The Star That Spins Faster Than Thought
PSR J1748-2446ad and the extraordinary physics of a neutron star rotating 716 times every second
Foreword
The Universe contains objects whose behaviour can appear almost contrary to ordinary experience. Among the most remarkable are neutron stars: the extraordinarily compact remnants left behind when massive stars undergo catastrophic stellar collapse. Within a sphere scarcely a few tens of kilometres across, matter is compressed to densities beyond anything that can be reproduced on Earth.
One such object is PSR J1748-2446ad, a millisecond pulsar associated with Terzan 5 in the direction of Sagittarius, approximately 18,000 light-years from Earth. It rotates at about 716 times per second. Its rotational period is only about 1.396 milliseconds.
This essay examines what that astonishing figure actually means, how such a star can acquire so much angular momentum, why its rotation places important constraints upon neutron-star physics, and what pulsars reveal about gravity, matter and the structure of the Universe.
This article is written in accordance with the spirit of Article 51A(h) of the Constitution of India, which calls upon citizens to develop “the scientific temper, humanism and the spirit of inquiry and reform”.
Approximate reading time: 8–10 minutes.
Translation: A translation option may be available through the translation facility of the blog. Machine translations may not always preserve the exact scientific terminology or nuances of the original English text.
Preface
Imagine an object roughly the size of a city, containing more mass than the Sun, turning upon its axis hundreds of times every second. Such a description sounds like science fiction, yet it is a measured property of a real astronomical object.
PSR J1748-2446ad is presently recognised as the fastest known spinning pulsar, with a measured spin frequency of approximately 716.36 hertz. In simpler terms, the neutron star completes more than seven hundred rotations every second. One minute would contain nearly 43,000 rotations.
The number is extraordinary, but the physics behind it is even more remarkable. The pulsar did not begin its life spinning at such a rate. Its present rotation is understood in the context of binary evolution, in which matter transferred from a companion star can also transfer angular momentum to the neutron star. This gradual process is commonly called recycling.
The Body
1. A Star Reduced to a Stellar City
A neutron star is the compact remnant of a massive star whose core has collapsed during a supernova. The collapse is so violent that atomic structure is largely destroyed. Matter is compressed into an extraordinarily dense state in which neutron-rich material dominates.
Neutron stars typically possess radii of only around ten to fifteen kilometres, although the precise radius depends upon their mass and the still incompletely known equation of state of ultra-dense matter.
In the particular case of PSR J1748-2446ad, the discovery observations established an important constraint: if the neutron star has a mass below two solar masses, its radius must be less than approximately 16 kilometres. Thus, the figure of 16 kilometres should be understood as an upper constraint rather than a direct measurement of the star's radius.
The scale is astonishing. A body with a radius of only a few kilometres can contain roughly a stellar mass. Its enormous gravitational field is a consequence of packing such a quantity of matter into such a small volume.
2. Seven Hundred and Sixteen Rotations Every Second
The measured spin frequency of PSR J1748-2446ad is approximately 716.36 hertz. Hertz means cycles per second; therefore, the neutron star rotates approximately 716 times in one second.
Its rotational period is about 1.396 milliseconds. Expressed as revolutions per minute, the rate is approximately 42,981 revolutions per minute.
These are not estimates produced merely by extrapolating from an astronomical model. The rotation is inferred from the highly regular periodic radio pulses emitted by the pulsar and measured with radio telescopes.
3. What Happens at the Equator?
Rotation becomes especially remarkable when its linear velocity at the equator is considered. The relationship is simple:
v = 2πR f
where v is the equatorial speed, R is the stellar radius and f is the rotational frequency.
If an upper radius of 16 kilometres is used, the equatorial surface speed is approximately 2.3 × 105, or about 24 per cent of the speed of light. The exact value depends upon the actual equatorial radius, which is not directly known.
This is an extraordinary velocity. Yet the star does not simply fly apart. Its intense gravity provides the force required to keep the material gravitationally bound while the star rotates.
4. The Balance Between Gravity and Rotation
Every rotating body experiences an outward tendency in its rotating frame. For an ordinary object, the relevant forces are familiar from everyday experience. For a neutron star, however, the conditions are extreme and general relativity becomes important.
If a neutron star rotates sufficiently rapidly, material at its equator can eventually reach the mass-shedding, or Keplerian, limit. At that point, the star can no longer retain material at its equatorial surface.
