R136a1 -Frequently Asked Questions
The European Southern Observatory
science media release
from 21 Jul 10, reporting on a
paper by Crowther et al. (accessible from arXiv:1007.3284) has received considerable international
interest. This page seeks to explain some of the issues
raised, following up enquires from members of the public and media,
and refers to scientific journal papers where appropriate.
What is all the fuss about? The lower mass
to stars is approximately 1/12th of the Sun's mass, whereas the upper
limit remains controversial. A new
analysis of hot, luminous young stars
in the biggest satellite galaxy of the Milky Way suggests the upper limit
is close to 300 solar masses, a factor of two higher than previously
thought. This is based on studies of previously known stars in the
clusters R136 (Large Magellanic Cloud, see Hubble image) and NGC 3603 (Milky Way, see
What is thought to be the upper stellar mass limit for
stars? Theoretically, a limit to the mass of stars higher than a wide
range of values have been proposed, ranging from
60 solar masses to 440 solar masses. Observationally, a firm limit
of 150 solar masses was inferred from studies of
another young star cluster, The
Arches Cluster (see Hubble image), close to the Galactic Centre. Normal,
stable stars may not exceed their Eddington limit - arising from the ratio
of radiation pressure to gravity. Zero age main sequence stars approach
this limit at very high masses, corresponding to 40 percent (at 150 solar
masses) and 55 percent (at 300 solar masses) of their maximum
Eddington luminosity, according to
evolutionary models of stars in the Large Magellanic Cloud. At its current
age, R136a1 has a luminosity equivalent to 80 percent of its Eddington
luminosity (since models suggest its luminosity has slightly increased
with respect to the value at birth and its mass has decreased by 20
How do you determine the 300 solar mass value for R136a1?
Reliable stellar masses requires knowledge of the temperature and
bolometric (total) luminosity of a star, its evolutionary state, plus
application of theoretical models describing how stars change their
physical properties as they age. For R136a1 the stellar temperature
was derived from stellar atmosphere fits to infrared and optical spectroscopic observations. Its stellar
luminosity then followed from its
infrared apparent magnitude, accounting for the 50 kpc (165,000 light
year) distance to the LMC and modest interstellar extinction. The deduced
stellar properties, plus its surface hydrogen abundance matched
evolutionary models calculated for rotating,
main-sequence LMC-metallicity stars.
Is the high mass for R136a1 reliable? Stellar atmosphere
models and stellar evolutionary models, used to calculate the
present and initial mass for R136a1, may have systematic uncertainties.
Fortunately, direct (model-independent) stellar masses can be obtained
if the star is a member of a binary system. NGC 3603 A1, another system
that has been analysed using identical techniques to R136a1, is an
eclipsing binary system whose components have been accurately measured. Comparisons between the direct,
dynamical masses for the components of NGC 3603 A1 and its theoretically
deduced masses are in excellent agreement, arguing against systematic
uncertainties for R136a1.
How do you deduce an initial mass for R136a1 from its current mass?
In addition to the physical properties deduced for R136a1, we have also
obtained its current rate of mass-loss through a stellar wind. This rate
of 0.00005 solar masses per year is corrected for wind clumping
(structure) and agrees closely with
theoretical wind calculations obtained for the
mass, temperature and luminosity derived for this star, that were
adopted in the evolutionary model calculations. R136a1 has an estimated
age of 1.5 million years, suggesting a loss of 75 solar masses, which is
reduced to 55 solar masses after accounting for the anticipated lower rate
of mass loss at earlier evolutionary phases.
