13.1 Supergiants
When stars run out of hydrogen to fuse in their cores, stars above about 10 times the mass of the Sun become supergiants and then undergo a supernova, becoming either a neutron star or a black hole.[1]
Stars over about 10 times the mass of the Sun are massive enough to continue fusion once they have used up all the helium in the shell around their core. Carbon and oxygen fuse into magnesium, neon, and other elements, and this extra radiation pressure causes these stars to expand even further, becoming red or blue supergiants.
Red supergiants are much larger than red giants, and so appear much brighter. Red supergiants can occasionally contract because different elements in the core fuse at different rates. When they contract, they become hotter and therefore bluer, and they are then known as blue supergiants. The most massive stars evolve directly into blue supergiants but many stars oscillate between the two phases.[2]
13.2 Supernova
Energy continues to be released by fusion until the star’s core is made of iron nuclei (made of 26 protons and 26 neutrons). Iron nuclei consume rather than release energy when they fuse, and so fusion ceases and the core suddenly collapses under the pull of gravity. Matter falls onto the inner core at about 1/5th of the speed of light and bounces off producing a shock wave. This is known as a core-collapse or Type II supernova, and for about a month the star will be brighter than a whole galaxy.[3]
In a Type II supernova, the force of gravity is strong enough to overcome electron degeneracy pressure. This allows electrons to merge with protons to become neutrons. In objects about 1.4 times as massive as the Sun, the core is prevented from collapsing further because of neutron degeneracy pressure and becomes a neutron star. In more massive objects, neutron degeneracy pressure is overcome and the star becomes a black hole. Walter Baade and Fritz Zwicky first proposed that supernovae could transform main sequence stars into neutron stars in 1934, just two years after the discovery of the neutron.[4]
Figure 13.1 |
Supernova SN 1987A. |
13.3 Neutron stars
Stars that were originally about 10 to 25 times the mass of the Sun become neutron stars.[5] Neutron stars are typically about 1.4 times the mass of the Sun and are about 20 km wide.[6] As well as being some of the densest objects in the known universe, neutron stars are also some of the fastest spinning objects, rotating up to thousands of times a second,[7] and have the highest magnetic fields ever observed.[8,9] The fact that neutron stars are some of the most extreme objects in the universe means they can be used as ‘natural laboratories’, where we can observe matter behaving in ways we could never replicate on Earth.
Neutron stars do not produce light in nuclear reactions. However, isolated neutron stars are visible because charged particles, like electrons, produce radiation when they are accelerated by the neutron star’s magnetic field.[10,11] This is known as synchrotron radiation, which is often in the radio spectrum. This means that isolated neutron stars can be observed with radio telescopes.
This radiation is emitted from the neutron star’s magnetic poles, and so it can travel past our line-of-sight as the star rotates. This makes the neutron star appear to pulsate, and so neutron stars that emit beamed radiation are known as pulsars.
Figure 13.2 |
An artist’s impression of a neutron star and its magnetosphere. |
The spin period of a pulsar can be determined by observing how the amount of light it emits changes as it rotates.[11] If a single magnetic pole is visible, then the amount of light increases as the pole appears on the neutron star’s horizon and rotates towards our line-of-sight. It then decreases as the pole disappears over the horizon and increases again when it reappears. The spin period of the pulsar can be determined by measuring the time between maximum outbursts of light.
Isolated pulsars that have particularly strong magnetic fields can also emit bursts of X-ray and gamma-ray radiation, and so they can be observed with X-ray and gamma-ray telescopes.[12,13] This occurs when the magnetic field lines twist, causing stress that can break the crust on the neutron star’s surface. These are known as magnetars.[14,15] Some magnetars emit large bursts of gamma and X-rays at irregular intervals. These are known as soft gamma repeaters (SGRs). SGRs were first discovered in 1979 by a variety of detectors,[16] and the theory behind them was explained in the 1990s.[17]
Neutron stars may also emit X-ray radiation if they are in a binary system and matter from another star falls towards the neutron star’s magnetosphere.[18] If the neutron star is spinning too fast then it will be expelled. This is known as the propeller mechanism because the same effect happens if you pour sand onto a horizontal propeller. If the propeller is slow enough then the sand can build up on the blade, if the propeller speeds up then eventually it will blow all the sand away. If the neutron star is slow enough then matter falls down the magnetic field lines at tremendous speeds, reaching free-fall velocities of up to 70% of the speed of light.[19] When the matter hits the magnetic poles, its velocity suddenly becomes zero and so its kinetic energy must be converted into other forms of energy.
