The Milky Way and Beyond (40 page)

The binary star system Epsilon Aurigae is of about third magnitude and has one of the longest orbital periods (27 years) among eclipsing binaries. It is located an estimated 2,000 light-years from Earth in the constellation Auriga. The primary star is a yellow-white star about 200 times the size of the Sun. The secondary star, once thought to be thousands of times the size of the Sun, is now believed to be a hot main-sequence star surrounded by an immense ring or shell of gas that eclipses the primary for two years.

Eta Carinae, which is also called Homunculus Nebula, is a peculiar red star and nebula about 7,500 light-years from Earth in the southern constellation Carina and is now known to be a binary star system. It is one of a small class of stars called luminous blue variables. The English astronomer Sir Edmond Halley noted it in 1677 as a star of about fourth magnitude. In 1838 Sir John Herschel observed it as a first-magnitude star. By 1843 it had reached its greatest recorded brightness, approximately −1 magnitude, or as bright as the brightest stars. Unlike the common types of exploding stars called novae and supernovae, it remained bright for several years. From about 1857 it faded steadily, disappearing to the unaided eye only about 1870. Since then it has varied irregularly about the seventh magnitude. The nebula around the star was formed during its 19th-century brightening and is an expanding shell of gas and dust, shaped like an hourglass with a disk at its centre.

In 2005 astronomers studying far-ultraviolet spectral observations of Eta Carinae made by spacecraft found that it is a binary star system with an orbital period of 5.52 years. Its A component has a temperature of about 15,000 K; its B component, about 35,000 K. The main star in Eta Carinae is about 100 times
more massive than the Sun. Its luminosity has been estimated as five million times that of the Sun. Flaring events producing not only visible effects but also X-ray, ultraviolet, and radio-wave effects have been observed. It is expected to become a supernova in the next several thousand years.

The isolated pulsar Geminga is about 800 light-years from Earth in the constellation Gemini and is unique in that about 99 percent of its radiation is in the gamma-ray region of the spectrum. Geminga is also a weak X-ray emitter, but it was not identified in visible light (as a 25th-magnitude object) until nearly two decades after its discovery in 1972. It was the first pulsar not detected at radio wavelengths. It pulsates with a period of 0.237 second, has a radius of about 10 km (6 miles), and probably originated in a supernova explosion about 300,000 years ago.

The seventh-magnitude star HD 209458, 150 light-years away in the constellation Pegasus, was the first star that had a planet detected by its transit across the star's face. The star, which has physical characteristics similar to those of the Sun, was shown in late 1999 to have a planet by detection of the planet's gravitational effects on the star's motion. Shortly afterward, astronomers independently confirmed the planet's presence by observing that HD 209458 changed in brightness with the same 3.5-day period predicted from the discovery data for the planet's orbit. Although the planet could not be seen directly, its passages between its star and Earth provided important information about its physical properties and atmosphere not otherwise available. The extrasolar planet is about 1.3 times the size of Jupiter but has only two-thirds of Jupiter's mass. It orbits surprisingly close to the star—about 10 stellar radii.

The star HR 8799 has the first extrasolar planetary system to be seen directly in an astronomical image. HR 8799 is a young (about 60 million years old) main-sequence star of spectral type A5 V located 128 light-years from Earth in the constellation Pegasus. Observations of this star taken by the Infrared Astronomical Satellite and the Infrared Space Observatory showed a disk of dust such as that expected in the last stages of planetary formation. In 2008 an international team of astronomers released images taken with the telescopes at the Keck and Gemini North observatories of three planets orbiting HR 8799. Observations taken over the period 2004–08 showed that the planets moved with the star and therefore were not background objects. The planets range in mass from 7 to 10 times that of Jupiter and orbit between
3.6 and 10.2 billion km (2.2 and 6.3 billion miles) from HR 8799. These planets are gas giants with temperatures of about 900 to 1,100 kelvins (600 to 800 °C, or 1,200 to 1,500 °F).

The Hyades is a cluster of several hundred stars in the zodiacal constellation Taurus. As seen from Earth, the bright star Aldebaran appears to be a member of the cluster, but in fact Aldebaran is much closer to the Earth than the Hyades' distance of about 150 light-years. Five genuine members of the group are visible to the unaided eye. Their name (Greek: “the rainy ones”) is derived from the ancient association of spring rain with the season of their heliacal (near dawn) rising.

Kepler's Nova was one of the few supernovae (violent stellar explosions) known to have occurred in the Milky Way Galaxy. Jan Brunowski, Johannes Kepler's assistant, first observed the phenomenon in October 1604; Kepler studied it until early 1606, when the supernova was no longer visible to the unaided eye. At its greatest apparent magnitude (about -2.5), the exploding star was brighter than Jupiter. No stellar remnant is known to exist, though traces of nebulosity are observable at the position of the supernova. Like Tycho's Nova, Kepler's served at the time as evidence of the mutability of the stars.

Composite image of Kepler's Nova, or Kepler's Supernova, taken by the Chandra X-ray Observatory
. NASA, ESA, R. Sankrit and W. Blair, Johns Hopkins University

Mira Ceti, which is also called Omicron Ceti, was the first variable star (apart from novae) to be discovered, lying in the southern constellation Cetus, and the prototype of a class known as long-period variables, or Mira stars. There is some evidence that ancient Babylonian astronomers noticed its variable character. In a systematic study in 1638, a Dutch astronomer, Phocylides Holwarda, found that the star disappeared and reappeared in a varying cycle of about 330 days. It thus acquired the name Mira
(Latin: “Miraculous”). Its brightness varies from cycle to cycle, but generally it is about magnitude 3 at maximum light and magnitude 9 at minimum. Mira is a binary; the red giant primary has a faint bluish white companion. In 2006 the ultraviolet satellite observatory Galaxy Evolution Explorer discovered that Mira had shed material into a cometary tail 13 light-years in length. Mira is about 350 light-years from Earth.

