Merging neutron stars and cool galaxies at Astronomy on Tap Seattle

One of the cool things about the Astronomy on Tap Seattle series of talks in pubs is access to scientists who are working on headline news. It happened at their October gathering at Peddler Brewing Company in Ballard. Jennifer Sobeck, a stellar astrophysicist in the Department of Astronomy at the University of Washington, was all set to give a talk titled, “A Hitchhiker’s Guide to the Galaxy: Bumming Around the Milky Way.” But a few days before the talk the news hit that LIGO and others had detected gravitational waves generated by merging neutron stars. Neutron stars are Sobeck’s thing, so the script went out the window and we learned about what happened.

Sobeck noted that neutron stars are what’s left behind when high-mass stars—around four to eight times the mass of the Sun—blow up in a supernova. Neutron stars are incredibly dense; the mass of the Sun packed into something 12 miles across. They have a crust, though light still gets through.
“Inside is just basically a soup,” Sobeck said. “It’s a hot mess.”

Everything inside is so compressed that scientists call it “degenerate.”

“There are no more atoms, there are no more molecules, those are all blown apart,” Sobeck explained. “It’s just like a soup of neutrons; there are just tons of neutrons, and the really cool thing is down in the center, they think the pressures are so high that you actually might get quarks.”

August discovery

Scientists knew they had detected a neutron star merger rather than the sort of black-hole mergers previously spotted by LIGO because the signals are different. The interesting thing about the detection of two neutron stars merging is that we could see it visually because the event created a kilonova, like a supernova, but smaller.

“It’s a little bit less on the explosion scale,” Sobeck said. “Kilonova means that you’re able to have electromagnetic radiation across the spectrum that a whole bunch of facilites were able to monitor.”

So when LIGO and VIRGO detected the gravitational wave, with the help of the Fermi gamma ray space telescope and the ESA’s Integral gamma-ray observatory they they were able to narrow down the location of the event and tell others to look there. When the optical observations came in, the kilanova was spotted in the galaxy NGC 4993.

“This has never been done before,” Sobeck noted. The detection occurred in mid-August of this year, and by the end of the month the visual was gone.

“This kilonova explostion lasted only for a period of only 15 days,” Sobeck said.

Observations were made not just in the visual, but across the spectrum from gamma rays to radio, and more than a dozen observatories were involved in the analysis.

“You’re getting a different piece of information from all of these parts of the spectrum,” Sobeck noted. “They all helped fill in that puzzle.”

The story in the media

Sobeck said the press went a little overboard with headlines such as collision “creates gold” (CNN)  and “Universe-shaking announcement” (New York Times), yet it’s true that the kilonova made some gold. Sobeck noted that hydrogen, helium, and a bit of lithium came from the Big Bang, but the rest of the elements were made in stars. But stars can only fuse elements as heavy as iron. To get the really heavy stuff called lanthanides you need a kilanova. The emitted light tells you what’s there. If you see blue light after a kilonova, that means there’s a high concentration of silver, cadmium, and tin. If the light is more red, then platinum, gold, mercury, or lead is present.

“This particular event went from blue very, very, quickly to red, and it stayed red most of the time,” Sobeck said. “Hence, we’ve got a bunch of gold on our hands.”

“We found out that neutron-star mergers do make elements,” she said. “We were right, so huzzah!”

All kinds of galaxies

Grace Telford, a graduate student studying astronomy and data science at the UW, stuck with her original topic of “A Whirlwind Tour of Galaxies: the Tiny, the Gigantic, and Everything in Between” for the October Astronomy on Tap. She noted that there are several ways to classify galaxies:

• Stellar mass or brightness
• Shape
• Star formation rate
• Nuclear activity

Stellar mass or brightness

This is pretty straightforward.

“Basically the more stars a galaxy has, the brighter it is,” Telford noted. There’s quite a range of sizes. The Milky Way is a pretty common-sized galaxy, and it’s hard to make them bigger. The largest are around 10 times the size of the Milky Way.” Smaller galaxies are plentiful.

