Blue Stragglers

Blue Stragglers in the Central Bulge

Bill Keel, Ray White III, Chris Conselice,
Cerro Tololo Inter-American Observatory



Larger image.




Located 13,400 ly away near the
Small Magellanic Cloud, 47
Tucanae is only observable from
the Southern Hemisphere (more
at Astronomy Picture of the Day
and Bill Keel's Globular Cluster
47 Tucanae).

In large globular clusters, a million stars may be crammed into a space that measures only a few dozen light-years across. By comparison, an equivalent volume in the Solar Neighborhood would contain only several hundred stars. As a result, stars in globular cores are much more likely io suffer gravitational interactions, where lower-mass stars and stellar remnants tend to be ejected from the cluster by energy transfer from higher mass stars (or binaries) which then fall in closer together towards the cluster center. Given the relatively slow motion of stars in globulars and enough time, some of their tightly packed stars, brown dwarfs, and stellar remnants (white dwarfs, neutron stars, and black holes) may eventually collide. Collisions with stellar remnants tend to obliterate main sequence dwarfs, but main sequence stars and brown dwarfs sometimes merge into a single, more massive star that is larger, hotter, and bluer than either pre-merger object (see some animations). It has been estimated that half of the stars in the central regions of some globular clusters may have undergone one or more collisions over the past 13 billion years (Michael M. Shara, Scientific American, November 2002).

Rex Saffer, Dave Zurek,
Villanova University,
STScI, NASA


Composite image with
with 47 Tucanae and
larger uncircled image.


Blue straggler stars
(such as BSS 19) are
formed from the merger
of smaller objects. Five
are circled in this image
of the crowded core of
globular cluster 47
Tucanae (more from
(STScI and Astronomy
Picture of the Day).

By August 1997, 21 anomalous, blue straggler stars (including BSS 19) had been found in globular cluster 47 Tucanae (STScI Press Release), the second largest and brightest globular of the Milky Way's halo (Shara et al, 1997). 47 Tucanae (NGC 104) is located in the southern part (0:24:5.19-72:4:49.9, ICRS 2000.0) of Constellation Tucana (see photo by David Malin), the Toucan -- southwest of Kappa Tucanae, roughly south of Epsilon, Zeta, and Beta1,2 Tucanae, west of the Small Magellanic Cloud, and north of Theta Octanis and Beta Hydri. Its several million stars are spread over a volume nearly 120 light years across, most of which may be around 13.5 +/- 2 billion years old (Hesser et al, 1987). Approaching Sol at about 12 miles (19 km) per second, the globular was discovered by the Abbé [Abbot] Nicholas Louis de La Caille (1713-1762) and cataloged in his list of southern nebulous objects.

CXC, NASA
(Grindlay et al, 2002)


Composite image with
full globular cluster.


The crowded core of
47 Tucanae is full
of unusual compact
binary star systems,
where a dwarf or
giant star is paired
with a white dwarf,
neutron star, or a
black hole. (more
at Astronomy Picture
of the Day and
Chandra X-Ray
Observatory Center).

Empirical evidence for stellar collisions is found in the crowded core of 47 Tucanae, which is full of unusual compact binary star systems (where a normal star is paired with a white dwarf, neutron star, or a black hole) in unusually large numbers rarely found outside of globular cores. The globular's core includes exotic objects such as milli-second pulsars, which are neutron stars that are rotating extremely rapidly (between 100 to nearly 1,000 times a second) as if they have the rotational energy of formerly two objects that recently merged. Most, or all, of the stellar-sized black holes that have formed over the lifetime of the cluster may have been ejected by slingshot encounters with binaries deep in the cluster core. (more from Astronomy Picture of the Day and the Chandra X-Ray Observatory).

Merged stars also stand out conspicuously as bluer and brighter, relatively massive stars in the crowded cores of many large globular clusters like 47 Tucanae. Since most members of ancient globulars are born at the roughly the same time, however, their relatively more massive stars should have already evolved out of existence. Hence, relatively young, more massive stars borned of mergers tend to stand out conspicously among their redder, smaller and dimmer elders, that have survived since the cluster's birth around 10 to 13 or more billion years ago. Many astronomers believe that the presence of anomalously hot and bright, blue stragglers in the dense cores of globulars provides strong evidence of stellar collisions resulting in merged stars (Paresce et al, 1991). Rex A. Saffer of Villanova University and colleagues have found a blue stragger in globular cluster NGC 6397 to be three times as massive as any ordinary star in that cluster, which they and other astronomers believe must have formed from a collisional merger (Sepinsky et al, 2000; and more information and images at APOD). Blue stragglers like BSS 19, however, can also form from the slow coalescence of contact binary stars, or of a star and brown dwarf. (More discussion from Charles Danforth's Blue Stragglers: A Study of Stellar Longevity).

STScI -- larger illustration
(BSS 19 was probably formed by the slow coalescence of contact binary stars -- more).

On October 2, 2006, a team of astronomers announced that they had found evidence that six of 43 blue stragglers in globular cluster 47 Tucanae formed by the slow coalescence of contact binary stars. The six blue stragglers contain less carbon and oxygen than the remaining 37, which suggests that the material at the surface of six came from deep inside their parent stars. In theory, however, this deep material can move to the surface of a blue straggler only during the mass transfer process occurring between two stars in a binary system, which is supported by numerical simulations (ESO press release).

ESO


Larger chart.


Six of 43 blue stragglers in 47 Tucanae
were found to have low abundances of
carbon and oxygen on their surface,
which is suggestive of formation by
slow coalescence rather than by
collision (more).

BSS 19 in 47 Tucanae

This star has about 1.7 +/- 0,4 times Sol's mass, which is nearly twice that (~0.9 Solar) of the oldest primordial hydrogen-burning cluster star in 47 Tucanae (Hesser et al, 1987). It's effective temperature and surface gravity correspond to a spectral and luminosity type of A7 V, for which the luminosity would be 10 to 12 times that of Sol's. BSS 19 is rotating rapidly, which means that it probably formed from the coalescence of a contact binary pair of old, smaller and dimmer stars -- instead of from a collision of previous unrelated stars (Shara et al, 1997). (A good example of a pre-merger pair of close binary stars in the Solar Neighborhood is the W Ursae Majoris type, shallow contact binary 44 (i) Boötis BC.)

© Carla Maceroni
(used with permission)


BSS 19 probably formed
from the slow coalescence
of a contact binary pair
of smaller and dimmer,
lower mass stars (catalog
of W Ursae Majoris,
late-type contact binaries).

As a star formed from a merger (of smaller stars, brown dwarfs, or a combination) will have hydrogen from its outer layers mixed into its core, it will have fresh fuel for core hydrogen fusion. The resulting higher stellar mass, moreover, will burn hotter (and bluer) and faster so that a star of two Solar masses will last only 800 million years, compared to a one-Solar-mass parent star (like Sol) that could have lived for 10 billion or more as a main sequence dwarf. Hence, blue stragglers like BSS 19 will burn out long before their dimmer and redder, ancient neighbors will.

It is likely that most planets that may have formed around BSS 19 or have been lost to interstellar space or the star itself from gravitational perturbations during its formation. Indeed, a recent search for Jupiter-class planets in a globular cluster seem to have found none (Gilliland et al, 2000). In any case, any developing carbon-based life on a developing Earth-type planet would have been subject to the intense and deadly radiation produced by the stellar merger of the original contact binary stars.