If R136a1 is a binary system, wouldn't the 150 solar mass limit still
We do not claim that R136a1, or any of the other stars that have
deduced initial masses in excess of 150 solar masses are single. However,
for the 150 solar mass limit to (approximlately) remain valid, individual
components would need to be of equal mass. A pair of equal mass stars in
a close orbit
would be expected to show rapid (Doppler) radial velocity variations over
the timescale sampled by our
infrared observations which were not detected. A pair
of equal mass stars in a intermediate orbit would each possess very
winds, interactions of which would lead to powerful X-rays, which are
not seen in R136a, but are detected elsewhere (such as R136c, NGC 3603
C). A pair of equal mass stars initially in a wide orbital
configuration would interact dynamically with other high mass stars
within the centre of R136a owing to a very high stellar high density,
leading to a hardening (reduced separation) of the binary system
components since they formed, again resulting in intermediate
separations leading to the expectation of
bright X-rays. If R136a1 were a binary system, with a secondary to primary
mass ratio of a quarter, the initial
primary mass would be reduced by about 10 percent from the single case.
Is R136a1 the biggest or the heaviest star known?
Neither. The biggest stars known are red supergiants, which are evolved
stars with initial masses in the range from 8 to approximately 20 solar
masses, that possess radii of up to 1500 solar radii. Weight depends on where the
measurement is made (people weigh much more on the Earth than the Moon)
whereas mass is independent of location. R136a1 is the highest mass
star known, but its high temperature results in a relatively modest radius
of 35 solar radii, explaining its blue appearance (warm stars are yellow
and cool stars are red).
How did R136a1 form?
The formation of high mass stars is poorly understood, in part because they form
in dense, dusty regions, far from the neighbourhood of the Sun
hindering direct observations, plus radiation pressure is
believed to generate outflows, hindering accretion (though see counter
arguments here). The current consensus suggests
that high mass stars form extremely rapidly in a few hundred thousand
years, potentially involving stellar
collisions (mergers) in extremely dense systems. Within a few parsec
(10 light years) of R136a1, the total stellar mass is thought to be
50,000 solar masses, so a total of approximately 100,000 stars have
formed, or are in the process of forming. Of these only a dozen stars
have inferred masses above 100 solar masses.
What is the significance of raising the upper mass limit from 150 to
300 solar masses?
Single, low and intermediate mass stars (up to 8 solar masses) are
believed to end their life peacefully as white dwarfs. Single high
mass stars (in excess of 8 solar masses) are believed to end
life violently once they develop iron cores, leading to the formation
of either a black hole or neutron star, and (in most cases) a
so-called core-collapse supernova explosion, up to a limit of 150 solar
masses. In contrast, stars - deficient in elements other than hydrogen or
helium - between 150 and 300 solar masses are predicted to end their
lifes as pair-instability supernovae, exploding prior to the development
of an iron core as exceptionally bright supernovae, without any stellar
remnant. The first stars in the early universe were believed to be
disproportionately massive, such that pair-instability supernovae are
thought to have been relatively common in chemical enrichment of
proto-galaxies. Closer to home, several bright supernovae have also been
claimed to be pair-instability supernovae, albeit
without observational support for very massive stars until now.
What is the lifetime of R136a1 and how will it die?
To date, our theoretical calculations have only been carried
out for the main-sequence evolution of R136a1, but a total lifetime
between 2.5 and 3 million years is anticipated, such that it
is approximately half way though its (very short) lifetime. Whether
R136a1 will explode as a (normal) core-collapse or (abnormal)
pair-instability supernova depends upon the mass of its core after
helium burning. According to independent calculations R136a1 may be composed of material
that is too rich in `metals' (elements other than hydrogen or helium)
to undergo a pair-instability supernova.
What about the Arches cluster?
Very high mass stars tend to be found in massive clusters, so the previous 150 solar mass
of limit was inferred from the highest mass, young
Milky Way cluster, the Arches, close to the Galactic Centre. More
recent infrared imaging, spectroscopic and dust extinction determinations suggests the
most massive stars in the Arches cluster have 200 solar masses,
far closer to that expected in such a high mass cluster if the
physical upper mass limit is 300 solar masses. Overall, a dozen
stars in the Arches, NGC 3603 and R136 clusters have derived initial
masses in excess of 150 solar masses, of which only two (R136c and
Melnick 34) are known or suspected massive binaries.
