# Nomenclature

Astronomy is perhaps the oldest science and astronomers did classify their object long before they did understand the underlying physics.

The nomenclature is therefore not always self explanatory. E.G. a pulsar does not pulsate, a planetary nebula has nothing to do with planets and is even larger as the whole solar system.

We present here some definitions hoping they are useful to somebody.

 Doppler shift: Let us observe an object emitting a signal at a known wavelength and let the object move towards us. We will measure the signal at a different slightly shorter wavelength. This shift is called Doppler shift and allows us to determine the radial speed of the object using the formula: (wavelength(obs) - wavelength(emit)) / wavelength(emit) = v(object)/v(light)back Early Stars: A hot star not a young star. back Equivalent width: In an absorption line less energy is transported than in the neighbouring continuum. The amount of this energy is proportional to the triangular area of the line. (Ok it is not really a triangle ;-)) Now convert this area to a rectangle with the heigt of the actual continuum. The width of this rectangle is the equivalent width. The equivalent width is independent of the instrumental response if your spectrum contains only the stellar signal. EW is positive for absorption lines and negative for emission lines. back Late Star: A cooler star, not an old one. back Keplerian Laws The three laws of Johannes Kepler (1533 - 1612) describe the motions of bodies in a central gravitaional field: I:   The orbits of the planets are ellipses, with the sun at one focus of the ellipse. II:  The line joining the planet to the sun sweeps out equal areas in equal times as the planet travels aroud the ellipse: III: The ratio of the squares of the revolutoinary periods for two planets is equal to the ratio of the cubes of their semimajor axes. back Luminosity sensitive: A spectral feature that chances significantly with luminousity. back Magnitude: The magnitude of a star is a measure for its brightness. But you know, astronomers are strange people. The larger the magnitude, the dimmer the star. You can see stars of 6th magnitude or brighter with your naked eye. The brightest star is of -27mag. It is - you guessed it - our sun. Measuring brightness, or should I say darkness, in terms of magnitude fits the optical sensation of our eye. Therefore it is a logarithmic measure of energy. 1 step in magnitude is a fraction of about 2.5 in energy. back Parsec: A parsec is the distance at which the astronomical unit will appear under an angle of 1 arc second. 1 parsec = 1pc = 3 ly back Photon: An electromagnetic wave transports energy in little packets called photon. back Wavelength: Light is an electromagnetic wave. The length of one wave is normaly measured in meters here nano meters 1 nm = 1.10-9 m. Astronomers use Ångstrøm. 10 Å = 1 nm. back