Novae, bursters, and X-ray sources

Many stars undergo violent changes in activity during their evolution as their chemical and physical makeups change. These stars—including novae, supernovae, neutron stars, and black holes—release tremendous amounts of electromagnetic radiation (including X-rays) during their violent phases.

Overview

The universe consists of perhaps one trillion galaxies. Each of these galaxies contains approximately 100 billion stars, assuming the universe is homogeneous. A star is a massive accumulation of hydrogen and helium atoms initially formed from the Big Bang explosion that created the universe between thirteen and fourteen billion years ago. An average star like the Sun is classified as a G2 star and contains a mass of more than one million Earths. Inside a star, the intense pressures and temperatures generated by the gravitational attraction of the hydrogen and helium atoms result in the fusion, or combining, of these atoms. Effectively, a star is 100 million hydrogen (fusion) bombs exploding every second. Hydrogen fuses to form helium during the star’s early life. Once hydrogen fuel is exhausted, helium is fused to form beryllium, lithium, and heavier elements. This process of thermonuclear nucleosynthesis continues as the progressively older star burns heavier and heavier elements.

Iron is the heaviest element that can be synthesized inside a star, which the star cannot burn. Once nothing is left in the star’s core except iron, the star may follow two possible routes, depending upon its star’s size. An average-size star, such as Earth’s Sun, will expand and contract, periodically throwing off outer layers of stellar matter, a phenomenon called a nova. Eventually, this star will cool and shrink into a dense, planet-sized white or brown dwarf. Larger stars will undergo a speedy collapse with the release of enormous amounts of matter and energy, enough to outshine an entire galaxy for several weeks. Such a collapse of a massive star is called a supernova. After the supernova explosion, all that remains of the original star is an incredibly dense neutron star or a black hole.

An average-size main sequence star, such as the Sun, proceeds in approximately ten billion years through this evolutionary development. The Sun is halfway through its life. More massive stars (for example, blue and red giants and supergiants) proceed through this evolutionary development much more quickly, sometimes in as short as ten million years.

All stars emit electromagnetic radiation during their lifetimes, which behaves as particles called photons and as waves consisting of crossed oscillating electric and magnetic fields. Electromagnetic radiation travels at the speed of light, approximately 300,000 kilometers per second. Various types of electromagnetic radiation exist, each with a particular frequency and wavelength. The electromagnetic radiation spectrum ranges from low-frequency, long-wavelength radiations to high-frequency, short-wavelength radiations. The electromagnetic radiation spectrum includes, in order from low to high frequency, radio waves, television waves, microwaves, infrared radiation, visible light waves, ultraviolet radiation, X-rays, and gamma rays. These radiations are produced and emitted from the thermonuclear fusion reactions inside stars. Certain stars exhibit intense bursts of various types of electromagnetic radiation during predictable time intervals. Many of these stars are called pulsating stars. They include Cepheid variables, RR Lyrae stars, binary or double-star systems, novae, supernovae, and the more exotic neutron stars and black holes.

Cepheid variables and the closely related RR Lyrae stars represent a class of stars that pulsate because they vary in their brightness, or luminosity, over time. These stars brighten, then dim in predictable, cyclic patterns that may last for several days for some stars or several weeks for others. Each such variable star has its own characteristic period—the time required for it to proceed through each cycle of brightening and dimming. Astronomers hypothesize that Cepheid variables are older main sequence stars that have used their hydrogen fuel and are shifting to helium fusion. This shift causes the stars to be thermodynamically unstable, resulting in periodic outbursts of energy (brightening).

Many main sequence stars exist as binaries, or double-star systems, in which two stars orbit around a common center of gravity. Many Sun-like stars exist in binary systems. As the two stars of a binary system orbit each other, occasionally, one will pass in front of the other relative to Earth, thereby eclipsing the view of the companion star. When the eclipse occurs, the combined light output from the two stars dims. Some examples of double stars within Earth’s galactic vicinity include Alcor-Mizar in the constellation Ursa Major (the Big Dipper) and Sirius A, B in the constellation Canis Major.

