Astronomy and related astronomical terms

Any light or object seen in the sky whose immediate identity is not apparent: Despite unsubstantiated claims, there is no evidence that UFOs are alien spacecraft. When investigated, the vast majority of sightings turn out to have been of natural or identifiable objects, notably bright stars and planets, meteors, aircraft, and satellites, or to have been perpetrated by pranksters.

A region of complete shadow resulting from the total obstruction or blocking of light: An umbra is the darkest portion of the shadow cast by an astronomical object during an eclipse, especially that which is cast on Earth during a solar eclipse.

An umbra can be tbe te main dark inner cone of shadow cast by one body onto another body during an eclipse.

Any point lying within the umbra will observe a total eclipse of the object.

The umbra is also the name given to the darkest part of a sunspot.

The totality of space, matter, and radiation that is potentially comprehensible: The universe is the whole cosmic system of matter and energy of which Earth, and therefore the human race, is a part.

Humanity has traveled a long road since societies imagined Earth, the Sun, and the Moon as the main objects of creation, with the rest of the universe being formed almost as an afterthought.

Today it is known that the Earth is only a small ball of rock in a space of unimaginable vastness and that the birth of the solar system was probably only one event among many that occurred against the backdrop of an already mature universe.

The universe is all of the space and its contents, the study of which is called "cosmology".

The universe has been determined to be mostly empty space, dotted with stars collected into vast aggregations called galaxies for as far as telescopes can see.

The most distant detected galaxies and quasars (quasi-stellar objects) lie ten billion light-years or more from the Earth, and are moving farther apart as the universe expands.

The third largest constellation in the sky in the north polar region: The seven brightest stars in the Ursa Major make up the familiar shape, or asterism, of the Big Dipper. The second star of the handle of the dipper, called "Mizar", has a companion star, "Alcor".

Two stars forming the far side of the bowl act as pointers to the north star, "Polaris". "Dubhe", one of them, is the constellation's brightest star.

The "Little Dipper" or small constellation of the northern hemisphere: Ursa Minor, or Little Bear is shaped like a dipper, with the bright north pole star "Polaris" at the end of the handle.

In theory, space that contains no matter: Tom read in the article that the term vacuum related to space devoid of any substances at all.

One of two zones of charged particles around the Earth's magnetosphere discovered in 1958 by U.S. physicist Jame Van Allen: Van Allen radiation belts relates to the atomic particles that came from the Earth's upper atmosphere and the solar wind, and are trapped by the Earth's magnetic field.

The inner belt lies above the equator, and contains protons and electrons from the solar wind.

One of the two regions of the Van Allen radiation belts is situated at about 1,900 miles, 3,000 kilometers, and 12,500 miles, 20,000 kilometers, above the equator, in which charged particles, trapped in the Earth's magnetosphere, oscillate between the magnetic poles.

The particles are caught from the solar wind, or produced by collisions between air molecules and cosmic rays.

A star whose brightness changes over time: A variable star is a star whose luminous output varies significantly with time.

Such variation may be regular; that is, eclipsing variable stars, or irregular, as with flare stars.

In addition, the variation can be intrinsic, because of changes within the star itself, or extrinsic, as the result of the interaction of one star with another.

A supernova remnant formed as a result of a supernova explosion about 30,000 years ago: Still expanding, the Veil Nebula will eventually become indistinguishable from interstellar gas.

The time when the Sun seems to cross the celestial equator: The vernal equinox occurs about March 21 in the northern hemisphere, and in September in the southern hemisphere.

The technique of linking together several radio telescopes spaced by hundreds and even thousands of miles: The purpose of the Very Long Baseline Interferometry is to achieve very high resolution observations of distant objects, such as quasars.

A zodiacal constellation of the northern hemisphere, the second largest in the sky: Virgo is represented as a maiden holding an ear of wheat, marked by the first-magnitude "Spica", Virgo's brightest star.

The sun passes through Virgo from late September to the end of October.

The orbits and positions of the planets: Wanderers could not be accurately accounted for before the invention of the telescope although star positions were known.

Understanding came with the revolutionary work of Galileo, Brahe, and Kepler which, together with Newton's contributions, finally swept away the Greek concept of an earth-centered universe and established the present model of the solar system.

The Greeks had simplified celestial mechanics according to the simple doctrine that "matter behaves according to nature."

The distance between two successive crests of a wave: A wavelength IS the span or length between successive peaks, or troughs, of a wave.

A type of star that has collapsed after exhausting its nuclear fuel: A white dwarf is a dense, small low-luminosity star of mass less than the Chandrasekhar limit (1.4 solar masses) left as the remnant of a supernova explosion.

Leftover heat causes a white dwarf to shine faintly.

A star may remain a giant or supergiant for several million years before all nuclear reactions cease.

Gravitational collapse then occurs with no outward pressure to stop it, and the final result may be a white dwarf.

Such a white dwarf is small, about the same size as the Earth, but has about one million times the density of water and the temperature at the surface is a hundred thousand degrees, yet the luminosity is quite low; about one-thousandth of the sun.