A concept known as frame-dragging from Einstein’s relativity gives scientists a new look into a whirling white dwarf.
A twist in the fabric of spacetime is causing the orbit of one stellar corpse to teeter around another stellar corpse, researchers report. And the relativistic corkscrew is helping astronomers reconstruct the final days of these two long-dead stars.
Einstein’s theory of relativity has many strange consequences. Time moves slower for those travelling at high speeds, and massive objects like the Sun deform the space-time in which they sit, causing light to “bend” around them.
When a massive object rotates, it will swirl, or drag, space-time around with it like a skirt swirling around a spinning dancer. This effect is small around a body like Earth, but more exaggerated around more massive, fast-spinning objects. Now, astronomers have used the frame-dragging effect to take some real-world measurements previously impossible.
General relativity is the foundation of modern gravitational theory. It explains the precise motion of the stars, planets and satellites, and even the flow of time. One of its lesser-known predictions is that spinning bodies drag space-time around with them.
The team of scientists measured the rotation speed of a white dwarf and confirmed previous clues about its unique history. This particular white dwarf (the remains of a sunlike star) is part of a binary system called PSR J1141-6545, which lies in the southern constellation Musca, the Fly. It’s an unusual system, with the quickly rotating white dwarf accompanied by a neutron star (a remnant left when a star more than eight times as massive as the sun goes supernova).
Of the pair, the white dwarf formed first and the pulsar later. And between those two events, mass from the star that would eventually explode to create the pulsar flowed onto the white dwarf, causing it to rotate much more quickly than it otherwise could.
Scientists measure frame-dragging around Earth by watching how the orbits of satellites around our planet change over time. We can’t send a satellite to orbit the white dwarf, so instead, the team calculated how much frame-dragging should have caused the pulsar’s orbit to precess, or change, over the 20 years since the system was discovered.
Previous research suggested that the white dwarf formed before the pulsar in this binary system. One prediction of such theoretical models is that, before the pulsar-forming supernova occurred, the progenitor of the pulsar shed nearly 20,000 Earth masses’ worth of matter onto the white dwarf for about 16,000 years, boosting its rate of spin.
The team determined that the pulsar’s orbit has precessed by about 93 miles using atomic clocks to precisely time the pulsar’s blips. From this change, they were finally able to calculate the rotation of the white dwarf: roughly once every 100 seconds.
That speed confirms that the white dwarf pulled mass off its companion star and spun up before the pulsar was created. And that gives researchers even more information about the system and how it formed, also shedding light on other systems like it.