# What is the final destiny of a neutron star?

We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

As I understand, neutron stars are born as extremely bright, extremely fast spinning cores of stars dying in a supernova. However, several websites tell me that within a course of a few years, the surface temperature of a neutron star falls from several trillion kelvin to only a few million kelvins. Furthermore, with the passage of time, the spinning speed of the neutron star also decreases considerably.

This raises the question: what is the final destiny of a neutron star? Does it always stay so horribly magnetic, hot and fast spinning or does it keep degrading into some form of cold, extremely dense star core with a much weaker magnetic field or do some of its features (specially the magnetic field strength and spin) stay at hightened levels forever (or at least several hundreds of billions of years)?

This raises the question: what is the final destiny of a neutron star?

Neutron stars cannot stay hot forever. Neutron stars cool because they radiate. (This is called radiational cooling.) Except for their gravitational field which distorts spacetime in the vicinity of a neutron star, most lone neutron stars slowly fade away over time, eventually becoming essentially invisible. One way of detecting those cold, lone neutron stars is to observe the gravitational lensing of stars behind them.

With regard to magnetic field and rotation, those too drop over time. A neutron star's rotation is what creates the magnetic field, but this magnetic field drains the rotation rate.

An alternate fate for neutron stars is to undergo gravitational collapse and form a black hole. This can happen in a number of ways. A massive neutron star can undergo collapse as a result of its slowing rotation rate. The initial rapid rotation staves off gravitational collapse, but that no longer works when the neutron star's rotation rate drops.

Some neutron stars are not isolated. They are instead members of multiple star systems. Neutron stars can draw material from a partner star and eventually become massive enough to undergo collapse. Finally, a few neutron stars orbit one another closely. The discover of this, the Hulse-Taylor binary, led to the 1993 Nobel Prize in physics. Those closely orbiting neutron stars emit gravitational waves, thereby causing the orbit to decay. Those neutron stars eventually collide, once again resulting in a gravitational collapse.

Neutron stars have extremely small heat capacities. That is because they consist largely of degenerate fermions and the heat capacity is further suppressed if, as expected, those fermions are in a superfluid state.

This has (at least) two consequences:

(a) they cool down extremely rapidly - neutrino emission processes are highly effective, in the first $10^5$ years or so of a neutron star's life, at reducing its interior temperature to a few $10^7$ K and the surface temperature to $<10^6$ K. After that, the dominant cooling process is photons emitted from the surface ($propto T^4$) and neutron stars rapidly fade from view thereafter.

(b) However, the low heat capacity also means that it is easy to keep a neutron star hot if you have any way of adding energy to it - such as viscous dissipation of rotation by friction, accretion from the interstellar medium or ohmic heating by magnetic fields.

No isolated neutron star surfaces have been measured with temperatures much below $10^6$ K - i.e. all observed isolated neutron stars are at young ages. The situation is summarised in section 5.7 of Yakovlev & Pethick (2004). Without any reheating, a neutron star would reach 100K in only a billion years - this is already utterly invisible. The reheating mechanisms must play some role for older neutron stars, but as Yakovlev & Pethick state: "Unfortunately, no reliable observational data on the thermal states of such stars are available". In conclusion, nobody really knows at the moment what the long-term ($>10^6$ years) fate of neutron stars is in terms of their temperature.

The situation with regard to spin and magnetic field is more secure. There are not the same mechanisms available to spin-up an isolated neutron star or regenerate their magnetic fields. Both are expected to decay with time and indeed the spin-down rate and magnetic field strength are intimately connected, because the spin-down mechanism is the emission of magnetic dipole radiation. The magnetic field decays through the generation of currents that then ohmically dissipate (providing a source of heat) or perhaps more rapidly via currents generated by the Hall effect or through ambipolar diffusion.

For pure magnetic dipole radiation, one predicts $dot{Omega} propto Omega^3$. For typical surface magnetic field strengths of $10^8$ T, pulsars spin down to periods of around a few seconds in less than a million years, at which point the "pulsar activity" switches off and we can't see them any more, unless they are in binary systems and accreting matter in order to spin them up again. Unfortunately, there is very little observational evidence to pin down how fast magnetic fields decay (because we don't see old, isolated neutron stars!). The decay of B-field cannot be very fast, certainly timescales are longer than $10^5$ years. Theoretical estimates of B-field decay timescales are more like billions of years. If this theory is right then neutron stars would continue to spin down very rapidly even after the pulsar mechanism has ceased.