When you drop a heavy object on Earth, for example, then its kinetic energy is mostly converted into sound, and that’s why heavier objects make more noise than lighter objects when you drop them. Some kinetic energy is also converted into heat. This is usually not noticeable if you drop a heavy object on Earth, but it makes a big difference on a neutron star, where things are dropped at close to the speed of light. All of this excess heat causes particles to scatter, emitting X-rays. This matter can build up in a process known as accretion, forming ‘mountains’ on the neutron star’s magnetic poles.
Figure 13.3 |
An artist’s impression of a neutron star in a low mass X-ray binary. Matter from the larger (low mass) star creates a disc around the neutron star known as an accretion disc. |
Figure 13.4 |
Matter from the accretion disc falls onto the neutron star’s magnetic poles creating beams of X-rays. |
Figure 13.5 |
Neutron stars in low mass X-ray binaries can rotate thousands of times a second. |
The surface of a neutron star is probably composed of a lattice of iron. Below this, nuclei increase in neutrons and even further down, neutrons exist free of nuclei in a superfluid.[20] A superfluid is a fluid that behaves as if there is no friction due to the laws of quantum mechanics (discussed in Book II). The core of a neutron star exists under such extreme forces that its composition is not known.
The Polish astronomer Aleksander Wolszczan and the Canadian astronomer Dale Frail discovered the first extrasolar planets around neutron star PSR B1257+12 in 1992[21] (discussed in Chapter 28). PSR B1257+12 is now thought to have at least three planets. These range from being about 2% as massive as Earth to about four times as massive.[22] Any life that may have existed on these planets would almost certainly have been destroyed by the supernova that created the neutron star.
13.4 Neutron stars in binaries
X-ray binaries are binary star systems composed of a compact object - usually a neutron star, sometimes a white dwarf or black hole - and a companion star, which could be a main sequence star.[18] X-rays are produced when matter is transferred from the companion star to the compact object.
There are generally two types of X-ray binaries: high mass X-ray binaries (HMXB) and low mass X-ray binaries (LMXB). In HMXB a compact object gains matter from a high mass companion star and in LMXB they gain matter from a low mass companion.[18]
There are very few intermediate mass X-ray binaries (IMXB) because IMXB have relatively short lifetimes, quickly losing enough mass to become LMXB.[23]
The first X-ray source discovered outside of our Solar System was an X-ray binary in the constellation of Scorpius known as Sco X-1.[24] Many similar sources were found in the 1960s and these were identified as X-ray binaries by 1967.[25]
13.4.1 Low mass X-ray binaries
Some of the fastest spinning neutron stars are in LMXB, which contain a low-mass companion star less than twice the mass of the Sun (a white dwarf or a luminosity class III-V, K-M-type star). LMXB form when a neutron star’s magnetic field has decayed and it has slowed enough that it rotates once every 10 seconds or so. This is so slow that it no longer produces observable radio waves. It’s then sped up again by the material that falls on it so that it can rotate up to thousands of times a second.[18]
In LMXB, material is loosely bound to the low-mass star, which is close enough to the neutron star that material constantly falls onto it. This material brings with it angular momentum, which causes the neutron star to speed up. There is so much material that a disc forms around the neutron star, which is known as an accretion disc.[18]
LMXB typically last for about 10 million to a billion years before there’s no longer enough matter to produce X-rays.
