R136a1 -Frequently Asked Questions
The European Southern Observatory science media release Stars just got bigger from 21 Jul 10, reporting on a MNRAS journal 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 limit 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 Hubble image).
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 percent).
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 hold? 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 power stellar 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 their 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 the combination 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
I have written about our detective work searching for monster stars in the Astronomical Society of the Pacific's Astronomy Beat column - access 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