## Neutron star

extremely small, extremely dense star, with as much as double the sun's mass but only a few miles in radius, in the final stage of stellar evolution stellar evolution,
life history of a star, beginning with its condensation out of the interstellar gas (see interstellar matter) and ending, sometimes catastrophically, when the star has exhausted its nuclear fuel or can no longer adjust itself to a stable configuration.
, 1893�, German-born American astronomer. From 1919 to 1931 he was on the staff of the Hamburg observatory from 1931 to 1958, at the Mt. Wilson observatory.
, 1898�, Swiss-American astrophysicist, b. Bulgaria, educated at Zürich. Associated with the California Institute of Technology after his arrival in the United States in 1925, he became professor of astrophysics in 1942 and emeritus professor in 1972.
. Click the link for more information. predicted the existence of neutron stars in 1933. The central core of a neutron star is composed of neutrons or possibly a quark-gluon plasma (see elementary particles elementary particles,
the most basic physical constituents of the universe. Basic Constituents of Matter

Molecules are built up from the atom, which is the basic unit of any chemical element. The atom in turn is made from the proton, neutron, and electron.
. Click the link for more information. ) there are no stable atoms or nuclei because these cannot survive the extreme conditions of pressure and temperature. Surrounding the core is a fluid composed primarily of neutrons neutron,
uncharged elementary particle of slightly greater mass than the proton. It was discovered by James Chadwick in 1932. The stable isotopes of all elements except hydrogen and helium contain a number of neutrons equal to or greater than the number of protons.
. Click the link for more information. squeezed in close contact. The fluid is encased in a rigid crystalline crust a mile or two thick. The outer gaseous atmosphere is probably only a few feet thick. The neutron star resembles a single giant nucleus nucleus,
in physics, the extremely dense central core of an atom. The Nature of the Nucleus
Composition

Atomic nuclei are composed of two types of particles, protons and neutrons, which are collectively known as nucleons.
. Click the link for more information. because the density everywhere except in the outer shell is as high as the density in the nuclei of ordinary matter. There is observational evidence of the existence of several classes of neutron stars: pulsars pulsar,
in astronomy, a neutron star that emits brief, sharp pulses of energy instead of the steady radiation associated with other natural sources. The study of pulsars began when Antony Hewish and his students at Cambridge built a primitive radio telescope to study a
. Click the link for more information. are periodic sources of radio frequency, X ray, or gamma ray radiation that fluctuate in intensity and are considered to be rotating neutron stars. A neutron star may also be the smaller of the two components in an X-ray binary star.

## Deciphering the lives of double neutron stars in radio and gravitational wave astronomy

Artist’s illustration of a double neutron star merger. Credit: LIGO, Sonoma State University, A. Simonnet.

Scientists from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) have described a way to determine the birth population of double neutron stars—some of the densest objects in the universe formed in collapsing massive stars. The recently published study observed different life stages of these neutron star systems.

Scientists can observe the merging of double neutron star systems using gravitational waves—ripples in the fabric of space and time. By studying neutron star populations, scientists can learn more about how they formed and evolved. So far, there have been only two double neutron star systems detected by gravitational-wave detectors however, many of them have been observed in radio astronomy.

One of the double neutron stars observed in gravitational wave signals, called GW190425, is far more massive than the ones in typical galactic populations observed in radio astronomy, with a combined mass of 3.4 times that of our Sun. This raises the question: why is there a lack of these massive double neutron stars in radio astronomy? To find an answer, OzGrav Ph.D. student Shanika Galaudage, from Monash University, investigated how to combine radio and gravitational-wave observations.

The birth, mid-life and deaths of double neutron stars

Radio and gravitational-wave astronomy enables scientists to study double neutron stars at different stages of their evolution. Radio observations probe the lives of double neutron stars, while gravitational waves study their final moments of life. To achieve a better understanding of these systems, from formation to merger, scientists need to study the connection between radio and gravitational wave populations: their birth populations.

Shanika and her team determined the birth mass distribution of double neutron stars using radio and gravitational-wave observations. "Both populations evolve from the birth populations of these systems, so if we look back in time when considering the radio and gravitational-wave populations we see today, we should be able to extract the birth distribution," says Shanika Galaudage.

The key is to understand the delay-time distribution of double neutron stars: the time between the formation and merger of these systems. The researchers hypothesized that heavier double neutron star systems may be fast-merging systems, meaning that they're merging too fast to be visible in radio observations and could only be seen in gravitational-waves.

