Cover image credit: ESO/L. Calçada
Two thousand years ago, Aristotle posited that the Sun and the planets of our Solar System are immaculate, divine bodies — a belief that stood its ground even into the 17th century. But Galileo, alongside other astronomers of his time, pointed his telescope at the heavens and uncovered a messier reality: the Sun, covered in darker, cooler splotches called sunspots, was far from the perfect orb that Aristotle imagined.
Dr. Jeff Bary, a professor of astronomy at Colgate University and the Amherst Physics and Astronomy Department’s colloquium speaker on September 30, studies stars like our Sun in their earliest stages of formation. These very young stars are important testbeds for theories of stellar evolution, but they too are not perfect orbs. As Bary explained throughout his talk, the starspots on these stars — or “sunspots on steroids” — present head-scratching challenges for astronomers who want to understand star formation.
“Trust the theorists”
The most important properties of a young star are its mass and age. Sadly, astronomers can’t just go up to a star and weigh it or ask it its age. Instead, they rely on the tools of stellar astrophysics to infer these properties from things they can observe.
One recipe for predicting a star’s mass and age calls for two ingredients: its luminosity (brightness) and surface temperature. Computers can crunch the complicated equations that govern a star’s interior, telling us how a star’s luminosity and temperature should change as it ages. These “evolutionary tracks” can be neatly visualized as paths on a plot where the star’s temperature is on the horizontal axis and its luminosity is on the vertical axis. This handy plot even has a special name: the Hertzsprung-Russell (H-R) diagram.
The particular path that a star will follow on the H-R diagram depends on its mass. Therefore, by simply placing a star with a known luminosity and temperature as a point on the H-R diagram, astronomers can figure out which track the star falls on, yielding its mass. Afterwards, it is a simple matter of determining how far the star lies along that track. Voilà — mass and age. As long as you “trust the theorists,” Bary said.
Bary is not one of those theorists, though. He’s an observational astronomer, meaning his job is to obtain these luminosity and temperature measurements and feed them into the theorists’ models. If his measurements are even slightly wrong, the entire method fails. Do we trust the observers, then?
Spotty observations
It is relatively straightforward to measure a star’s luminosity — just add up the light counts your telescope receives from the star. Temperature is trickier. Once again, a few equations and a few bright minds, this time from the 20th century, save the day.
At the turn of the century, many astronomers (including women who worked at the Harvard College Observatory) studied stars by splitting up starlight into its component colors, forming a spectrum. Stars were eventually sorted into spectral classes based on common features in their spectra. The physics behind these features, however, remained nebulous.
The Indian astrophysicist Meghnad Saha later discovered the missing physics link: a star’s spectrum, down to the finest details, depends on its surface temperature according to an exact physical law. In other words, spectral class is an extremely reliable proxy for temperature.
Today, astronomers routinely derive temperatures by taking advantage of stellar spectra. By building up a catalog of template spectra, which are theoretical spectra representing each spectral class, researchers can find the best-fit template spectrum for any given star. The fit indicates the spectral class of the star, and hence its temperature.
This strategy, although clever, makes a major assumption: the temperature of the star in question is uniform across its surface. Essentially, it presupposes that stars are perfect, Aristotelian orbs… when they’re not.
As Bary explained, some stars can be really spotty. A famous example, the star HD 12545, has starspots that are larger than the entire Sun!
These massive starspots are the fly in the ointment for the tried-and-true template fitting method. Starspots are significantly cooler than the rest of the star, so what appears to be a cool star could really be a hot star with lots of starspots. Since evolutionary models don’t take care of starspot physics, the resulting mass and age estimates will be wrong.
A better approach should correct for the unwanted effect of large starspots on a star’s apparent temperature. Bary spent the rest of the talk discussing a model he and his team developed to resolve this issue.
Doubling up
Instead of fitting a single template spectrum to his data, Bary’s new technique takes a combination of two template spectra of different spectral classes. In effect, one spectrum represents the cooler starspots of a star, and the other its hotter, unblemished surface. The relative weights of the two spectra can be tweaked by controlling a variable called the spot-filling factor, which represents the percentage of the star’s surface covered by starspots. This process can be thought of like finding a weighted average.
Thus, Bary’s model isolates the spotted and non-spotted regions of a star’s surface, which helps to more accurately constrain the temperature of the latter. This model also has the nice benefit of estimating how spotty the star is.
After applying this model on a set of young stars, Bary and his team found a notable change in mass and age estimates compared to previous models. Specifically, the spot-correcting method predicts both higher masses and higher ages.
While starspots today won’t inspire the next scientific revolution, they still throw our assumptions about nature into question. Like all good astronomy research, Bary’s work continues to disentangle the messiness of the cosmos one star at a time.
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