The Cosmic Odyssey Of Neutron Stars
By Siri Paramesh
Cosmofluencer (Season 2)
The cosmos contains many enigmatic things born from the remnants after the demise of massive stars. These mysterious objects beckon us to unravel the mysteries of the universe. Let us navigate the frontiers of Astrophysics and delve into the unique properties and phenomena that define these stellar remnants.
What are Stars?
Stars are celestial objects primarily composed of hydrogen and helium undergoing nuclear fusion in their cores. The energy produced by nuclear fusion counteracts the gravitational forces trying to collapse the star, resulting in a stable and often long-lived astronomical body. There are various types of stars, classified based on factors such as brightness, size, colour and behaviour.
1. Main Sequence Star
A normal star forms from a clump of dust and gas. When the clump’s core heats up to millions of degrees, nuclear fusion starts. Fusion releases energy that heats the star, creating pressure that pushes against the force of its gravity. A star is born. Scientists call a star that is fusing hydrogen to helium in its core a main sequence star.
2. Red Giant Stars
When a main sequence star less than eight times the Sun’s mass runs out of hydrogen in its core, it starts to collapse because the energy produced by fusion is the only force fighting gravity’s tendency to pull matter together. But squeezing the core also increases its temperature and pressure, so much so that its helium starts to fuse into carbon, which also releases energy. Hydrogen fusion begins moving into the star’s outer layers, causing them to expand. The result is a red giant, which would appear more orange than red.
3. Neutron Stars
Neutron stars are stellar remnants that pack more mass than the Sun into a sphere about as wide as a small city. A teaspoon of its material would weigh as much as a mountain on Earth!
A neutron star forms when a main sequence star with between about 7 and 20 times the Sun’s mass runs out of hydrogen in its core. The very central region of the star – the core – collapses, crushing together every proton and electron into a neutron. (Heavier stars produce stellar-mass black holes.)
Let's Dive Deeper into Neutron Stars
The concept of neutron stars was first proposed by Lev Landau, an influential Soviet physicist, who independently suggested the possibility of stars composed mainly of neutrons in 1932. The theoretical groundwork for neutron stars was further laid by Fritz Zwicky and Walter Baade. They explored the consequences of supernova explosions, suggesting that these events could lead to the formation of extremely dense remnants. They proposed the existence of neutron stars, less than two years after the discovery of the neutron by James Chadwick.
When a star more massive than our Sun runs out of fuel at the end of its life, its core collapses while the outer layers are blown off in a supernova explosion. What is left behind depends on the mass of the original star. If it’s roughly 7 to 20 times the mass of our Sun, we are left with a neutron star. If it starts with more than 20 times the mass of our Sun, it becomes a black hole. In 1997 Hubble provided the first direct look, in visible light, at an isolated neutron star. The telescope’s results showed the star is very hot (670 000 degrees Celsius at the surface), and can be no larger than 28 kilometres across. These results proved that the object must be a neutron star because no other known type of object can be this hot, small, and dim.
In 2017 the telescope also observed for the first time the source of gravitational waves created by the merger of two neutron stars. This merger gave rise to an event known as a kilonova — something predicted by theory decades ago — that results in the ejection of heavy elements such as gold and platinum into space.
Structure of a Neutron Star
The structure and composition of neutron stars can be analysed through studies of their oscillations. Asteroseismology, a study applied to ordinary stars, can reveal the inner structure of neutron stars by analysing observed spectra of stellar oscillations.
According to studies, the matter at the surface of a neutron star is composed of ordinary atomic nuclei crushed into a solid lattice with a sea of electrons flowing through the gaps between them. The “atmosphere” of a neutron star is hypothesized to be at most several micrometres thick, and its dynamics are fully controlled by the neutron star’s magnetic field. Below the atmosphere, one encounters a solid “crust”. This crust is extremely hard and very smooth (with maximum surface irregularities on the order of millimetres or less), due to the extreme gravitational field.
Types of Neutron Stars
A. Magnetar
A magnetar is a neutron star with a particularly strong magnetic field, about 1,000 times stronger than a normal neutron star. That’s about a trillion times stronger than Earth’s magnetic field and about 100 million times stronger than the most powerful magnets ever made by humans. Scientists have only discovered about 30 magnetars so far.
B. Pulsar
In 1967, a Cambridge Ph. D Student, Jocelyn Bell found something strange in radio Astronomy data. A faint blip coming from 1 part of the sky that repeated every 1.3 seconds with great precision. The discovery was named LGM-1 Little Green Men. It was only through an important study of supernovas the astronomers got to know that the pulses they saw were the beams of spinning neutron stars sweeping past Earth every 1.3 seconds. These were termed PULSARS.
Most of the roughly 3,000 known neutron stars are pulsars, which emit twin beams of radiation from their magnetic poles. Those poles may not be precisely aligned with the neutron star’s rotation axis, so as the neutron star spins, the beams sweep across the sky, like beams from a lighthouse. To observers on Earth, this can make it look as though the pulsar’s light is pulsing on and off.
C. Magnetar + PULSAR
There are now 6 known neutron stars that are both Magnetars and pulsars.
Some Infographics on Neutron Stars
This diagram of a pulsar shows a neutron star with a strong magnetic field (field lines shown in blue) and a beam of light along the magnetic axis. As the neutron star spins, the magnetic field spins with it, sweeping that beam through space. If that beam sweeps over Earth, we see it as a regular pulse of light.
(Credit: NASA/Goddard Space Flight Center Conceptual Image Lab)
Conclusion and Ongoing Observations and Discoveries (2000s - Present)
Ongoing research explores the diverse population of neutron stars, including magnetars (neutron stars with extremely strong magnetic fields) and other exotic variants.
Here are some recent discoveries about neutron stars:
PSR J0901-4046
This neutron star was discovered in May 2022. It has a long period of radio emission and spin properties that are different from other neutron stars.
The newly discovered neutron star, PSR J0901-4046, has characteristics of pulsars, magnetars, and fast radio bursts.
It is different from known pulsars, which have spin periods between 1.4 milliseconds and 23.5 seconds. PSR J0901-4046 has unique radio pulses that occur at regular intervals. It’s unclear how its radio emission is generated, and it challenges the current understanding of how pulsars evolve.
Mega neutron stars
In January 2023, NASA scientists discovered that two neutron stars can collide to form a mega neutron star. This star is so massive and dense that it can only survive for a short time before collapsing into a black hole.
Advances in observational technology, including space-based telescopes and ground-based detectors, continue to enhance our understanding of neutron stars. The story of neutron stars is far from reaching its final chapter. It unfolds many unexpected discoveries with the ongoing research. In conclusion, the fascinating realm of neutron stars remains a dynamic field of exploration, with advancements in observational technologies taking our understanding to new heights.