White Dwarfs in Death Spiral Could Unravel Century-Old Mystery
White dwarfs are the collapsed corpses of stars like our Sun that ran out of fuel. Now, astronomers have found a pair of binary white dwarfs orbiting each other once every seven minutes, the fastest orbit ever found for this type of system. Known as ZTF J1539+5027 (or J1539), this pair of stellar corpses could help unlock the secrets of gravitational waves — mysterious ripples in space time first proposed by Albert Einstein in 1915.
Every clear night, the Zwicky Transient Facility (ZTF), operated by Caltech at the 48-inch telescope at Mount Palomar, searches for celestial objects which move, blink, or vary in brightness. Promising candidates are then forwarded to the Kitt Peak National Observatory in Arizona, where they are studied in more detail. The 2.1 meter telescope at Kitt Peak was equipped with a new instrument, the Electron Multiplying Demonstrator (KPED) to quickly identify varying brightness levels seen from stars.
“As the dimmer star passes in front of the brighter one, it blocks most of the light, resulting in the seven-minute blinking pattern we see in the ZTF data,” Caltech graduate student Kevin Burdge explained.
The J1539 system is so small, it would fit inside within the orbit of Saturn. The pair is found 8,000 light years from Earth, seen in the constellation of Boötes. Orbiting their common center of gravity once every 6.91 minutes (six minutes and 54 seconds), the system constantly loses energy as it creates gravitational waves. As this happens, the stellar corpses spiral in toward one another in a death dance.
A Glimpse of the Future
In roughly five billion years, our own Sun will run out of available hydrogen fuel, and start to collapse in on itself. This will result in helium fusing for the first time, and the sun will expand back out, becoming a red giant star. As it does so, our parent star will swallow the planets Mercury and Venus, and it may consume the Earth. Eventually, the Sun will shrink down to become a white dwarf, a cool stellar remnant the size of Earth.
A vast number of stars in our galaxy have partners, forming binary star systems. When these stars expand into red giants, they can engulf their companion, and spiral close together. The result of this can be a white dwarf binary. These systems are thought to be fairly common, but only a small number have yet been discovered.
This particular system is so close together, astronomers calculate the pair will collide about 100,000 years in the future. Archival data was examined, confirming the upcoming collision.
These binary white dwarf systems like J1539 are testing grounds in understanding gravitational waves, first predicted by Albert Einstein in his general theory of relativity. These ripples in spacetime were first detected in 2015 by the Laser Interferometer Gravitational-wave Observatory (LIGO). Those waves were seen coming from a pair of black holes as they collided, 1.3 billion light years from Earth.
The first evidence of dark matter within galaxies was discovered by famed astronomer Vera Rubin at the 2.1 meter telescope on Kitt Peak. In continuous operation since 1964, this telescope is also noted for finding the first gravitational lens by a galaxy, the first pulsating white dwarf, and this same instrument was used to examine the frequency of binary systems for stars like our Sun.
Today, the nature of dark matter (and dark energy) remains the greatest mysteries of the Universe.
Eyes on the Prize
“Einstein’s gravitational theory, which is said to be the greatest single achievement of theoretical physics, resulted in beautiful relations connecting gravitational phenomena with the geometry of space; this was an exciting idea.” — Richard P. Feynman
The discovery of these rapidly-revolving white dwarfs was made using just a small portion of data which will be coming from the ZTF survey. survey has, so far, returned data from just one percent of the one billion targets it is scheduled to examine. The study of gravitational waves is a cutting-edge branch of astronomy, and telescopes on the ground and above the Earth are being utilized to study the phenomenon.
“Over the next years, LIGO will be putting general relativity to its most stringent tests ever, it will be discovering new sources of gravitational waves, and we will be using telescopes on the ground and in space to search for light emitted by these catastrophic events,” Fiona Harrison, Caltech physicist, stated.
In 2034, NASA plans to launch the Laser Interferometer Space Antenna (LISA), on a quest to better understand gravitational waves. LISA, a trio of spacecraft, should detect J1539 within one week of its launch.
“I think the most exciting prospect of this discovery, is seeing that this pair of white dwarfs was far from the prototypical pair we expected, but instead exhibits very exotic behavior that is still difficult to explain,” Burdge states in a Behind the Paper article published in Nature.
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