Black holes & Co. / Elementary tour part 1: Neutron stars and pulsars

Stars that are between five and forty times as massive as our sun end their lives in a spectacular way – not with a whimper, but a bang! Once their nuclear fuel is exhausted, there is an gigantic explosion called a supernova, in which the outer layers of the star are ejected into space, leading to a sudden dramatic increase in the star’s brightness.

Simultaneously, the star’s core region collapses. Within seconds, the core’s density increases to a point where the star’s plasma can no longer resist the pressure. The electrons and protons unite to form electrically neutral neutrons, and the core region evolves into an unbelievably dense ball of nuclear matter, no more than 20 kilometres (13 miles) in diameter, but with a mass greater than that of our sun: A neutron star is born. A pinhead’s worth of neutron star matter has more than twice the mass of the Queen Mary II, the world’s largest cruise ship.

A figure skater’s rotation speed increases dramatically when the arms and legs are pulled close to the body axis for a pirouette. By the same laws of physics, the contraction of a slowly rotating stellar core can lead to a neutron star that rotates a few hundred times every second. The interplay of rotation, magnetic field and a cloud of elementary particles which surrounds the neutron star can lead to an exciting consequence: The star can act like a cosmic light-house, with two narrow beams of radio waves pointing in opposite directions, sweeping in a circle with each rotation of the star. If the star is oriented in a way that one of those beams sweeps across the earth, then (as with a light-house) terrestrial astronomers will see a regular blinking, as sketched in the following animation:

To radio astronomers, this type of neutron star appears as a pulsar, an object from which they receive a highly regular chain of radio pulses.

Due to their compactness, neutron stars are ideal cosmic laboratories for testing general relativity (especially when, as pulsars, they send out highly regular signals for astronomers to analyze): Close to a compact body, gravity is very strong, and under such conditions, the interesting effects of general relativity are very large. For instance, binary systems involving a neutron star can be used to test the influence of gravity on light (i.e., the radio light of the pulses) to high precision. Also, from such systems we have the first indirect evidence for the existence of gravitational waves (cf. PSR1913+16).

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Definitioner

PSR1913+16

Ein bestimmter Doppelstern, der aus zwei einander umkreisenden Neutronensternen besteht, von denen einer ein Pulsar ist, von dem wir hier auf der Erde regelmäßige Radiopulse auffangen können. Aus Sicht der Allgemeinen Relativitätstheorie ist das System nicht nur interessant, weil sich an den Pulsen Effekte wie die relativistische Lichtlaufzeitverzögerung mit sonst nicht erreichbarer Präzision nachweisen lassen, sondern vor allem, weil er den ersten indirekten Nachweis der Existenz von Gravitationswellen liefert: Die Umlaufzeit der Doppelsternpartner umeinander nimmt mit der Zeit exakt so ab, wie man es im Rahmen der Allgemeinen Relativitätstheorie, aufgrund der Abstrahlung von Gravitationswellen, erwarten würde.

waves (wave)

In a general sense: any travelling pattern, whether or not it involves matter being transported as well. Simple examples are water-waves - wave crests and troughs travelling over a water surface, and a Mexican wave in a football stadium, with fans alternately standing up and sitting down - the pattern moves throught the stadium, not the fans themselves.

An especially simple form for a wave is a sinus wave, a regular pattern of wave crests and troughs.

supernova

Highly energetic explosion that ends the life of stars with more than about ten solar masses. In this explosion, the outer layers of the stars are ejected into space, while the core regions collapse to form a neutron star or even a black hole.

sun

The central (and most massive) body of our solar system; the star closest to us; a ball of gas with a radius of ca. 700000 km (for comparison the radius of the earth: 12756 km) and a mass of 1.989·10³⁰ kilograms [see exponential notation]. In the interior of the sun, nuclear fusion processes run their course; they are responsible for the sun's impressive brightness.

star

A cosmic gas ball that is massive enough for pressure and temperatur in its core to reach values where self-sustained nuclear fusion reactions set in. The energy set free in these reactions makes stars into very bright sources of light and other forms of electromagnetic radiation. Once the nuclear fuel is exhausted, the star becomes a white dwarf, a neutron star or a black hole.

stars (star)

speed

An object's average speed is the distance it moves during a given period of time, divided by the length of the time interval. If you make the time interval infinitely small, the result is the object's speed at one particular moment in time. The notion of speed can be applied to waves in different ways; for instance, for a simple wave, the phase speed is the speed at which any given wave crest or wave propagates through space. See also the more general entry velocity.