GW190425 and the fast-merging channel

The study found mild support for a fast-merging channel, indicating that heavy double neutron star systems may not need a fast-merging scenario to explain the lack of observations in radio populations. "We find that GW190425 is not an outlier when compared to the broader population of double neutron stars," says study co-author Christian Adamcewicz, from Monash University. "So, these systems may be rare, but they're not necessarily indicative of a separate fast-merging population."

In future gravitational wave detections, researchers can expect to observe more double neutron star mergers. "If future detections reveal a stronger discrepancy between the radio and gravitational-wave populations, our model provides a natural explanation for why such massive double neutron stars are not common in radio populations," adds co-author Dr. Simon Stevenson, an OzGrav postdoctoral researcher at Swinburne University of Technology.

## Mineral Physics

### 2.07.1 Introduction

The past two decades in modern astronomy have seen astounding success in discovering planets orbiting stars beyond the solar system. Starting with the first detections of planets orbiting neutron stars ( Wolszczan and Frail, 1992 ) and those orbiting sunlike stars ( Marcy and Butler, 1996 Mayor and Queloz, 1995 ), over 900 confirmed extrasolar planets or ‘exoplanets’ are now known. These planets have been discovered by a variety of techniques. By far, the largest number has been identified by the radial velocity method, which involves observing the wobbling of the host star due to the gravitational pull by the planet along the line of sight of the observer. The amplitude of the radial velocity of the stellar motion places a lower limit on the planetary mass, while the period gives the orbital period of the planet. Another major approach to exoplanet detection has been the transit method, whereby the passage of the planet in front of the host star is seen in the temporary dimming of the stellar light as observed from Earth. The transit method ushered in a major advance in the characterization of exoplanets as it allows the measurement of the planetary radius and the orbital inclination of the system, which conclusively determines the planetary mass when radial velocity observations are available. One of the greatest successes in discovering exoplanets have been achieved by the Kepler space telescope ( Batalha et al., 2013 Borucki et al., 2010 ), which has discovered (as of July 2013) 134 confirmed exoplanets and over 3200 exoplanet candidates with a wide range of exoplanetary properties. Exoplanets have also been detected by other methods such as pulsar timing ( Wolszczan and Frail, 1992 ), gravitational microlensing ( Gaudi et al., 2008 ), and direct imaging ( Kalas et al., 2008 ), covering different regions of exoplanet parameter space. For further details, see Chapter 10.21 .

The numerous exoplanet discoveries via different methods have also led to the estimation of occurrence rates of exoplanets over a wide range of planetary masses and radii, ranging from gas giant planets to sub-Neptune and even terrestrial-size planets in the solar neighborhood ( Fressin et al., 2013 Howard et al., 2012 ). The available statistics indicate a planetary mass (size) function increasing with decreasing planet mass (size), namely, that giant planets are comparatively rare ( Howard et al., 2012 Figure 1 ). It is estimated that 15% of sunlike stars host at least one planet of 3–30 Earth masses (ME), and an additional 14% of such stars have planets with 1–3 ME ( Howard, 2013 ). Correlations of the planet occurrences with the metallicities of the host stars have also suggested that formation of giant planets is positively correlated with the metallicities of the stellar hosts ( Fischer and Valenti, 2005 ), whereas low-mass planets are uncorrelated with the stellar metallicity ( Buchhave et al., 2012 ). Besides the statistical correlations, exoplanets detected to date span a wide diversity in orbital parameters (orbital periods, separations, eccentricities, inclinations, etc.), masses, radii, and temperatures, much wider than the phase space spanned by planets in our solar system. In recent years, it has even become possible to observe the atmospheres of a wide range of exoplanets (e.g., Seager and Deming, 2010 ).

Figure 1 . Average metallicity (proportion of elements heavier than H and He) for planet-hosting stars and a number of observed planets plotted as a function of planetary radius. The red symbols show the average metallicity of the host stars with planets of different radii grouped in 1.33 Earth radii and 4 Earth radii bins. The bin size is indicated by the length of the horizontal line, and the uncertainty in the average metallicity is given by the standard deviation. The gray histogram gives the number of planets in each bin.

Reproduced from Buchhave et al. (2012) An abundance of small exoplanets around stars with a wide range of metallicities. Nature 486: 375–377, with permission.

The latest exoplanet surveys are increasingly focusing on discovering low-mass planets around nearby stars, with the ultimate aim of discovering Earth analogs in nearby stellar systems. To date, masses and radii have already been measured for over a dozen super-Earths, defined as those planets with masses between 1 and 10 ME (see the succeeding text). Understanding the interiors and atmospheres of these super-Earths represents a new frontier in exoplanet science. There are no super-Earth analogs in our solar system. Limited data available at present suggest that super-Earths may span a wide range of possible compositions from iron-rich super-Mercuries and molten lava planets to water worlds with thick steamy envelopes and planets dominated by carbon. It is not yet known what fraction of super-Earths are scaled-up versions of terrestrial planets and which are scaled-down versions of ice giants such as Neptune or lie somewhere in the continuum between these compositions.

