LIGO detects gravitational waves from neutron star merger

Carl Sagan’s famous quote says “The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of starstuff.” In that famous quote, Sagan makes reference to the whole universe started off with hydrogen and helium, all stars produce helium, and then stars over a certain mass threshold produce carbon, nitrogen, oxygen and lots of heavier elements – which are also the source of the life. The star stuff is inside us – every living thing on Earth.

But, even stars are aren’t powerful enough to create heavy elements like silver, gold, and cesium. Since the 1950s, scientists have wondered: where do most of the elements in the periodic table come from?

On October 16, 2017, a team of thousands of LIGONotes 1 scientists announced that the Advanced LIGO and Advanced VirgoNotes 2 gravitational-wave detectors made their first observation of a binary neutron star inspiral. This was not the first time that the gravitational waves produced from two colliding neutron stars were detected. But, for the first time, the scientists were able to detect the event’s location in the sky and witnessed the event with optical and electromagnetic telescopes.

Two Neutron Stars Collide - explosion
The merger of two neutron stars is a extremely powerful event. Gravitational waves (pale arcs) bleed away orbital energy, causing the stars to move closer together and merge. As the stars collide, some of the debris blasts away in particle jets moving at nearly the speed of light, producing a brief burst of gamma rays (magenta). In addition to the ultra-fast jets powering the gamma-rays, the merger also generates slower moving debris. An outflow driven by accretion onto the merger remnant emits rapidly fading ultraviolet light (violet). A dense cloud of hot debris stripped from the neutron stars just before the collision produces visible and infrared light (blue-white through red). The UV, optical and near-infrared glow is collectively referred to as a kilonova. Later, once the remnants of the jet directed toward us had expanded into our line of sight, X-rays (blue) were detected.

LIGO and VIRGO first detected gravitational waves that generated by black holes that had crashed into one another on August 17, 2017. Just two seconds later, NASA’s Fermi Gamma-ray Space Telescope measured a burst of high-energy radiation from the explosion.

But the recent detection, which named GW170817 by the scientists, was very different. This time, the source isn’t a black hole merger – it is the collision of two neutron stars – which means the event is visible to the optical telescopes. The signal was also much stronger than the August’s event, which suggested it was much closer to Earth. What’s more – it lasted 100 seconds, whereas the black hole signals lasted just a few. So, for the first time, scientists have directly detected gravitational waves in addition to light from the spectacular collision of two neutron stars. This marks the first time that a cosmic event has been viewed in both gravitational waves and light.

Neutron stars are the smallest and densest stars known to exist. A neutron star is the collapsed core of a large star having between 10 and 29 solar masses. Despite an average neutron star typically has a radius on the order of 10 kilometers (6.2 mi), they can have masses of about twice that of the Sun. The inside pressure inside so immense, the only things that can exist under these conditions are neutrons, because the protons fused with electrons. So, the merge of two neutron stars is a very big, powerful event. The announced merger, witnessed for the first time by the astronomers, created a massive explosion more powerful than around a thousand supernovaeNotes 3 – that’s why the explosion after the merger called “kilonova”. As the neutron stars spiraled in, they produced gravitational waves.

What’s more, after the explosion, many of the remaining neutrons merged together to form elements – including heavy elements that cannot be produced after supernovae, like gold and platinum. And that’s what exactly happened during the GW170817 event: after analyzing the color and quality of the light coming from the afterglow of the explosion, and we were lucky that the merger was quickly located, scientists confirmed the creation of gold and platinum. Observations revealed the event forged roughly 50 Earth masses’ worth of silver, 100 Earth masses of gold, and 500 Earth masses of platinum.

The video titled “Doomed Neutron Stars Create Blast of Light and Gravitational Waves”, published by NASA

We are made of starstuff

Carl Sagan, Cosmos: Stars – We Are Their Children

Notes

  1. LIGO, the Laser Interferometer Gravitational-Wave Observatory is a large-scale physics experiment and observatory to detect cosmic gravitational waves and to develop gravitational-wave observations as an astronomical tool. Two large observatories were built in the United States with the aim of detecting gravitational waves by laser interferometry. These can detect a change in the 4 km mirror spacing of less than a ten-thousandth the charge diameter of a proton, equivalent to measuring the distance to Proxima Centauri with an accuracy smaller than the width of a human hair. It has so far made four detections of gravitational waves, which predicted in 1916 by Albert Einstein on the basis of his theory of general relativity. Gravitational waves are ripples in the curvature of spacetime that are generated in certain gravitational interactions and propagate as waves outward from their source at the speed of light.
  2. Virgo is a large interferometer designed to detect gravitational waves predicted by the general theory of relativity. Virgo is a Michelson interferometer that is isolated from external disturbances: its mirrors and instrumentation are suspended and its laser beam operates in a vacuum. The instrument’s two arms are three kilometers (1.86 miles) long and located near Pisa, Italy.
  3. A supernova is an astronomical event that occurs during the last stellar evolutionary stages of a massive star’s life, whose dramatic and catastrophic destruction is marked by one final titanic explosion. This causes the sudden appearance of a “new” bright star, before slowly fading from sight over several weeks or months. In Latin, nova means “new”, referring astronomically to what appears to be a temporary new bright star. Adding the prefix “super-” distinguishes supernovae from ordinary novae, which are far less luminous. The word supernova was coined by Walter Baade and Fritz Zwicky in 1931. The “star stuff” what made us, which Carl Sagan refers, were delivered in the universe by supernovae.

Sources

Leave a Reply