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The quest for the most massive star: astronomers are conducting a frenetic search for our galaxy's most massive star. (Our Galaxy's Biggest, Brightest Brutes.

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Stars are not born equal. Those with large masses dominate their surroundings; only they can simultaneously make gas shine to create beautiful nebulae, push around nearby material, and form a rich stew of heavy chemical elements. With masses tens of times that of the Sun and with light outputs millions of times solar, these stellar masters hold the power of life and death. They destroy fragile objects such as protoplanetary disks and trigger the birth of other stars. And after their brief but flamboyant lives, they die in titanic supernova explosions.

So if you want to understand the universe, you need to understand these massive objects. Moreover, because the most massive stars produce the greatest impact, the record holders constitute an astronomical "holy grail."

Astronomers have other reasons to study the most massive stars. Every stellar model needs to be tested, and what's a better laboratory than extremely massive stars? Indeed, the lowest mass for a star is a well-known limit: 8% of the Sun's mass. Below that, stellar cores lack the necessary pressures and temperatures to sustain nuclear fusion. But theories are less clear-cut on the upper boundary. For the moment, astronomers can only surmise that a limit exists. Above a certain mass, nuclear reactions in a stellar core become so powerful that they destroy the star. Calculating this exact mass depends on our knowledge of nuclear and stellar physics, but is thought to be around 100 to 120 solar masses for stars that have formed recently in our Milky Way Galaxy. It remains to be tested if our educated guesses are correct.

Weighing Without a Scale

Measuring a star's mass is no easy job. After all, astronomers cannot visit a star and put it on a scale. Weighing must be done from afar. How can this be done?

One strategy is to model stellar spectra. When astronomers obtain a star's spectrum, they are measuring chemicals and conditions in the star's photosphere. The atmosphere's physical state depends crucially on its temperature and its pressure, which in turn are linked to the star's gravity, which in turn depends on its radius and mass.

Astronomers can use basic physics to model stellar atmospheres. For a given set of parameters (temperature, radius, mass), scientists can calculate the state of each chemical element in the atmosphere, thereby deriving its potential signature in a stellar spectrum. Astronomers can then compare a real spectrum to the modeled one, find the best fit, and then indirectly infer the stellar parameters, including the mass. To find the record holder, one can simply imagine modeling many different stars, until the most massive object is found.

But models are always calibrated by observations. Searching for the most massive objects--by nature outside the tested limits of the model--implies extrapolation, which is never precise, nor always correct. The modeling of stellar spectra can thus only provide an approximate mass.

Another possibility is to measure a star's luminosity. For adult (main-sequence) stars, basic stellar physics dictates that their luminosities are strongly correlated with their masses. For example, a 40-solar-mass star is five times more luminous than a 20-solar-mass star. Estimating masses now appears trivial: observe many different luminous stars, preferentially in several filters, measure their luminosities, and then compute their masses. But simplicity is often deceptive and many details render the task much more complicated than first thought.

For example, this mass-luminosity relation is only valid for the absolute luminosity, while a direct measurement yields only an apparent brightness. To determine the former from the latter, astronomers must correct for distance, reddening by dust, and the short energy range sampled by the observations--three parameters that are not always known with precision.

Astronomers also need to be certain that the measured luminosity applies to only one star. Errors can be frequent where stars are packed closely together in a cluster. One famous example is R136 in the Large Magellanic Cloud. Based on its high luminosity, it was once considered to be a single "superstar" with a whopping 1,000 to 2,500 solar masses. But in 1991, the Hubble Space Telescope confirmed earlier results from speckle imaging that R136 was a tight cluster composed of hundreds of stars. Sure, it contained some very massive stars, but they were not as massive as previously claimed.

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The same error was made on a smaller scale for Pismis 24-1, when recent Hubble observations showed that it consists of two stars. The mass of the famous Pistol Star, often mentioned as the record-holder with an original mass of 200 Suns, is thus to be taken with caution: it could be off by a factor of two.

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If observing one star doesn't give precise information, measuring several can yield reliable statistics. Astronomers have known for decades that the mass distribution of stars is not random: massive stars are less common that low-mass stars. When expressed mathematically, this distribution is called Salpeter's law, after the late Cornell University astronomer Edwin Salpeter. For each star in our galaxy with a mass between 60 and 120 solar masses, there are 250 objects with 1 to 2 solar masses, and 5,600 stars with one-fifth to one-tenth of the Sun's mass.

