CSI Universe: Unraveling the mysteries of Tabby's Star and supernovae

The universe is full of mysteries; that’s one of the reasons that astronomy is so interesting! We dug into a couple of puzzling phenomena at the most recent gathering of Astronomy on Tap Seattle at Peddler Brewing Company in Ballard. The session was dubbed “CSI: Universe,” and Brett Morris, one of the co-hosts of Astronomy on Tap Seattle and a Ph.D. candidate at the University of Washington, gave a talk about the star KIC 8462852, more commonly called Tabetha Boyajian’s star, thank goodness. His talk was titled, “The Weirdest Star Gets Weirder.”

You helped

Citizen scientists were the first to notice that there was something odd about Tabby’s Star. The Kepler Space Telescope was searching for exoplanets by watching for slight but regular dips in a stars brightness, a possible indication of a planet in orbit around a distant star. Morris noted that it can be difficult to write a computer algorithm to filter out noise in the data, so they enlisted the help of the public through the website PlanetHunters.org.

Brett Morris (Photo: Greg Scheiderer)

“What you can do on this website is help scientists look for things that are weird,” Morris said. People identify objects that don’t look right, then professional astronomers check them out. “Through this process they found a whole bunch of stars that misbehave.”

One of them was Boyajian’s.

“If we look at its colors, if we look at its spectrum, it behaves like all the other F-stars,” Morris said, “and so we were a little bit puzzled when we started looking at data.”

There were dips in light from Tabby’s Star, all right. There were smaller dips early in the mission that never really matched up. Then in March 2011 there was a huge dip of 15 percent of the star’s light, and it lasted for days, not hours as most transits do. Then in February 2013 there was an even bigger reduction in brightness of 20 percent. Nobody has come up with a plausible explanation for this.

“Whatever this is, this thing’s big,” Morris said.

No easy answer

An astounding array of possible explanations have been thrown out there. Examples include an object like Saturn with rings that could cause variations in the light curve, a passing comet, debris from a huge planetary impact like the one thought to have formed our Moon, and Tabby’s Star’s indigestion from having just swallowed a whole planet. The one in vogue at present is that a family of 10 to 20 comets, all giving off material, are creating these odd light curves. Morris doesn’t quite buy this one, either.

“The more bodies that you imagine being there, the easier it is to fit a light curve,” he said. “If you just keep adding new parameters into your model, eventually it will fit.”

“If you invoke weirdly shaped objects, you can fit it perfectly,” Morris added. “If you invoke the kinds of objects that we expect are most likely, it’s a lot harder. We really don’t know what this star is doing.”

Some have wondered if something between us and Tabby’s Star, maybe interstellar gas or dust, caused the strange light curves. Morris himself investigated this one. Back in May he got a Tweet—he said this is mostly how astronomers communicate these days!—noting that Tabby’s Star’s brightness was changing. He used the Apache Point Observatory to look for signs of absorption from interstellar gas or dust. But the spectra didn’t change even though the star was changing.

“We’re slowly ruling things out,” Morris said. “It’s not something in our solar system, it’s not something between us and the star; it’s got to be something near the star, but we don’t know what near the star could be doing this.”

As for wild speculation that the strange light curves could be caused by a Dyson Sphere or other “alien megastructure”:

“Extraordinary claims require extraordinary evidence, and I do not have any evidence to suggest that we can make a claim as extraordinary as that,” Morris said. He and a team of undergraduates at the University of Washington continue to work on the puzzle.

Coroner for the Stars

The second talk of CSI: Universe came from Prof. Melissa Graham of the UW, who does work on supernovae. These mark the death of a star, and Graham’s job is to figure out whodunnit.

Melissa Graham (Photo: Greg Scheiderer)

Graham pointed out that a star is considered alive if it’s in hydrostatic equilibrium; that is, when atomic fusion in the star’s core supports the star by counteracting gravity. Sometimes the death of a star is from natural causes. A typical star will fuse hydrogen and helium into carbon, then gradually fuses neon, oxygen, and heavier elements until eventually a core of iron forms. Graham said this means trouble, because fusing iron into something heavier is not exothermic; it doesn’t release energy.

“If you end up with a core of iron, your hydrostatic equilibrium suffers because you are losing out on that fusion in the core,” she said. “The core collapses because it can’t support itself anymore, the outer layers fall onto the inner layers, and you end up with a supernova explosion.”

Material blows away and leaves neutron star behind.

“That’s death by natural causes,” Graham said.

Type 1a supernovae are more interesting to stellar criminologists. These involve a white dwarf star, which is the remnant of a smaller star that doesn’t have enough mass to fuse carbon and oxygen into anything heavier.

“The carbon and oxygen core shrinks under its own self-gravity, and the outer layers are lost, which causes a really pretty planetary nebula,” Graham said. “The star is now supported by electron degeneracy pressure.”

This means the star isn’t alive because it’s not fusing elements.

“It’s more of a zombie star,” Graham said. “It’s died once and continues to live.”

The usual suspects

It’s a suspicious death when you see one of these explode. Graham rounded up the usual suspects: It could be a binary companion, such as a red giant or a sun-like star or another white dwarf. Sometimes it could be a pair of white dwarfs with a third companion star. A type 1a supernova also might from from a white dwarf’s impact with a primordial black hole or comet.

One way to figure this out is to simply look at the scene of the crime.

“Once this white dwarf star explodes, the other companion star would still be there,” Graham said. A companion would heat up and get brighter, so it might be detectable. Interstellar dust and gas may also light up from the energy of a supernova. Looking back at the scene later might detect such material that is at significant distance from the event. Graham is using the Hubble Space Telescope to check to find out if this is happening. She’s also looking forward to the completion of the Large Synoptic Survey Telescope, which is expected to find some ten million supernovae over its 10-year mission. With so many new examples we will, “really start to understand how these carbon-oxygen white dwarfs die,” Graham said.