An American astrophysicist, Andrea Ghez, was awarded the Nobel Prize in physics Tuesday for discovering that stars at the center of our galaxy are hurtling through space around a supermassive black hole.

Ghez, 55, shared half the prize with German astrophysicist Reinhard Genzel, who independently observed the astonishing acceleration of stars in the galactic center. Astrophysicists now believe that supermassive black holes reside at the center of all galaxies and played a role in the formation of galaxies from the primordial soup of matter in the early universe.

The other half of the prize went to Roger Penrose, a British mathematical physicist cited for his discovery that the existence of black holes is one of the bizarre implications of Albert Einstein’s general theory of relativity, in which gravity is associated with the curvature of space and time.

Ghez, a professor at UCLA, became the fourth woman to win a physics Nobel, following Marie Curie in 1903, Maria Goeppert Mayer in 1963 and Donna Strickland in 2018.

Dawn was still many hours away in California when Göran K. Hansson, secretary general of the Royal Swedish Academy of Sciences, phoned Ghez to tell her that she had been laureated. About an hour later, she spoke by phone to reporters in Stockholm, discussing the thrills of her research and her hopes that this new recognition will inspire more women to enter the field of physics.

Asked what she thought when she first saw signs that something mysterious was lurking at the center of the galaxy, she said: “I think the first thing is doubt. You have to prove to yourself you’re really seeing what you think you’re seeing. Doubt and excitement.”

She added, “We have no idea what’s inside the black hole, and that’s what makes these things such exotic objects.”

Ghez has received many honors, including a MacArthur Foundation “genius” award. She was the first woman to receive the Royal Swedish Academy’s Crafoord Prize. A graduate of MIT, where she majored in physics, and the California Institute of Technology, where she received her doctorate, she has been on the UCLA faculty since 1994.

On Tuesday, she told reporters she feels particularly passionate these days about the teaching side of her profession.

“I take very seriously the responsibility of being the fourth woman to win the Nobel Prize,” Ghez said. “I hope I can inspire other young women into the field. It’s a field that has so many pleasures, and if you’re passionate about the science, there’s so much to be done.”

This year’s physics Nobel honored the theoretical side of black holes — Penrose’s work — and the observational side, the investigations of Ghez and Genzel. There is no shortlist for a Nobel, and laureates find out they have won only when they get the early-morning phone call from Sweden. This year, as has happened in the past, the announcement was delayed briefly while the academy attempted to reach one of the laureates.

The fact that this year’s prize would somehow involve black-hole physics was hinted at by Hansson in his opening statement: “This year’s prize is about the darkest secrets of the universe.”

The usually packed room at the academy was mostly empty amid restrictions in place because of the coronavirus pandemic. Hansson said this year there would be no in-person Nobel celebration in Stockholm in December.

Genzel, 68, is a professor at the Max Planck Institute for Extraterrestrial Physics near Munich and also has an appointment at the University of California at Berkeley. Penrose, 89, is an emeritus professor at the University of Oxford who a half-century ago collaborated with Stephen Hawking to develop theories about the existence and nature of black holes.

Penrose did not invent the term “black hole,” but, the academy said Tuesday in its scientific brief describing the prize, “It was after Penrose’s discoveries that ‘black hole’ finally stuck as the name for this exotic gravitational anomaly.”

The academy’s brief cites four of Hawking’s papers, including one co-authored with Penrose. Hawking, who died in 2018, never won a Nobel Prize. Several scientists opined Tuesday that Hawking probably would have shared a Nobel with Penrose had he lived. The academy does not award prizes posthumously.

“The one piece of sadness is that Stephen Hawking is not alive to share the theory half of the prize with Roger Penrose,” David Spergel, director of the Center for Computational Astrophysics in New York, said Tuesday in an email.

In announcing the prize, the academy cited an article Penrose wrote in 1965, a decade after Einstein’s death, in which he said black holes really exist. “His groundbreaking article is still regarded as the most important contribution to the general theory of relativity since Einstein,” the academy wrote.

University of Chicago physicist Michael S. Turner on Tuesday called Penrose “a brilliant mathematician who turned his amazing skills to understanding Einstein’s theory at a time when there were still doubts about even the mathematical reality” of black holes.

Turner said Einstein did not fully understand the implications of his own theory. “It took another generation of brilliant physicists to figure it all out, not because of Einstein’s limitations but because of the richness of the theory,” Turner said.

Black holes are among the strangest features of the universe. They are formed from collapsed stars, with their matter so compressed by gravity that, according to the equations of general relativity, space becomes infinitely curved. Light cannot escape the gravity well. In 2019, scientists revealed the first direct image of a black hole — a supermassive black hole at the center of Messier 87, a galaxy in the constellation Virgo.

Ghez and Genzel, backed by teams of researchers and using some of the world’s largest telescopes, separately published findings in the 1990s and 2000s that provided observational support for the existence of a supermassive black hole — or something acting suspiciously like one — in the center of our own galaxy in a region known as Sagittarius A*.

The tremendous speed at which stars move in that region suggests they are influenced by the gravity of a supermassive object. What that object is, exactly, is unknown, but as the Swedish Academy put it in announcing the prize, “a supermassive black hole is the only currently known explanation.”

While they did not observe the black hole directly, they instead scrutinized individual stars whose motion implied the presence of something creating a powerful gravitational field. Our sun makes a full orbit of the galaxy over the course of about 230 million years, but near the center of the galaxy, a few speed-demon stars have orbits of less than 20 years, including one of just 11.5 years described in a 2012 paper in the journal Science co-authored by Ghez.

The mysterious “something” at the galactic center appears to have the mass equal to 4 million suns.

The observations of the stars in the galactic center were technically challenging, even with huge telescopes employed by Ghez in Hawaii and Genzel in Chile. The core of the galaxy is crowded with stars, and the scientists needed to pick out individual stars amid the swarm. The distances involved are immense — about 26,000 light-years — and the motions of those faraway stars hard to detect. The observations took many years, even decades.

Abundant dust interfered with the view, so the scientists had to observe in the dust-penetrating near-infrared portion of the spectrum. And they had to find a way, through what is known as adaptive optics, to correct for the distortions created by Earth’s atmosphere.

Tuesday’s announcement came as something of a surprise to the physics community, simply because the academy usually rotates the prize through areas of the sprawling field, which covers everything from the smallest subatomic particle to the vastness of the universe. But for the second year in a row, the academy honored work in cosmology and astrophysics.