Astronomers can observe a group of stars, estimate the stellar masses through the mass-luminosity relation (however imprecise that might be), and then check if Salpeter's law correctly applies. To understand the population of the rare massive stars, one needs a very large stellar group--such as the Arches cluster, a grouping of about 100 hot, luminous stars (and thousands of cooler ones) near the galactic center. The statistical analysis clearly shows a shortage of extremely high-mass stars: 20 to 30 stars with more than 130 solar masses should be present that are not detected. This result implies that stars with more than about 130 solar masses cannot form.

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Similar studies based on the simultaneous analysis of several clusters have derived comparable upper limits of 150 solar masses. But if astronomers have proven that the limit exists, they do not have a precise value yet. Statistically, 130 or 170 solar masses are as acceptable as the mean value of 150.

The Solution

In fact, there is only one proven method for precisely determining the mass of a star: studying eclipsing spectroscopic binaries.

About half of stars reside in binaries, in which two stars revolve around a common center of gravity. The spectra of such systems combine the spectrum of each star. Spectral lines, which are the signatures of chemical elements, therefore appear double. The positions of these lines change over time. This movement, linked to the Doppler effect, reflects the orbital motion. The lines of the approaching star are shifted toward the blue, whereas those of its receding companion are shifted toward the red. The shifts are reversed after half a cycle.

Astronomers can then apply Kepler's and Newton's laws to measure the velocities and thus determine the orbital parameters: the distances of the stars to their common center of mass, the orbital eccentricity, the period, and each star's mass. For massive stars, the precision achieved for these values depends mainly on the quality of the spectra. Today, it can reach 1 kilometer per second, or about 2,200 miles per hour. (Compare this to the 1-meter-per-second precision achieved in exoplanet searches of low-mass stars observed with very-high-resolution spectrographs.)

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But the velocity curve alone doesn't yield the actual stellar distances and masses. These parameters have to be multiplied by a factor depending on the orbit's inclination to our line-of-sight. The orbital plane could be tipped in any direction, so the masses determined by this method are thus only lower limits. To eliminate that uncertainty, astronomers study eclipsing binaries. The stars in these systems periodically occult each other, which means that the orbital plane is nearly perfectly aligned with our line-of-sight.

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The most promising targets are the rare stars of spectral type 02 and 03, which are the hottest and most luminous known. But studies of these objects have not fulfilled our hopes. Astronomers have measured 03 stars with 50 to 60 solar masses, but this is quite far from the mean value of 150 estimated by cluster statistics.

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In fact, the most massive known stars came from an unexpected direction: Wolf-Rayet stars. In principle, these objects correspond to evolved 0 stars that have exhausted their hydrogen fuel and are now burning helium and heavier elements in their cores. Since hot stars possess strong stellar winds (a scaled-up version of the solar wind), they eject tens of solar masses--perhaps half their initial mass--during their lives. Indeed, WR stars are generally much less massive than 0 stars.

The classification of WR stars relies only on the peculiar appearance of their spectra, which results from the presence of a very dense outflow. But in the past few years, astronomers have detected "false" WR stars. These deviant objects are still burning hydrogen in their cores: they are thus super-0 stars rather than their evolved descendants. They just happen to eject large amounts of material, much more than "normal" 0 stars, thereby mimicking the characteristics of genuine WR stars.

In 1996 a Belgian team led by Gregor Rauw studied the very massive Wolf-Rayet star WR22 and measured a minimum mass of 72 Suns. This surprising result was confirmed by a team led by Jorg Schweickhardt, but with a downward revision to 55 solar masses. This conclusion is still to be secured--the second team had more spectra but of a lesser quality.

In 2004 the same Belgian team unveiled the incredible properties of the truly astonishing binary WR20a, a system little studied until then. It's now known to contain two stars of 82 and 83 solar masses--with an uncertainty of only 7%. Each one of these objects beats the previous record by a large amount.