Low mass stars are typically older than high mass stars. This is because massive stars undergo nuclear fusion faster, and so run out of fuel quicker. Stars over eight times as massive as the Sun can fuse all of their material within a million years, whereas stars like the Sun will last for about 10 billion years. This means that LMXB are found in parts of the Galaxy that contain older stars, like in the Galactic bulge and in globular clusters.[26]
LMXB have also been observed in other galaxies including the Large Magellanic Cloud (LMC),[27] an irregular galaxy that is gravitationally bound to our own, and M31, also known as the Andromeda Galaxy.[28] This is the closest spiral galaxy to our own.
13.4.2 High mass X-ray binaries
High mass X-ray binaries (HMXB) are composed of a neutron star and another star that is over eight times as massive as the Sun.[29] There are two types of HMXB:
- Supergiant X-ray binaries (SGXB), and
- Be X-ray binaries (BeXB).
Figure 13.6 |
An artist’s impression of a supergiant X-ray binary (SGXB). |
Figure 13.7 |
An artist’s impression of a Be X-ray binary (BeXB). |
SGXB contain supergiant stars (luminosity class I-II, O-B-type stars), which can either lose material in the same way that the stars in LMXB do or through strong stellar winds. These can reach velocities of up to 2000 km/s, which is over 7 million km/hour[30]. Either way, material tends to constantly fall onto the neutron stars in SGXB. The neutron stars in SGXB tend to rotate about once every 10 minutes and complete one orbit every 7 days or so.[18]
BeXB contain OBe stars (luminosity class III-V, Oe-Be-type stars). These are massive blue stars that are orbited by a disc of material known as a circumstellar disc. The circumstellar disc contains material that’s been expelled from the star and can extend for a distance of over 1 AU (where 1 AU is the distance between the Earth and the Sun). The neutron star can sometimes pass within this disc, and this causes material to fall onto it.[18]
The orbital period of the neutron star can then be determined from the time between X-ray outbursts and is typically tens to hundreds of days. The circumstellar disc can change in size, however, sometimes enlarging so that the neutron star is always within the disc.[18]
Neutron stars in BeXB typically rotate faster if they have shorter orbital periods.[31] This is because accreted material makes them spin faster, and systems with short orbital periods accrete more often and have less time to slow down between accretion events. The fastest neutron stars in BeXB tend to rotate once every second, and the slowest once every hour.
Massive stars have relatively short lifetimes and rapidly lose material, and so HMXB tend to only last for about 100,000 years or so. This means that HMXB are mostly found in star-forming regions in the Galactic plane.[26]
There are about 70 known SGXB in the Milky Way and at least two outside of the Galaxy, one in the LMC (LMC X-4), and one (SMC X-1) in the Small Magellanic Cloud (SMC), an irregular dwarf galaxy gravitationally bound to the LMC.[18]
There are a roughly even number of SGXB and BeXB in the Galaxy, but there are many more BeXB outside of the Milky Way, with about 70 BeXB in the SMC and about 10 in the LMC.[18]
Figure 13.8 |
A diagram of a typical Be X-ray binary (from the side) to scale with the radius of the neutron star’s magnetosphere ×20. |
Figure 13.9 |
A diagram of a typical Be X-ray binary (from the top) to scale with the radius of the neutron star’s magnetosphere ×20. |
13.4.3 The magnetic field of neutron stars
Neutron stars contain charged particles like electrons and protons, and they are some of the fastest spinning objects in the universe. This means they should have extremely high magnetic fields.
The volume of space around an object that’s controlled by that object’s magnetic field is known as the magnetosphere, and the magnetospheres of neutron stars can extend for thousands of times their radius.[8] Some neutron stars have been found to have magnetic fields so high that they cannot be described using classical physics and quantum field theories (discussed in Book II) must be used. This is because when magnetic fields get this high they affect the space around them. Atoms become elongated so that they are hundreds of times thinner and can form new chemical bonds, photons can split in two or merge together, and images are distorted and magnified.[32] These magnetic fields are 10,000 billion times larger than the magnetic field of the Earth at the Earth’s surface and cannot be found anywhere else in the universe.
The magnetic fields of neutron stars can be determined directly using scattering physics[33] or indirectly using accretion physics.[34]
Scattering physics and the quantum limit
In BeXB, once matter passes through the neutron star’s magnetosphere it’s channelled by the magnetic field lines to the neutron star’s magnetic polar caps.
