Four Stars Found in Amazingly Tight Bunch
A to-scale schematic of the quadruple stellar system overlayed for comparison with a diagram of the solar system's planetary orbits.
Credit: Evgenya Shkolnik

AUSTIN, Texas — A quartet of stars has been discovered in an intimate cosmic dance, swirling around each other within a region about the same as Jupiter's orbit around the sun.

Astronomers say a gaseous disk might have once engulfed and pushed the stars into their tight orbits.

Though bright, the stellar system was thought to be a single star dubbed BD -22°5866. Now, research presented here today at a meeting of the American Astronomical Society reveals the pinpoint of light is a rare system of four closely orbiting stars. The group is located about 166 light-years from the sun. In our sky, they are just south of the constellation Aquarius.

Each of the stars is about half as massive as the sun and older than 500 million years. The sun, by comparison, is 4.6 billion years old.

Since most stars form as part of a multiple-star system, the new findings could have implications for understanding the evolution of stars.

Evgenya Shkolnik of the University of Hawaii's Institute for Astronomy and NASA Astrobiology Institute and colleagues spotted the foursome while surveying hundreds of nearby low-mass stars with the Keck I telescope and the Canada-France-Hawaii telescope, both on the summit of Mauna Kea.

At the time of the observations, two of the stars were orbiting each other at 300,000 mph (483,000 kilometers per hour), taking under five days to complete an orbit. The other couple had an orbit speed of 120,000 miles per hour (193,000 kilometers per hour) and takes about 55 days for a complete jaunt around their common gravitational midpoint in space.

The first pair has an orbit radius of at most .06 astronomical units (AU), where one AU is the average distance between Earth and the sun. The second pair has a maximum radius of .26 AU.

The two pairs also promenade each other in less than nine years with a maximum radius of just 5.8 AU. Jupiter, to compare, is 5.2 AU from the sun.

The researchers say that fewer than 1 in 2,000 stars observed might be involved in such intimately bound systems.

"The extraordinarily tight configuration of this stellar system tells us that there may have been a single gaseous disk that forced them into such small orbits within the first 100,000 years of their evolution," Shkolnik said, "as the stars could not have formed so close to one another."

In fact, the spin energy of the more rapidly rotating pair, mixed with the gravitational interaction between the two pairs, has pushed the other pair farther away over the years.

"At one point early in its history, it was even closer than we see now," Shkolnik told SPACE.com.

The research has been submitted to the Astrophysical Journal Letters.