ASTR 103 - Astronomy

Glossary - O

OB association

 

objective

Main lens or mirror of the telescope.

objective prism

Thin, large prism placed in front of the telescope objective. It produces a spectrum of each star in the field of view of the telescope.

oblateness

 

oblate spheroid

Solid formed by rotating an ellipse about its short axis.

obscuration (interstellar)

 

Occam's Razor

Rule of medieval philosopher William of Occam that one should not burden the explanation of a phenomenon with more than a minimum number of assumptions or involved interpretations.

occultation

Transitory blocking of an object's light by the passage of a larger intervening body. Examples include the Moon passing over a star or planet and the shadow of Jupiter passing over one of its satellites.

opacity

Reduction in the intensity of light as it passes through the layers of a medium (in stars through the gaseous layers).

open cluster

Somewhat loose assemblage of stars, numbering dozens to hundreds, with various degrees of central condensation. In our Galaxy it is also known as a galactic cluster.

open universe

A cosmological model of the Universe that at an infinite time in the future will possess an infinite volume with hyperbolic geometry; it also has no center and no boundaries.

opposition

Position of a superior planet when it is closest to Earth. At this time it is 180^o from the Sun's direction.

orbit

Path of a body subjected to the gravitational force of another body.

organic compound

Compound that contains carbon.

O star

Stars of spectral type O are very hot blue stars with surface temperatures of about 35,000 K, whose spectra are dominated by the lines of singly ionized helium.

outgassing

Venting of volatile gases from the heated interior of a solid body.

oxides

Chemical compounds containing oxygen.

ozone

Ultraviolet-absorbing layer of O₃ molecules between 20 and 35 kilometers high.