Hefty Stars Prompt Birth Debate

A new study found more massive stars than expected in an intensely star-forming region. The results beg the question whether the process of star formation really is universal.

The Tarantula Nebula, a luminous hub of giant stars-to-be in the Southern Hemisphere sky, sits more than 160,000 light-years away in another galaxy. If we moved it as close as the Orion Nebula, our nearest (but much smaller) example of massive star formation, it would span the area of 60 full Moons and appear so bright that it would cast its own shadows.

This star formation region, also called 30 Doradus, is the biggest and brightest in the entire Local Group of galaxies. It may serve as a good example of what starbirth looked like in the universe’s turbulent early years.

That’s why recent results are so surprising: Fabian Schneider (University of Oxford, UK) and colleagues published details on hundreds of the nebula’s stars in the January 5th Science, revealing extra stars with at least 60 Suns’ worth of mass. The excess of massive stars suggests that their formation isn’t a universal process.

Tarantula Nebula

The Hubble Space Telescope imaged the central region of the Tarantula Nebula.
NASA / ESA / Danny LaCrue

How Do Stars Form?

The process of star formation begins when a cloud of gas meets one basic condition: its own gravity overcomes internal pressure. In other words, a cloud can only begin to fall in on itself when it’s big and dense enough.

But while the minimum condition is well understood, the maximum remains a mystery. A single gas cloud won’t collapse into a single, gigantic star — it fragments. A single gas cloud will produce many smaller stars and only a few bigger ones, just as you’ll typically see many more pebbles than boulders when you hike a mountain trail.

The details of this process are difficult to understand, and for more than a decade, astronomers have been debating whether it’s really universal. Do differing conditions, such as the presence of turbulence or more pristine gas, affect how stars form, or is the process pretty much the same wherever we look?

Made in the Tarantula Nebula

Schneider’s team used the Fibre Large Array Multi Element Spectrograph on the Very Large Telescope in Chile to take spectra of about 800 stars in the Tarantula Nebula. A star’s spectrum reveals many of its characteristics, including its temperature, luminosity, surface gravity, and rotational velocity. Focusing 247 single stars with at least 15 times the Sun’s mass, the astronomers calculated the regions initial mass function, basically determining how many massive stars there are compared to smaller ones.

To their surprise, Schneider and colleagues found more massive stars than previous studies would have predicted. Not only that, but they found the nebula gave birth to truly gigantic stars: the birth weight of the largest star they observed was equivalent to 200 Suns, and even more massive stars (with up to 300 solar masses) might exist in the central (unobserved) regions of the nebula. These findings run against previous results showing a maximum stellar mass of 150 Suns.

It could be that the type of stars that form depends on where they’re born. The intense conditions in starbursting regions like the Tarantula Nebula might give rise to more massive stars than quieter areas of a galaxy. Nearby newborns may heat the stellar nursery, so that more massive regions must collapse to give birth to stars in the first place.

Or the more massive stars might be a product of their pristine location. Compared to the Sun, the Tarantula Nebula has 40% as many heavy elements polluting its gas. The presence of heavy elements cools gas, so gas lacking these elements remains hot. Hotter gas exerts more pressure against the pull of gravity, so more gas is required to collapse into a star.

Tarantula Nebula and its surroundings

The Tarantula Nebula, also called 30 Doradus, is situated just over the center line of this image, which shows the star-forming region amidst its surroundings in the Large Magellanic Cloud.
ESO / R. Fosbury (ST-ECF)

However, it’s important to realize that the calculations these astronomers did are far from straightforward. Massive stars live fast and die young, and many such stars in the Tarantula Nebula have already exploded as supernovae. Meanwhile, gravitational interactions in this chaotic environment may have ejected smaller stars. Every study of stellar mass distributions must correct for these and other factors, says Nathan Bastian (Liverpool John Moores University). Since different studies use different techniques, they’re difficult to compare.

“I will be convinced on the non-universality of the initial mass function when the authors apply the same techniques to other regions and find significant variations,” contends Bastian, who has suggested that the star formation process really is universal. “While this is a very exciting result, I wouldn’t be too surprised if it was found to be due to analysis systematics in a few years’ time.”

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