space

In a strict sense: Space as we know it from everyday life: the totality of all locations in which objects can sit, with three dimensions. In a more general sense used by mathematicians, all kinds of sets of points are spaces - a line for instance, which has but a single dimension, or a two-dimensional surface, but also higher-dimensional spaces. Also, in such more general spaces, geometry can be different from the standard Euclidean geometry taught in high schools - such spaces can be curved.

second

In the International System of units: the basic unit of time. Defined as a certain multiple of the oscillation period of electromagnetic radiation set free in a certain transition within the electron shell of atoms of the type Cesium-133.

radio waves

Variety of electromagnetic radiation with frequencies of a few thousand to a few billion oscillations per second, corresponding to wave-lengths of a few kilometres to a few centimetres. True to their name, these are the electromagnetic waves that bring radio and TV programs from the broadcast towers to our personal antennas and receivers. Cosmic radiowaves also make for interesting observations - see radio astronomy.

pulsar

Rotating neutron star from which regular pulses of radiation reach the Earth. Behind those pulses is the fact that the pulsar sends out narrowly focussed beams of radiation that, due to the pulsar's rotation, sweep through space like the beam of a light-house. An animation illustrating this effect can be found on the page Neutron stars and pulsars in the chapter Black holes & Co. of Elementary Einstein.

pulsars (pulsar)

PSR1913+16

A specific binary system consisting of two orbiting neutron stars, one of which is a pulsar from which we here on Earth receive regularly spaced radio pulses. From the point of view of general relativity, the system is interesting not only because, using the pulses, one can measure effects such as Shapiro delay with impressive precision, but because it has given us the first indirect proof for the existence of gravitational waves: the orbital period of the two stars becomes slightly shorter with each orbit, exactly in the way predicted by general relativity due to the energy the system radiates away in the form of gravitational waves.

protons (proton)

Particle that carries positive electric charge and is comparatively massive; the atomic nuclei consist of protons and neutrons. Protons are not elementary particles, they are compound particles consisting of quarks that are bound together through the strong nuclear interaction . Collectively, protons, neutrons and a number of similar particles are called baryons.

pressure

A measure for the strength of the resistance with which matter (for instance a gas) resists attempts to decrease the volume it occupies.

point

Elementary "building block" of geometrical entities such as surfaces or more general spaces. For instance, a surface is the set of all its points, of all possible locations on the surface, and all geometrical objects in that surface are defined by the points that belong to them - for instance, a line on the surface is the set of (infinitely many) points.

plasma

State of matter in which the atoms are largely or even completely separated into electrons and atomic nuclei, which fly around in a highly energetic mixture. Compare the other states of matter: solid state, gas, liquid.

neutron

Particle that is electrically neutral and comparatively massive; the atomic nuclei consist of neutrons and protons. Neutrons are not elementary particles, they are compound particles consisting of quarks that are bound together through the strong nuclear interaction . Collectively, neutrons, protons and a number of similar particles are called baryons. Neutron stars are mainly made of neutrons.

neutrons (neutron)

neutron star

Final stage of massive stars that explode as a supernova. In the explosion process, the core of the star collapses to form a compact object with roughly 1.4 solar masses that mostly consists of nuclear matter, predominantly of neutrons. For astronomers, neutron stars are of interest as there exists a variety called pulsars from which they receive highly regular pulses of electromagnetic radiation. For relativists, they are interesting as the typical effects of general relativity are very pronounced in objects that compact (compare PSR 1913+16, double pulsar PSR J0737-3029A/B).

matter

In general relativity: All contents of spacetime that contribute to its curvature: particles, dust, gases, fluids, electromagnetic and other fields. In particle physics: All elementary particles with half-integer spin, such as electrons and quarks, as well as their composites such as protons and neutrons, in contrast with force particles.

mass

In classical physics, mass plays a triple role. First of all, it is a measure for how easy it is to influence the motion of a body. Imagine that you're drifting in emtpy space. Drifting by are an elephant and a mouse, and you give each of them a push of equal strength. The fact that the mouse abruptly changes its path, while the elephant's course is as good as unaltered, is a sure sign that the mass (or, in the language of physics, the inertia or inertial mass) of the elephant is much greater than that of the mouse. Secondly, mass is a measure of how many atoms there are in a body, and of what type they are. All atoms of one and the same type have the same mass, and adding up all those tiny component masses, the total mass of the body results. Thirdly, in Newton's theory of gravity, mass determines how strongly a body attracts other bodies via the gravitational force, and how strongly these bodies attract it (in this sense, mass is the charge associated with the gravitational force). In special relativity, one can also define a mass that is a measure for a bodies resistance to changing its motion. However, the value of this relativistic mass depends on the relative motion of the body and the observer. The relativistic mass is the "m" in Einstein's famous E=mc² (cf. equivalence of mass and energy). The relativistic mass has a minimum for an observer that is at rest relative to the body in question. This value is the so-called rest mass of the body, and when particle physicists talk of mass, this is usually what they mean. Just as in classical physics, the rest mass is a kind of measure for how much matter the body is made up of - with one caveat: For composite bodies, the energies associated with the forces holding the body together contribute to the total mass, as well (another consequence of the equivalence of mass and energy). In general relativity, mass still plays a role as a source of gravity; however, it has been joined by physical quantities such as energy, momentum and pressure.

