Big news rocked the universe earlier this month when astronomers announced the detection and study of the collision of two neutron stars. Telescopes in Hawaii and the University of Hawaii’s Institute for Astronomy played key roles in the discovery. Let’s take a look at why the news excited astronomers.
Albert Einstein predicted gravitational waves in his theory of general relativity in 1916. Gravitational waves transport energy, but unlike light we cannot see gravitational waves. Picture the waves as ripples in space, much like the ripples that appear when a rock is thrown in water.
Astronomers built the Laser Interferometer Gravitational Wave Observatory (LIGO) to detect these waves. The waves are detected by measuring the very tiny changes in the distance or time between a laser beam that is split and then recombined. The gravitational waves appear as beats in the combined beam. Using LIGO, astronomers have detected five observations of gravitational waves since September 2016.
The first four detections were caused by the collision of pairs of black holes. Each time, two black holes orbiting each other collided and combined into one larger black hole. When the black holes merged, the collision released a tremendous amount of energy, more energy than the combined power of all the light released by all the stars in the observable universe put together. Yet, by the time the gravitational waves traveled 1.3 billion light years (7.6 followed by 21 zero miles), the LIGO lasers were moved by one thousand the width of a proton.
The latest detection was different. Two neutron stars, the leftover cores of long dead massive stars, collided. Their collision created a kilonova, which gave off light and gravitational waves, both of which were detected on Earth. The beat generated at LIGO by the event lasted for 100 seconds. The UH Pan-STARRS telescope followed up with observations in the sky that confirmed the event as a kilonova.
When studying the light from the collision, astronomers unexpectedly detected heavy elements like gold and silver. While elements heavier than iron are common on Earth, these elements are very rare in the universe.
Astronomers believed that all the heavy elements in the universe were created in supernova. A supernova occurs when a massive star reaches the end of its life and can no longer create the energy needed to counteract gravity’s pull. The end result is a gigantic explosion, sometimes brighter than the star’s entire galaxy.
The supernova creates and ejects elements like gold, lead and silver, spreading them throughout space. The heavier elements end up in stars and planets when the next generation of each is formed. Neutron stars and black holes are the remnants of supernova, the leftover bits of the star’s core. With the discovery of heavy elements in the remnants of the neutron star collision, astronomers now have a new research path to follow.
The discovery opens up a new branch of astronomy known as multi-messenger astronomy. Using the combination of gravitational waves and light, astronomers have a whole new perspective on the universe.