Life of a Star
A star begins life as a large, relatively cool mass of gas in a nebula, such as the Orion Nebula (left). As gravity causes the gas to contract, the nebula’s temperature rises, eventually becoming hot enough to trigger nuclear reactions in its atoms and form a star. A main sequence star (middle) shines because of the massive, fairly steady output of energy from the fusion of hydrogen nuclei to form helium. The main sequence phase of a medium-sized star is believed to last as long as 10 billion years. The Sun is just over halfway through this phase. Stars eventually use up their energy supply, ending their lives as white dwarfs, which are extremely small, dense globes, or in the case of larger stars, as spectacular explosions called supernovas. A supernova is shown within the Large Magellanic Cloud at the bottom right of the rightmost photo.
Astrophysics, the branch of astronomy that seeks to understand the birth, evolution, and end states of celestial objects and systems in terms of the physical laws that govern them. For each object or system under study, astrophysicists observe radiations emitted over the entire electromagnetic spectrum and variations of these emissions over time (see Electromagnetic Radiation; Spectroscopy; Spectrum). This information is then interpreted with the aid of theoretical models. It is the task of such a model to explain the mechanisms by which radiation is generated within or near the object, and how the radiation then escapes. Radiation measurements can be used to estimate the distribution and energy states of the atoms, as well as the kinds of atoms, making up the object. The temperatures and pressures in the object may then be estimated using the laws of thermodynamics.
Models of celestial objects in equilibrium are based on balances among the forces being exerted on and within the objects, with slow evolution taking place as nuclear and chemical transformations occur. Cataclysmic phenomena are interpreted in terms of models in which these forces are out of balance.
THE STUDY OF STARS
The H-R diagram compares the brightness of a star with its temperature. The diagonal line running from the upper left to the lower right is called the Main Sequence. Stars lying on the Main Sequence are blue when they are bright and red when they are dim. The Sun lies in the center of the Main Sequence.
Stars are among the best understood celestial objects. If the light of a star is dispersed into its wavelength spectrum, the relative intensities at various wavelengths yield considerable information about the star. The surface temperature can be estimated, using the laws of thermal radiation.
If the distance of the star is known, its luminosity can be found by summing the observed intensities over all wavelengths. Its radius can then be found using the fact that the luminosity is the product of the energy emitted per unit area (which depends only on the surface temperature) and the total surface area.
If the spectrum of a star is studied under high resolution, many dark lines are seen at specific wavelengths. These lines are due to the absorption of light from deeper layers by atoms in the cooler layers above. The kinds of atoms present in the star can then be identified by comparing stellar absorption lines with those produced in the laboratory by known gases, and the temperature and pressure of the atmosphere as well as the relative abundances of the chemical elements can be calculated.
Most stars are classified as part of a “main sequence” in which both temperature and luminosity increase with mass. Some stars are much brighter and hence much larger than main-sequence stars of the same temperature, and are called red giants. Many stars are much fainter and hence much smaller than main-sequence stars of the same temperature, including white dwarfs (1 percent the size of the Sun) and neutron stars (0.001 percent the size of the Sun). Black holes emit no light, but absorb all light that passes within a few kilometers (see Black Hole).
Theoretical models of stellar interiors have been calculated based on the theory that an equilibrium exists between the force of gravity, which tends to cause the star to collapse, and the pressure of superheated gases, which tend to expand. High stellar temperatures also drive a flow of heat from inside the star to the outside. If the star is to be in equilibrium, this heat loss must be compensated by the energy released by nuclear reactions in the core. As various nuclear fuels are exhausted, the star slowly evolves, and the core contracts to increasingly higher densities.
For stars of low mass, this process ends when the outer layers are gently ejected to form a planetary nebula; the core then cools down to form a white dwarf. More massive stars become unstable; as they evolve, this core suddenly collapses to form a neutron star or black hole, and the energy thereby released ejects the outer layers at very high speed, forming a supernova (see Supernova).
THE STUDY OF GALAXIES
A collision between two spiral galaxies that began millions of years ago created the so-called Antennae galaxies, named for the antenna-like arms thrown out by the encounter. The two galaxies are merging together, causing billions of new stars to form in the blue regions. Our Milky Way and the Andromeda galaxy will collide in a similar way billions of years from now.
Galaxies are giant systems of stars at very great distances from each other. Many galaxies also contain interstellar material in the form of diffuse gas and dust particles, permeated by weak magnetic fields in which are trapped energetic charged particles called cosmic rays (see Cosmic Rays; Galaxy).
Elliptical galaxies are spheroidal in shape and have little interstellar matter; spiral galaxies are highly flattened rotating disks composed of interstellar matter and large numbers of massive stars, as well as the less massive stars that are also common in ellipticals. The matter in the disk forms a spiral pattern, usually with two spiral arms.
Galaxies M86 and M84
The elliptical galaxies M86 (center) and M84 (right) are members of the Virgo cluster of galaxies, located about 50 million light-years away from our smaller cluster, the Local Group. Elliptical galaxies are populated by older stars and contain little interstellar matter. They are usually the brightest galaxies.
In the nucleus of some galaxies active sources of relativistic particles (particles with speeds approaching that of light) emit radio waves and X rays as well as visible light. This phenomenon is observed in both elliptical and spiral galaxies; objects called quasars seem to be extreme forms of such activity, with luminosities ranging up to 100 times that of all the stars in the galaxy. At present the explanation of the energy source in active galaxies is unknown (see Quasar; Radio Astronomy).
Theoretical models of galaxies are based on the exchange of matter and energy between stars and interstellar matter. When a galaxy forms, it consists at first entirely of interstellar matter; but stars then form from this gas. From the supernovas occurring among these stars, matter enriched in heavy elements is injected back into space. Thus, interstellar matter is progressively enriched with heavy elements, which then become part of new generations of stars. In ellipticals, the process is largely complete, and little interstellar matter remains. In spirals, however, much interstellar matter remains; in these galaxies the rate of star formation is much higher in the spiral arms than in the core. Apparently, spiral density waves compress interstellar matter to form dark clouds, and these subsequently collapse to form new stars.
THE STUDY OF THE UNIVERSE
Cosmology seeks to understand the structure of the universe. Modern cosmology is based on the American astronomer Edwin Hubble's discovery in 1929 that all galaxies are receding from each other with velocities proportional to their distances. In 1922 the Russian astronomer Alexander Friedmann proposed that the universe is everywhere filled with the same amount of matter. Using Albert Einstein's general theory of relativity to calculate the gravitational effects, he showed that such a system must originate in a singular state of infinite density (now called the big bang) and expand from that state in just the way Hubble observed. Most astronomers today interpret their data in terms of the big bang model, which in the early 1980s was further refined by the so-called inflationary theory, an attempt to account for conditions leading to the big bang. According to the theory, the big bang occurred about 14 billion years ago. The discovery in 1965 of cosmic background microwave radiation, a faint glow or radio transmission almost identical in all directions, fulfilled a prediction of the big bang model that radiation created in the big bang itself should still be present in the universe.
Theorists now believe that the universe will continue to expand forever. There does not appear to be enough mass in the universe for the attraction of gravity to slow and eventually reverse the universe’s expansion. Observations of extremely distant supernovas indicate instead that the universe’s expansion is accelerating. Astronomers have invented the name “dark energy” for the cause of this expansion, but they do not know what dark energy is.