B00B7H7M2E EBOK (45 page)

10. The ‘Leviathan of Parsontown’, built by Lord Rosse in the 1840s and through which he observed the spiral nebulae. It remained for more than half a century the largest telescope in the world.

11. Henrietta Swan Leavitt (1868–1921). Her discovery of Cepheid variable stars and how to use them as stellar yardsticks led to the first measurements of distances outside the Milky Way Galaxy.

12. Before and after photos of Supernova 1987A in the Large Magellanic Cloud. The image on the left was taken in 1969; that on the right in February 1987, about a week after the supernova appeared.

13. The Andromeda galaxy, dominant spiral galaxy of the Local Group, with two companion galaxies – M32 (the bright dot close to Andromeda on the left) and NGC 205 (the bright dot on the right).

14. Albert Einstein visits Edwin Hubble at Mt. Wilson in 1930. Hubble’s astronomical observations and Einstein’s theories both indicated that the universe must be expanding.

15. Grote Reber’s radio telescope in Wheaton, Illinois (1937). Reber spent $4,000 of his own savings on designing and building this device in his mother’s back yard.

16. This image of the Milky Way Galaxy in ‘near infrared’ wavebands, from observations of the Cosmic Background Explorer (COBE), clearly shows the thin, flattened disk and central bulge.

17. The Hubble Space Telescope, NASA’s orbiting observatory, linked to the Space Shuttle Endeavour during the December 1993 repair mission.

GLOSSARY

absolute magnitude:
How bright a star looks from a distance of ten parsecs (32.6 light-years). More technically: The amount of light received from a star that is ten parsecs away. The absolute magnitude of a star doesnʼt change with distance. Just as a 100-watt light bulb is still a 100-watt light bulb no matter how much its brightness appears to change with distance, a starʼs absolute magnitude remains the same no matter how much its
apparent
magnitude changes with distance.

absolute zero:
The lowest possible temperature, at which a substance contains no heat energy.

absorption lines:
Dark lines in a spectrum produced when light from a distant source passes through cooler gas closer to the observer.

acceleration:
The rate at which the speed of an object is changing.

action at a distance:
The phenomenon of an object exerting a force on a second object across empty space, without the intervention of anything physical.

aether:
Aristotleʼs fifth element, of which he thought stars and planets were made.

angular size:
Astronomers describe the apparent size of an object in the sky in terms of its angular size. For example, if two lines are drawn from an observer on the Earth to the opposite edges of the Moon, the angle formed by the two lines where they meet at the observer is about ½ degree. Another way of putting that is to say that the Moon “subtends” an angle of ½ degree; or that it has an angular size of ½ degree. The angular size of an object canʼt be converted into its true physical size unless the distance to the object is known.

In calculating angular size: A circle has 360 degrees; each degree is divided into 60 minutes of arc or arcminutes; each minute of ark is divided into 60 seconds of arc or arcseconds.

apparent magnitude:
How bright a star looks as viewed from the Earth. More technically: The amount of light received from a star as observed from the Earth. The apparent magnitude of a star changes with distance.

apparent size:
As opposed to true size, apparent size is the size of a heavenly body as viewed from the Earth. The apparent sizes of the Sun and Moon are the same; their true sizes are not.

arcminute:
See angular size.

arcsecond:
See angular size.

astronomical telescope
: A telescope based on an optical system first described by Kepler that uses only convex lenses. Keplerʼs telescope inverted the image, but after that problem was sorted out, the astronomical telescope replaced the so-called Dutch telescope (the kind Galileo had used) in most serious astronomical work by the mid-seventeenth century. It provided a much larger field of view at equal magnifications.

atom:
A unit of ordinary matter. The center of the atom is the nucleus, made up of protons and neutrons. Electrons orbit the nucleus.

bender:
A massive body or galaxy or cluster of galaxies that is responsible for the bending of paths of light passing near it.

Big Bang:
The state of enormous heat and density in which the universe probably began, and from which the universe has expanded and cooled to its present state; not necessarily a singularity.

Big Crunch:
The collapsed state in which the universe might end.

binary star (or binary system):
Double star system in which the two stars are bound together gravitationally and orbit their common center of mass.

black hole:
The classical definition is a region of space-time from which nothing can escape unless it can travel at a speed greater than the speed of light.

blueshift:
Displacement of the spectral lines in light coming from distant stars and galaxies that are moving toward Earth.

brightness fluctuation method:
Technique used to calculate the distances to galaxies by measuring the unevenness in the brightness of the surface of the central bulge or near the center.

Cepheid variable:
Pulsating variable star whose period of brightness variation is directly related to its absolute magnitude.

Chandrasekhar limit:
About 1.4 times the mass of the Sun. The maximum possible mass of a stable cold star, above which it must collapse.

charge-coupled device (CCD):
A digital imaging system.

closed universe:
Cosmological model in which the universe eventually stops expanding and collapses. A universe in which omega equals more than 1.

Copernican astronomy:
The Sun-centered astronomy introduced by Nicolaus Copernicus.