A nova is an eruptive, older star that ejects outer layers of gas as it slowly shrinks in size. One theory describing novae maintains that a nova could result from a binary system in which one star pulls matter away from the other, thereby creating instability in the thief star (the one receiving matter from the other star) and an occasional ejection of matter from this same star. Such a nova associated with a binary system would release large amounts of energy and matter. X-ray emissions have been detected from nova outbursts in binary systems.

A supernova—the catastrophic collapse and explosion of a massive star—releases in an instant more energy than all the stars in any given galaxy. This energy emission includes high-frequency electromagnetic radiation, such as X-rays and gamma rays. Approximately 1 10⁵⁰ ergs of energy are released in a supernova explosion. Following the supernova explosion, the remains of the collapsed star may be either a neutron star or a black hole, which are superdense objects that exert strong electromagnetic and gravitational fields. According to theory, whether a star becomes a neutron star or a black hole depends upon the mass of the pre-supernova star. A star having a mass at least 1.4 times greater than the mass of the Sun should become a neutron star following a supernova explosion. If a star has a mass at least ten times greater than the mass of the Sun, then the star should contract infinitely into a black hole after a supernova explosion. Numerous neutron stars have been located. Black holes are strongly inferred from images from the Hubble Space Telescope and Chandra X-Ray Observatory. Supermassive black holes are believed to power many galactic cores, including the Milky Way’s.

Neutron stars form when the entire mass of the star collapses to perhaps a sixteen-kilometer radius, thereby producing incredible densities that crush matter. The density and pressure are so great that atoms cease to exist, and protons and electrons fuse to produce neutrons. The neutron star, therefore, is one gigantic atomic nucleus composed solely of neutrons. If the neutron star is rotating, it is called a pulsar because it pulsates. Certain regions of the rotating star are hot spots of radiation emission, including X-rays. As the pulsar rotates, the hot spots appear periodically with the associated radiation emission. Neutron stars are believed to exist at the centers of such supernova remnants as the crab nebula in Taurus, Cassiopeia A, and Supernova 1987A. Neutron stars also appear to be associated with certain binary star systems. In such systems, one star is visible because of its light emission, but the neutron star companion is invisible because it does not emit visible light. The neutron star pulls matter away from its companion star, occasionally ejecting the matter with an intense burst of X-rays.

A black hole is an even more extreme case of superdensity than a neutron star. A black hole is hypothesized to occur when a supernova remnant at least ten times more massive than the Sun collapses to infinite density, thereby creating a hole, or vortex, in space and time that permanently captures any matter or energy that enters the black hole’s intense gravitational field. Not even light can escape from a black hole; however, black holes are intense emitters of X-rays. Furthermore, black holes should cause intense X-ray outbursts from binary systems where a black hole draws matter away from a companion star.

Numerous stellar X-ray sources have been detected, primarily from the experiments of the American cosmic-ray astronomers Riccardo Giacconi and Herbert Friedman. These scientists and others launched X-ray-detecting telescopes into orbit to locate X-ray stars that were undetectable from beneath the Earth’s atmosphere. In 1962, Giacconi discovered the first extrasolar X-ray source, Scorpius X-1. Scorpius X-1 is one of several X-ray sources found within binary star systems. Several others include Centaurus X-3, Cygnus X-1, and Hercules X-1. Each of these binary star systems consists of two objects, one being a visible light-emitting star, the other being an invisible X-ray emitter. Scientists believe that the invisible X-ray emitter is pulling matter away from the visible companion star. There is considerable debate over whether the invisible X-ray emitters are neutron stars or black holes. In several cases, estimates of the invisible companion’s mass within such binary systems favor a black-hole explanation.