magnetic field

The magnetic force is a force by which electric currents (i.e. moving electric charges) act on each other; the magnetic field is the associated field. All phenomena related to the magnetic force or magnetic field are subsumed under the heading of magnetism. Magnetic fields cannot be understood separate from electric fields - their complete description is possible only within the more general context of electromagnetism.

light

Light in the strict sense of the word is electromagnetic radiation the human eye can detect, with wave-lengths between 400 and 700 nano metres. In relativity theory and in astronomy, the word is often used in a more general sense, encompassing all kinds of electromagnetic radiation. For instance, astronomers might talk about "infrared light" or "gamma light"; in this context, light in the stricter sense is referred to as "visible light". Within classical physics, the properties of light are governed by Maxwell's equations; in quantum physics, it turns out that light is a stream of energy packets called light quanta or photons. In the context of relativistic physics, light is of great interest, and for a number of reasons. First of all, the speed of light plays a central role in both special and general relativity. Also, there are a number of interesting effects in general relativity which are associated with the propagation of light, namely deflection, the Shapiro effect and the gravitational redshift.

lead

Chemical element with the symbol Pb; each lead nucleus contains 82 protons. Lead nuclei, stripped of their electrons, are among the types of heavy ions which are brought into collision in particle accelerators such as the Relativistic Heavy Ion Collider in order to recreate the state of matter in the early universe shortly after the big bang.

gravity

See gravitation

gravitational waves

Distortions of space geometry that propagate through space with the speed of light, analogous to ripples on the surface of a pond propagating as water waves. For more informations about gravitational waves, please consult the chapter Gravitational waves of Elementary Einstein. Selected aspects of gravitational wave physics are described in the category Gravitational waves of our Spotlights on relativity.

gravity (gravitation)

In classical physics: An action-at-a-distance force by which all bodies that possess mass attract each other (see Newtonian theory of gravity), synonym: gravitational force. In Einstein's general theory of relativity: The fact that matter that possesses mass, energy, pressure or similar properties distorts spacetime, and that this distortion in turn influences whatever matter might be present. An introduction to the basic ideas of general relativity is provided by the section General relativity of Elementary Einstein. More information about the nature of gravity in general relativity can be found in the spotlight text Gravity: From weightlessness to curvature.

general relativity (general theory of relativity)

Albert Einstein's theory of gravity; a generalization of his special theory of relativity. For information about the concepts and applications of this theory, we recommend the chapter general relativity of our introductory section Elementary Einstein. Further information about many different aspects of general relativity and its applications can be found in our section Spotlights on relativity.

field

A field describes how a physical quantity is distributed in space and time. For instance, the area where electric forces act on a test particle is subject to an electric field. Or the gravitational forces which act on the mass of a test body define a gravitational field. In general a field contains energy, occupies space and can change over time.

elementary particles (elementary particle)

Within the commonly accepted models of physics, certain particles are not built of even more fundamental sub-particles - they are themselves elementary. Examples for elementary particles are electrons, quarks and neutrinos, while particles such as protons or neutrons consist of sub-units and are thus not elementary. The study of elementary particles is called particle physics, under which keyword some more information about the theoretical framework of elementary particles can be found.

electrons (electron)

Low-mass elementary particle with negative electric charge. The atoms that are the constituents of everyday matter each consists of a nucleus surrounded by a shell of electrons.

density

In a stricter sense synonymous with "mass density": The average density of matter in a region of space is the total mass of all matter contained in that region, divided by the region's volume. More generally, density can refer to other physical quantities as well. The energy density, for instance, is the total sum of energy localized in a region divided by that region's volume.

binary (binary star)

A system consisting of two stars in orbit around each other. From a relativistic point of view, there are binaries that are of special interest, namely those in which at least one of the partners is a neutron star or a black hole. Potentially, such systems are effective sources of gravitational waves.

Black holes & Co. / Elementary tour part 1: Neutron stars and pulsars

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