The Greatest Fireworks Show in the Universe
By Amanda Jermyn

On August 17th, scientists made history when LIGO, the Laser Interferometer Gravitational-Wave Observatory, detected the clearest gravitational wave ever recorded, signaling the collision of two neutron stars. Dr. David Reitze of the California Institute of Technology and executive director of LIGO called it “the greatest fireworks show in the universe.”

Last year LIGO made history with the first ever detection of gravitational waves, a feat that was honored with this year’s Nobel Prize in physics. While that detection resulted from the collision of black holes, this most recent discovery, of merging neutron stars, provides us with far more information. Black holes are so dense that light cannot escape their gravity, but the light from neutron stars can, allowing astronomers to observe the aftermath of their collision in a wide range of electromagnetic signals.

Neutron stars form from stars with a mass greater than eight times that of our sun. When they have burned all their nuclear fuel they experience a gravitational collapse which results in a violent supernova explosion. What remains after the collapse is a super-dense object about 12 miles in diameter but with a mass greater than our sun. When the core collapses it creates so much pressure that its protons and electrons combine to form subatomic particles called neutrons, hence the name neutron star. While very dense, neutron stars are not as dense as black holes, so they are still capable of emitting light.

While the latest discovery was only announced on October 16th, the excitement began on August 17th when the detector in Hanford, Washington picked up a signal indicating the detection of a gravitational wave, which was later confirmed by the detector in Livingston, Louisiana. This involves two antennas detecting the minute squeezing and stretching of space caused by a gravitational wave. The signal is then converted into sound. From the high frequency of the sound or “chirp” emitted on August 17th, astronomers could tell that the activity involved something lighter than a black hole. Two seconds later, the Fermi Gamma-Ray Space Telescope orbiting the Earth detected a brief gamma-ray burst of a type that astronomers suspect is the result of neutron stars colliding.

The discovery sent astronomers scrambling to locate the suspected collision. Fortunately, the European Virgo antenna had recently joined the gravitational wave network, and the faint signal it detected helped point astronomers in the right direction. Once they knew where to look, the fireball from the explosion was soon located, and astronomers were able to observe the radiation emitted in its aftermath. We now know that it happened in a galaxy about 130 million light years away in the constellation Hydra.

The picture that emerged was of two neutron stars circling each other and eventually spiraling so close that they collided, creating a great fireworks display called a kilanova, and sending gravitational waves through the fabric of space.

Studying the aftermath of the explosion has confirmed what astronomers have long suspected, that the heavier elements such as platinum, gold and uranium are formed during such collisions. The merger of two neutron stars frees up the neutrons stored inside them, allowing them to combine with atoms in the surrounding interstellar medium to form these heavy elements. In fact, analysis of data from this collision indicates that it produced a cloud of gold dust many times more massive than the Earth.

The importance of the discovery was highlighted by Harvard astronomer Edo Berger: “With this merger we can see all the expected signatures of the formation of these elements, so we are solving this big open question in astrophysics of how these elements form. We had hints of this before, but here we have a really nearby object with exquisite data, and there is no ambiguity.” If only the alchemists of old had lived to see this day.

Join the Springfield Stars Club on Tuesday, November 28th at 7:00pm at the Springfield Science Museum for a talk by Paul Cardone on “Cassini’s Grand Finale,” the spectacular end to the Cassini mission to Saturn. Cardone is an amateur astronomer and telescope maker who is active in astronomy outreach locally. Refreshments will be served, and the public is welcome. The meeting is free of charge for members, with a suggested donation of $2 per non-member.

Also, on Friday, December 1st at 7:30pm, the Stars Club and the Springfield Science Museum will host “Stars over Springfield,” an astronomy adventure for the whole family. Paul Cardone will give another presentation on “Cassini’s Grand Finale.” A fee of $3 for adults and $2 for children under 18 will be charged.