Neutron star collision reveals new information about the universe

Imagine the universe 133 million years ago. Deep in a corner of the Hydra constellation, two massive stars known as neutron stars met their end as they merged together in an explosive event. For 133 million years, the rays and waves released from this explosion traveled across the galaxy until it intercepted the Earth in August of 2017.  

Dr. Davide Lazzati, an associate professor of physics, who leads the computational astrophysics group at Oregon State University, accurately predicted some of the results of this neutron star merger in a paper published July 9 in “Monthly Notices of the Royal Astronomical Society”. Lazzati uses sophisticated computing programs to predict how the universe might behave. 

“I have a particular interest in the ‘violent universe’, or explosive phenomena,” Lazzati said. “The death of massive stars, also known as supernovae.”

Neutron stars are incredibly massive, according to Lazzati. They form from the core of an enormous dying star; depending on conditions in the core, giant star collapses could result in black holes or neutron stars.

“When a very big star dies, it explodes much more powerfully than a normal star,” Lazzati said.  “The news of the day is when two neutron stars merge and explode in a different way.

According to Lazzati, he connects to a NASA supercomputer known as Pleides to calculate what theoretical cosmic events might look like. The computer program uses what is known as “relativistic hydrodynamics”, which allows it to incorporate universal laws pertaining to gravity, thermodynamics and general and special relativity.  

“I want to simulate an explosion, so I build a star in my computer,” Lazzati said. 

Energy is then added to the core of the star, according to Lazzati. Throughout the process, the Pleides computer code tracks what occurs.

In early July, Lazzati and his lab published their original theoretical predictions concerning short, gamma ray bursts, according to Lazzati. Rumors of the neutron star burst came a few weeks later as Lazzati and his team were putting finishing touches on a second draft that included computer simulations. 

“The paper said, ‘We think we will see not only a gravitational wave come from the two stars merging, but there will also be a gamma ray flash no matter the orientation,” Lazzati said. “Saying we could see these gamma rays from the side, that was new.”

Lazzati’s published theory was cutting-edge, as it suggested that observers on Earth would be able to observe this event no matter where their detectors were in relation to the rays. According to Lazzati, the current theory hypothesized that scientists wouldn’t be able to detect gamma ray emissions unless they were beamed directly in Earth’s pathway. 

“We were lucky because it was published and less than two months later this rumor starts to circulate that maybe something had been seen,” Lazzati said. “So we published a second paper saying that, ‘Hey, maybe we aren’t so crazy after all,’ and so the first paper was almost entirely pen and paper analytic, there was no computer simulations. We were adding computer simulations when the rumors came out.”

Gravitational waves were first theorized by Einstein, and their existence was confirmed by the Laser Interferometer Gravitational-Wave Observatory in 2015. The LIGO research network is a collaborative effort among scientists to understand more about the universe. 

According to Ben Farr, an assistant professor of physics at the University of Oregon, the event was measured using incredibly sensitive technologies that can detect changes in length smaller than the width of an atomic nucleus. Farr works with LIGO as a data analyst, and specializes in extracting new properties associated with the sources of gravitational waves.

“It’s like looking for a needle in a haystack,” Farr said. “Our signals are buried in very appreciable noise. To properly find what we’re looking for, we need to know what the needle looks like.” 

LIGO is composed of L-shaped tunnels that run over 2 ½ miles, according to Farr. Lasers in these tunnels measure any disturbances to their length. Most disturbances are earth based noise, but sometimes they are astronomical, in the form of gravitational waves. 

LIGO first detected gravitational waves in 2015, according to Farr. 

“People are incredibly excited, to say the least,” Farr said. “This is what we call a multi-messenger event, where we have detected this source in gravitational waves and in another sense, which is light. This is the start of a whole new area of astronomy.”

“Multi-messenger events” are events that provide multiple forms of data, according to Farr. In this case, not only did scientists gather data pertaining to gamma ray origins and gravitational wave behavior, but also gathered information on the rest of the electromagnetic spectrum, as well as insight into how some periodic heavy metals are formed. 

Information gathered from this multi-messenger event can apply to not only astronomers interested in stars but to planetary scholars as well. 

According to Dr. Randall Milstein,  astronomer-in-residence of the Oregon NASA Space Grant Consortium and an astronomer instructor at OSU, new areas to explore include new techniques in measuring astronomical distances based on location and brightness.  

Astronomers have a variety of techniques used to measure astronomical distances from Earth, according to Milstein. 

“When we measure parallax angles, it allows us to measure astronomically, relatively short distances. If we want to measure farther, we use a star of a known brightness, named Cepheid Variable,” Milstein said. “We can then compare similar Cepheid stars calculating distance from observed brightness.”

If the object is farther out, say in another galaxy, astronomers turn to the greater brightness of 1A supernovea to calculate distance, according to Milstein. If the object is even farther away, astronomers turn to using Hubble’s Equation, a mathematical formula.

“The recent observation of of colliding neutron stars offers a measuring step between 1A supernovae and Hubble’s equation,” Milstein said. “It’s another tool in the toolbox offering additional accuracy in measuring cosmic distances.”

Milstein is a planetary astronomer by discipline, but the new information learned from this cosmic event affects even his work. The neutron star collision yields information that applying to many different pathways of thinking, which is why so many scientists are excited about this event, according to Milstein.

“What I like to see is the excitement of my colleagues who are in astrophysics,” Milstein said. “That excitement helps me understand the importance of the discovery, and gets me interested in a topic I may not have looked into unless this had happened.”

According to Farr, scientists have also confirmed origins of some of the known heavy metals. The neutron star collision resulted in an explosion known as a kilonova, which created massive amount of selected heavy metals, such as platinum and gold. 

The kilonova that occurred immediately after the collision likely made more than earth’s mass of gold, according to Farr. 

According to Lazzati, this cosmic event leaves much to be explored and interpreted in the next upcoming months. What many are excited about is the further thinking that can occur with the confirmed data.

“Now that we know what data we have we can try to refine the model with what we know. We’re not shooting in the dark anymore,” said Lazzati. “Before we were going for a big target, so we were shooting with a cannon. Now we know where to go, so we can go with a more precise blade.”