New challenge to theory of
black hole formation
27 October 2010
Astronomers from Europe , using the Very Large Telescope in Chile,
have for the first time demonstrated that a magnetar — an unusual type
of neutron star — was formed from a star that started its life with at
least 40 times as much mass as the Sun.
Up till now, a star as massive as this was expected to evolve into a
black hole, not a magnetar. The result presents great challenges to
current theories of how stars evolve and raises a fundamental question:
just how massive does a star really have to be to become a black hole?
The magnetar in question was found in the extraordinary star cluster
known as Westerlund 1, located 16 000 light-years away in the southern
constellation of Ara (the Altar). Westerlund 1 is 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
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. They are formed when a certain type of star undergoes a supernova
explosion: the outer layers get blown away and the core collapses into a
rapidly spinning ball of ultra-dense neutrons.
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
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.
This artist's impression shows the magnetar in
the very rich and young star cluster Westerlund 1. Credit:
"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
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
"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 Clark.
 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
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.