It may be tempting to think that the stars that light up the sky each night are timeless. Astronomy professor John Thorstensen knows better. He has spent the last forty years studying cataclysmic binary pairs of closely orbiting binary stars: a smoldering dead star known as a white dwarf, and a star in its prime, like our Sun, pulled out. into the orbit of a dead star.

The white dwarf gradually removes the hot gas from its companion, forming a ring that can glow at random intervals. To an observer on the ground they seem like a single entity, sometimes appearing brighter, then dimmer.

“This is a reminder that stars have life cycles and evolve over time,” Thorstensen says. “Even the stars are not eternal.”

Amateur astronomers love catastrophe binaries, also known as catastrophic variables, or CVs for short, because their periodic bursts are bright enough to be seen with an ordinary telescope. Their intensity is the main reason why Thorstensen went on to go on with his life’s work when he came to Dartmouth in 1980. More than 10,000 have been counted so far, but only a few have been described in detail.

Today, astronomers have calculated orbital periods at about a thousand CVs. The orbital period, which usually lasts a few hours, gives a rough idea of ​​how close the two stars are and where they are in their evolution. Thorstensen has identified many disasters of her own throughout her career, and has identified hundreds of disasters, including 30 “short-term” resumes in the 2020 survey.

In recent years students have joined Thorstensen in search of the Astronomy Foreign Studies Program. Students spend the winter semester attending lectures by Dartmouth professors of astronomy at the University of Cape Town and immersing themselves in South African culture. One of the highlights is a one-week visit to the South African Astronomical Observatory in Sutherland, a remote location four hours north of Cape Town. Students’ round-trip transportation to and from the observatory, as well as accommodation, was provided by generous donations from Claudia and Jay Weed.

On one of the most productive visits, Thorstensen and four students returned this spring with new details on seven disasters that occurred about 1,000 to 3,000 light-years from Earth. His observations are published at: Astronomy Journal .

Exploring South Africa’s dazzling night sky

Located at the far end of Africa and washed by the fierce Southern Ocean, South Africa is known for its stunning and often distant landscapes. Perhaps it is less famous for its starry sky. The national observatory is located on a desert plateau that is home to the largest optical telescope in the southern hemisphere and many small instruments.

On the way there, the settlements are getting further and further away from each other. The lack of light as soon as the sun goes down is striking. Students would leave the dormitory with “night meals” every evening. After a one-mile hike, they reached the observatory, turned on the equipment, and waited.

“When the sun goes down, it gets so dark you can’t see your hands,” says Chase Alvarado-Anderson ’23, an astronomy student from Dallas. “Then your eyes get used to it and it snaps into place instantly. The sky looks like a black canvas covered with white paint.”

Abigail Burroughs, an astronomer in Washington DC, was equally impressed. “The sky explodes before your eyes when you get used to the dark,” she says. “Every time you look at a certain spot, you see more and more stars. We saw the Southern Cross, the Large and Small Magellanic Clouds, and the Galactic Plain in the early morning.”

COVID-19 delayed the trip for two years. As senior year approached, none of the students were sure they could go. Alvarado-Anderson was at the MDM Observatory in Kitt Peak, near Tucson, which Dartmouth operates as part of a consortium of schools. Burroughs never made it beyond Europe. This winter the stars align. In February of last year, after preparing the prerequisites for a long time, they flew to Cape Town.

Thorstensen was also ready. From a catalog of 10,000 targets, he chose resumes that looked relatively bright, obscure, and could be observed with relatively modest equipment. The telescope they used in the study, dating from the 1940s, had a 1.9-metre mirror, which was very small compared to the observatory’s famous SALT telescope. But he still had enough power to collect the necessary measurements.

An instrument mounted on a telescope, such as a spectrograph, that separates light into different colors or wavelengths, just as a prism separates white light into a rainbow. The amount of light available at each wavelength tells you what the star is made of. All stars contain hydrogen, which glows deep red when seen to emit the alpha spectral line, and it is this element that allows us to measure their velocities. As the star moves away from the observer, its light spreads out to a longer, redder wavelength; As they move towards the observer, the wavelengths get shorter and shift towards the blue.

The faster the star, the greater the change in wavelength due to the Doppler effect, an event that causes the siren to decrease as the ambulance or fire truck moves away from you. The students were able to track their speed by measuring the varying wavelengths of their target résumé, which allowed them to calculate their orbital periods.

They found that all seven of the disasters were typical, orbiting each other within hours, and typically traveling at more than 160 kilometers per second. Four of the stars they studied were consistently bright in a class known as variable novae; others were dwarf novae with small flares.

“These systems have similar physics but can look very different depending on what type of secondary star you have,” says Burroughs.

Each measurement can take hours. If the humidity got too high, the students had to turn off the telescope.

Alvarado-Anderson returned from the trip with his first published article, his knowledge of astronomy programs such as IRAF, PyRAF, and ds9, and advice on how to pass the graduate school qualifying exam. Now he was on his way to Stanford to get his PhD. Alvarado-Anderson, doctor of planetary science, says the trip reaffirms his love of space and the stars.

It also changed his perception of white dwarfs, which are at the center of all resumes. He had previously seen the dense core of a dead star that had run out of fuel. Today he sees a star that has come back to life by sucking gas from its neighbor.

“It’s very interesting to think of the process as rebirth,” he says.