Mizar, also called Zeta Ursae Majoris, was the first star found (by the Italian astronomer Giovanni Battista Riccioli in 1650) to be a visual binary—i.e., to consist of two optically distinguishable components revolving around each other. Later, each of the visual components was determined to be a spectroscopic binary; Mizar is actually a quadruple star. Apparent visual magnitudes of the two visual components are 2.27 and 3.95. Set in the middle of the Big Dipper's handle, Mizar (Arabic: “Veil,” or “Cloak”) makes a visual double with the fainter Alcor (Arabic: “Faint One”); however, the two are three light-years apart and thus are not gravitationally bound to each other. The ability to separate the dim star Alcor from Mizar 0.2° away with the unaided eye may have been regarded by the Arabs (and others) as a test of good vision.

Nova Herculis, or DQ Herculis, was one of the brightest novae of the 20th century, discovered Dec. 13, 1934, by the British amateur astronomer J.P.M. Prentice, in the northern constellation Hercules. It reached an apparent visual magnitude of 1.4 and remained visible to the unaided eye for months. At its centre was found an eclipsing binary pair of small stars, revolving around each other with a period of 4 hours and 39 minutes. One component is a rapidly spinning white dwarf star accreting material from its companion.

Nova Persei, or GK Persei, was a bright nova that attained an absolute magnitude of -9.2. Spectroscopic observations of the nova, which appeared in 1901, provided important information about interstellar gas. The shell thrown off by the exploding star was unusually asymmetrical, and a bright nebulosity near the star appeared to be expanding incredibly fast, at practically the speed of light. This apparent speed is thought to have been an effect of reflection within a preexisting dark nebula around the star. From this phenomenon, sometimes called a light echo, it is possible to calculate the distance of the nova from Earth, about 1,500 light-years.

Omega Centauri (NGC 5139) is the brightest globular star cluster. It is located in the southern constellation Centaurus. It has a magnitude of 3.7 and is visible to the unaided eye as a faint luminous patch. Omega Centauri is about 16,000 light-years from Earth and is thus one of the
nearer globular clusters. It is estimated to contain several million stars; several hundred variables have been observed in it. There is some evidence for a black hole at the centre of Omega Centauri that is 40,000 times as massive as the Sun. The English astronomer John Herschel in the 1830s was the first to recognize it as a star cluster and not a nebula.

Pleione is a star in the Pleiades, thought to be typical of the shell stars, so called because in their rapid rotation they throw off shells of gas. In 1938 sudden changes in the spectrum of Pleione were attributed to the ejection of a gaseous shell, which by 1952 had apparently dissipated. Pleione is a blue-white star of about the fifth magnitude. Some astronomers conjecture that it may have been brighter in the past; it would then have made a seventh bright star in the Pleiades cluster, which is named for seven mythological sisters.

Pollux is the brightest star in the zodiacal constellation Gemini. A reddish giant star, it has an apparent visual magnitude of 1.15. The stars Castor and Pollux are named for the mythological twins. Pollux is also called Beta Geminorum and is 33.7 light-years from Earth. In 2006, a planet, Pollux b, was discovered. Pollux b has nearly three times the mass of Jupiter, orbits Pollux every 590 days, and is at an average distance of 253 million km (157 million miles). Pollux is the brightest star with a known extrasolar planet.

Praesepe, which is also known as the Beehive, is an open cluster of about 1,000 stars in the zodiacal constellation Cancer and is located about 550 light-years from Earth. Visible to the unaided eye as a small patch of bright haze, it was first distinguished as a group of stars by Galileo. It was included by Hipparchus in the earliest known star catalog,
c
. 129 BCE.

The name Praesepe (Latin: “Cradle,” or “Manger”) was used even before Hipparchus' time. The name Beehive is of uncertain but more recent origin.

Procyon is the brightest star in the northern constellation Canis Minor (Latin: “Lesser Dog”) and one of the brightest in the entire sky, with an apparent visual magnitude of 0.41. Procyon lies 11.4 light-years from Earth and is a visual binary, a bright yellow-white subgiant with a faint, white dwarf companion of about the 10th magnitude. The name apparently derives from Greek words for “before the dog,” in reference to the constellation.

Ras Algethi, which is also called Alpha Herculis, is a red supergiant star, whose diameter is nearly twice that of Earth's
orbit. It lies in the constellation Hercules and is of about third magnitude, its brightness varying by about a magnitude every 128 days. It is 380 light-years from Earth. The name comes from an Arabic phrase meaning “the kneeler's head,” referring to the Arabic name of the constellation.

Regulus, or Alpha Leonis, is the brightest star in the zodiacal constellation Leo and one of the brightest in the entire sky, having an apparent visual magnitude of about 1.35. It is 77 light-years from Earth. The name Regulus, derived from a Latin word for king, reflects an ancient belief in the astrological importance of the star.

Rigel (Beta Orionis) is one of the brightest stars in the sky, intrinsically as well as in appearance. A blue-white supergiant in the constellation Orion, Rigel is about 870 light-years from the Sun and is about 47,000 times as luminous. A companion double star, also bluish white, is of the sixth magnitude. The name Rigel derives from an Arabic term meaning “the left leg of the giant,” referring to the figure of Orion.

Scorpius X-1 is the brightest X-ray source in the sky and the first such object discovered in the direction of the constellation Scorpius. Detected in 1962, its X-radiation is not only strong but, like other X-ray sources, quite variable as well. Its variability exhibits two states, one at higher output with great variability on a time scale of minutes and another at lower output with the variability correspondingly lessened.