Morbid threesome: a millisecond pulsar (left foreground) is orbited by a hot white dwarf star (centre), both of them orbited by a more distant and cooler white dwarf (top right) (Image: Bill Saxton; NRAO/AUI/NSF)

Objects: A fast-pulsing star and two white dwarfs

Location: A rare triple-star system

The three friends had been through a lot over the years. One had an explosive personality that in the early days nearly drove the friends apart, but has since settled down to ticking away the time. The two others have a complicated relationship with the clock-watcher, one preferring to stick close by, while the other keeps its distance. Together, the trio are attempting to put Einstein to the test.

Systems composed of three ordinary stars are common throughout the galaxy. But because exotic stars like pulsars or white dwarfs form in violent explosions that often shove companions away, they tend to go it alone. But now astronomers have found the first example of a triple system containing only the strange corpses of burned-out stars, and they plan to use it to probe the nature of gravity.


Both pulsars and white dwarfs are the embers left behind when an ordinary star dies. White dwarfs form relatively gently from the cores of small stars that slough off their outer layers of gas and dust once their internal furnaces burn out.

Pulsars form violently when massive stars explode in supernovas, leaving ultra-dense balls of neutrons behind. These stellar corpses emit beams of radio waves from their poles that sweep the sky periodically as the star spins, like a lighthouse. This regular “tick” serves as a precise cosmic clock when the beams sweep past Earth. If the clock ticks out of time, it indicates that the pulsar is changing direction in space, indicating that it may have a cosmic companion pulling it to and fro.

Three’s company

Scott Ransom of the US National Radio Astronomy Observatory in Charlottesville, Virginia, and his colleagues were conducting a pulsar survey with radio telescopes when they found a pulsar that at first seemed fairly ordinary. The pulsar appeared to flash every 2.73 milliseconds, with a variation in arrival time of around 2.5 seconds. That suggested the pulsar was in a circular orbit with a white dwarf, and the pair whirled around each other every 1.5 days. Such a binary setup is much less common than a solo pulsar, but not unheard of.

But further examination of the timing data revealed this system was something much more unusual. The team found longer-term variation in the arrival time, a difference of 150 seconds over the course of an Earth year. That suggests the binary is accompanied by a second white dwarf with a circular orbit much further out.

“These things are way less than one in a million – these are one in a billion type systems,” says Ransom.

Unlikely tale

Ransom thinks the system is so rare because the sequence of events needed to produce it is extremely unlikely. First, you need a system of at least three ordinary stars. The most massive star then explodes in a supernova, creating a neutron star. At least two other stars must survive the explosion and continue orbiting the neutron star, likely in very elliptical orbits.

After a billion years, the outer star becomes a white dwarf, ejecting mass onto the inner binary system. Another billion years pass, and finally the inner star also becomes a white dwarf. It feeds material to the neutron star, which speeds up its rotation. “People have done many simulations, and usually these types of system don’t survive,” says Ransom.

The resulting gravitational forces produce a complex pulsar dance, which could allow astronomers to test an aspect of Einstein’s theory of general relativity, which governs how massive bodies interact via gravity. Although Einstein’s theory has passed the many tests it has been put to so far, it does not agree with quantum mechanics, the other great pillar of modern physics, which describes the behaviour of minuscule particles. If it eventually proves impossible to unify the two theories, one of them may have to fall, and this triple system could turn out to be a nail in relativity’s coffin.

Leaning tower

A longstanding idea in theories of gravity is the equivalence principle, which says objects fall at the same rate in a gravitational field such as that found on a planet, regardless of their compositions. Galileo is apocryphally said to have tested the principle by dropping two different weights from the Leaning Tower of Pisa.

Einstein’s relativity includes a stronger version of the principle, which says the result is also true for bodies held together by their own gravity, such as stars. The dense pulsar has a stronger self-gravity than its closer white dwarf companion, but general relativity says both should “fall” around the outer white dwarf at the same rate.

The density and closeness of the rare stars in this triplet offer the best laboratory for testing this principle so far, Ransom says. Measuring the orbital periods and masses of all three objects could help reveal deviations from Einstein’s theory, and possibly point the way to a more fundamental theory of gravity.

“By measuring to very high precision, we’ll be able to test this strong equivalence principle,” says Ransom.

Journal reference: Nature, DOI: 10.1038/nature12917