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Alpha Crucis
A
Accretion
The accumulation of dust and gas to make large bodies. It is one of the most important processes
in the formation of the
planets. The early Earth, for example, is thought to have grown by this process, with meteorites and small planetesimals colliding with it regularly. The
effect of the collisions
was to heat the surface
of the Earth and at the
same time cover it with a
blanket of dust that kept the
heat in. In this way the Earth could have become hot enough
to begin melting after growing to less than 15% of its final volume. All this may have occurred within a few hundred million years after the Sun was formed.
One or more planetesimals that struck the Earth are thought to
have been about the same size as
the early Earth. Some astronomers have suggested that the Moon could have been formed when a large chunk of the Earth was blasted away at the time of this collision. This might account for why the rocks on the Moon are similar to those found in the Earth’s mantle.
Once accretion had caused the Earth’s rocks to heat up to melting point, the metals in the rocks would have begun to sink toward the center of the Earth to form the core. At
the same time, this would have released enough gravitational
energy to generate yet more
heat. In this way the core became even hotter. Radioactive decay of elements brought to the planet during accretion could have added to the heating.
Mercury Venus
Mars
Moon
Adrastra
The second closest, and one of the largest, of Jupiter’s satellite rocks, some 25km across, and on the
edge of the Jovian ring. It orbits 129,000km above the planet’s surface and is probably important in keeping the rocks of the ring
in place, acting as a “shepherd moon.”
Albedo
The amount of light reflected from an object. The higher the albedo, the more light it reflects, and the brighter it looks. Many features on the planets in the Solar System are seen in patterns of light and dark. By comparing the way the landscapes reflect light, scientists can estimate some properties of these remote surfaces.
Alpha Centauri
A bright triple star and the closest bright star system to the Sun, at about 4.3 light-years distance. The two brighter stars revolve around each other every 80 years, with the third revolving around them far more slowly. The brightest star is similar to the Sun. One of the other
stars is more red than the Sun, and the third is a red dwarf star.
Alpha Centauri appears from Earth as the third brightest
star (the brighter stars being Sirius and Canopus). It is
visible to anyone in the night sky who is looking from south of 40°N latitude.
Alpha Crucis
The brightest star in the Crux constellation. It is actually a double star, some 510 light- years from the Earth.
Jupiter
Earth
Uranus
Pluto
Saturn
Neptune
 Accretion—The Earth and all of the other planets may have formed from dust that clumped together as the Solar System formed.
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