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Every star has a story to tell

March 17, 2021

Even when she was a little girl, physics professor and astronomer Sharon Montgomery was mesmerized by space – specifically the stars in space.

She'd lie out on a football field in Middleport, N.Y, with her dad, now a retired chemist, and stare up at the stars.

"The interest started really early," she said.

She'd even look through his homemade telescope. Montgomery laughs as she recalls that it was a really poor telescope, but the equipment wasn't as important as the interest that it piqued.

One day Montgomery would look at those same stars with much better and more powerful telescopes at some of the premier observatories in the world.

Starting in 2010 at McDonald Observatory in west Texas and then later in South Africa, Montgomery and her collaborator, Barry Welsh of the University of California, Berkeley, would observe the same stars night after night. And during some of these observations, she and Welsh would make some discoveries that are quite literally out of this world.

They discovered many comets in several different star systems.

Discovering and studying comets is important because it answers questions about star formation and our own planet. Montgomery said some scientists believe that Earth's water was delivered by falling comets.

She best explains the role of comets like this: Every solar system is a construction site, and comets are the debris left over at the site.

"The comets are the nails, the hammers," she said. "The comets are the space junk." But they also are clues as to what came before.

Astronomers look for comets by observing starlight – "A big comet tail alters the starlight," Montgomery said. She likens it to a streetlight that is altered by fog when we see it. The gas around the comet alters the starlight, she said.
"We look for that subtle diminishing of light that changes night to night," she said.

The star systems Montgomery and Welsh were observing are "young star systems, so they're still kind of junky," only 10 to 100 million years old.

That may not seem very young, but the oldest stars in the universe are about 13 billion years old. The Milky Way (our home galaxy) contains stars nearly this old, too. Montgomery's star systems are young, even compared to the solar system, which formed about four and a half billion years ago. If the universe's oldest stars are octogenarians, the sun is a young adult, and Montgomery's "young and junky" star systems are infants, less than half a year old.

Montgomery and Welsh ascertained the comets they discovered had never been discovered before by completing an exhaustive literary search. Also, the list of stars possessing detected comets is extremely short – in the ball park of 15, Montgomery said. She and her collaborator have discovered all but about five of them.

Also, the presence of comets probably means that planets are forming, she said.

"Comets falling into the star are likely signs of a large, massive planet because, like planets, comets will generally orbit the star in stable, circular orbits. In the case of comets, they are generally expected to reside far from the star. However, a massive Jupiter-like planet will tug on these more distant comets and occasionally pull them inward.

Without at least one planet, it's hard to see why large numbers of comets would be on death marches toward the star," Montgomery said.

Montgomery said it's likely that these comets no longer exist since they probably fell into the stars. This also means these comets won't be given names.

Evidence of the large amount of debris once commonplace in our solar system can be found on the cratered face of our moon, she said. The rocky worlds in the solar system were absolutely pummeled by debris about 4 billion years ago. Today these comets are safely sequestered far from the sun in a space called the Oort Cloud, which may contain trillions of comets. Only rarely does a comet careen into the inner part of solar system now, and that's why comets like Halley's Comet are a big deal, Montgomery said.

In other words, our solar system has had plenty of time to clean up its construction site, reducing the number of comets we are likely to see.

"Most of the large bits of debris were swept up by the young planets as they orbited the sun," Montgomery said. "Gravity will also help to clean up a messy construction site since the star and any planets will pull debris toward them. Even if the comet does not fall into the star or the planet, a narrow miss is likely to change the comet's orbit so much that it will be slingshot right out of the star system."

Montgomery is an astronomer, but she's also known as a stellar spectroscopist. Stellar spectroscopists spread starlight into a rainbow of colors and then perform an "archaeological dig in some respects" of the starlight, she said. They sift carefully through the starlight, hunting for subtle unexpected artifacts of comets.

She and Welsh, as well as another group in France, are currently the only scientists hunting for comets by examining starlight.

When viewing any star, Montgomery said the real magic is in the giant telescopes themselves, which bundle the faint starlight in such a way that we can see it.

"Everything we know about stars comes from the starlight. That's all we get from Earth," Montgomery said.

But whatever we do manage to get from Earth, it's important because "every star has a story to tell," Montgomery said.

Last Updated 3/19/21