Edward van den Heuvel

A very energetic supernova associated with the |[gamma]|-ray burst of 29 March 2003

Over the past five years evidence has mounted that long-duration (>2 s) γ-ray bursts (GRBs)—the most luminous of all astronomical explosions—signal the collapse of massive stars in our Universe. This evidence was originally based on the probable association of one unusual GRB with a supernova, but now includes the association of GRBs with regions of massive star formation in distant galaxies, the appearance of supernova-like ‘bumps’ in the optical afterglow light curves of several bursts and lines of freshly synthesized elements in the spectra of a few X-ray afterglows. These observations support, but do not yet conclusively demonstrate, the idea that long-duration GRBs are associated with the deaths of massive stars, presumably arising from core collapse. Here we report evidence that a very energetic supernova (a hypernova) was temporally and spatially coincident with a GRB at redshift z = 0.1685. The timing of the supernova indicates that it exploded within a few days of the GRB, strongly suggesting that core-collapse events can give rise to GRBs, thereby favouring the ‘collapsar’ model.

Models for the formation of binary and millisecond radio pulsars

The peculiar combination of a relatively short pulse period and a relatively weak surface dipole magnetic field strength of binary radio pulsars finds a consistent explanation in terms of (i) decay of the surface dipole component of neutron-star magnetic fields on a timescale of (2–5) × 106 yr, in combination with (ii) spin-up of the rotation of the neutron star during a subsequent mass-transfer phase.The four known binary radio pulsars appear to fall into two different categories. Two of them, PSR 0655 + 64 and PSR 1913 + 16, have short orbital periods (<25 h) and high mass functions, indicating companion masses 0.7M⊙ (∼1 (± 0.3) M⊙ and 1.4 M⊙, respectively). The other two, PSR 0820 + 02 and PSR 1953 + 29, have long orbital periods (117d), nearly circular orbits, and low, almost identical mass functions of about 3×10-3 M⊙, suggesting companion masses of about 0.3M⊙. It is pointed out that these two classes of systems are expected to be formed by the later evolution of binaries consisting of a neutron star and a normal companion star, in which the companion was (considerably) more massive than the neutron star, or less massive than the neutron star, respectively. In the first case the companion of the neutron star in the final system will be a massive white dwarf, in a circular orbit, or a neutron star in an eccentric orbit. In the second case the final companion to the neutron star will be a low-mass (∼ 0.3 M⊙) helium white dwarf in a wide and nearly circular orbit.In systems of the second type the neutron star was most probably formed by the accretion-induced collapse of a white dwarf. This explains in a natural way why PSR 1953 + 29 has a millisecond rotation period and PSR 0820 + 02 has not.Among the binary models proposed for the formation of the 1.5-millisecond pulsar, the only ones that appear to be viable are those in which the companion disappeared by coalescence with the neutron star. In such models the companion may have been a red dwarf of mass 0.03M⊙, a neutron star, or a massive (>0.7M⊙) white dwarf. Only in the last-mentioned case is a position of the pulsar close to the galactic plane a natural consequence. In the first-mentioned case the progenitor system most probably was a cataclysmic-variable binary in which the white dwarf collapsed by accretion.

X-Ray Binaries and Stellar Evolution

Observational evidence suggests that most — if not all — binary X-ray sources are neutron stars. The evolutionary status and possible formation mechanisms of the type I (massive) and type II (low-mass) X-ray binaries are discussed. The difference between the “standard” massive X-ray binaries and the Be/X-ray binaries is ascribed to a somewhat different evolutionary history and status, and possible reasons for the existence of short- and long-period X-ray pulsars are discussed. Type II X-ray sources in globular clusters were most probably formed by capture processes; their formation rate inferred from the observations indicates that only a small fraction (≲ 1 to 10 percent) of the originally formed neutron stars have remained in their clusters. Type II sources in the galactic bulge may also have formed from cataclysmic binaries in which a white dwarf was driven over the Chandrasekhar limit by accretion.

Our Strange Universe

Astronomical observations of the past century have shown that we live in an expanding universe that originated a long but finite time ago in an incredibly dense and hot initial state called the Big Bang. Apart from matter, also space is an essential ingredient of our universe, and the observed expansion of the universe implies that the amount of space increases in the course of time. In the past there was less space and in the future there will be more. This appears strange and opposite to our daily experience, from which we know space to be a fixed quantity, such as the volume of our room. Physics tells us, however, that space—even if it is pure vacuum (completely empty space that contains no atoms or molecules)—is an essential ingredient of the universe, that contains hidden particles and energy, and can expand or contract. A strange discovery made in 1998, thanks to the measurements of the brightness of very distant exploding stars, is that the empty space of the universe contains the bulk (about 70 %) of all energy of the universe. This energy manifests itself by a mysterious force, still not understood, that causes the expansion of the universe to accelerate. The remaining about 30 % of the energy of the universe (according to Einstein, mass and energy are equivalent) manifests itself as the mass of “real” matter, which exerts gravitational attraction. Of this real matter, only about one sixth is ordinary matter, consisting of atoms and molecules, and five sixth is mysterious Dark Matter, which does exert gravitational attraction, but whose nature is still completely unknown.

