The Solar System | Star Formation | Celestial Bodies | Spectral Classes


A system of about 100 billion stars. Our Sun is a member of the Milky Way Galaxy, which is sometimes just designated by capitalization: Galaxy. There are billions of galaxies in the observable universe. Exactly when and how galaxies formed in the Universe is a topic of current astronomical research.


An old star, about the same size as the Sun, that has exhausted its available nuclear fuel and collapsed to about the size of the Earth. They are not very luminous but are hot enough to radiate white or blue-white light. The final stage of life for all stars like the Sun is the white dwarf stage.


Burnt-out core of an old star that no longer emits light, generally believed to follow the white dwarf stage.


A small dark star, smaller than a planet and has a mass equivalent to less than one-tenth of the Sun's mass. A type of failed star - one that did not get hot enough to start the nuclear process.


Massive low-density stars with a diameter up to 100 times greater than that of the Sun. They are very bright and can be seen from vast distances. Giant and supergiant stars represent stages in the lives of stars after they have burned most of their internal hydrogen fuel. Stars swell as they move off the main sequence, becoming giants and for more massive stars supergiants


Neutron stars are the collapsed cores left behind by supernova explosions. Pulsars are a special type of neutron star. Pulsars and neutron stars form when the remnant of a star left after a supernova explosion collapses until it is about 10 km in radius. At that point, the neutrons in the core resist being pressed together further and balance the gravitational force to stop the core collapsing. At that point, the star is so dense that a teaspoonful has the mass of a billion tonnes. Neutron stars become pulsars when the magnetic field of a neutron star directs a beam of radio waves out into space. The star is so small that it rotates from one to a few hundred times per second. As the star rotates, the beam of radio waves sweeps out a path in space. If Earth is in the path of the beam, radio astronomers see the rotating beam as periodic pulses of radio waves. This pulsing is the reason these stars are called pulsars.


A quasi-stellar object - an astronomical object that is very bright for its size and distance from Earth and emits huge amounts of energy, sometimes equal to the energy output of an entire galaxy.

Quasars are believed by most astronomers to be the energetic nuclei of very distant galaxies. For reasons not yet known, they have brightened so much that they mask the light from their underlying galaxies. Often they occur in extremely distant clusters of galaxies. The spectral lines of quasars display very large red shifts, which would indicate that these objects are traveling away from earth's galaxy at speeds in the range of 80 percent of the speed of light. Their apparent great speed also means that they are among the most distant of cosmological objects.


A pulsating variable star. This type of star undergoes a rhythmic pulsation as indicated by its regular pattern of changing brightness as a function of time. The period of pulsation has been demonstrated to be directly related to a Cepheid's intrinsic brightness making observations of these stars one of the most powerful tools for determining distance known to modern day astronomy.


The death explosion of a massive star, resulting in a sharp increase in brightness followed by a gradual fading. At peak light output, supernova explosions can outshine a galaxy. The outer layers of the exploding star are blasted out in a radioactive cloud. This expanding cloud, visible long after the initial explosion fades from view, forms a supernova remnant the core often collapses to form a neutron star.


Between the inner and outer planets of the Solar System lie the asteroid belts. These consist of about 40 000 rocks most of which are only a few metres across. The largest, Ceres, is 936m in diameter.

Other asteroids in the solar system include the Aten asteroids between Venus and Earth, Sallit 1 between Mars and Jupiter, the two Trojan groups in the same orbital path as Jupiter and Chiron between Saturn and Uranus.


A chunk of frozen gasses, ice, and rocky debris that orbits the Sun. A comet nucleus is about the size of a mountain on earth. When a comet nears the Sun, heat vaporizes the icy material producing a cloud of gaseous material surrounding the nucleus, called a coma. As the nucleus begins to disintegrate, it also produces a trail of dust or dust tail in its orbital path and a gas or ion tail pointing away from the Sun. Comet comas can extend up to a million miles from the nucleus and comet tails can be millions of miles long. There are thought to be literally trillions of comets in our solar system out past Neptune and Pluto, but only once per decade or so does one become near and bright enough to see easily without binoculars or a telescope.


Meteors are the millions of tiny particles that orbit the Sun. Most of those that enter the Earth's atmosphere burn up leaving a bright trail. The larger bodies that reach the surface are called meteorites.

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