is too severe for the current generation of ground-
based telescopes to resolve them."
Globular clusters are massive systems of up to 1 million stars packed into a spheroid
about 20 light-years in diameter. They also are among the oldest stellar systems in the
Milky Way Galaxy. Stars speeding through the extremely crowded cluster core are far more
likely to collide or experience other dynamic interactions with their neighbors than stars
in the sparse neighborhood of the Sun. These processes can produce a zoo of stellar
animals, such as X-ray binaries, pulsars, blue stragglers, and other exotic species, all
of which have actually been observed in globular cluster cores.
47 Tuc (NGC 104) is one of the intrinsically largest and brightest globular clusters. A
naked eye object, it is just west of the Small Magellanic Cloud. Blue stragglers were
first discovered in its core by the Hubble telescope's Faint Object Camera in 1991.