PSR J1748-2446ad is therefore of considerable importance because its observed rotation rate places it close to the range in which centrifugal effects become decisive for neutron-star structure. The precise theoretical limit depends upon the star's mass and upon the equation of state describing matter at nuclear and supra-nuclear densities.
It is consequently inaccurate to regard 716 rotations per second as a universal absolute maximum for every possible neutron star. Theoretical models permit higher rates under some conditions. Rather, the importance of PSR J1748-2446ad lies in its being the fastest known observed pulsar and in how strongly its rotation tests models of dense matter and stellar stability.
5. How Does a Neutron Star Become Such a Rapid Spinner?
The answer is closely connected with its binary companion. PSR J1748-2446ad belongs to an eclipsing binary system. Its companion has a minimum mass of approximately 0.14 times the mass of the Sun, and the orbital period is about 26 hours.
The system provides an excellent illustration of the process known as recycling.
A newly formed neutron star may rotate relatively slowly compared with a millisecond pulsar. If it subsequently exists in a close binary system, matter from its companion can be transferred towards the neutron star. As this matter falls inward, it carries angular momentum with it.
The effect can be compared, cautiously, with a person sitting on a rotating platform and drawing mass towards the centre: the distribution of angular momentum changes, and the rotation can become faster. In a binary neutron-star system, however, the actual process involves accretion physics, magnetic fields, radiation and strong gravitational fields, making the astronomical mechanism vastly more complicated.
Over a very long period, the transfer of angular momentum can spin the neutron star up to millisecond periods. The result is a millisecond pulsar.
6. Why Is It Called a Pulsar?
A pulsar is a rotating neutron star whose electromagnetic radiation is observed as periodic pulses. The neutron star possesses a powerful magnetic field and rotates about an axis that is generally not perfectly aligned with its magnetic axis.
Radiation associated with the magnetic poles can sweep across space as the star rotates. If the beam crosses the Earth, astronomical instruments can detect a regular series of pulses.
The effect resembles the beam of a lighthouse sweeping across a dark landscape. The star itself does not switch on and off hundreds of times per second; rather, its rotating emission geometry causes the observer to receive pulses at extremely regular intervals.
7. A Clock in the Cosmos
The remarkable regularity of pulsars gives astronomers a natural timing standard. Their pulses can be timed with extraordinary precision, allowing changes in the arrival times to be studied.
Pulsar timing has become an important tool for investigating fundamental physics. In suitable systems, timing measurements can reveal the effects of orbital motion, relativistic gravity and the propagation of gravitational disturbances.
Pulsar timing arrays take this principle further by monitoring many pulsars across the sky. Tiny correlated variations in pulse arrival times can provide evidence of a very low-frequency gravitational-wave background. Thus, pulsars can function collectively as a kind of galactic-scale timing instrument.
8. A Window into Matter We Cannot Reproduce
The significance of PSR J1748-2446ad extends beyond its spectacular rotation rate. Neutron stars provide one of the few natural laboratories in which matter is subjected to densities far beyond those encountered in ordinary terrestrial environments.
The relationship between mass, radius, rotation and stability depends upon the equation of state of dense matter. Different theoretical descriptions predict different internal structures. Observations of rapidly rotating neutron stars therefore help to narrow the range of physically acceptable models.
A pulsar spinning 716 times per second is consequently not merely an astronomical curiosity. It is a stringent test of our understanding of matter under extreme conditions.
9. Terzan 5: A Remarkable Stellar Environment
PSR J1748-2446ad lies in the crowded stellar system known as Terzan 5, in the direction of the Galactic bulge. For many years Terzan 5 was classified as a globular cluster and became famous for its unusually large population of millisecond pulsars.
There is an important modern qualification. Observations made with the James Webb Space Telescope and the Hubble Space Telescope have shown that Terzan 5 contains multiple distinct stellar populations, including younger populations. NASA reported in June 2026 that these observations establish that Terzan 5 is not a globular star cluster in the conventional sense in which it had long been classified. It is now better understood as a relic stellar system associated with the formation of the Milky Way's bulge.
This recent development is itself a useful reminder that scientific knowledge is not a fixed catalogue of facts. Astronomical classifications can change when better observations reveal that an object is more complex than previously believed.
