A decades-old prediction has been verified: astronomers have discovered a “cataclysmic” pair of stars

Astronomers have discovered a pair of stars with an extremely short orbit. A star similar to our sun and a white dwarf orbit each other every 51 minutes

An artist’s illustration shows a white dwarf (right) orbiting a large sun-like star (left) in a very short orbit, forming a “cataclysmic” binary system. Credit: M. Weiss/Center for Astrophysics Harvard & Smithsonian

Astronomers at MIT and other institutions have discovered a pair of stars with an extremely short orbit. In fact, they seem to orbit each other every 51 minutes. The system appears to belong to an elusive class of binary stars called “cataclysmic variables,” in which stars like our Sun closely orbit a white dwarf—the dense, hot core of a stratospheric star.

A variable pair Cataclysmic happens when the two stars get closer, over billions of years, and as a result the white dwarf begins to accumulate, or eat, material from its companion star. This process can emit huge, variable flashes of light that centuries ago astronomers assumed were the result of some unknown type of cataclysm (catastrophe).

The newly discovered system, which the team has designated ZTF J1813+4251, is a cataclysmic variable pair with the shortest orbit discovered to date. Unlike other such systems observed before, the astronomers caught this pair when the stars eclipsed each other many times, and this allowed the team to measure properties of each.

A white dwarf is what stars similar to our sun become after they have used up their nuclear fuel. Near the end of its nuclear burning phase, this type of star sheds most of its outer material, forming a planetary nebula. Only the hot core of the star remains. This core becomes a very hot white dwarf, with a temperature exceeding 100,000 Kelvin. The typical mass of a white dwarf is half the mass of the Sun, but it is only slightly larger than Earth. This means that the density of a white dwarf is nearly 200,000 times that of Earth, so it is a collection of the densest matter, second only to a neutron star.

With these measurements the researchers ran simulations of what the system is probably doing today and how it will develop in the next hundreds of millions of years. They concluded that the stars are now undergoing a transformation, and the Sun-like star is orbiting the voracious white dwarf and “donating” most of its hydrogen atmosphere to it. The sun-like star will eventually empty into a very dense helium-rich core. In 70 million years, the stars will move even closer and the very short orbit will reach only 18 minutes, before they begin to expand and move away.

Decades ago, scientists at MIT and elsewhere predicted that pairs of such cataclysmic variables would change into very short orbits. This is the first time that such a variable system has been observed directly.

“This is a rare case where we have caught such a system during the transformation from a hydrogen to helium storage,” says Kevin Burdge from the Department of Physics at MIT. “People predicted that these objects would change to very short orbits, but long debated whether they could shorten enough to emit detectable gravitational waves. This discovery ends the debate.”

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