Since then, the quest has intensified, thanks to the combined efforts of Belgian, Canadian, and Argentinean teams. The results for various eclipsing binaries are generally less precise than for WR20a, but they are all very encouraging: HD 15558 contains two objects of 152[+ or -]51 and 46[+ or -]11 solar masses; NGC 3603-A1 possesses two stars of 114[+ or -]30 and 84[+ or -]15 solar masses; WR25 consists of two stars of 75[+ or -]7 and 27[+ or -]3 solar masses; and R145 has two components of 140[+ or -]37 and 59[+ or -]26 solar masses. These values need to be confirmed, but they show that astronomers are inching closer to the symbolic figure of 100 solar masses.

Theoretical work is needed to check if these observations fit the current stellar models. It's possible that a more massive star than the ones mentioned above could exist and have no binary companion at all. In this case, there's no possibility to ascertain its mass precisely and so astronomers must accept defeat.

Even More?

Is 100 solar masses the final record? Maybe not. Over the past decade, a new type of star has emerged at the forefront of stellar astrophysics: Population III. These were the first stars born in the universe, when the cosmos was only a few hundred million years old (S&T: May 2006, page 30). Because they formed from gas clouds consisting of pure hydrogen and helium, computer simulations strongly suggest that these objects did not follow Salpeter's law. They could have been very massive--"very" meaning several hundred solar masses, maybe up to a few thousand! Other researchers have also speculated about the early existence of dark-matter-powered stars that could have had up to 10,000 solar masses (March issue, page 26).

Population III stars could be the sources of distant gamma-ray bursts. Our present telescopes cannot see far enough back in time to study such objects, but astronomers are confident that the next generation of instruments, such as NASA's James Webb Space Telescope, the Thirty Meter Telescope, and the Extremely Large Telescope, will help confirm or deny this bold hypothesis. ?

RELATED ARTICLE:

SUPER STAR CLUSTER R136 resides near the center of the Tarantula Nebula (NGC 2070) in the Large Magellanic Cloud. Astronomers once thought that R136 was a single super star of several thousand solar masses. But Hubble Space Telescope images such as this clearly resolve it into a cluster populated by thousands of luminous stars.

RELATED ARTICLE: The metal-mass connection.

In astronomy parlance, elements heavier than hydrogen and helium are called "metals." Low-metallicity stars produce very weak stellar outflows during their formation, since the few electrons in hydrogen and helium means the infalling material is less easily driven off by radiation pressure. These stars can thus accumulate much higher mass than stars that form today in our galaxy, whose interstellar nurseries are laden with metals produced and ejected by previous generations of stars.

RELATED ARTICLE: The birth of massive stars.

Astronomers have known for decades that low-mass stars form and grow by accreting nearby gas. Such a process has problems forming massive stars, however. Before they are completely formed, these objects begin to shine so brightly that the sheer pressure of their radiation pushes away infalling material--prohibiting further growth. This threshold occurs at about 10 solar masses.

But stars much more massive than that clearly exist. Theorists can get around the problem in one of two ways. First, they have developed models in which a star accretes gas at its equator and emits most of its light from its poles. Second, they can invoke the merger of two or more lowmass stars into a very massive star near the centers of dense clusters. These formation scenarios lack clear-cut limits, and there are no observations yet that could help constrain them since we have never witnessed the birth of a truly massive star.--Y. N.

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RELATED ARTICLE: What about Eta Carinae?

Eta Carinae is certainly one of the most luminous and massive stars in our galaxy. Based on its extraordinarily high luminosity of about 6 million Suns, astronomers inferred that it may contain about 120 solar masses. But this guesstimate could be way off because the star is shrouded in mystery...literally. This Hubble Space Telescope image shows Eta Carinae embedded in the Homunculus Nebula, which contains many solar masses of gas and dust ejected by the star. This material makes direct observations of the star virtually impossible. There's strong evidence for a binary companion, but astronomers have yet to ascertain its exact properties. Current estimates suggest the two stars contain about 90 and 30 solar masses.--Robert Naeye

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Yael Naze is an astronomer at the FNRS/Universite de Liege, Belgium, who studies massive stars. As a writer and public lecturer in her free time, she tries to share her passion for these intriguing objects--and other marvels of the sky.
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Author:Naze, Yael
Publication:Sky & Telescope
Geographic Code:1USA
Date:May 1, 2010
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