An international team of astronomers has discovered a binary system of dwarf stars that orbit around each other faster than any eclipsing binary seen before. These objects take less than 7 minutes – 6.91 to be precise – to go around each other.
The binary system is known as ZTF J1539+5027, J1539 for short. It is made of two white dwarfs, the dense core of small dying stars. Each of these objects is made of so-called electron degenerate matter, so despite being pretty heavy, they are relatively small. To put it this way, the brighter, heavier one (which appears the smallest of the two) packs 61 percent of the mass of our Sun into an object with a radius of 10,900 kilometers (6,800 miles), not that much bigger than Earth.
The dimmer, lighter one is about one-third of the mass of its companion, but as it's less condensed it’s a bit “fluffier” with a radius of 21,850 kilometers (13,600 miles). That’s slightly smaller than the planet Neptune. As reported in Nature, the two white dwarfs are 78,000 kilometers (48,500 miles) apart. The whole system would fit snuggly inside Saturn.
Researchers discovered the stars using Caltech’s Zwicky Transient Facility (ZTF). The ZTF is designed to spot unusual and changing events in the cosmos, and these white dwarfs certainly fit the bill. The system is a so-called eclipse binary because seen from Earth, the two stars periodically pass in front of each other, changing the amount of light we receive.
“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,” lead author Kevin Burdge, a Caltech graduate student, said in a statement.
The discovery of this record-breaking pair will help researchers understand how these systems form. They will also be of interest to a different branch of astronomical research: gravitational waves. Gravitational waves are ripples in space-time produced by massive objects moving.
As these two stars orbit each other, they emit gravitational waves, slowly losing energy. For this reason, over time, their orbit will decay. They will spiral closer and closer together until they collide. Their gravitational wave emission is currently too weak to be detected by current observatories, but this will change in the next 15 years.
The European Space Agency is planning to build a gravitational wave detector in space called LISA (Laser Interferometer Space Antenna). Based on the estimation from the system and the expected capability of LISA, the gravitational emission of J1539 should be right near the peak of sensitivity for the observatory.
LISA won’t launch until 2034 and this gives ZTF and other observatories time to discover more such systems before we look at their gravitational emission.