Vesta

Vesta (astronomy), asteroid orbiting the Sun between the orbits of Mars and Jupiter at a mean distance of 2.36 astronomical units (about 353 million km/about 219.4 million mi) and having a diameter of approximately 385 km (about 240 mi). Vesta is the third largest asteroid and was the fourth asteroid to be discovered. German astronomer Heinrich Olbers found it in 1807. In 1999 the National Aeronautics and Space Administration (NASA) spacecraft Deep Space 1 made observations that suggested that asteroid 9969 Braille, which passes close to Earth, and Vesta are composed of similar materials. Scientists used the information to theorize about the relationship between Vesta and the many meteorites that have struck Earth over time.

Redshift

Redshift, change, or shift, in the light radiated by an object, such as a star or galaxy, that indicates the object’s motion. Scientists have used redshifts to measure the velocities (speed and direction) of distant galaxies. Knowing the velocities of galaxies helps astronomers understand how the universe is changing. This knowledge allows scientists to interpret the distant past of the universe and to predict the universe’s distant future.

Redshift only occurs when an object is moving. Another mechanism can also redden the light of astronomical objects, but it is not considered to be the same as redshift. Dust particles between stars are just the right size to scatter light with short wavelengths more than they scatter light with long wavelengths. As the light of a star passes through a cloud of dust on the light’s way to Earth, more of the long, red wavelengths get through the dust than the short, blue wavelengths do. This makes the star appear redder than it really is, but the light that reaches Earth is the true red light of the star and has not actually shifted. See also Interstellar Matter.

Milky Way

Milky Way, the large, disk-shaped aggregation of stars, or galaxy, that includes the Sun and its solar system. In addition to the Sun, the Milky Way contains about 400 billion other stars. There are hundreds of billions of other galaxies in the universe, some of which are much larger and contain many more stars than the Milky Way.

The Milky Way is visible at night, appearing as a faintly luminous band that stretches across the sky. The name Milky Way is derived from Greek mythology, in which the band of light was said to be milk from the breast of the goddess Hera. Its hazy appearance results from the combined light of stars too far away to be distinguished individually by the unaided eye. All of the individual stars that are distinct in the sky lie within the Milky Way Galaxy.

From the middle northern latitudes, the Milky Way is best seen on clear, moonless, summer nights, when it appears as a luminous, irregular band circling the sky from the northeastern to the southeastern horizon. It extends through the constellations Perseus, Cassiopeia, and Cepheus. In the region of the Northern Cross it divides into two streams: the western stream, which is bright as it passes through the Northern Cross, fades near Ophiuchus, or the Serpent Bearer, because of dense dust clouds, and appears again in Scorpio; and the eastern stream, which grows brighter as it passes southward through Scutum and Sagittarius. The brightest part of the Milky Way extends from Scutum to Scorpio, through Sagittarius. The center of the galaxy lies in the direction of Sagittarius and is about 25,000 light-years from the Sun (a light-year is the distance light travels in a year, about 9.46 trillion km or 5.88 trillion mi).

Topics:

• Structure
• Types of Stars
• Rotation

Structure of Milky Way

Galaxies have three common shapes: elliptical, spiral, and irregular. Elliptical galaxies have an ovoid or globular shape and generally contain older stars. Spiral galaxies are disk-shaped with arms that curve around their edges, making these galaxies look like whirlpools. Spiral galaxies contain both old and young stars as well as numerous clouds of dust and gas from which new stars are born. Irregular galaxies have no regular structure. Astronomers believe that their structures were distorted by collisions with other galaxies.

Astronomers classify the Milky Way as a large spiral or possibly a barred spiral galaxy, with several spiral arms coiling around a central bulge about 10,000 light-years thick. Stars in the central bulge are close together, while those in the arms are farther apart. The arms also contain clouds of interstellar dust and gas. The disk is about 100,000 light-years in diameter and is surrounded by a larger cloud of hydrogen gas. Surrounding this cloud in turn is a spherical halo that contains many separate globular clusters of stars mainly lying above or below the disk. This halo may be more than twice as wide as the disk itself. In addition, studies of galactic movements suggest that the Milky Way system contains far more matter than is accounted for by the visible disk and attendant clusters—up to 2,000 billion times more mass than the Sun contains. Astronomers have therefore speculated that the known Milky Way system is in turn surrounded by a much larger ring or halo of undetected matter known as dark matter.

Milky Way: Types of Stars

The Milky Way contains both the so-called type I stars, brilliant, blue stars; and type II stars, giant red stars. Blue stars tend to be younger because they burn furiously and use up all of their fuel within a few tens of millions of years. Red stars are usually older, and use their fuel at a slower rate that they can sustain for tens of billions of years. The central Milky Way and the halo are largely composed of the type II population. Most of this region is obscured behind dust clouds, which prevent visual observation. Astronomers have been able to detect light from this region at other wavelengths in the electromagnetic spectrum, however, using radio and infrared telescopes and satellites that detect X rays. Such studies indicate compact objects near the galactic center, probably a massive black hole. A black hole is an object so dense that nothing, not even light, can escape its intense gravity. The center of the galaxy is home to clouds of antimatter particles, which reveal themselves by emitting gamma rays when they meet particles of matter and annihilate. Astronomers believe the antimatter particles provide more evidence for a massive black hole at the Milky Way’s center.

Observations of stars racing around the center also suggest the presence of a black hole. The stars orbit at speeds up to 1.8 million km/h (1.1 million mph)—17 times the speed at which Earth circles the Sun—even though they are hundreds of times farther from the center than Earth is from the Sun. The greater an object’s mass, the faster an object orbiting it at a given distance will move. Whatever lies at the center of the galaxy must have a tremendous amount of mass packed into a relatively small area in order to cause these stars to orbit so quickly at such a distance. The most likely candidate is a black hole.

Surrounding the central region is a fairly flat disk comprising stars of both type II and type I; the brightest members of the latter category are luminous, blue supergiants. Imbedded in the disk, and emerging from opposite sides of the central region, are the spiral arms, which contain a majority of the type I population together with much interstellar dust and gas. One arm passes in the vicinity of the Sun and includes the great nebula in Orion.