The Milky Way and Beyond (39 page)

Maffei I is a large elliptical galaxy. At about 3,000,000 light-years' distance, it is close enough to belong to what is called the Local Group of galaxies, of which the Milky Way Galaxy is a member. Maffei II has a spiral structure and is about three times farther away than Maffei I.

Virgo A (catalog numbers M87, and NGC4486, ) is a giant elliptical galaxy in the constellation Virgo whose nucleus provides the strongest observational evidence for the existence of a black hole. Virgo A is the most powerful known source of radio energy among the thousands of galactic systems comprising the so-called Virgo cluster. It is also a powerful X-ray source, which suggests the presence of very hot gas in the galaxy. A luminous gaseous jet projects outward from the galactic nucleus. Both the jet and the nucleus emit synchrotron radiation, a form of nonthermal radiation released by charged particles that are accelerated in magnetic fields and travel at speeds near that of light. Virgo A lies about 50 million light-years from the Earth.

In 1994 the Hubble Space Telescope obtained images of Virgo A that showed a disk of hot, ionized gas about 500 light-years in diameter at a distance of
about 60 light-years from the galaxy's centre. The disk's gases are revolving about the nucleus at a speed of about 550 km per second, or about 1.9 million km (1.2 million miles) per hour, a velocity so great that only the gravitational pull of an object with a mass six billion times that of the Sun would be capable of holding the disk together. This supermassive object could occupy a region as small as the galactic nucleus only if it were a black hole. Gravitational energy released by gas spiraling down into the black hole produces a beam of electrons accelerated almost to the speed of light; the bright gaseous jet that emanates from Virgo A is thought to be radiation from this beam of electrons.

The Virgo cluster is the closest large cluster of galaxies; it is located at a distance of about 5 × 10
⁷
light-years in the direction of the constellation Virgo. More than 2,000 galaxies reside in the Virgo cluster, scattered in various subclusters whose largest concentration (near the famous system M87 [Virgo A]) is about 5 × 10
⁶
light-years in diameter. Of the galaxies in the Virgo cluster, 58 percent are spirals, 27 percent are ellipticals, and the rest are irregulars. Although spirals are more numerous, the four brightest galaxies are giant ellipticals, among them Virgo A. Calibration of the absolute brightnesses of these giant ellipticals allows a leap to the measurement of distant regular clusters.

S
cattered throughout the sky are a myriad of stars and clusters of stars. Some, such as Antares and Spica, are prominent in the night sky or have unusual properties, such as Mira Ceti or Geminga. Others, including HD 209458 or 61 Cygni, have been prominent in the history of astronomy.

In 1838, German astronomer Friedrich Wilhelm Bessel obtained a distance of 10.3 light-years for 61 Cygni, the first star whose distance from Earth was measured. The European Space Agency satellite Hipparcos made much more accurate distance measurements than ground-based telescopes had accomplished and obtained a distance to 61 Cygni of 11.4 light-years. The star is a visual binary, the components of which revolve around each other in a period of 659 years, and is located in the northern constellation Cygnus. They are of fifth and sixth magnitudes.

Alcor (Arabic: “Faint One”) is a star with apparent magnitude of 4.01. Alcor makes a visual double with the brighter star Mizar in the middle of the handle of the Big Dipper (Ursa Major); 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. The pair have also been called the Horse and Rider.

Aldebaran (Arabic: “The Follower”) is a reddish giant star in the constellation Taurus. Aldebaran (also called Alpha Tauri) is one of the 15 brightest stars, with an apparent visual magnitude of 0.85. Its diameter is 44 times that of the Sun. It is accompanied by a very faint (13th magnitude) red companion star. Aldebaran lies 65 light-years from Earth. The star was once thought to be a member of the Hyades cluster, but in fact Aldebaran is 85 light-years closer to Earth. Aldebaran was probably named “The Follower” because it rises after the Pleiades cluster of stars.

Algol, or Beta Persei, is the prototype of a class of variable stars called eclipsing binaries, the second brightest star in the
northern constellation Perseus. Its apparent visual magnitude changes over the range of 2.1 to 3.4 with a period of 2.87 days. Even at its dimmest it remains readily visible to the unaided eye. The name probably derives from an Arabic phrase meaning “demon,” or “mischief-maker,” and the Arabs may have been aware of the star's variability even before the invention of the telescope.

The first European astronomer to note the light variation was the Italian Geminiano Montanari in 1670; the English astronomer John Goodricke measured the cycle (69 hours) in 1782 and suggested partial eclipses of the star by another body as a cause, a hypothesis proved correct in 1889. The comparatively long duration of the eclipse shows that the dimensions of the two stars are not negligible in comparison with the distance between them. A third star, which does not take part in the eclipses, revolves about the other two with a period of 1.862 years.

Antares is a red, semiregular variable star, with apparent visual magnitude about 1.1, the brightest star in the zodiacal constellation Scorpius and one of the largest known stars, having several hundred times the diameter of the Sun and 10,000 times the Sun's luminosity. It has a fifth-magnitude blue companion. Antares (also called Alpha Scorpii) lies about 600 light-years from the Earth. The name seems to come from a Greek phrase meaning “rival of Ares” (i.e., rival of the planet Mars) and was probably given because of the star's colour and brightness.

