How Big Is Betelgeuse Really?

Earlier this year, furor surrounded Betelgeuse — the star had “fainted,” dimming more than expected in its usual cycle of brightness changes, leading some so far as to suggest the star might go supernova sometime soon. But even as astronomers begin to get a handle on the red giant’s unexpected behavior, they’re still struggling to understand its basic properties — namely its size and distance.

Meridith Joyce (Australian National University) and colleagues reported in the Astrophysical Journal (arXiv preprint available here) that Betelgeuse is actually smaller, and therefore closer, than previously thought. Not everyone agrees with the results. Nevertheless, the study represents a new — and needed — approach toward understanding this enigmatic giant.

Betelgeuse's Song

As a red giant star, Betelgeuse isn’t quite stable. It has run out of hydrogen to fuse in its core and is relying on helium fusion to hold off gravitational collapse. As it does so, pressure waves (that is, sound waves) move through the star so that it slowly pulses: The star swells, contracts, and swells again. Such pulses can help astronomers “listen” to a star’s interior structure, providing a view we would never otherwise see.

Using historical data collected by amateur astronomers of the American Association of Variable Star Observers, as well as archival observations from an imager aboard the Coriolis spacecraft, Joyce’s team assembled a light curve that shows how Betelgeuse’s brightness varies over time.

“In the case of both Betelgeuse and T UMi last year, having more than 100 years of visual data was critical,” Joyce explains. Her team was able to identify a pulsation with a period of 185 days. If we could hear the singing of this pulsing star, this 185-day cycle would be the first overtone, a pulsation occurring in the star’s outer layers on a resonant frequency.

Inputting this overtone into a computer simulation called Modules for Experiments in Stellar Astrophysics (MESA), Joyce modeled Betelgeuse’s outer layers, using the star’s swell-and-contract rhythm to determine its girth: between 702 and 880 times the Sun’s diameter. That’s enormous — yet it’s smaller than we thought. If Betelgeuse were in place of the Sun, the new estimate would have it extend two-thirds of the way to Jupiter instead of all the way.

Betelgeuse’s size on the sky is already known to some degree — while a point of light in most telescopes, infrared detectors can work together to resolve the star’s tiny spot 42 milliarcseconds across on the sky. If the distance to the star is known, then that angular diameter translates to its size. But Joyce’s team worked backwards: They compared the size calculated in their simulation to the angular diameter of the star, giving the distance to the star as between 500 and 636 light-years.

Dealing with Uncertainty

Betelgeuse’s distance has long been uncertain, and thus so has its size. For many stars, parallax is the go-to technique for measuring distance. Parallax is the apparent movement of a nearby star against more distant background sources over time, in the same way that a finger held at arm’s length appears to move as you peek at it with first one eye and then the other.

But Betelgeuse is so big, it’s not a point on the sky, the way most stars are. And it seems to be slightly asymmetric, perhaps due to ejections and/or interaction with its surroundings. That complicates parallax measurements.

The Hipparcos satellite was the first to measure Betelgeuse’s parallax in 1997, but right away astronomers knew something was off. The apparent movement of the star on the sky  disagreed with radio measurements of its position.

Because calculating the star’s parallax depends on knowing not just its position but the apparent movement of its position on the sky, astronomers knew the distance was probably not quite right. “The 5-parameter solution is a simultaneous solution, so you can't trust one parameter if two of the others are wrong,” explains Graham Harper (University of Colorado, Boulder).

Recently, Harper set about correcting for this discrepancy. He combined radio measurements from the Very Large Array, Atacama Millimeter/submillimeter Array, and the e-Merlin array with revised Hipparcos measurements published in 2007, arriving at a distance between 620 and 880 light-years. Due to the complications involved in making the calculation, the range of possible values is “not small,” Harper says. If the distance were wrong, that would indicate the size is wrong too.

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Joyce emphasizes that while different, the range in values her team found is within the range Harper reported: “This can be described as slightly different but statistically consistent,” she says.

“The new [study] uses completely independent methods that place much trust in numerical simulations of the structure of the star (which is not at all well-known) to see how the surface oscillates,” Harper says. “Such a completely independent approach is to be highly commended because the parallax method is going to remain problematic.”

Indeed, Joyce says, the ambiguity surrounding traditional parallax measurements is likely to remain in the near-future. (The European Space Agency’s Gaia satellite, which is determining parallaxes, and thus distances, to over a billion stars in the Milky Way, is so sensitive it can’t even observe bright Betelgeuse.)

Nevertheless, Harper urges caution with the new results. “You always need a ground-truth when developing new techniques. All the assumptions and uncertainties (known and unknown) can add up.”

Andrea Dupree (Center for Astrophysics, Harvard & Smithsonian) agrees. “I would be conservative and wait for some confirming calculations.  But it is an interesting result.”

If the result pans out, it has some implications: A smaller Betelgeuse is likely at a slightly earlier stage of its lifetime, putting off any potential supernova. “It's burning helium in its core at the moment, which means it's nowhere near exploding,” Joyce says. “We could be looking at around 100,000 years before an explosion happens.”