Orbiting X-ray telescopes, such as Uhuru (meaning “freedom” in Swahili) and the Einstein X-Ray Observatory, first detected hundreds of stellar X-ray sources. One particular class of X-ray emitters releases enormous bursts of X-rays over a period of minutes, hours, or days. Some of these X-ray bursters are associated with globular clusters, large accumulations of stars located around the halos of most galaxies, especially surrounding galactic centers. Other X-ray bursters appear to be associated with binary star systems containing neutron stars. In the latter case, the tremendous X-ray emissions most likely result from the attraction of matter from the visible star to the invisible companion, which is probably a neutron star or a black hole. Other stellar X-ray sources include galactic nuclei and quasars. Galactic nuclei, the centers of galaxies, are compact clusters of billions of stars from which X-rays emanate. The X-ray emissions could come from the combined stellar atmospheres of these billions of stars. An alternative hypothesis is that a supermassive black hole exists at the center of each galaxy. Because of the massive clustering of stars in galactic nuclei, this hypothesis seems plausible. There is strong evidence that black holes exist at the centers of the Milky Way galaxy.

Quasars, quasi-stellar radio sources, are intense emitters of all forms of electromagnetic radiation, including visible light, radio waves, and X-rays. Quasars are believed to be the oldest, most distant objects detected in the universe. They may represent the first major structures produced from the Big Bang explosion that created the universe, or they may represent vestiges of the Big Bang itself. One theory describing quasars maintains that they are supermassive black holes one billion times more massive than the Sun. Such black holes would be emitters of enormous quantities of energy and radiation.

X-ray sources appear to emanate predominantly from unstable processes that occur during certain phases of a star’s life or during certain interactions between stars. The X-ray and other radiation emissions usually occur in intense bursts, which follow predictable, cyclical periods. All stars emit some radiation from thermonuclear fusion, but some (variables, binaries, novae, supernovae, bursters, neutron stars, and black holes) release large quantities at specific times. Research continued with even more sophisticated space-based observatories, principally the Hubble Space Telescope, Chandra X-Ray Observatory, and XMM-Newton. As a result of Hubble images, black holes moved from the realm of the theoretical to become accepted astrophysical objects to be studied across the electromagnetic spectrum.

Applications

The discovery of high-level X-ray emitting objects in the universe helps astronomers to understand the nature and evolution of the universe, the evolution of stars, and the fate of Earth’s star, the Sun. X-rays and other high-frequency electromagnetic radiations are emitted by a variety of stellar sources (for example, stars, planets, galaxies, quasars, and interstellar hydrogen gas). These X-ray emissions are natural releases of energy from various chemical and physical processes.

Astronomers use a variety of sophisticated instruments to detect and map the sources of electromagnetic radiation emissions. Optical telescopes are used to measure visible light-emitting objects, such as stars, planets, and galaxies. Radio telescopes are used to detect the radio emissions from the same objects. For X-rays, however, detector telescopes must be launched above the Earth’s atmosphere. Ground-based telescopes cannot detect X-rays because X-rays are absorbed by the atmosphere.

Optical and radio telescopes can detect variable stars, binary systems, novae, supernovae, neutron stars, galaxies, and quasars. However, X-ray detectors are needed above Earth’s atmosphere to determine whether these objects release X-rays and the quantity of radiation released. From the late 1950s through the early 1980s, astronomers at the Naval Research Laboratory in Washington, DC; at American Science and Engineering, Inc., in Cambridge, Massachusetts; and at the National Aeronautics and Space Administration (NASA), Goddard Space Flight Center in Greenbelt, Maryland, worked diligently at producing orbiting X-ray satellites. Among the principal scientists who were involved in this work were Giacconi and Herbert Gursky of American Science and Engineering, Inc., and Friedman of the Naval Research Laboratory. While initially attempting to map the location of radiation hot spots coming from the Sun and from around the Earth-Moon system (for example, the Van Allen radiation belts), they developed suborbital rocket-borne X-ray detectors to locate extraterrestrial X-ray sources. Friedman led the efforts with high-altitude rocket launches during which the X-ray detectors mapped X-ray emissions from the Sun.

In the summer of 1962, Giacconi, Gursky, and their colleagues launched a suborbital rocket above Earth’s atmosphere. Aboard the rocket, a tiny X-ray detector located the first extrasolar X-ray source, Scorpius X-1, in the constellation Scorpius. Scientists have discovered that Scorpius X-1 is part of a binary system. Giacconi, Gursky, Friedman, and other astronomers followed this breakthrough with more suborbital X-ray telescope launches, locating many more X-ray emitters in the process.

