The Milky Way and Beyond (31 page)

A major component of the interstellar medium, or the warm ionized medium (WIM), is the diffuse ionized gas, dilute interstellar material that makes up about 90 percent of the ionized gas in the Milky Way Galaxy. It produces a faint emission-line spectrum that is seen in every direction. It was first detected from a thin haze of electrons that affect radio radiation passing through the Milky Way Galaxy. Similar layers are now seen in many other galaxies. The American astronomer Ronald Reynolds and his collaborators have mapped ionized hydrogen and a few other ions (N
⁺
, S
⁺
, and O
⁺⁺
). The total power required for the ionization is amazingly large: about 15 percent of the luminosity of all O and B stars. This energy output is about equal to the total power provided by supernovae, but the latter radiate most of their energy either in nonionizing radiation or in providing kinetic energies to their expanding shells. Other potential energy sources fall far short.

Unlike H II regions, the diffuse ionized gas is found far from the galactic plane as well as close to it. Pulsars (spinning neutron stars emitting pulsed radio waves) occasionally reside at large distances from the plane and emit radio
waves. The electrons in the diffuse ionized gas slow these waves slightly in a manner that depends on the frequency, allowing observers to determine the number of electrons per square metre (1 square metre = 10.8 square feet) on the path to the pulsar. These observations show that the diffuse ionized gas extends more than 3,000 light-years above and below the galactic plane, which is much farther than the 300-light-year thickness of distributions of molecular clouds, H II regions, and O and B stars.

On average, the densities of the electrons are only about 0.05 per cubic cm (a fifth of the average density in the galactic plane), and only 10 to 20 percent of the volume is occupied by gas even at this low density. The rest of the volume can be filled by very hot, even lower density gas or by magnetic pressure. In the diffuse ionized gas, the comparatively low stages of ionization of the common elements (O
⁺
, N
⁺
, and S
⁺
) are much more abundant relative to higher stages (O
⁺⁺
, N
⁺⁺
, and S
⁺⁺
) than in typical diffuse nebulae. Such an effect is caused by the extremely low density of the diffuse ionized gas; in this case, even hot stars fail to produce high stages of ionization. Thus, it seems possible to explain the peculiar ionization of the diffuse ionized gas with ionization powered by O and B stars, which are mostly found in the plane of the Milky Way Galaxy. Apparently the stars are able to ionize passages through the clouds enveloping them so that a substantial part of the ionizing radiation can escape into the regions far from the galactic plane.

Nearly all nebulae are beautiful objects when seen up close, through a telescope. Some of the most notable nebulae also are fascinating subjects when considered in depth, with attention to detail.

Cassiopeia A is the strongest source of radio emission in the sky beyond the solar system, located in the direction of the constellation Cassiopeia about 9,000 light-years from Earth. Cassiopeia A, abbreviated Cas A, is the remnant of a supernova explosion caused by the collapse of a massive star. The light from the event is estimated to have reached Earth about 1667, which makes Cas A the youngest known supernova remnant in the Milky Way Galaxy. Although the explosion must have been very powerful, no contemporary record exists of its having been observed, so the explosion may have happened behind an interstellar dust cloud. Today the remnant is also weakly observable at visible, infrared, and X-ray wavelengths, and it appears as an expanding ring of material approximately five arc minutes in diameter. The expansion rate of the remnant has been used to estimate how long ago the explosion occurred.

The Coalsack is a dark nebula in the Crux constellation (Southern Cross). Easily visible against a starry background, it is perhaps the most conspicuous dark nebula. Starlight coming to Earth through it is reduced by 1 to 1.5 magnitudes. The Coalsack is about 500 light-years from Earth and 50 light-years in diameter. It figures in legends of peoples of the Southern Hemisphere and has been known to Europeans since about 1500. The Northern Coalsack, in the constellation Cygnus, is similar in nature and appearance but somewhat less prominent.

The Great Rift is a complex of dark nebulae that seems to divide the bright clouds of the Milky Way Galaxy lengthwise through about one-third of their extent. From the constellation Cygnus, the rift reaches through Aquila and Sagittarius, where the centre of the Galaxy lies hidden behind it, to Centaurus. The clouds of dark material making up the Great Rift are several thousand light-years from the Earth.

The largest known emission nebula in terms of angular diameter as seen from Earth is the Gum Nebula, which extends about 35° in the southern constellations Puppis and Vela. A complex of diffuse, glowing gas too faint to be seen with the unaided eye, it was discovered by the Australian-born astrophysicist Colin S. Gum, who published his findings in 1955. The Gum Nebula lies roughly 1,000 light-years from Earth and is about 1,000 light-years in diameter. It may be the remnant of an ancient supernova—i.e., violently exploding star.

