Archaeologists and paleontologists study lost civilizations and fossils, respectively, but astrophysicist Heloise Stevance studies the past using a different approach. By analyzing signals in the sky captured by astronomers, such as light from exploding stars, Stevance conducts stellar genealogy by rewinding the clock on the signal by billions of years. She examines the past lives of dying and dead stars and finds their intriguing life stories full of drama while working at the University of Auckland in New Zealand.
In August 2017, scientists observed the collision of two neutron stars, which are the remnants of dead stars, in a distant galaxy. This phenomenon, known as a neutron star merger, was detected by the gravitational waves' ripples in space-time and the light from the resulting explosion. The discovery was the first and only time such an event was observed using gravitational waves.
Studying Neutron Star Merger
Based on the signals, the scientists determined that the neutron stars were 1.1 to 1.6 times the mass of the Sun and that these collisions produce heavier natural elements such as gold and platinum. However, the signals posed more questions than they answered. It is currently unknown how frequently neutron star mergers occur or if they are responsible for creating all heavy elements in the Universe. To answer these questions and others such as the age of the Universe, astrophysicists need to observe more of these mergers. This is where stellar genealogy can assist. In a study published in Nature Astronomy in January, Heloise Stevance and her team examined the past of the neutron stars involved in a collision.
They studied the billions of years during which the stars were fusing hydrogen in their cores as a binary star system. By obtaining a better understanding of binary star systems and their evolution, the researchers aim to develop a more systematic approach to search for and comprehend these merger events. According to the analysis by Stevance and her team, the two neutron stars involved in the collision were the remnants of a star that was 13 to 24 times the mass of the Sun and another star that was 10 to 12 times the mass of the Sun.
These stars began shining 5 to 12.5 billion years ago when only 1 percent of their composition consisted of elements heavier than hydrogen and helium. The study also explored the interactions between the stars before they became neutron stars by burning out their fuel. Initially, the stars were tens of millions of kilometers apart, with each star surrounded by a stellar envelope composed of gas. Over time, one star's envelope engulfed the other at least twice, meaning their outer gasses merged to become a shared envelope. The level of detail that Stevance and her team were able to deduce about the two distant neutron stars, particularly given that the astrophysicists only directly witnessed their violent demise, is impressive.
Complex Temperature and Solutions
According to Wired, the team combined observations of the neutron stars with insights gleaned from studying other stars and galaxies, utilizing a complex mathematical model that describes the temperature, chemical composition, and other features of 250,000 different types of stars. This model includes information about the stars' interiors and surfaces, as well as how these properties change as stars burn fuel and eventually die. The model can also simulate entire galaxies, each containing multiple collections of stars with different ages and chemical compositions.
Essentially, the team was able to reconstruct a city from a pile of dust, utilizing their extensive knowledge of stellar properties and interactions to piece together the stars' pasts. To piece together the past of the merged neutron stars, Stevance and her team used their mathematical model to replicate the observed data for the stars. By doing so, they were able to determine the most likely scenarios of what occurred before the two stars merged. For instance, the team concluded that the stars shared an envelope multiple times based on the time it took for them to collide.
The merging of envelopes creates a drag force that slows down the stars' orbit, causing them to spiral in towards each other and rapidly reduce the distance between them. To merge as quickly as they did, the stars had to share envelopes multiple times. The study of the neutron star merge is based on a model of stars that was developed 15 years ago by Stevance's colleagues to study celestial objects in distant galaxies. This model is built on star models from the 1970s and is the result of decades of research in astronomy.
Stevance's Model
The work highlights how scientific discoveries often emerge from a long and indirect process involving many researchers working on different questions over time. The open-source nature of the work by Stevance and her team means that other researchers can use their framework to study other types of stellar activity, such as supernovae. By studying more of these various types of explosions and their contribution to the formation of heavy elements, astrophysicists can gain a better understanding of the origin of elements in the Universe, including those that were eventually used to form the Earth.
The knowledge gained from Stevance's model and the study of neutron star mergers could guide future searches for these mergers and help astrophysicists calculate the rate of expansion of the Universe. This is because the gravitational wave signal and emitted light from the mergers can be used to estimate the distance from Earth to the neutron stars and how fast they are moving away, respectively. This information can help reconcile the two conflicting rates for the Universe's expansion calculated by different methods.
Indeed, the study of the Universe has been an exciting and ongoing endeavor, and there is still much more to discover and understand. The detection of gravitational waves from neutron star mergers and the work of astrophysicists like Stevance and her team continue to provide valuable insights into the origins and evolution of our Universe.
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