“A dwarf galaxy is something that is at least a hundred times less massive than our Milky Way,” Telford said, and they can go a lot smaller.

Way out at the small end of the chart are ultra faint dwarf galaxies, which can’t really be seen because they’re too faint. They can’t be detected at long distances.

A recently discovered type is called an ultra diffuse galaxy. This may be the same size as the Milky Way but have 100 times fewer stars, all held together by dark matter.

“This is an open area of research,” Telford said. “It’s hard to explain how to form these wierdo galaxies that are not very massive at all, but huge.”

Shape

The three main shapes of galaxies are elliptical, spiral, and irregular. Spirals may come with a large central bulge or a bar. Irregular galaxies tend to be small.

Star formation rate

It’s in star formation rate that galaxies really differentiate themselves, Telford said. Galaxies that emit a lot of blue light have lots of young stars and new star formation. Galaxies that look red are “quenched.” Their stars are older, and there’s little new star formation.

In between red and blue is the “green valley” of galaxies. They don’t actually emit green light, but they’re in transition from blue to red.

An interesting type is the “starburst” galaxy. These are galaxies that somehow stumble into a source of gas that wasn’t available to them before.

“They have the ability to form stars at a very high rate relative to the normal amount of star formation for a galaxy of its size,” Telford explained. “As a result, you have a lot of these massive young stars that are dying and exploding as supernovae and injecting a lot of energy into the gas.”

These objects are short-lived, they exhaust their gas in a hurry, at least in astronomical terms—in between 100 million years and a billion years.

Nuclear activity

Most galaxies have supermassive black holes, which can create jets of energy.

“Sometimes these black holes eat a lot of gas really quickly and then they blow out a whole bunch of energy,” Telford explained. These jets are nuclear activity. Galaxies with active galactic nuclei are most typically found in the green valley, though they’re in other types as well.

Telford gave a plug for Galaxy Zoo, where you can go looking for these differing types of galaxies and actually participate in citizen science.

Visiting Vesta and Ceres

The Dawn spacecraft has found a lot of surprises at Vesta and Ceres. Debra Buczkowski
a geologist and planetary scientist at the Johns Hopkins University Applied Physics Lab, gave a talk recently at the Museum of Flight discussing some of the findings from the mission.

Dr. Debra Buczkowski, a geologist
and planetary scientist at the Johns
 Hopkins University Applied Physics
Lab, spoke about the findings of the
Dawn mission to Vesta and Ceres
recently at the Museum of Flight.
(Photo: Greg Scheiderer)

Vesta was Dawn’s first stop, entering orbit around the asteroid on July 15, 2011. Scientists expected to find volcanoes on Vesta. Buczkowski explained that this expectation traces back to meteorites found on Earth that are know to be from Vesta. These are known as HEDs: “howardite–eucrite–diogenite.” These closely resemble igneous rocks found on Earth, and those are made from volcanic activity. But the volcanoes aren’t there.

Before Dawn arrived at Vesta the Hubble Space Telescope showed that Vesta wasn’t spherical, but rather was significantly flattened out at its south pole. Scientists speculated that this was because of an enormous impact, and that proved to be correct. Dawn observed a huge impact crater, now called Rheasilvia Basin, the rim of which is almost as wide as Vesta itself.

“It really should have broken the asteroid apart,” Buczkowski said of the impact that created the basin, which has a huge central peak. Dawn also found a second impact crater, Veneneia Basin, which is almost as large.

Another surprise finding from Dawn is that Vesta is fully differentiated.

“Most of the asteroids are just kind of chunks of rock with one kind of rock all the way through,” Buczkowski explained. “Not Vesta; Vesta actually has a core, it has a mantle, and it has a crust.”
Vesta’s core is about half the diameter of the asteroid itself, about 220 kilometers.

