### Spotting a neutron star collision

This exercise is a part of Educator Guide: Neutron Star Crash Seen for First Time / View Guide

Based on the article Neutron star crash seen for first time”:

1. This article summarizes breaking news in the world of science. Create a Snapchat post that summarizes the article to share with your friends.

Possible student response: Draw your own image of a neutron star collision (you can refer to the guide cover image for inspiration) and add text such as “Two neutron stars collide!” Label the electromagnetic waves and gravitational waves being emitted. Create stamps of different heavy metal elements and place them on the image so they appear to be emitted from the crash.

2. What is a neutron star? How massive were the neutron stars that collided?

Possible student response: A neutron star is an ultradense, neutron-rich core of a dead star. When some stars grow old they explode and the elements within it compress. The neutron stars had masses between 1.17 and 1.60 times that of the sun.

3. Where were gravitational waves from the neutron star collision detected?

Possible student response: Gravitational waves were detected by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) in Louisiana and Washington state. In both locations, LIGO detected a burst of approximately 100 seconds of gravitational waves, the strongest and longest signal yet detected. Only a faint gravitational wave signal was recorded by LIGO’s sister experiment in Italy, Advanced Virgo.

4. How did scientists determine the direction from which the gravitational waves came? Where were the colliding neutron stars located?

Possible student response: Gravitational wave detectors are more sensitive to gravity waves coming from certain directions relative to the orientation of the detector. With three gravitational wave detectors (two LIGO detectors plus the Virgo detector in Italy), scientists can compare the gravitational wave signals arriving at each detector to determine which direction the waves came from. LIGO’s two detectors in the United States sensed a strong signal, but the Virgo detector in Italy only sensed a weak signal, indicating that the waves came from a certain area of the sky to which Virgo is not very sensitive. A new speck of glowing visible light in that area of the sky glimpsed by telescopes, helped scientists pinpoint the colliding stars’ location: galaxy NGC 4993, 130 million light-years from Earth in the constellation Hydra.

5. What corresponding electromagnetic radiation was detected from the neutron star collision?

Possible student response: Just 1.7 seconds after LIGO received the gravitational wave signal, NASA’s Fermi space telescope detected gamma rays in the same region of the sky. Other telescopes spotted a glow of visible and infrared light starting about 11 to 12 hours after the collision. More than a week later, as those wavelengths faded away, X-rays were emitted, followed by radio waves.

6. How did scientists conclude that the colliding celestial bodies were neutron stars and not black holes?

Possible student response: Black holes that scientists had previously recorded merging were much more massive — tens of times the mass of the sun — than the crashing bodies that were recorded in this merger. Also, the recent collision emitted various types of light. A black hole collision is not expected to produce light. Both of these observations led scientists to determine that the colliding celestial bodies were neutron stars and not black holes.

7. What elements were produced by the neutron star collision, and by what process? What was significant about this discovery?

Possible student response: The collision produced heavy elements including gold (about 10 to 100 times the Earth’s mass in gold!), silver, platinum and uranium. In a chain of reactions called the r-process, atomic nuclei combine with neutrons and undergo radioactive decay, transforming into new elements. Astrophysicists had never directly witnessed the r-process or the creation of these heavy metal elements, and debated whether the r-process occurred in supernovas or neutron star mergers. Though researchers can’t yet say whether or not the r-process also occurs in supernovas, scientists now know that when neutron stars collide, a large amount of heavy metal elements are produced via the r-process.

8. What did astrophysicists learn about short gamma-ray bursts? Why is this discovery significant?

Possible student response: The colliding neutron stars emitted a burst of high-energy light called short gamma rays. Similar short gamma-ray bursts are detected about 50 times a year, so it now appears that those come from neutron star collisions too. Up until this recent merger, scientists were uncertain of where short gamma-ray bursts came from.

9. How did astrophysicists use their observations of the neutron star collision to learn more about the expansion of the universe?

Possible student response: By measuring the distance of the collision using gravitational waves and comparing that with how much the universe expansion stretched light from the neutron stars’ galaxy, astrophysicists measured the expansion rate of the universe. Scientists previously measured this property, known as the Hubble constant, through other means. But they got two different results: 67 and 73 kilometers per second per megaparsec. The new measurement indicates that distantly separated galaxies are spreading apart at about 70 km/s per megaparsec, right in the middle of the two previous results. Future detections of neutron star mergers could improve the measurement.