If it is so bright why didn't astronomers know this already?
Both 30 Doradus (containing R136) and NGC 3603 star forming regions
can be accessed from the southern sky through a small telescope, but are
too far away to be seen unaided with the naked eye. Very massive stars
are incredibly rare - perhaps no more than a few more massive than
150 solar masses exist in the Milky Way out of over 100 billion stars
(typical stars have masses a factor of two lower than the Sun).
R136a1 lies at a projected distance of 5000 AU
(1AU = Earth-Sun distance) from the second brightest star R136a2,
corresponding to 0.1 arcsec (Moon diameter from Earth is 1800 arcsec) so
it was only possible to isolate the two stars from
high resolution infrared spectroscopic and imaging observations, while stellar atmospheric models have also improved
over the past decade. Earlier
studies claimed much lower masses due to
of less sophisticated atmospheric models and uncertain corrections for dust absorption to
optical/ultraviolet Hubble Space Telescope images and spectroscopy
What's with the terrible name? Number 136 in the
Radcliffe catalogue of bright 'stars' in the Large Magellanic Cloud
(satellite galaxy of Milky Way) was subsequently resolved into three
sources, a, b and c. At one point, R136a was claimed to possess a mass as great as 2500 solar masses,
but soon thereafter was proven incorrect, once it was established that it
was in fact a star cluster, in which 7 components were originally
idenfied, labelled as R136a1 (brightest) through R136a7 (faintest).
These ground-based datasets have been refined through Hubble images,
intially with WFPC2 and more recently with WFC3.
See Name that Star! (The Guardian).
Who has been involved in the study? How was it funded?
Team members include Prof Paul Crowther (Sheffield, atmospheric models,
led work), Dr Olivier Schnurr (AIP, binaries), Dr Raphael Hirschi
(Keele/Tokyo, overview of evolutionary models), Ms Norhasliza Yusof
(Malaya, evolutionary model calculations), Dr Richard Parker (Sheffield,
cluster simulations), Dr Simon Goodwin (Sheffield, cluster dynamics), Dr
Hasan Kasin (Malaya, supervision of Ms Yusof). Primary funding was
provided by the UK's Science
and Technology Facilities Council which supported Dr Schnurr
(postdoc) and Dr Parker (PhD studentship) plus access to ground (ESO)
and space (ESA) facilities.
What do experts not involved in the work think?
A number of external experts within this field have commented upon these
results in media reports from Science NOW (Nolan Walborn and Scott Kenyon), New Scientist (Phil Massey and Mark Krumholz), NY Times (Phil Massey and Mark Krumholz)
Discover News (Don Figer). The UK astronomer royal, Prof
Martin Rees has also given his opinion in the Guardian.
Reactions within the scientific literature includes: arXiv:1008.1014 (Phil Massey)
Media reports included BBC Science, BBC Radio 4 Today, Associated Press, Science NOW, New Scientist, The Guardian, The Times, Telegraph, The Independent, Channel 4 News, Sky
and Telescope, Sky News, Al Jazeera English,
National Public Radio, Science News, Mirror, Newsday, COSMOS, NBC, Astronomy, CNN, USA Today, National Geographic, Daily Mail, Astronomy Now, Nature blog, Le Figaro, Spiegel, El Pais,
Pagina/12, Malaysian star
The Monster star also featured in the August 2010
edition of BBC's Sky at Night
I have written about our detective work searching for monster stars in
Astronomical Society of the Pacific's Astronomy Beat column
the story Monster Stars: How Big Can They Get? (pdf format).
Discover Magazine featured Mammoth Star is the biggest one ever seen as one of
their Top 100 Science Stories of 2010, which was also included
in the science news highlights of 2010 from BBC Science News
and the Heftiest stars discovered was from Science News' 2010 Science News of the Year: Atom & Cosmos