“This is probably why Vesta did not fall apart when the Rheasilvia Basin formed, because it has this huge, massive core,” Buczkowski said.

The surface of Vesta was found to have lots of fractures, features larger that Earth’s Grand Canyon that look like faults. Buczkowski said they did a lot of computer modeling to see if an object the size of Vesta with a core the size of Vesta’s could develop fractures on the crust.

“The stresses that result from that huge impact kind of get redistributed because of the giant core,” she said of the findings. “Instead of being focused around the crater, they move to the equator and fracture at the equator. If we do this same model without the giant core, there’s no fracturing at the equator. So it’s because of the giant core that we have these huge fractures.”

Buczkowski said that was a little disappointing because they were hoping for volcanoes or magma-driven geology. While they didn’t find volcanoes, there is evidence of moving magma that didn’t break through to the surface. Rather, it pushed some of the surface upward, forming mounds.

On to Ceres

Dawn departed Vesta in September 2012 after spending about 14 months in orbit. As Dawn approached the dwarf planet Ceres there was much speculation about extremely bright spots on its surface that were found in Hubble images. Other observations had detected water vapor on Ceres. Since Ceres is relatively large but not dense, scientists were expecting to find ice. But there was more rock and less ice than anticipated. What they did find, Buczkowski said, was evidence of volcanism.

This image from NASA’s Dawn spacecraft shows Occator Crater
on Ceres, with its signature bright areas. Dawn scientists have
found that the central bright spot, which harbors the brightest
material on Ceres, contains a variety of salts. (Image
 credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA)

“We’re not expecting magma on Ceres,” she said. “Ceres isn’t dense enough for the kind of magma that we’re used to here on Earth, made out of silicate rocks. This is something called cryomagma; it is basically ice with a little bit of rock.”

The biggest and brightest of the bright spots, named Cerealia Facula, is in the crater Occator. Many of the craters on Ceres are fractured, even on the crater floors, and the many bright spots on Ceres are associated with these fractures.

“What it’s looking like is that we’re having cryomagmatic activity underneath (Occator) crater,” Buczkowski said, “and what’s coming up out of these fractures is a pyroclastic spray, and the water, the volatiles in that, is sublimating away and all it’s leaving is the sodium carbonates.” Those are the bright spots we see all over Ceres.

Dawn also found that Ceres is covered in ammoniated phyllosilicates.

“Ammonia is interesting,” Buczkowski explained. “We don’t expect to find ammonia this close to the Sun, it’s usually something that’s found further out in the solar system.” They’re still studying whether Ceres may have formed further from the Sun and migrated in, or if the ammonia somehow made its way to Ceres from the outer solar system.

It turns out that Ceres had quite a few volcanoes, though most of them have now collapsed. There’s one that hasn’t, known as Ahuna Mons, that stands about five kilometers tall. It’s a cryovolcano.

“The volcano that we thought would be on Vesta is on Ceres,” Buczkowski noted. Ahuna Mons may be younger than the others, and also may collapse over time.

Like Vesta, Ceres was found to be differentiated, though only partially so.

“There’s a rocky core, there’s a volatile-rich mantle, and there’s a muddy slurry, a mud ocean” below the crust, Buczkowski said.

Ceres is now considered a dwarf planet, while Vesta still has asteroid status because of its lopsided shape from the giant impact. Buczkowski figures Vesta deserves dwarf-planet status, too. Whatever you call them, she thinks they’re fascinating to study because they’re kind of a bridge between the asteroids and rocky planets.

“These are more involved bodies than just plain, old asteroids,” Buczkowski said. “They’re not just chunks of rock floating in space. They’re actually like little mini-planets. They’ve got a lot of planet-like properties.”

Though they’re pretty small, they can teach us a lot.

“They’re interesting to us because they tell us a lot about how Earth and the other planets formed,” Buczkowski said. “Studying these little protoplanets we actually are looking back to the beginning of the solar system.”