The secret's in the stars

The universe is expanding, and we may finally know just how quickly

This is a crucial way to understand in more detail how the universe evolved.

Courtesy of Arecibo Observatory/University of Central Florida - William Gonzalez and Andy Torres.

When two stars cross paths, their gravitational tug pulls them closer to one another until they are locked in a bright and violent dance. The two stars eventually merge, a process which could take a billion years, to become one.

Most of these star mergers tend to be pretty equal, with both stars being of similar masses. However, astronomers recently reported the merger of a rather odd pairing, an asymmetrical binary star system.

The new discovery is detailed in a study published Wednesday in the journal Nature, and could help astronomers resolve a disagreement over the rate of expansion of the universe.

Pulsars act as a lighthouse in the cosmos. SA/JPL-Caltech

The stars in question are neutron stars. One of which is a pulsar, a fast-rotating neutron star, or the super-dense remains of a star that exploded in a supernova. These stars emit electromagnetic radiation in the form of bright, narrow beams that sweep across the cosmos in a round motion as the star itself spins.

If you are observing the stars from a distance, it will look as though they are pulsating in flashes of light, which is how they got their name.

The team of astronomers behind the new finding first spotted the asymmetrical pair in 2014 using the Arecibo Observatory in Puerto Rico. The two stars are interlocked and in about half a billion years, they will collide together. A rare event that we will most likely miss.

However, they didn't realize just how massive they are until two years later.

Robert Ferdman, a researcher at the University of East Anglia's School of Physics, and lead author of the study, describes to Inverse exactly how they came to the finding.

"We had to keep track of these pulses for at least one and a half to two years before we got a handle on the masses," Ferdman says. "Once we did, we realized we had two neutron stars that are actually quite different in mass."

One of the stars is much larger than the other, measuring at 1.62 times the mass of the Sun, while the other one measures at 1.27 solar masses. It may not seem like much of a difference, but the usual mass ratio between two merging stars falls somewhere around 0.9, while this pair's mass ratio is a whopping 0.78.

"It is well outside the norm," Ferdman says.

In 2017, astronomers detected the merger of two neutron stars for the first time in an event known as GW170817. When the stars collide, they produce a strong burst of gravitational waves, or ripples in the fabric of space-time.

This recently discovered asymmetrical binary system will merge in around 470 million years, and the gravitational influence of the larger neutron star will distort the shape of its smaller companion by stripping away large amounts of matter before they merge. This will in turn release a larger amount of hot material than if the two stars were on equal playing fields, mass wise.

An artist's impression of the newly discovered pulsar.Courtesy of Arecibo Observatory/University of Central Florida - William Gonzalez and Andy Torres.

"You’re seeing those mergers in the making," Ferdman says. "It’s a precursor to what will happen to these types of systems that we do detect."

The team of researchers behind the new study believe that there is a whole population of these mismatched mergers taking place in the universe, and that they can help resolve an age old question about cosmic expansion.

Since the Big Bang birthed the universe around 13.7 billion years ago, the cosmos has continued to expand. The universe expansion rate is measured through the Hubble Constant, which is calculated by comparing the galaxies' distances to the apparent rate of recession away from Earth.

In order to do so, the two major ways of determining the Hubble Constant are either by looking at the light from nearby variable stars, or the cosmic microwave background, which is electromagnetic radiation, in different galaxies.

"These two methods are at odds with each other right now, they don't agree entirely with each other," Ferdman says.

Therefore, the study suggests using the gravitational waves and electromagnetic light emitted by these neutron star mergers to measure the Hubble Constant instead.

"This is entirely independent of the way that these two are measured, it’d be a way of breaking the tie," he adds. "Depending on how bright these things turn out to be, if we get 15-20 then we can get a precise enough measurement of the Hubble Constant to favor one method over the other for the expansion rate of the universe."

The team is currently scouring the cosmos, looking for more of these odd neutron star pairings to use their explosive light as a guide.

Abstract: The discovery of a radioactively powered kilonova associated with the binary neutron-star merger GW170817 remains the only confrmed electromagnetic counterpart to a gravitational-wave event1,2 . Observations of the late-time electromagnetic emission, however, do not agree with the expectations from standard neutron-star merger models. Although the large measured ejecta mass3,4 could be explained by a progenitor system that is asymmetric in terms of the stellar component masses (that is, with a mass ratio q of 0.7 to 0.8)5 , the known Galactic population of merging double neutron-star systems (that is, those that will coalesce within billions of years or less) has until now consisted only of nearly equal-mass (q > 0.9) binaries6 . The pulsar PSR J1913+1102 is a double system in a fve-hour, low-eccentricity (0.09) orbit, with an orbital separation of 1.8 solar radii7 , and the two neutron stars are predicted to coalesce in 470−11 +12 million years owing to gravitational-wave emission. Here we report that the masses of the pulsar and the companion neutron star, as measured by a dedicated pulsar timing campaign, are 1.62 ± 0.03 and 1.27 ± 0.03 solar masses, respectively. With a measured mass ratio of q = 0.78 ± 0.03, this is the most asymmetric merging system reported so far. On the basis of this detection, our population synthesis analysis implies that such asymmetric binaries represent between 2 and 30 per cent (90 per cent confdence) of the total population of merging binaries. The coalescence of a member of this population ofers a possible explanation for the anomalous properties of GW170817, including the observed kilonova emission from that event.
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