Knowledge of the mineralogy of super-Earth planets is essential to understanding their interior structure, thermal behavior, and long-term evolution. The mineralogy will depend on bulk composition and the pressures and temperatures of the interior. The large sizes of super-Earth planets mean that interior pressures are very high, posing a severe challenge for laboratory experiments attempting to reproduce super-Earth conditions. Quantum mechanical theoretical methods offer an effective alternative means to evaluate interior mineralogy, but such methods require testing and confirmation by experiment. New laboratory techniques for extending the accessible pressure range are undergoing rapid development. Combined with the fast pace of advances in astronomical detection, observation, and modeling of extrasolar planets, it is clear that the study of super-Earth planets, while in its infancy today, will expand rapidly in the coming years. This chapter reviews our current understanding of mineral behavior at extreme pressure (P)–temperature (T) conditions and its possible implications for super-Earth planets. It is expected that future developments will likely require substantial revisions in this understanding, but much progress has been made to date, and a summary of this progress is undertaken here. Other recent review articles discussing possible internal structures and mineralogy of exoplanets can be found elsewhere ( Sotin et al., 2010 Valencia, 2013 ).

## What is the final destiny of a neutron star? - Astronomy

The concept of neutron star maximum mass is revisited. In particular we show that when the dynamical processes occuring in the first few seconds after the neutron star birth are considered, the concept of neutron star maximum mass, as introduced by Oppenheimer and Volkoff, is partially inadequate. We show that both the maximum mass concept and the final stages of the evolution of massive stars depend on the composition of the neutron star material. In particular, we find two different scenarios depending on the absence or presence of negatively charged hadrons among the constituents. In the first scenario, we show that the Oppenheimer Volkoff mass M_OV_ does not represent the boundary between the value of the masses of neutron stars and black holes. In fact, we find a mass range in which both a neutron star and a black hole may exist. In the second scenario we show that, contrary to the standard view, it is possible to have a supernova explosion (accompained by nucleosynthesis and neutrino emission) followed by the delayed formation of a black hole. The latter mechamism could explain the lack of any observational evidence for a neutron star in the remnant of the supernova 1987A.

## Neutron star

Our editors will review what you’ve submitted and determine whether to revise the article.

Neutron star, any of a class of extremely dense, compact stars thought to be composed primarily of neutrons. Neutron stars are typically about 20 km (12 miles) in diameter. Their masses range between 1.18 and 1.97 times that of the Sun, but most are 1.35 times that of the Sun. Thus, their mean densities are extremely high—about 10 14 times that of water. This approximates the density inside the atomic nucleus, and in some ways a neutron star can be conceived of as a gigantic nucleus. It is not known definitively what is at the centre of the star, where the pressure is greatest theories include hyperons, kaons, and pions. The intermediate layers are mostly neutrons and are probably in a “superfluid” state. The outer 1 km (0.6 mile) is solid, in spite of the high temperatures, which can be as high as 1,000,000 K. The surface of this solid layer, where the pressure is lowest, is composed of an extremely dense form of iron.

Another important characteristic of neutron stars is the presence of very strong magnetic fields, upward of 10 12 gauss (Earth’s magnetic field is 0.5 gauss), which causes the surface iron to be polymerized in the form of long chains of iron atoms. The individual atoms become compressed and elongated in the direction of the magnetic field and can bind together end-to-end. Below the surface, the pressure becomes much too high for individual atoms to exist.

The discovery of pulsars in 1967 provided the first evidence of the existence of neutron stars. Pulsars are neutron stars that emit pulses of radiation once per rotation. The radiation emitted is usually radio waves, but pulsars are also known to emit in optical, X-ray, and gamma-ray wavelengths. The very short periods of, for example, the Crab (NP 0532) and Vela pulsars (33 and 83 milliseconds, respectively) rule out the possibility that they might be white dwarfs. The pulses result from electrodynamic phenomena generated by their rotation and their strong magnetic fields, as in a dynamo. In the case of radio pulsars, neutrons at the surface of the star decay into protons and electrons. As these charged particles are released from the surface, they enter the intense magnetic field that surrounds the star and rotates along with it. Accelerated to speeds approaching that of light, the particles give off electromagnetic radiation by synchrotron emission. This radiation is released as intense radio beams from the pulsar’s magnetic poles.

