Dying stars build gigantic ‘cocoons’ that shake the fabric of space-time

Since the first direct detection of space-time ripples known as gravitational waves was announced in 2016, astronomers regularly hear the sound of black holes throughout the universe. Projects like the Laser Interferometer Gravitational Wave Observatory (better known as LIGO) have detected nearly 100 collisions between black holes (and sometimes neutron stars), shaking the fabric of the cosmos and sending invisible waves through space.

But new research shows that LIGO could soon hear another kind of jolt in space: cocoons of turbulent gas thrown up by dying stars. Researchers at Northwestern University used cutting-edge computer simulations of massive stars to show how these cocoons can produce “impossible to ignore” gravitational waves, according to research presented this week at the 242nd meeting of the American Astronomical Society. Studying these waves in real life could provide valuable information about the violent deaths of giant stars.

When massive stars run out of fuel, they collapse into black holes, spewing out huge jets of ultrafast particles at the same time. The team of astronomers simulated these final stages of a star’s life, thinking that the jets could lead to gravitational waves, but something else took center stage. “When I calculated the gravitational waves in the neighborhood of the black hole, I found another source that interrupted my calculations: the cocoon,” said the lead researcher. Mineral Gottliebun, an astronomer at Northwestern’s Center for Interdisciplinary Research and Exploration in Astrophysics, said in a statement. The cocoon is a turbulent mass of gas, formed when the outer layers of the collapsing star interact with high-powered jets released from within. To produce gravitational waves, you need something massive that moves asymmetrically, like the moving material of the cocoon.

“A jet starts deep within a star and then works its way out,” Gottlieb said. “It’s like when you drill a hole in a wall. The rotating bit hits the wall and debris spills out of the wall. The bit gives energy to that material. Similarly, the jet passes through the star, causing the star’s material to heat up. they rise and spill. These debris form the warm layers of a cocoon.” Based on Gottlieb’s calculations, LIGO should easily detect the waves created by the cocoon during its next series of observations. In addition, the cocoons emit light, so astronomers can obtain information about them with gravitational waves and telescopes at the same time, an exciting feat known as multi-message astronomy.

If LIGO does observe a cocoon in the near future, it will surely be an interesting new look into the interiors of stars and the end of their lives. It could also be the first time LIGO has succeeded in detecting gravitational waves from an individual object, rather than the interactions between two binary objects orbiting each other. “To date, LIGO has only detected gravitational waves from binary systems, but one day it will detect the first non-binary source of gravitational waves,” Gottlieb said. “Cocoons are one of the first places to look for this type of source.”