Astronomy and related astronomical terms

1. A two-dimensional representation of the night sky as it would appear to a specific observer at one particular latitude and time.
2. The modern equivalent of the astrolabe, but more convenient to use.

1. A gas consisting of electrons and ions; called "the fourth state of matter" because the temperature is too high for whole atoms to exist.
2. The so-called "fourth state of matter", consisting of ions and electrons in equilibrium.

Such a state can be arrived at in regions of very high temperatures; such as, those which exist within stars.

An open star cluster about 400 light-years away from earth in the constellation Taurus, represented as the Seven Sisters of Greek mythology.

Its brightest stars (highly luminous, blue-white giants only a few million years old) are visible without the need of a telescope, but there are many fainter ones.

A celestial body that orbits the sun, has a roughly spherical shape, is farther away from the sun than Neptune, and shares its orbit with other objects.

Similar distant bodies in the solar system will be called plutoids according to a decision by the International Astronomical Union (IAU) in 2008.

The International Astronomical Union has decided that any object which is orbiting the sun beyond the orbit of Neptune, and large enough for its own gravity to pull it into a spherical shape, will henceforth be classified as a plutoid.

The solar system's only other known dwarf planet is the asteroid Ceres. It is also big and spherical, but because it orbits the sun in the asteroid belt between the orbits of Jupiter and Mars, it is not yet considered to be a plutoid.

The brightest star of the Little Dipper in the constellation Ursa Minor, near the celestial north pole.

Because it always indicates due north from an observer anywhere on the earth, Polaris is important for navigation.

The degree to which the electric and magnetic components of electromagnetic radiation are confined to one configuration.

One of two groups that stars and stellar clusters can be divided into, according to age, position in space, and chemical composition.

Population I stars are relatively young, very luminous stars belonging to the spiral arms of galaxies. Typical members are O-type stars, Delta Cephei stars, and open clusters.

One of two groups that stars and other celestial bodies can be divided into.

Population II stars are typically old, metal-deficient stars found in the center of the galaxy, and also following highly elliptical orbits that take them far from the galactic disk.

Typical Population II members are long-period variable stars, globular clusters, and type II cepheid variables.

1. A slow change in the direction of the tilt of earth's axis, which results in an apparent change in the position of the stars.
2. Slow wobble of the earth on its axis, like that of a spinning top.

The gravitational pulls of the sun and moon on the earth's equatorial bulge cause the earth's axis to trace out a circle on the sky every 25,800 years.

The position of the celestial poles is constantly changing because of procession, as are the positions of the equinoxes (the points at which the celestial equator intersects the sun's path around the sky).

The precession of the equinoxes means that there is a gradual westward drift in the ecliptic or the path that the sun appears to follow and in the coordinates of objects on the celestial sphere.

1. A measure of the force exerted on a surface.
2. The force exerted by something pressing or squeezing an area.

The tires on a car are under pressure because air, forced into them, pushes against their rubber walls.

A wedge-shaped piece of glass used to split light into a spectrum.

A cloud of gas in the sun's atmosphere at a lower temperature than its surroundings; probably coronal material under the influence of the sun's magnetic field.

The movement of a star in the celestial sphere which is more noticeable in near stars than in distant stars; in practice, the movement of the latter is negligible.

1. An elementary particle with a mass about 1,836 times that of the electron and a positive charge, equal and opposite to that of the electron.
2. An atomic particle with high mass and a positive charge, present in the nuclei of all atoms.

The nucleus of a hydrogen atom is a single proton.

The stage in the evolution of a star between fragmentation and the zero age main sequence state, where nuclear reactions begin.

Stars are believed to be born in groups from the collapse of large, cold clouds of interstellar material composed primarily of hydrogen gas.

A protostar becomes a proper star when it begins fusing hydrogen into helium in its core. A star of one solar mass fuses its hydrogen for about ten billion years.

According to this analysis, the sun, which is about 4.6 billion years old, is a middle-aged star.