A pulsar and a neutron star with a huge size difference have been discovered in a binary star system. Find out why this could unlock the mysteries of the cosmos.
I remember walking to school when I was in Grade 2. My friend and I were looking at the Sun, and she asked me, “What will happen when the Sun goes out?”
I replied, “I think it gets really, really cold, and everything freezes solid, even the air!” She said, “I” m glad I won’t be around for that.”
She was right. The Sun is good for another five billion years or so. In its old age, it will expand into a red giant star and engulf all the inner planets, including Earth.
Sun Become a Red Giant Star and Engulf Inner Planets
After that, it will shrink down into a tiny white dwarf star. In the end, there will be nothing left of the Sun but compressed carbon in the form of an enormous diamond.
That’s only one of the ways stars can come to an end. For example, stars that are ten to twenty-five times the mass of the Sun go out with a bang in a supernova explosion.
After the massive blast, the supernova remnant collapses under its own gravity. Even the massive star’s atoms collapse, fusing protons and electrons into neutrons. What’s left is called a neutron star.
Star’s Atoms Collapse, Fusing Atoms into Neutrons
Neutron stars are incredibly dense. They’re only about ten kilometres in diameter on average, but their average mass is about 1.4 times the mass of the Sun.
To picture this extreme density, imagine taking all of the Sun and compressing it to the size of an average city. If you prefer, a spoonful of a neutron star’s material would weigh a billion tons.
There’s a fascinating sub-category of rotating neutron stars called pulsars. Pulsars act like a lighthouse, sending out very precisely timed jets of charged particles at regular intervals from their highly magnetic poles.
Pulses of Intense Light or Radio Waves
These jets produce pulses of intense light or radio signals. The first pulsar was discovered by Jocelyn Bell Burnell and Antony Hewish in 1967.
They detected intense radio sources that arrived in bursts coming from the same location in the sky. The blasts were precisely 1.33 seconds apart.
It took them a while to figure out what these signals were, There was even some idle speculation that they might come from intelligent life somewhere.
Designated the Signal LGM-1 for “Little Green Men”
At one point, when the team was in a silly mood, they designated the signal LGM-1, with LGM standing for “little green men.” Of course, we now know that pulsars are naturally occurring.
Pulsars are essential reference points for astronomers. Their pulses are very regular, sometimes even more reliable than an atomic clock.
Last week, the journal Nature published a research paper that put neutron stars and pulsars into the spotlight. Astronomers at the University of East Anglia announced their discovery of a binary system comprised of a neutron star and a pulsar.
Pulsar is Much Larger than the Neutron Star
It’s a unique binary system because the pulsar is much larger than the neutron star. This, combined with the fact that the pulsar and the neutron star will eventually merge in half a million years, might unlock some of cosmology’s biggest mysteries.
The team discovered the pulsar and its partner using the massive 305-metre radio observatory at Arecibo in Puerto Rico. Lead researcher Dr. Robert Ferdman explained, “Back in 2017, scientists at the Laser Interferometer Gravitational-Wave Observatory (LIGO)/Virgo collaboration first detected the merger of two neutron stars. The event caused gravitational-wave ripples through the fabric of space-time, as predicted by Albert Einstein over a century ago.”
Connecting that 2017 detection and his team’s discovery, Dr. Ferdman continued, “Most theories about this event assumed that neutron stars locked in binary systems are very similar in mass. Our new discovery changes these assumptions. We have uncovered a binary system containing two neutron stars with very different masses.”
“Two Neutron Stars with Very Different Masses”
When the LIGO/Virgo team encountered the first two merging neutron stars, they emitted gravity waves and a powerful flash of light. Awe-inspiring as these events were, they were also expected.
What astronomers didn’t expect was the volume of matter ejected from the collision, and the brightness of the light burst. Even though the merger took place in a distant galaxy 130 light-years away, the researchers picked up the gravity waves and detected the flash.
This more recent discovery that binary neutron stars can be asymmetrical in size seems to reconcile these anomalies. In the case of the neutron star collision, a discrepancy in the mass of the two companion stars would explain the extra light and debris.
“a Very Plausible Explanation”
As Dr. Ferdman put it, “Although <the collision> can be explained by other theories, we can confirm that a parent system of neutron stars with significantly different masses, similar to the <newly discovered> system, is a very plausible explanation.”
We can’t say whether the unexpected matter and energy from the neutron star merger occurred because of a size discrepancy. However, this new discovery tells us that a disparity like that would explain it.
There’s another reason why this discovery is exciting. The bright flash that appeared when the neutron stars merged was visible in optical telescopes.
Bright Flash Was Visible in Optical Telescopes
That means that future events like this will be visible as well. Dr. Ferdman outlined why that realization is exciting.
“This may also allow for a completely independent measurement of the Hubble constant – the rate at which the Universe is expanding. The two main methods for doing this are currently at odds with each other, so this is a crucial way to break the deadlock and understand in more detail how the Universe evolved.”
We’ve written about the Hubble constant several times in these pages. The discrepancy in the methods for measuring the Hubble constant is relatively small–something like 67.4 (km/sec)/Mps) versus 74 (km/s)/Mps, or roughly 10%.
Discrepancy in the Hubble constant is roughly 10%
If the discrepancy was getting narrower as methods and tools improved, we might just chalk it up to differences in our measurement techniques. In that case, we would just wait until better processes closed the gap.
The trouble so far is that as methods have improved, the gap doesn’t close. The better the data gets, the more stubborn those two numbers seem to be.
So Professor Ferdman’s proposed third way would allow a different perspective to weigh in on the debate. At this point, nobody knows what the numerical result from such an approach would say.
Third Way Would Allow a Different Perspective
Even so, just the process of measuring the Hubble constant differently will provide additional information that presents the question in a different light. That in itself would be worthwhile.
We can’t tell the whole story of the origin and fate of the Universe without knowing how quickly it is expanding. That’s the entire point of the Hubble constant.
Pinning down the precise value of the Hubble constant would unlock a series of unsettled questions in cosmology. The next step is for astronomers to be vigilant in locating more twin neutron stars and, when they get the chance, observing them when they merge.
We always have more to learn if we dare to know.
How colliding neutron stars could shed light on Universal mysteries
Asymmetric mass ratios for bright double neutron-star mergers
Newborn Stars Bringing Forth Solar Systems
Hubble Constant – How Fast Are We Going?
Are We Inside a Hubble Bubble?