The Horsehead Nebula (catalog number IC 434) is an H II region in the constellation Orion. The nebula consists of a cloud of ionized gas lit from within by young, hot stars; a dark cloud containing interstellar dust lies immediately in front. The dust absorbs the light from part of the ionized cloud. A portion of this dark cloud has a shape somewhat resembling a horse's head. The nebula is located 400 parsecs (1,300 light-years) from the Sun. It has a
diameter of approximately 4 parsecs (13 light-years) and a total mass of about 250 solar masses.

The Lagoon Nebula (catalog numbers NGC 6523 and M8) is an H II region located in the constellation Sagittarius at 1,250 parsecs (4,080 light-years) from the solar system. The nebula is a cloud of interstellar gas and dust approximately 10 parsecs (33 light-years) in diameter. A group of young, hot stars in the cloud ionize the nearby gas. As the atoms in the gas recombine, they produce the light emitted by the nebula. Interstellar dust within the nebula absorbs some of this light and appears almost to divide the nebula, thus producing a lagoonlike shape.

The North American Nebula (NGC 7000) is an ionized-hydrogen region in the constellation Cygnus. The nebula is a cloud of interstellar gas ionized from within by young, hot stars. Interstellar dust particles in part of this cloud absorb the light emitted by recombining atoms. The shape of the nebula roughly resembles that of North America, with the dusty region being shaped like the Gulf of Mexico. The North American Nebula is approximately 520 parsecs (1,700 light-years) from the Sun. It has a diameter of about 30 parsecs (100 light-years) and a total mass equal to about 4,000 solar masses.

The bright diffuse Orion Nebula is faintly visible to the unaided eye in the sword of
the hunter's figure in the constellation Orion. The nebula lies about 1,350 light-years from Earth and contains hundreds of very hot (O-type) young stars clustered about a nexus of four massive stars known as the Trapezium. Radiation from these stars excites the nebula to glow. It was discovered in 1610 by the French scholar Nicolas-Claude Fabri de Peiresc and independently in 1618 by the Swiss astronomer Johann Cysat. It was the first nebula to be photographed (1880), by Henry Draper in the United States.

Images of the nebula continued to improve, and technological advances in the late 1980s enabled scientists to photograph infrared-emitting objects in the Orion Nebula that had never before been observed optically. The Hubble Space Telescope in 1991 revealed the sharpest details yet available of known features of the nebula, including what appeared to be a jet (an energetic outflow) related to the birth of a young star.

R Monocerotis (NGC 2261) is a stellar infrared source and nebula in the constellation Monoceros (Greek: Unicorn). The star, one of the class of dwarf stars called T Tauri variables, is immersed in a cloud of matter that changes in brightness erratically, reflecting or re-radiating energy from the star.

The Ring Nebula (catalog numbers NGC 6720 and M57) is a bright planetary nebula in the constellation Lyra, about 2,300 light-years from the Earth. It was discovered in 1779 by the French astronomer Augustin Darquier. Like other nebulae of its type, it is a sphere of glowing gas thrown off by a central star. Seen from a great distance, such a sphere appears brighter at the edge than at the centre and thus takes on the appearance of a luminous ring. It is a popular object for amateur astronomers.

The Trifid Nebula (catalog numbers NGC 6514 and M 20) is a bright, diffuse nebula in the constellation Sagittarius, lying several thousand light-years from the Earth. It was discovered by the French astronomer Legentil de La Galaisière before 1750 and named by the English astronomer Sir John Herschel for the three dark rifts that seem to divide the nebula and join at its centre. Of about the ninth magnitude optically, the Trifid is also a radio source.

T
he Milky Way Galaxy is just one of many galaxies, the systems of stars and interstellar matter that make up the universe. Many galaxies are so enormous that they contain hundreds of billions of stars.

Nature has provided an immensely varied array of galaxies, ranging from faint, diffuse dwarf objects to brilliant spiral-shaped giants. Virtually all galaxies appear to have been formed soon after the universe began, and they pervade space, even into the depths of the farthest reaches penetrated by powerful modern telescopes. Galaxies usually exist in clusters, some of which in turn are grouped into larger clusters that measure hundreds of millions of light-years across. These so-called superclusters are separated by nearly empty voids, and this causes the gross structure of the universe to look somewhat like a network of sheets and chains of galaxies.