How Much Mass Makes a Black Hole? Astronomers
Challenge Current Theories
ScienceDaily (Aug. 19, 2010) — Using
ESO's Very Large Telescope, European astronomers have for the first time
demonstrated that a magnetar -- an unusual type of neutron star -- was formed
from a star with at least 40 times as much mass as the Sun. The result
presents great challenges to current theories of how stars evolve, as a star
as massive as this was expected to become a black hole, not a magnetar. This
now raises a fundamental question: just how massive does a star really have to
be to become a black hole?
To reach their conclusions, the astronomers looked in detail at the
extraordinary star cluster Westerlund 1, located 16 000 light-years away in the
southern constellation of Ara (the Altar). From previous studies, the
astronomers knew that Westerlund 1 was the closest super star cluster known,
containing hundreds of very massive stars, some shining with a brilliance of
almost one million suns and some two thousand times the diameter of the Sun (as
large as the orbit of Saturn).
"If the Sun were located at the heart of this remarkable cluster, our night
sky would be full of hundreds of stars as bright as the full Moon," says Ben
Ritchie, lead author of the paper reporting these results.
Westerlund 1 is a fantastic stellar zoo, with a diverse and exotic population
of stars. The stars in the cluster share one thing: they all have the same age,
estimated at between 3.5 and 5 million years, as the cluster was formed in a
single star-formation event.
A magnetar is a type of neutron star with an incredibly strong magnetic field
-- a million billion times stronger than that of the Earth, which is formed when
certain stars undergo supernova explosions. The Westerlund 1 cluster hosts one
of the few magnetars known in the Milky Way. Thanks to its home in the cluster,
the astronomers were able to make the remarkable deduction that this magnetar
must have formed from a star at least 40 times as massive as the Sun.
As all the stars in Westerlund 1 have the same age, the star that exploded
and left a magnetar remnant must have had a shorter life than the surviving
stars in the cluster. "Because the lifespan of a star is directly linked to its
mass -- the heavier a star, the shorter its life -- if we can measure the mass
of any one surviving star, we know for sure that the shorter-lived star that
became the magnetar must have been even more massive," says co-author and team
leader Simon Clark. "This is of great significance since
there is no accepted
theory for how such extremely magnetic objects are formed."
The astronomers therefore studied the stars that belong to the eclipsing
double system W13 in Westerlund 1 using the fact that, in such a system, masses
can be directly determined from the motions of the stars.
By comparison with these stars, they found that the star that became the
magnetar must have been at least 40 times the mass of the Sun. This proves for
the first time that magnetars can evolve from stars so massive we would normally
expect them to form black holes. The previous assumption was that stars with
initial masses between about 10 and 25 solar masses would form neutron stars and
those above 25 solar masses would produce black holes.
"These stars must get rid of more than nine tenths of their mass before
exploding as a supernova, or they would otherwise have created a black hole
instead," says co-author Ignacio Negueruela. "Such huge mass losses before the
explosion present great challenges to current theories of stellar evolution."
"This therefore raises the thorny question of just how massive a star has to
be to collapse to form a black hole if stars over 40 times as heavy as our Sun
cannot manage this feat," concludes co-author Norbert Langer.
The formation mechanism preferred by the astronomers postulates that the star
that became the magnetar -- the progenitor -- was born with a stellar companion.
As both stars evolved they would begin to interact, with energy derived from
their orbital motion expended in ejecting the requisite huge quantities of mass
from the progenitor star. While no such companion is currently visible at the
site of the magnetar, this could be because the supernova that formed the
magnetar caused the binary to break apart, ejecting both stars at high velocity
from the cluster.
"If this is the case it suggests that binary systems may play a key role in
stellar evolution by driving mass loss -- the ultimate cosmic 'diet plan' for
heavyweight stars, which shifts over 95% of their initial mass," concludes
 The open cluster Westerlund 1 was discovered in 1961 from Australia by
Swedish astronomer Bengt Westerlund, who later moved from there to
become ESO Director in Chile (1970-74). This cluster is behind a huge
interstellar cloud of gas and dust, which blocks most of its visible light. The
dimming factor is more than 100 000, and this is why it has taken so long to
uncover the true nature of this particular cluster.
Westerlund 1 is a unique natural laboratory for the study of extreme
stellar physics, helping astronomers to find out how the most massive stars in
our Milky Way live and die. From their observations, the astronomers conclude
that this extreme cluster most probably contains no less than 100 000 times the
mass of the Sun, and all of its stars are located within a region less than 6
light-years across. Westerlund 1 thus appears to be the most massive compact
young cluster yet identified in the Milky Way galaxy.
All stars so far analysed in Westerlund 1 have masses at least 30-40 times
that of the Sun. Because such stars have a rather short life -- astronomically
speaking -- Westerlund 1 must be very young. The astronomers determine an age
somewhere between 3.5 and 5 million years. So, Westerlund 1 is clearly a
"newborn" cluster in our galaxy.
The research will soon appear in the research journal Astronomy and
Astrophysics ("A VLT/FLAMES survey for massive binaries in Westerlund 1:
II. Dynamical constraints on magnetar progenitor masses from the eclipsing
binary W13," by B. Ritchie et al.). The same team published a first study of
this object in 2006 ("A Neutron Star with a Massive Progenitor in Westerlund 1,"
by M.P. Muno et al., Astrophysical Journal, 636, L41).