The Origin of the Matter in the Universe

Where did the protons, neutrons and electrons in the hot Big Bang come from? Gamow and his collaborators had, without giving an explanation for it, assumed that everything started with neutrons and a very large amount of radiation energy. In 1953 Alpher, Herman and Follin had, just as the later workers mentioned in the last chapter, assumed a mixture was present of neutrons, protons, electrons, neutrinos and anti-neutrinos, plus very much heat radiation. But where did these particles come from? The answer is: they originated from the photons of this heat radiation of the Big Bang.

Is the Universe Open, Closed or Flat? The Horizon Problem, the Flatness Problem and Inflation

We would very much like to know if our universe will keep expanding forever, as in the case of an open or flat universe, or whether the expansion will after some time halt and thereafter will reverse sign and become contraction, leading to a final “big crunch”. Crucially for answering this question is knowledge of how large the present mean density of matter and energy in the universe is (energy has an equivalent mass, given by Einstein’s formula E = mc2). As depicted in Fig. 8.4, open as well as flat and closed universes all start expanding from a very small and compact beginning. If for a closed universe the time when it reaches its maximum size is still very far in the future, it will at present be very difficult to distinguish the expanding Friedmann solution for this state from a flat or open one. The curves that depict the increase with time of the distances between galaxies for these three solutions are then at the present time very close to each other. To determine the precise shape of the curve that represents the increase of the distance between galaxies with time requires very accurate measurements of galaxy distances and redshifts, particularly for very distant galaxies, where the shapes of the curves for the three different Friedmann solutions begin to deviate from each other.

The Sun’s Backyard: Our Solar System

There are eight planets in our solar system. The originally ninth planet Pluto, since 2006 is no longer counted as a planet, but was moved by the International Astronomical Union to the category of dwarf planets , of which several more have been discovered at the outer edge of the solar system.

Ripples in the Cosmic Microwave Background Radiation

The Cosmic Microwave Background Explorer satellite (COBE) of NASA, launched in 1989, measured with unprecedented precision the intensity of the cosmic microwave background radiation (CMB) at many wavelengths and in all directions over the sky. The spatial resolution (sharpness) of the microwave optics in the satellite was 7°—14 times the diameter of the full moon. This means that the temperature of the background radiation was measured in separate areas of the sky with this size. The result is a sky map on which one can see that (small) temperature differences between such areas on the sky exist. These are depicted in Fig. 14.1. Before this map could be obtained, the observations first had to be corrected for the Doppler effect produced by the motion of the sun with respect to the background radiation, with a speed of about 390 km/s (see Fig. 9.14). This Doppler effect causes the temperature on one half of the sky to be slightly higher than average and on the other half to be slightly lower than average. The motion of the sun relative to the background radiation is a combination of the motion in its orbit around the Galactic centre and the motion of our Galaxy relative to the average background of distant galaxies. The last-mentioned motion, with a velocity of a few hundred km/s, is probably caused by the attraction of the other galaxies in the Local Group and of the Virgo Cluster (see Chap. 6), or of the local super cluster of which the Local Group and the Virgo Cluster are members.

Intelligent Life Elsewhere in the Universe

One of the most fundamental questions of all science is whether elsewhere in the universe intelligent civilisations have developed, intelligent being defined as having consciousness and a scientific/technological level at least comparable to that of present humanity. A positive as well as a negative answer to this question will have far-reaching consequences, for philosophy as well as religion. If we are the only ones in the universe, then the universe has apparently been created especially to allow us to come into being, and then we are extremely special: in a way “at the centre of the universe”. On the other hand, if intelligent life is found elsewhere, we are much less special. The latter case will fit well with the Copernican principle, which states that Earth, Sun and Milky Way do not occupy special privileged positions in the universe: Earth is just one of the eight planets orbiting the Sun, the Sun is just one of some 200 billion stars orbiting around the centre of the Milky Way, and our Milky Way galaxy is just one among hundreds of billions of galaxies in the observable part of the expanding universe. We do, however, not know if the Copernican principle also holds with respect to intelligent life. In this chapter we discuss the probability that elsewhere intelligent life could exist, and possible ways to get into contact with such life.

The Big Bang as Origin of the Universe

Lemaitre’s idea of the Big Bang was that of a “Primeval Atom” of unimaginably high density and temperature. He thought of matter with a density similar to that of atomic nuclei (some 1014 times the density of water) and consisting of protons and electrons. However, in 1931 the knowledge of the physics of such matter (nuclear physics) was still so poor that he could not further support his model with reliable physical calculations. He speculated that his Primeval Atom would have fragmented and decayed, finally producing all known kinds of atomic nuclei which combined with the electrons and made atoms. At a later stage these atoms would have assembled into stars, planets and galaxies.