Many binary X-ray sources, such as Hercules X-1, contain neutron stars. Cosmic objects of this kind emit X-rays by compression of material from companion stars accreted onto their surfaces.

Neutron stars are also seen as objects called rotating radio transients (RRATs) and as magnetars. The RRATs are sources that emit single radio bursts but at irregular intervals ranging from four minutes to three hours. The cause of the RRAT phenomenon is unknown. Magnetars are highly magnetized neutron stars that have a magnetic field of between 10 14 and 10 15 gauss.

Most investigators believe that neutron stars are formed by supernova explosions in which the collapse of the central core of the supernova is halted by rising neutron pressure as the core density increases to about 10 15 grams per cubic cm. If the collapsing core is more massive than about three solar masses, however, a neutron star cannot be formed, and the core would presumably become a black hole.

The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Adam Augustyn, Managing Editor, Reference Content.

## Neutron stars

When the mass of the remnant core lies between 1.4 and about 2 solar masses, it apparently becomes a neutron star with a density more than a million times greater than even that of a white dwarf. Having so much mass packed within a ball on the order of 20 km (12 miles) in diameter, a neutron star has a density that can reach that of nuclear values, which is roughly 100 trillion (10 14 ) times the average density of solar matter or of water. Such a star is predicted to have a crystalline solid crust, wherein bare atomic nuclei would be held in a lattice of rigidity and strength some 18 orders of magnitude greater than that of steel. Below the crust, the density is similar to that of an atomic nucleus, so the residual atomic cores lose their individuality as their nuclei are jammed together to form a nuclear fluid.

Although neutron stars were predicted in the 1930s, it was not until the late 1960s that observers accidentally discovered a radio source emitting weak pulses, each lasting about 0.3 second with a remarkably constant period of approximately 1.337 seconds. Other examples of such an object, dubbed a pulsar for “pulsating radio star,” were soon found.

A large body of evidence now identifies pulsars as rotating magnetized neutron stars. All the energy emitted in the pulses derives from a slowing of the star’s rotation, but only a small fraction is released in the form of radio-frequency pulses. The rest goes into pulses observed elsewhere in the electromagnetic spectrum and into cosmic rays, with perhaps some going into the emission of gravitational energy, or gravity waves. For example, the pulsar at the centre of the Crab Nebula, the most well-known of modern supernovas, has been observed not only at radio frequencies but also at optical and X-ray frequencies, where it emits 100 and 10,000 times, respectively, as much radiation as in the radio spectrum. The slowing of the pulsar’s spin also supplies the energy needed to account for the nonthermal, or synchrotron, emission from the Crab Nebula, which ranges from X-rays to gamma rays.

Pulsar radiation is polarized, both linearly and circularly, and can be understood in terms of a rotating star having a powerful magnetic field of a trillion gauss. (By contrast, Earth’s magnetic field is about 0.5 gauss.) Various mechanisms have been proposed whereby charged particles can be accelerated to velocities close to that of light itself. Possibly most, if not all, galactic cosmic rays originate from supernovas and remnant pulsars.

Modern observations have recorded sudden changes in the rotation rates of pulsars. The Vela pulsar, for instance, has abruptly increased its spin rate several times. Such a period change or “ glitch” can be explained if the pulsar altered its radius by about one centimetre this sudden shrinkage of the crust is sometimes called a “starquake.” Pulsar phenomena apparently last much longer than the observable supernova remnants in which they were born, since well more than 2,500 pulsars have been cataloged and only a few are associated with well-known remnants. Even so, the statistics of pulsars are likely to be observationally biased, since signals from pulsars at great distances in the Galaxy become distorted by ionized regions of interstellar space.

All stars seem to evolve through the red-giant phase to their ultimate state along a straightforward path. In most instances, especially among low-mass stars, the distended outer envelope of the star simply drifts off into space, while the core settles down as a white dwarf. Here the star (really the core) evolves on the horizontal branch of the Hertzsprung-Russell diagram to bluer colours and lower luminosities. In other cases, in which the mass of the star is several solar masses or more, the star may explode as a supernova. Even for these more massive stars, however, if the residual mass in the core is less than 1.4 solar masses (the Chandrasekhar limit), the stellar remnant will become a white dwarf. The matter in such a dwarf becomes a degenerate gas, wherein the electrons are all stripped from their parent atoms. Gas in this peculiar state is an almost perfect conductor of heat and does not obey the ordinary gas laws. It can be compressed to very high densities, typical values being in the range of 10 million grams per cubic cm (i.e., about 10 million times the density of water). Such a white dwarf no longer has any source of energy and simply continues to cool down, eventually becoming a black dwarf.