Barnard's star is the third nearest star to the Sun (after Proxima Centauri and Alpha Centauri's A and B components considered together), at a distance of about 6 light-years. It is named for Edward Emerson Barnard, the American astronomer who discovered it in 1916. Barnard's star has the largest proper motion of any known star—10.25 seconds of arc annually. It is a red dwarf star with a visual magnitude of 9.5; its intrinsic luminosity is only 1/2,600 that of the Sun.

Because of its high velocity of approach, 108 km (67 miles) per second, Barnard's star is gradually coming nearer the solar system and by the year 11,800 will reach its closest point in distance—namely, 3.85 light-years. The star is of special interest to astronomers because its proper motion, observed photographically between the years 1938–81, was thought to show periodic deviations of 0.02 seconds of arc. This “perturbation” was interpreted as being caused by the gravitational pull of two planetary companions having orbital periods of 13.5 and 19 years, respectively, and masses of about two-thirds that of Jupiter. However, this finding has not been supported by results from other methods of detection.

Beta Lyrae is an eclipsing binary star, the two component stars of which are so close together that they are greatly distorted by their mutual attraction; they exchange material and share a common atmosphere. Beta Lyrae is a member of a class of binary systems known as W Serpentis stars. It is of about third magnitude and lies in the northern constellation Lyra.

The variable character of Beta Lyrae was discovered in 1784 by the English amateur astronomer John Goodricke. Its period of about 13 days is increasing by about 19 seconds per year, probably because the stars are steadily losing mass to a continually expanding gaseous ring surrounding them.

The fourth-magnitude star Beta Pictoris is located 60 light-years from Earth in the southern constellation Pictor and is notable for an encircling disk of debris that might contain planets. The star is of a common type somewhat hotter and more luminous than the Sun. In 1983 it was discovered to be an unexpectedly strong source of infrared radiation of the character that would be produced by a disk of material surrounding the star. The disk was later imaged and found to have a width roughly 2,000 times the Earth-Sun distance (2,000 astronomical units [AU]). Observations with the Hubble Space Telescope revealed the disk to be warped and its inner regions to be relatively clear. A likely, but not exclusive, explanation for these characteristics is that one or more extrasolar planets exist there. The outer part of the disk shows rings, possibly caused by a passing star. In 2008, infrared images showed a possible planet orbiting Beta Pictoris. This planet is estimated to have a mass eight times that of Jupiter and to be at a distance from Beta Pictoris of 8 AU.

Debris disk surrounding the star Beta Pictoris, in an image gathered by the European Southern Observatory's 3.6-metre (140-inch) telescope at La Silla, Chile. The warping seen in the disk's bright inner region may be indirect evidence for one or more orbiting planets
. European Southern Observatory

Canopus, or Alpha Carinae, is the second brightest star (after Sirius) in the night sky, with a visual magnitude of -0.73. Lying in the southern constellation Carina, 310 light-years from Earth, Canopus is sometimes used as a guide in the attitude control of spacecraft because of its angular distance from the Sun and the contrast of its brightness among nearby celestial objects. The Syrian Stoic philosopher Poseidonius (
c
. 135–50 BCE) used sightings of Canopus near the horizon in his estimation of the size of Earth.

Capella (also called Alpha Aurigae) is the sixth brightest star in the night sky and the brightest in the constellation Auriga, with an apparent visual magnitude of 0.08. Capella (Latin: “She-Goat”) is a spectroscopic binary comprising two G-type giant stars that orbit each other every 104 days. It lies 42.2 light-years from Earth.

Castor, which is also called Alpha Geminorum, is a multiple star having six component stars, in the zodiacal constellation Gemini. The stars Castor and Pollux are named for the twins of Greek mythology. Castor's combined apparent visual magnitude is 1.58. It appears as a bright visual binary, of which both members are spectroscopic binaries. An additional two component stars form an eclipsing binary system of red dwarfs revolving around each other in less than a day and orbiting the four main stars in a period of 14,000 years. The system is 51.5 light-years from Earth.

Cor Caroli, which is also called Alpha Canum Venaticorum, is a binary star located 110 light-years from Earth in the constellation Canes Venatici and consisting of a brighter component (A) of visual magnitude 2.9 and a companion (B) of magnitude 5.5. It is the prototype for a group of unusual spectrum-variable stars that show strong and fluctuating absorption lines of silicon, chromium, strontium, or certain rare earths. Europium apparently is concentrated around one magnetic pole, chromium around the other. Cor Caroli (“Heart of Charles”) was named by Sir Edmond Halley for King Charles II of England.

The binary star system Cygnus X-1 is a strong source of X-rays and provided the first major evidence for the existence of black holes. Cygnus X-1 is located about 7,000 light-years from Earth in the constellation Cygnus. The primary star, HDE 226868, is a hot supergiant revolving about an unseen companion with a period of 5.6 days. Analysis of the binary orbit led to the finding that the companion has a mass greater than seven solar masses.
(The mass has been determined from subsequent observations to be nearly nine solar masses.) A star of that mass should have a detectable spectrum, but the companion does not; from this and other evidence astronomers have argued that it must be a black hole. The X-ray emission is understood as being due to matter torn from the primary star that is being heated as it is drawn to the black hole.

Delta Cephei is the prototype star of the class of Cepheid variables and is in the constellation Cepheus. Its apparent visual magnitude at minimum is 4.34 and at maximum 3.51, changing in a regular cycle of about five days and nine hours. Its variations in brightness were discovered in 1784 by the English amateur astronomer John Goodricke, and periodic changes in radial velocity (now attributed to pulsation) were established in 1894.