In 1970, the first orbiting X-ray satellite, the Cosmic X-Ray Explorer, also called Uhuru, was launched from Kenya by Giacconi, Gursky, and other NASA astronomers and engineers. It detected and mapped the locations of hundreds of new X-ray sources during the early 1970s. It was followed in 1978 by the Einstein X-Ray Observatory, a more sensitive orbiting X-ray telescope that mapped even more X-ray sources outside the solar system during its three-year operational lifetime. Einstein was followed in 1999 by the Chandra X-Ray Observatory. The data collected from these and other astronomical projects fueled the theories of astrophysics and cosmology—specifically, the theories on stellar evolution and black holes. For the first time, many decades-old ideas could be tested. The existence of neutron stars was verified, X-ray emissions from the gas exchange between binary stars and novae could be substantiated, and considerable evidence supporting the existence of black holes within certain X-ray-emitting binaries was obtained. X-ray astronomy caused a boom in the astronomy and astrophysics community, and it emerged as an important branch of astronomy and physics.

The existence of black holes had been hypothesized for more than two hundred years by numerous philosophers and astronomers. In the first half of the twentieth century, their existence was championed by the theoretical physicists Subrahmanyan Chandrasekhar, Fritz Zwicky, and John A. Wheeler. Black-hole physics was developed in meticulous detail by the theoretical physicists Stephen W. Hawking, Igor Novikov, and Kip Thorne, who explained the particle and energy emissions from these objects.

Black holes are the final stage of stellar evolution for very massive stars. Most cosmological theories that describe the structure and evolution of the universe support the inevitable formation of black holes. These theories include Albert Einstein’s theory of general relativity. The universe may be teeming with countless trillions of black holes, with several million in the Milky Way alone. Black holes may eventually accumulate all the mass of the universe within many trillions of years. This scenario is just one of many proposed end states of the universe, one in which all matter is absorbed into black holes. Even then, black holes should gradually evaporate their energy into space, leaving nothing but an energy-dominated universe with practically no matter left. By contrast, a more accepted end-state would have the universe continue to expand and suffer a cold, entropic death. The existence of X-ray sources in novae, binary systems, and galactic nuclei provides evidence supporting or rejecting theories describing the origin and evolution of the universe.

In the distant future, major X-ray bursters may be sources of considerable concern for human space travelers. High-frequency radiations, such as X-rays and gamma rays, penetrate many materials and living tissue, producing mutations and sometimes death. X-ray astronomy will help astronomers to understand the universe and to live safely in space.

Context

X-ray astronomy has enabled scientists to see the universe from a different perspective. The fact that many stellar objects release intense bursts of X-rays in predictable cycles has expanded the interpretation of how stars develop and interact with one another. Intense bursts of X-rays are emitted from stars undergoing instabilities in their development or from violent interactions between pairs of stars, especially if one star in the pair is a neutron star or a black hole. Low quantities of X-rays are emitted from a variety of stable objects, including the solar flares of the Sun and the atmospheres of gaseous planets, such as Jupiter.

Among the most intense X-ray emitters are binary star systems, in which one member of the pair is a nova, neutron star, or black hole. Also, supernovae are intense X-ray emitters. Variable stars and novae by themselves are X-ray emitters but not intense emitters. Large quantities of X-rays are released from these objects in all directions when there is a violent transfer of matter away from one star onto the surface of another or when matter is ejected from a supernova at incredible speeds, temperatures, and pressures.

X-ray emissions from normal stars and from unstable interactions between stars are usually associated with intense magnetic fields on the star’s surface. The interactions of matter with these fields cause the rearrangement of electrons within atoms with a corresponding release of energy, such as X-rays. The magnitude of the interaction determines the quantity of the X-ray burst.

With X-ray bursts coming from violent stellar interactions, it is interesting that quasars are such intense X-ray bursters, possibly reflecting a violent era of the early universe when matter and energy were ejected from black holes. The same situation may be seen as matter gravitationally contracts and accretes into new black holes. X-ray bursts could represent telltale signs of black holes, which are scattered throughout the universe.

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