Two stars spiraling toward catastrophe test Einstein’s gravity

Astronomers have observed a pair of stars trapped in a death spiral, and their dance of destruction reveals more about how gravity works.

The system, called ZTF J2130, is located about 4,000 light years away. Astronomers have known about this system for some time, but this is the first time they’ve seen it so clearly.

The two stars that make up this system will soon merge. Their spiral motion is consistent with theoretical predictions, meaning that as future observations become more sophisticated, researchers will be able to use the system to test their understanding of gravity, the authors say in a study submitted for publication in the journal Astronomy & Astrophysics in October.

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This is a very old system. One of the stars is a white dwarf, the white, hot remnant core of a star like our Sun. The other is known as a subdwarf, a small star near the end of its life. The two stars are so close that they orbit once in less than 40 minutes. In fact, they have already started kissing. Their mutual gravity is so strong that they are stretched and distorted, causing material from the subdwarf to flow into its white dwarf companion.

Stars are so massive and moving so fast that they emit gravitational waves, ripples in the fabric of space-time that were first predicted by Albert Einstein and confirmed to exist in 2015. This emission of gravitational waves robs the system of energy, bringing the two stars closer each year.

Astronomers took painstaking efforts to measure the orbital period as accurately as possible, combining data from the Oskar Luening telescope at Germany’s Hamburg Observatory and Spain’s CAHA observatory in Germany and Spain. They found that the orbit was slowly decaying. Every second that passes, the orbital period decreases by about two trillionths of a second.

This is consistent with calculations based on current theoretical understanding of gravity. But scientists have been eager to overcome Einstein’s theory of general relativity for more than a century, so any chance to test it is immediately appealing.

Astronomers have discovered that the upcoming gravitational-wave observatory, known as the Laser Interferometer Space Antenna (LISA), should be able to directly measure gravitational waves emanating from the system. The European Space Agency plans to launch LISA in the 2030s, so this great pair will be around for another decade.

When the stars finally merge, they create a supernova-level explosion that is bright enough to be seen with the naked eye. In the meantime, you should test gravity before enjoying the fireworks show.