If a relatively low amount of material falls to the surface, then this material forms a mound covering an area of about 1 km2. When material hits the surface, its kinetic energy is converted into heat. This causes matter on the surface to become more energetic and emit X-ray photons via blackbody radiation. These photons travel in the opposite direction to the falling matter, in a ‘pencil’ beam.[35]
Figure 13.10 |
‘Pencil’ beam geometry. |
Figure 13.11 |
‘Fan’ beam geometry. |
If a relatively large amount of material falls onto the pole, then it blocks the ‘pencil’ beam radiation, creating a shock wave about 1 km from the surface. This means that radiation can only escape from the sides, in a ‘fan’ beam. X-ray radiation in a fan beam consists of blackbody radiation, and radiation caused by bremsstrahlung and cyclotron emission.[36-39]
Bremsstrahlung (German for ‘braking radiation’) occurs when charged particles, like electrons, are slowed down and deflected by particles of the opposite charge, like protons. The electron loses energy, emitting it in the form of an X-ray photon.
Figure 13.12 |
Bremsstrahlung. |
Cyclotron emission is produced by charged particles, like electrons, which emit X-ray photons when they are accelerated by the magnetic field. This is the same mechanism as the synchrotron radiation that occurs in isolated pulsars. The difference between cyclotron and synchrotron radiation is how fast the electron is travelling, where during synchrotron radiation the electron is travelling close to the speed of light.
Figure 13.13 |
Synchrotron radiation. |
Figure 13.14 |
Compton scattering. |
These X-ray photons do not always make it straight out into space and can bump into other electrons, losing energy. This is known as Compton scattering. When this happens, it leaves gaps in the X-ray spectrum, which are known as absorption lines[40] (discussed in Book II). These gaps correspond to the amount of energy the electron ‘takes’ from the photon, and because the electrons are involved in cyclotron emission, the amount of energy that the electron takes depends on the strength of the magnetic field. The magnetic field places electrons into quantised positions at quantised energies known as Landau orbits, after the Russian physicist Lev Landau.
This energy (Ecyc) is found using,
| Ecyc = nħ eBmec | (13.1) |
Here B is the magnetic field, and n, ħ, e, and me are constants (discussed in Book II). n is the quantum number corresponding to the Landau level, where the lowest energy level is at n = 1, ħ = h/2π, where h is Planck’s constant, e is the elementary charge, and me is the electron mass.
This equation shows that electrons in stronger magnetic fields have higher energies than electrons in weaker magnetic fields. Neutron stars that exhibit these gaps tend to have magnetic fields of up to 1013 (10,000 billion) Gauss (this is 1,000 million Tesla). For comparison, the magnetic field on the surface of the Earth is about 0.4 Gauss (0.00004 Tesla). If neutron stars have magnetic fields much higher than 1013 Gauss, then Ecyc becomes too high for these lines to be visible with X-ray telescopes.
The stronger the magnetic field, the closer the electrons orbit to the magnetic field line. When electrons orbit so close that the orbital radius is equal to their de Broglie radius (discussed in Book II), the field can no longer be described using classical physics. When this happens, the cyclotron energy equals the electron rest energy.
| Ecyc | = | mec2 | (13.2) |
| nħ eBmec | = | mec2 | (13.3) |
| B | = | mec3ħe | (13.4) |
| = | 4.4×1013 Gauss |
This means that the effects of quantum mechanics are important above 4.4×1013 Gauss. This is known as the quantum critical field.
Accretion physics: beyond the quantum limit
The magnetic field of neutron stars can also be determined indirectly, using accretion physics. This means determining how the neutron star’s rotational velocity is changing. Accreted material makes the neutron star speed up, like in LMXB, but the drag of the magnetic field slows the neutron star down.[8]
By working out how much material is accreted – from the neutron star’s luminosity – physicists can predict how much this should speed the neutron star up, and then compare this to its actual acceleration. The difference is due to the drag of the magnetic field, and the higher the magnetic field the more it slows the neutron star down. Accretion physics allows physicists to measure magnetic fields above the quantum critical field.
