Two dead stars have been found 7800 light-years away from Earth, orbiting each other at incredibly high speeds. Their orbit is so close that astronomers hope to find gravitational waves from the pair in some years when sensitive tools are used. Although we are quite used to observe things on a cosmic level occurring very slowly, the white dwarf binary, ZTF J1539+5027 or J1539, in short, has an orbital period of only 6.91 minutes, shortest for an eclipsing binary. With this close orbit, the binary system could fit inside Saturn. The findings have been published in Nature journal.
White dwarfs are dead stars, the progenitor of which had a mass no greater than 10 times of the Sun. If the star is more massive, it converts to a neutron star on its death. On being larger, a black hole is obtained.
When the Sun runs out of hydrogen, it will turn to be a red giant, fusing helium and carbon till their depletion. After this, the outer layers will be blown away, and the remaining shining ultradense core which is the corpse of the dead star is termed as the white dwarf.
The two stars in J1539 have gone through this process. The primary star has 60 percent mass of the Sun in a core of Earth’s size while the secondary star has only 20 percent mass of the Sun however larger than its companion, hence having a lesser density. This also means that it is less bright so when it moves between its brighter companion and us, it fully obscures the latter from our view so the binary is an eclipsing one. Astronomers found it through the Zwicky Transient Facility of Caltech at Palomar Observatory.
Kevin Burdge, Caltech physicist explained that when the dimmer star moves in front of the brighter one, it blocks the maximum amount of light creating a seven-minute blinking pattern.
Till now, the gravitational waves have been detected in the final moments of a collision between the massive objects such as black holes or neutron stars. J1539 is relatively lighter and not yet at the condition for stars to merge. However, they are growing closer at the rate of 26 centimetres per day which means that it will take a minimum of 130,000 years for the orbital period to be 5 minutes. In that situation, the mass transfer from the secondary to the primary star will rise.
There are two possibilities. If a stable mass transfer takes place it will result in the separation of two stars, creating an AM Canum Venaticorum star where the primary star steadily accretes matter from its companion. However, if a stable mass transfer does not occur, the stars will merge creating a R Coronae Borealis variable, a low mass analogue of type la supernova. However, we might not be around to see the final result of the ever decaying orbit of J1539.
Astronomers believe that the Laser Interferometer Space Antenna (LISA) once finished will be able to detect the gravitational waves. However, LISA will not launch until 2034 and J1539 will be the strongest signal for it to detect.
Journal Reference: Nature journal