The Milky Way and Beyond (21 page)

Because pulsars slow down so gradually, they are very accurate clocks. Since pulsars also have strong gravitational fields, this accuracy can be used to test theories of gravity. American physicists Joseph Taylor and Russell Hulse won the Nobel Prize for Physics in 1993 for their
study of timing variations in the pulsar PSR 1913+16. PSR 1913+16 has a companion neutron star with which it is locked in a tight orbit. The two stars' enormous interacting gravitational fields affect the regularity of the radio pulses, and by timing these and analyzing their variations, Taylor and Hulse found that the stars were rotating ever faster around each other in an increasingly tight orbit. This orbital decay is presumed to occur because the system is losing energy in the form of gravity waves. This was the first experimental evidence for the existence of the gravitational waves predicted by Albert Einstein in his general theory of relativity.

Some pulsars, such as the Crab and Vela pulsars, are losing rotational energy so precipitously that they also emit radiation of shorter wavelength. The Crab Pulsar appears in optical photographs as a moderately bright (magnitude 16) star in the centre of the Crab Nebula. Soon after the detection of its radio pulses in 1968, astronomers at the Steward Observatory in Arizona found that visible light from the Crab Pulsar flashes at exactly the same rate. The star also produces regular pulses of X-rays and gamma rays. The Vela Pulsar is much fainter at optical wavelengths (average magnitude 24) and was observed in 1977 during a particularly sensitive search with the large Anglo-Australian Telescope situated at Parkes, Australia. It also pulses at X-ray wavelengths. The Vela Pulsar does, however, give off gamma rays in regular pulses and is the most intense source of such radiation in the sky.

Pulsars also experience much more drastic period changes, which are called glitches, in which the period suddenly increases and then gradually decreases to its pre-glitch value. Some glitches are caused by “starquakes,” or sudden cracks in the rigid iron crust of the star. Others are caused by an interaction between the crust and the more fluid interior. Usually the interior is loosely coupled to the crust, so the crust can slow down relative to the interior. However, sometimes the coupling between the crust and interior becomes stronger, spinning up the pulsar and causing a glitch.

Some X-ray pulsars are “accreting” pulsars. These pulsars are in binaries; the neutron star accretes material from its companion. This material flows to the magnetic polar caps, where it releases X-rays. Another class of X-ray pulsars is called “anomalous.” These pulsars have periods of more than five seconds, sometimes give off bursts of X-rays, and are often associated with supernova remnants. These pulsars arise from highly magnetized neutron stars, or magnetars, which have a magnetic field of between 10
¹⁴
and 10
¹⁵
Gauss. (The magnetars also have been identified with another class of objects, the soft gamma-ray repeaters, which give off bursts of gamma rays.)

Some pulsars emit only in gamma rays. In 2008 the Fermi Gamma-ray Space Telescope discovered the first such pulsar within the supernova remnant CTA 1; since then, it has found 11 others. Unlike radio pulsars, the gamma-ray emission does not come from the particle beams at
the poles but arises far from the neutron star surface. The precise physical process that generates the gamma-ray pulses is unknown.

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.

A black hole can be formed by the death of a massive star that exceeds about two solar masses. When such a star has exhausted its internal thermonuclear fuels at the end of its life, it becomes unstable and gravitationally collapses inward upon itself. The crushing weight of constituent matter falling in from all sides compresses the dying star to a point of zero volume and infinite density called the singularity. Details of the structure of a black hole are calculated from Albert Einstein's general theory of relativity. The singularity constitutes the centre of a black hole and is hidden by the object's “surface,” the event horizon. Inside the event horizon the escape velocity (i.e., the velocity required for matter to escape from the gravitational field of a cosmic object) exceeds the speed of light, so that not even rays of light can escape into space.

The radius of the event horizon is called the Schwarzschild radius, after the German astronomer Karl Schwarzschild, who in 1916 predicted the existence of collapsed stellar bodies that emit no radiation. The Schwarzschild radius (
R
_g
) of an object of mass
M
is given by the following formula, in which
G
is the universal gravitational constant and
c
is the speed of light:

R
_g
= 2
GM
/
c
²
.

The size of the Schwarzschild radius is proportional to the mass of the collapsing star. For a black hole with a mass 10 times as great as that of the Sun, the radius would be 30 km (18.6 miles).

Black holes cannot be observed directly on account of both their small size and the fact that they emit no light. They can be “observed,” however, by the effects of their enormous gravitational
fields on nearby matter. For example, if a black hole is a member of a binary star system, matter flowing into it from its companion becomes intensely heated and then radiates X-rays copiously before entering the event horizon of the black hole and disappearing forever. One of the component stars of the binary X-ray system Cygnus X-1 is a black hole. Discovered in 1971 in the constellation Cygnus, this binary consists of a blue supergiant and an invisible companion 8.7 times the mass of the Sun that revolve about one another in a period of 5.6 days.