The energy output of a white dwarf is so small that the object can go on shining mainly by radiating away its stored energy until virtually none is left to emit. How long this might take is unknown, but it would seem to be on the order of trillions of years. The final stage of this kind of low-mass star is typically a ball not much larger than Earth but with a density perhaps 50,000 times that of water.

The Sun is destined to perish as a white dwarf. But, before that happens, it will evolve into a red giant, engulfing Mercury and Venus in the process. At the same time, it will blow away Earth’s atmosphere and boil its oceans, making the planet uninhabitable. None of these events will come to pass for several billion years.

The first white dwarf to be recognized was the companion to Sirius. It was originally detected by its gravitational attraction on the larger, brighter star and only later observed visually as a faint object (now called Sirius B), about 10,000 times fainter than Sirius (now called Sirius A) or 500 times fainter than the Sun. Its mass is slightly less than that of the Sun, and its size a little less than that of Earth. Its colour and spectrum correspond roughly to spectral type A, with a surface temperature of about 25,000 K. Hence, the energy emission per unit area from the surface must be much greater than that of the Sun. Because Sirius B is so faint, its surface area and thus its volume must be very small, and its average density is on the order of 100,000 times that of water.

Another well-known white dwarf, designated BD + 16°516, is paired with a much cooler K0 V dwarf in an eclipsing system. The two stars, whose centres are separated by 2,092,000 km (about 1,300,000 miles), revolve around each other with a period of 12.5 hours. The white dwarf produces pronounced excitation and heating effects in the K-type star’s atmosphere. The white dwarf’s mass is about 0.6 that of the Sun, but its diameter is only 16,000 km (10,000 miles) hence, its density is about 650,000 times that of water.

## What are neutron stars?

Neutron star. Credit: NASA

Thrilled physicists and astronomers announced Monday the first-ever observation of the merger of two neutron stars, one of the most spectacularly violent phenomena in the Universe.

We asked Patrick Sutton, head of Cardiff University's gravitational physics department, who contributed to the discovery.

A: You can think of them as the collapsed, burnt-out cores of dead stars.

When large stars reach the end of their lives, their core will collapse, the outer layers of the star blown off. You're left with an extremely exotic object, this neutron star.

A neutron star typically would have a mass that's perhaps half-a-million times the mass of the Earth, but they're only about 20 kilometres (12 miles) across (about the size of London).

A handful of material from this star would weigh as much as Mount Everest.

They are very hot, perhaps a million degrees, they are highly radioactive, they have incredibly intense magnetic fields. They are arguably the most hostile environments in the Universe today.

Q: Why do neutron stars merge?

A: It's very common for stars. in the Universe to actually be formed in pairs by a given gas cloud.

If the stars are large enough, then at the end of their life they explode and they leave behind neutron star cores, and the neutron stars will continue orbiting each other.

As they orbit, they give off gravitational waves and the waves carry away energy and so the stars slowly fall closer and closer together.

As they get closer together they orbit faster and faster and the gravitational wave emission speeds up.

You get a runaway process where the two stars in the last few moments of their life, they'll be orbiting each other several hundred times per second, so moving at very close to the speed of light, and eventually they will merge.

A: Because we don't understand exactly the mechanics of how these neutron stars work on the interior, it's not certain what the final fate is.

If the stars are heavy enough, we're sure they will collapse to form a black hole and some of the remaining matter. will form what is called an accretion disk orbiting just around the black hole.

It may be that if the stars are light enough, that they will actually form a single, very heavy neutron star instead of a black hole. That may be stable and stay as a neutron star forever, or it may be unstable and eventually collapse into a black hole.

## Contents

Recent theoretical research has found the mechanisms by which the quark stars with "strange quark nuggets" [10] may decrease the objects' electric fields and the densities from previous theoretical expectations, causing such stars to appear nearly indistinguishable from ordinary neutron stars. This suggests that many, or even all, known neutron stars might be the strange stars. However, the investigating team of Jaikumar, Reddy, and Steiner (2006) [10] made some fundamental assumptions that led to uncertainties in their results significant enough that the question is not settled. More research, both observational and theoretical, remains to be done on strange stars in the future. [10]

Other theoretical work contends that :

A sharp interface between quark matter and the vacuum would have very different properties from the surface of a neutron star. [11]

Addressing key parameters like surface tension and electrical forces that were neglected in the original study, the results show that as long as the surface tension is below a low critical value, the large strangelets are indeed unstable to fragmentation and strange stars naturally come with complex strangelet crusts, analogous to those of neutron stars. [11]

For a strange star's crust to collapse, it must accrete matter from its environment in some form.

