Buzz
Kilonovas occur when two neutron stars collide, unleashing a spectacular cosmic display. NASA, ESA, and A. Feild (STScI)When a massive star exhausts its fuel and perishes, it may end with a dramatic explosion known as a supernova.
However, supernovas are not the only grand explosions in space. Enter the 'kilonova.' It's 1,000 times more brilliant than a nova (which happens when a white dwarf erupts), yet not as bright as a supernova. Kilonovas are sparked by the collision of two stellar remnants. These events create the most intense electromagnetic bursts in the cosmos, dispersing gold across the universe.
Stellar Husks
Neutron stars are the remnants of massive stars that have exploded in supernovae. These incredibly dense objects, no bigger than a dozen miles across, are primarily made up of neutrons. Despite their small size, they have a mass greater than that of the sun and generate intense magnetic fields. To give you an idea of their density, just a teaspoon of neutron star matter weighs a staggering 1 billion tons (907 million metric tons).
The matter inside neutron stars doesn’t behave like ordinary matter. The immense gravitational forces squeeze everything into a 'degenerate' state. The pressure is so extreme that quantum mechanics is the only force keeping their mass from collapsing into a black hole.
A collision between two neutron stars is a catastrophic event, one that sends shockwaves through space. On August 17, scientists witnessed such a collision’s aftermath, thanks to the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the U.S. and the Virgo observatory in Italy. They detected a faint and unusual signal originating from the galaxy NGC 4993, located 130 million light-years away.
Multi-messenger Astronomy
Up until that point, gravitational wave detectors had only detected black hole mergers occurring billions of light-years away. So, when a faint signal was observed at a much closer range, it came as a surprise. After analyzing the characteristic 'chirp' in the gravitational wave signal—an increase in frequency as two massive objects spiral toward each other and merge—scientists determined that the signal, named GW170817, wasn't from a black hole merger but rather the merger of two neutron stars. These stars, with masses of just 1.1 and 1.6 times that of the Sun, had become gravitationally bound, spinning closer and closer until they collided.
When the detection was made, NASA's Fermi Gamma-Ray Observatory and the European INTEGRAL space telescope both observed a powerful gamma-ray flash emanating from NGC 4993. This phenomenon is known as a short gamma-ray burst (GRB).
While scientists had speculated that short GRBs might result from colliding neutron stars, only with the aid of gravitational wave detectors could this theory be proven. This event marks the first time that both gravitational and electromagnetic waves have been measured from a single cosmic occurrence, directly linking a GRB with a neutron star merger and opening up a new frontier in cosmic research known as "multi-messenger astronomy."
Kilonova!
The gravitational waves enabled scientists to link the gamma-ray burst with the neutron star collision, but the question remains: what exactly triggered the GRB?
The merger of the two neutron stars that caused GW170817 was nothing short of catastrophic. As these massive objects rapidly spun towards each other and collided, enormous amounts of superheated neutron star matter were expelled into space. This set the stage for a spectacular kilonova explosion.
Neutron stars, primarily made up of neutrons, also contain protons, which are essential components of atomic nuclei. After the collision, the resulting chaos sent a flurry of subatomic particles into space. The environment was so extreme that pieces of radioactive neutron star material began to fuse, forming new elements. This process, called rapid neutron capture ('r-process'), allowed neutrons to bind with the newly formed elements before they could decay. The immense energy produced by this reaction erupted in the form of gamma-ray radiation, giving rise to the GRB detected from 130 million light-years away.
Follow-up investigations of the chaotic site, conducted by the Hubble Space Telescope, Gemini Observatory, and ESO Very Large Telescope, provided spectroscopic evidence for the r-process. What's remarkable is that in the remnants of the kilonova explosion, significant amounts of heavy elements, including gold, platinum, lead, uranium, and silver, were created.
For years, scientists have questioned how elements heavier than iron come into being in the universe (lighter elements are typically created in stars through stellar nucleosynthesis). Now, we have observational proof that these violent kilonova events are also responsible for the creation of the universe’s heaviest and most valuable elements.
Editorial note: This article was corrected on Oct. 20, to rectify an inaccuracy introduced by the editor, misstating the brightness of kilonovas. Supernovas are, in fact, the brightest, followed by kilonovas and novas, respectively.
Gravitational waves travel at the speed of light, but GW170817 was detected by LIGO and Virgo just moments before the gamma-ray burst (GRB) was spotted by Fermi and INTEGRAL. According to NASA, this sequence of events occurred because the neutron star merger happened first, emitting gravitational waves, while the kilonova followed closely behind, unleashing gamma rays into the universe.