Systems with magnetic fields this high were thought to be unusual, but it has recently been shown that they may be common in BeXB.[8] If many neutron stars in BeXB do have magnetic fields over the quantum critical field, then this might mean many more isolated neutron stars also have magnetic fields this high.
If isolated neutron stars have magnetic fields over the quantum critical field, then their magnetic fields can usually only be measured when they undergo a magnetar-like outburst. The BeXB sources have not been observed to undergo these outbursts, and this suggests that perhaps these outbursts are not common for neutron stars with magnetic fields this high, and that there is a population of isolated neutron stars with extremely high magnetic fields that cannot be observed.
X-ray telescopes
X-rays are unable to penetrate the water in the atmosphere, and so nobody knew if there were X-rays in space until it was possible to send detectors high above the surface of the Earth. In 1929, the American geophysicist Edward Hulburt devised an experiment to send an X-ray detector into the upper atmosphere in a rocket, before letting it parachute back down.[41] This method could be used to detect ultraviolet radiation and X-rays.
After World War II, captured German rockets were made available to American scientists, such as Herbert Friedman, who used this method to directly detect X-rays from the Sun in 1949.[41] The British Skylark rocket program produced high-quality X-ray images shortly after the launch of Sputnik.[42] The Sun does not emit many X-rays and until 1962, it was thought that most stars would only emit faint X-ray radiation.
In 1962, Riccardo Giacconi, Herb Gursky, Bruno Rossi, and Frank Paolini used an Aerobee 150 rocket to detect X-rays from the Sun and Moon while working at MIT.[24] To their surprise, their results showed that X-rays are emitted in small amounts from all over the sky, and more X-rays were being emitted from a source in the constellation of Scorpius than had ever been observed elsewhere. This source was designated Sco X-1 and was later identified as an LMXB.[43]
The X-ray sky was mapped more thoroughly in the years that followed, with X-ray detectors raised into the upper atmosphere in balloons. The longer the detector remained above the atmosphere, the better the results, and so Giacconi and others worked on a project to launch an X-ray telescope into orbit, culminating in NASA’s Uhuru satellite.
X-ray telescopes focus X-rays at a detector, where the X-rays are stopped, and their position and energy are recorded. X-rays travel through most things, and will only reflect from a mirrored surface if it’s made of a heavy element like gold, and only if they hit it at a very shallow angle. In order to focus, many parabolic and hyperbolic mirrors are needed, and so they are made of thin materials like gold foil.[44] The first detectors were photographic plates. Other X-ray detectors include proportional counters, X-ray CCDs, micro-channel plates, and calorimeters[45] (discussed in Book II).
Proportional counters were used in the 1962 discovery of the first X-ray source outside of the Solar System and on Uhuru, the first orbiting X-ray astronomy satellite, which was launched by NASA in 1970. Uhuru detected 339 X-ray sources, including X-ray binaries, supernova remnants, active galaxies, and clusters of galaxies.[46] Giacconi later worked on the first fully imaging X-ray telescope, the Einstein Observatory, which was launched in 1978 and detected X-ray jets from active galaxies.[47]
Many X-ray telescopes were launched into space from the early 1980s to the 2010s. These included EXOSAT (the European X-ray Observatory Satellite, 1983-1986), which observed cataclysmic variables, Ginga (1987-1991), which observed black holes, and ROSAT (the Röntgen Satellite, 1990-1999), which detected over 150,000 X-ray sources including comets.
RXTE (the Rossi X-ray Timing Explorer, 1995-2012), BeppoSAX (1996-2003), Chandra (1999-present), and XMM-Newton (the X-ray Multi-Mirror Mission - Newton, 1999-present) were all launched in the 1990s. The most recent X-ray telescopes to be launched include Swift XRT (the Swift X-ray telescope, 2004-present), Suzaku (2005-2015), AGILE (the Astrorivelatore Gamma ad Immagini ultra LEggero, 2007-present), and NuSTAR (the Nuclear Spectroscopic Telescope Array, 2012-present).