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Aurora
A glow in the ionosphere of a planet caused by the interaction between the planet’s magnetic field and the solar wind (the flow of charged particles from the Sun).
The name for the aurora
in the highest latitudes of the Northern Hemisphere on Earth is the Aurora Borealis, also called the Northern Lights; the Aurora Australis, or Southern Lights,
is a similar effect in the Earth’s Southern Hemisphere.
The size of the auroras varies with the amount of solar activity. When the activity is intense, the auroras can be seen from latitudes greater than 40°, but during periods of less activity they may only be visible at above 60°.
Auroras are often a green, white, red, or blue color. They
can look like luminous curtains, arcs and bands, and patches that continually move and change shape. (See also: Magnetosphere.)
 Aurora—A typical curtain of light produced where the solar wind comes into the influence of the Earth’s magnetic field.
Axis, axes
The center of rotation of a spinning object. Most planets spin counterclockwise around an axis; only Venus spins clockwise. Most axes of rotation are more or less at right angles to the plane
of the orbit; only Uranus has its axes more or less in the plane of its orbit, and so it is described as “spinning on its back”!
B
Barnard’s Star
The second closest star to the Sun. Some six light-years away, it is 2,000 times as bright as the Sun. It is named after E. E. Barnard, who discovered it in 1916.
Beta Centauri
A binary star, Beta Centauri or Hadar is the 11th brightest star in the night sky. It is about 300 light- years from the Earth.
Betelgeuse
The common name of the star Alpha Orionis. It is the tenth brightest star in the sky and about 1,400 light-years away. The star
is probably 50,000 times as bright as the Sun. Betelgeuse shrinks and swells periodically; and as this happens, it also changes brightness.
Big Bang
One of the most fundamental questions of astronomy is how the Universe came into being. The “Big Bang” theory, or model, is the most commonly accepted idea about the evolution of the Universe. According to this theory, the Universe began from a tiny mass of material that was unbelievably compressed and unbelievably hot.
The Universe is thought to
have begun with a massive expansion of this material between 10 and 12 billion years ago. This is known as the Big Bang, and it might have looked like a fantastic, expanding fireball.
As soon as the Universe began expanding, the material became less dense and much cooler. The first atoms formed within a few seconds of the Big Bang. They were hydrogen, then helium, and then lithium.
Some billion years later the Universe was cool enough for other atoms to form. Scientists think
that they can detect the remains of the early Universe by looking at the radiation (called background radiation) that is found throughout the Universe.
The Big Bang theory explains how matter and radiation could come about, but it cannot explain why the Big Bang itself came about.
The term Big Bang was coined by Sir Fred Hoyle (born 1915).
Big Bang
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