The release of even small amounts of its matter causes a cascading effect on the star's crust. This is thought to result in a massive release of magnetic energy as well as electron and positron pairs in the initial phases of the collapsing stage. This release of high energy particles and magnetic energy in such a short period of time causes the newly released electron / positron pairs to be directed towards the poles of the strange star due to the increased magnetic energy created by the initial secretion of the strange star's matter. Once these electron / positron pairs are directed to the star's poles, they are then ejected at relativistic velocities, which is proposed to be one of the causes of FRBs.

Theoretical investigations have revealed that quark stars might not only be produced from neutron stars and powerful supernovae, they could also be created in the early cosmic phase separations following the Big Bang. [12]

If these primordial quark stars can transform into strange quark matter before the external temperature and pressure conditions of the early universe renders them unstable, they might become stable, if the Bodmer–Witten assumption holds true. Such primordial strange stars could survive to this day. [12]

## University of California, San Diego Center for Astrophysics & Space Sciences

Because iron is the most tightly bound nucleus (the "break-even point" between fusion and fission) the star is no longer able to produce energy in the core via further nuclear burning stages. Nuclear reactions will continue, however, because of the extremely high temperatures in the massive star's core. These further reactions have a devastating effect on the star, because they take energy out of the core. At such high temperatures and densities the gamma-ray photons present in the core have sufficient energy to destroy the heavy nuclei produced in the many stages of nuclear reactions, e.g.:

Here is a NASA supernova animation of a supernova explosion and formation of a pulsar. The final death of a massive star is a Type II supernova. As described here there are two types of supernovae. A white dwarf star in a binary star system may accrete material from its companion star. If the white dwarf exceeds the 1.44M limit for support by electron degeneracy, it will collapse and produce a Type Ia supernova. Type I and II supernovae can be distinguished by their light curves and spectral emission lines. Here is some more supernova background and further links here.

A supernova explosion may for a short period of time shine as brightly as the hundreds of billions of stars in its galaxy. Here's an example - a time series of photos of SN 1998S in the galaxy NGC 3877 taken in March--June 1998.

For the first time astronomers have observations of the star that exploded, the supernova precursor, known as Sk -69 202 from a catalog of stars in the LMC by N. Sanduleak. Sk -69 202, shown in the photograph above was a B3I sopergiant with an atmospheric temperature, T = 16,000K, a luminosity of about 100,000L , and an estimated mass of about 20M . An unexpected aspect of this is that Sk -69 202 was a blue supergiant (note temperature and spectral type) rather than a red supergiant.

in Centaurus in the southern sky. - The Crab Supernova in Taurus recorded by Chinese and Native American astronomers. - Tycho's Supernova, studied in detail by Tycho Brahe. - Kepler's Supernova, shown here in a historical illustration

plus other probable & possible Milky Way supernovae.

According to theory, the core of the star that remains after a supernova explosion is a tiny (R

10km) remnant of extremely high density neutrons, supported by neutron degeneracy -- a neutron star. The existence of Neutron Stars was predicted by Baade & Zwicky (1934) and Oppenheimer (1939). But how to detect such a small remnant?

As with many things in astronomy, the discovery came in unexpected fashion with the discovery of Pulsars. In 1967, Cambridge graduate student Jocelyn Bell (now Burnell) and her advisor, Anthony Hewish, were using a special radio telescope to look for radio scintillation, fluctuations in the signals from distant radio sources producted by turbulence in the interplanetary and interstellar plasma similar to the twinkling of stars caused by atmospheric seeing. On November 28, Bell discovered a source with an exceptionally regular pattern of radio flashes. These radio flashes occurred every 1 1/3 seconds like clockwork. Puzzled by what sort of object could produce such a regular pattern, the source was initially dubbed "LGM" - standing for little green men, because the only source that they could imagine that could be so regular was some sort of extraterrestrial technical civilization. After a few weeks, however, three more rapidly pulsating sources were detected, all with different periods. They were dubbed "pulsars."

What were the pulsars? From the short pulse duration and the rapid pulse rate, astronomers concluded that the pulsars must be exceedingly small objects. The radio pulses must come either from radial pulsations of a star, or from a rotation of a beam of light, like a lighthouse beam. Normal stars or even white dwarfs are too big to pulsate that fast, and rotation rates of several times per second would cause even the most compact stars to fly apart.