10. The Fastest Known Does Not Mean the Fastest Possible
PSR J1748-2446ad holds a remarkable observational distinction: it is the fastest known spinning pulsar, rotating at approximately 716 Hz.
Yet astronomy must distinguish carefully between fastest observed and fastest physically possible. The latter depends upon the internal composition of neutron stars, their mass, their radius, their equation of state and the mechanisms that may limit their rotation.
The absence of an even faster observed pulsar may therefore have several explanations. Such objects may be intrinsically rare, they may be difficult to form, or their radio beams may simply not sweep across the Earth. Astronomical surveys can detect only those objects whose signals reach our instruments in a sufficiently favourable manner.
11. The Extraordinary Lesson of a Dead Star
A neutron star has no continuing nuclear-burning life comparable with that of an ordinary star like the Sun. It is a stellar remnant. Yet its existence demonstrates that the end of a star's conventional life does not mark the end of its physical significance.
Within a remarkably small sphere, gravity, nuclear physics, electromagnetism, rotation and relativity all meet. The star's pulses cross interstellar space and arrive at Earth carrying information about a body that cannot be visited, photographed as an ordinary surface and sampled directly.
PSR J1748-2446ad therefore represents something considerably more significant than a record-breaking spinner. It is a natural experiment conducted by the Universe itself.
A star that once lived as part of a massive stellar system ended its ordinary stellar life in catastrophic collapse. What remained became a compact remnant, acquired angular momentum from a companion and ultimately reached a rotation rate of more than seven hundred turns every second.
The spectacle is almost beyond imagination, but the science is precise: a period of approximately 1.396 milliseconds, a frequency of about 716.36 hertz, and a rotation approaching 43,000 revolutions per minute. Behind those numbers lies a profound lesson. Nature can compress a stellar history into an object scarcely larger than a city and make that object one of the most precise clocks available to astronomy.
The star may be a remnant, but its signal is very much alive. Every pulse is a reminder that even the ashes of a star can continue to illuminate the deepest questions of physics.
Glossary
- Angular momentum
- A physical quantity associated with rotational motion. In an isolated system it is conserved, although it can be transferred between bodies.
- Binary system
- Two astronomical objects gravitationally bound to one another and orbiting their common centre of mass.
- Equation of state
- A physical description relating quantities such as pressure, density and energy under specified conditions. For neutron stars it describes the behaviour of extremely dense matter.
- Hertz (Hz)
- The SI unit of frequency, equal to one cycle per second.
- Mass-shedding limit
- The rotational limit at which material at a star's equator can no longer remain gravitationally bound to the stellar surface.
- Millisecond pulsar
- A pulsar with a rotational period of only a few milliseconds. Many are thought to have been spun up through prolonged accretion from a companion.
- Neutron star
- An extremely compact stellar remnant formed principally from the collapsed core of a massive star after a supernova.
- Pulsar
- A rotating neutron star whose periodic electromagnetic emission is detected as regular pulses.
- Recycling
- The evolutionary process by which a neutron star in a binary system can be spun up by the transfer of matter and angular momentum from its companion.
- Terzan 5
- A dense stellar system in the direction of the Galactic bulge, long classified as a globular cluster and now recognised from recent observations as a more complex relic stellar system.
References & Further Reading
- Hessels, J. W. T., Ransom, S. M., Stairs, I. H., Freire, P. C., Kaspi, V. M. & Camilo, F. (2006), A Radio Pulsar Spinning at 716 Hz, Science, 311, 1901–1903. arXiv:astro-ph/0601337
- National Radio Astronomy Observatory, A Radio Pulsar Spinning at 716 Hertz. NRAO
- NASA Science (2026), NASA Webb, Hubble Reveal History of Relic of Milky Way's Formation. NASA Science
- NASA, 'Extreme' Telescopes Find the Second-fastest-spinning Pulsar. NASA
- SIMBAD Astronomical Database, PSR J1748-2446ad. SIMBAD
- Cipolletta, F. et al. (2015), Fast Rotating Neutron Stars with Realistic Nuclear Matter Equation of State. arXiv
Copyright
© Dhinakar Rajaram 2026. All rights reserved.
This article is an original work prepared for publication on the author's blog. The scientific facts have been checked against astronomical literature and institutional sources. Reproduction, republication or substantial adaptation without permission is not authorised.
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