Some black holes apparently have nonstellar origins. Various astronomers have speculated that large volumes of interstellar gas collect and collapse into supermassive black holes at the centres of quasars and galaxies. A mass of gas falling rapidly into a black hole is estimated to give off more than 100 times as much energy as is released by the identical amount of mass through nuclear fusion. Accordingly, the collapse of millions or billions of solar masses of interstellar gas under gravitational force into a large black hole would account for the enormous energy output of quasars and certain galactic systems. One such supermassive black hole, Sagittarius A*, exists at the centre of the Milky Way Galaxy. In 2005, infrared observations of stars orbiting around the position of Sagittarius A* demonstrated the presence of a black hole with a mass equivalent to 4,310,000 Suns.

Supermassive black holes have been seen in other galaxies as well. In 1994 the Hubble Space Telescope provided conclusive evidence for the existence of a supermassive black hole at the centre of the M87 galaxy. It has a mass equal to two to three billion Suns but is no larger than the solar system. The black hole's existence can be inferred from its energetic effects on an envelope of gas swirling around it at extremely high velocities.

The existence of another kind of nonstellar black hole has been proposed by the British astrophysicist Stephen Hawking. According to Hawking's theory, numerous tiny primordial black holes, possibly with a mass equal to that of an asteroid or less, might have been created during the big bang, a state of extremely high temperatures and density in which the universe is thought to have originated 13.7 billion years ago. These so-called mini black holes, like the more massive variety, lose mass over time through Hawking radiation and disappear. If certain theories of the universe that require extra dimensions are correct, the Large Hadron Collider (the world's most powerful particle accelerator) could produce significant numbers of mini black holes.

O
ur Sun is not part of a multiple star system. It floats alone in space, serene and solitary. However, many stars can be found in groups called star clusters. There are two general types of these stellar assemblages. They are held together by the mutual gravitational attraction of their members, which are physically related through common origin. The two types are open (formerly called galactic) clusters and globular clusters.

Open clusters contain from a dozen to many hundreds of stars, usually in an unsymmetrical arrangement. By contrast, globular clusters are old systems containing thousands to hundreds of thousands of stars closely packed in a symmetrical, roughly spherical form. In addition, groups called associations, made up of a few dozen to hundreds of stars of similar type and common origin whose density in space is less than that of the surrounding field, are also recognized.

Four open clusters have been known from earliest times: the Pleiades and Hyades in the constellation Taurus, Praesepe (the Beehive) in the constellation Cancer, and Coma Berenices. The Pleiades was so important to some early peoples that its rising at sunset determined the start of their year. The appearance of the Coma Berenices cluster to the naked eye led to the naming of its constellation for the hair of Berenice, wife of Ptolemy Euergetes of Egypt (3rd century BCE); it is the only constellation named after a historical figure.

Open clusters are strongly concentrated toward the Milky Way. They form a flattened disklike system 2,000 light-years thick, with a diameter of about 30,000 light-years. The younger clusters serve to trace the spiral arms of the Galaxy, since they are found invariably to lie in them. Very distant clusters are hard to detect against the rich Milky Way background. A classification based on central concentration and richness is used and has been extended to nearly 1,000 open clusters. Probably about half the known open clusters contain fewer than 100 stars, but the richest have 1,000 or more. The largest have apparent diameters of several degrees, the diameter of the Taurus cluster being 400 arc minutes (nearly seven arc degrees) and that of the Perseus cluster being 240 arc minutes.

The linear diameters range from the largest, 75 light-years, down to 5 light-years. Increasingly, it has been found that a large halo of actual cluster members surrounds the more-noticeable core and extends the diameter severalfold. Cluster membership is established through common motion, common distances, and so on. Tidal forces and stellar encounters
lead to the disintegration of open clusters over long periods of time as stars “evaporate” from the cluster.

Stars of all spectral classes from O to M (high to low temperatures) are found in open clusters, but the frequency of types varies from one cluster to another, as does concentration near the centre. In some (O or OB clusters), the brightest stars are blue, very hot spectral types O or B. In others, they are whitish yellow, cooler spectral type F. High-luminosity stars are more common than in the solar neighbourhood, and dwarfs are much more scarce. The brightest stars in some open clusters are 150,000 times as bright as the Sun. The luminosity of the brightest stars at the upper end of the main sequence varies in clusters from about −8 to −2 visual magnitude. (Visual magnitude is a magnitude measured through a yellow filter, the term arising because the eye is most sensitive to yellow light.)