The answer to the nature of the pulsars came with the discovery of a pulsar in the direction of the Crab nebula , with a pulse rate of 30 times per second -- then the most rapid pulsar known. Continued observation of the Crab Pulsar showed that it was slowing down -- its period was increasing by 38 nanoseconds per day. This first confirmed that the pulses are produced by rotation a pulsating object pulses only at its natural frequency. More importantly, it simultaneously solved two mysteries: the nature of pulsars and why the Crab Nebula continues to shine so brightly 1000 years after the supernova explosion.

The Pulsar in the Crab Nebula

Spinning objects have rotational energy of motion (kinetic energy)just as moving objects have translational energy of motion. A simple calculation showed that a rapidly spinning neutron star slowing down by 38 nanoseconds per day releases almost exactly the energy which is being radiated by the Crab Nebula, just the p[lace where Baade & Zwicky would predict that a neutron star might have been formed. Although the details of how the pulsar's rotational energy is transformed into the luminous energy of the nebula, this agreement was too good to be coincidence --- astronomers were certain that the elusive neutron stars had been discovered! Anthony Hewish later shared in the Nobel Prize for the discovery of pulsars.

Conservation of Angular Momentum - as a spinning object decreases its size, conservation of angular momentum dictates that its rate of spinning must increase. This is the physics that causes a spinning ice skater to "spin-up" as he or she pulls arms and legs inward. A star like the sun, rotating once per month would rotate about 1000 times per second when contracted down to 10 km in size.

Conservation of Angular Momentum

The detailed mechanisms by which pulsars produce their emission from radio waves through x-rays (up even to gamma rays in some cases) is complex and not fully understood, but the basic idea --- that pulsars produce a Lighthouse Effect due to an intense magnetic field whose axis is misaligned with its rotational axis --- is generally accepted. (Remember that the earth's magnetic "North Pole" is not at the true nort pole, but in northern Canada.) One popular view is that the rapid rotation and intense magnetic field of the neutron star generate strong electric fields, which accelerate charged particles (principally electrons because they are less massive) near the magnetic poles where the magnetic field is most intense. The charged particles, accelerated along the curved magnetic field lines produce a type of light called curvature radiation. (Say in unison: "Whenever charged particles are accelerated electromagnetic radiation (light) is produced.")

Whatever the detailed mechanism, radiation near the neutron star poles produces strong, narrow beams of light which sweep around the sky like a tilted lighthouse, as shown below. If the earth lies in the path of the beam we see a pulsar. (This idea has the added attraction that it explains why we don't see pulsars in all supernova remnants.)

Energy is transported from the spinning neutron star into the nebula by the magnetic field. HST has provided dramatic evidence of the interaction of the Crab Pulsar and the supernova remnant. This Picture of the Day and this HST Press release show the ripples produced by the pulsar moving outward through the nebula, also shown in this MPEG movie.

Here is an excellent and detailed Introduction to Neutron Stars by Cole Miller at Chicago.

Listen to the sounds of pulsars from the Princeton Pulsar Group and also at England's Jodrell Bank Radio Observatory (includes Crab Pulsar).

• Astronomy Pictures of the day of Neutron Stars
• New Pulsar discovery.
• Binary Pulsar which resulted in the award of the Nobel Prize in Physics to Joe Taylor and Russell Hulse, its discoverers. More about the binary pulsar next lecture.
• From extremely precise timing experiments, radio astronomers have detected shifts in pulsar periods caused by the orbital motion of the neutron star and accompanying planets.
• Bursting Pulsar discovered by RXTE.
• Recently the discovery of stars which produce bursts of gamma-rays with pulsar-like periodicities has suggested the existence of a new class of pulsar which are inferred to have exceptionally high magnetic fields, as high as 10 15 gauss (1000 times the magnetic field of a typical pulsar). The discoverers have dubbed these pulsars magnetars. Here is a detailed account by one of the discoverers. (The existence of such high magnetic fields and of magnetars remains controversial).

Summary: End Points of Stellar Evolution
Remnant Progenitor
Mass
Remnant
Mass
Size Density Means of
Support
Final Stage
White Dwarf M* 3
(1 Volkswagen/cm 3 )
e - degeneracy Planetary Nebula
Neutron Star 8M 3
(All Volkswagens/cm 3 )
n degeneracy Supernova
Black Hole M* > 20M MBH > 3M 0
Rgrav = 2GM/c 2
none ?

Prof. H. E. (Gene) Smith
CASS 0424 UCSD
9500 Gilman Drive
La Jolla, CA 92093-0424

Last updated: 16 April 1999