The "A to Zee" of Astronomee.

By: Wolf Lange

Sources include: Collins Dictionary of Astronomy 2^nd Edition, Burnhams Celestial Handbook Revised and Enlarged Edition, Patterns in the Sky by Julian DW Staal and the Amateur Astronomers Handbook by JB Sedgwick.

Falling star – popular name for meteor.

False cross – popular name for "skewed" cross like formation; former constellation Argo now divided between Carina and Vela. The 4 stars making up the ‘false cross" are all 2^nd magnitude.

Field of view - often found inscribed on binoculars. It refers to the area made visible by the optical system of an instrument at a particular setting. It is expressed in the form of its angular diameter e.g. 6.3º. This diameter increases with decreased magnification depending on the eyepiece used in a telescope but it will normally be fixed for binoculars.

Fireball – a bright visual Meteor of magnitude greater than –10. About 50 000 to 100 000 occur in the earth’s atmosphere each year. It will show up brighter than any object seen in our night sky with the exception of the sun and the moon.

FIRST – abbreviation for Far Infrared and submillimeter Space Telescope. An orbiting telescope under design for ESA and presently scheduled for launch into earth orbit in 2007. It will primarily be used for interstellar studies. It is planned to be a 4.5-meter radiatively cooled Infrared Cassegrain telescope feeding instruments working at wavelength of 100um to 1mm.

First point of Aries – another name for the vernal Equinox, which over 2000 years ago, when Hipparchus first used the term, lay in the constellation of Aries. Due to westerly Precession of the Equinoxes, the vernal Equinox now lies in Pisces and will subsequently move to Aquarius. Likewise the autumnal Equinox once lay in Libra but is now located in Virgo. NOTE: these seasonal references are for the Northern Hemisphere.

Flares - sudden short-lived brightening of small areas of the sun’s upper Chromosphere / inner Corona that are optically visible usually only in the Monochromatic light of certain strong Fraunhofer lines. (See also Fraunhofer later-on under F). They represent massive explosive release of energy – in the form of particles and radiation – that causes temporary heating of the surrounding medium and may accelerate electrons, protons and heavier ions to high velocities.

Flyby – a trajectory that takes a spaceprobe close to a planet or satellite, but does not permit it to enter an orbit about the body or land on it. (Sorry for those that waited in the departure lounge!)

Focal Length - symbol f it presents the distance between the centre of a reflecting surface or refracting medium to the focal point or focus. With a telescope, microscope and binoculars this would be the point where usually the observer’s eye would be. The focal plane is the plane through the focus, at right angles to the optical axis, in which the image of a distant object will be formed.

Fomalhaut (a. PsA) – a white star that is brightest in the constellation Piscis Austrinus. The infrared satellite IRAS has detected a shell of cool (about 50 K) material around this star that is interpreted as being due to solid material orbiting the star and may be a planetary system.

Fornax (Furnace) – an inconspicuous constellation in the Southern Hemisphere fairly near Orion, the brightest stars being of 3^rd and 4^th magnitude.

FR I and FR II – abbreviation for Fanaroff-Riley types I and II. A simple but powerful classification made in 1974 of Radio Galaxies that contain hot spots and diffuse emission.

Fraunhofer lines – (absorption lines in the solar (photospheric) spectrum, first studied in detail by Joseph von Fraunhofer in 1814. He catalogued over 500 lines and labelled the more striking ones with letters. Over 25 000 lines have now been identified. These lines show the presence of ionised calcium, neutral hydrogen, sodium, magnesium and iron.

F stars – stars of spectral type F that are white with a surface temperature of about 6000 to 7400 Kelvin. Examples are Canopus, Polaris and Procyon.

Wolf Lange