Glossary

Absolute Magnitude ( M _v ) - The Relative brightness of each star by comparing them if they were all the same distance away at 10 parsec ( 32.6 light years ) M _v = m _v +5 -5 Log ( 1 / Parallax ). Also see Apparent Magnitude.

Apparent Magnitude ( m _v ) - The Visual brightness of a star viewed with the naked eye. The Brightest stars are 1st magnitude, Dimmest are 6th magnitude. Each unit of magnitude is 2.512 times greater or smaller than the previous one ( 100 to the 5th root ). For example, a star of m _v= 2.3 is 2.512 times brighter than one of m _v = 3.3 while a star of m _v = 7.0 is 2.512 times dimmer than one of m _v = 6.0. Also see Absolute Magnitude.

Brightness - See Apparent Magnitude.

Celestial Sphere - A two-dimensional imaginary illusion that is used in describing the exact location of all celestial objects (planets, stars, galaxies, etc). All stars (etc.) are very far away, some further than others but to the eye they all appear to be equally distant. So imagine that the Earth is at the center of a hugh sphere, maybe inside of a monster basketball, and time and all motions of the Earth have stopped. All of the objects in the sky are now marked ( maybe with white paint) on the inside of this giant, dark sphere. Now the coordinate lines of the Earth (which are imaginary but yet real also) such as the equator, north & south pole, horizontal lines of latititude (see Declination) and vertical lines of longitude (see Right Ascension) are projected outward and also marked (with white paint) on the inside of the sphere. Remember that this sphere with all of the marks (stars, etc.) & lines (coordinates) on it never moves or spins. Now start the Earth back to its normal motion and imagine the Earth rotating and moving around the Sun against the Backdrop of the inside of the now marked sphere. This explains the motions and positions of all the stars, etc. that we see.

Declination (d - delta) - Equivalent to degrees of Latitude on Earth. A "+" (plus) Declination starts at 0 (zero) degrees at the Celestial Equator and goes up to +90 degrees which is the Celestial North Pole. A "-" (minus) Declination starts at 0 (zero) degrees at the Celestial Equator and goes down to -90 degrees which is the Celestial South Pole. For example the cooridinates (position) for a certain star can be expressed as a Declination of -42d14m37s. This locates the vertical position (+ or -) and is used in conjuction with the Right Ascension to define the exact location on the Celestial Sphere. An example of a complete set of coordinates (RA & Dec.) is 12h25m13s-42d14m37s. Also see Right Ascension.

Distance (d) - ( pc - parsecs ) = 1 / Parallax.

Parallax  (Trigonometric) ( p - pi ) ( arc seconds ) - An angular value derived from a method to determine the distance (d) to the nearby stars Used are the radius of the Earth's orbit which is also the distance to the Sun called 1 AU (astronomical unit) and one-half of the displacement of the star against the background of more distant stars when viewed from points in Earth's orbit six months apart.  Tangent p = 1 AU / d. Since the distances involved are very large, a more convenient unit called a parsec  is used ( 1 pc = 3.26 light years = 206,265 AU ) thus   d = 1 / Parallax.

Proper Motion ( m - mu ) - ( "/yr - arc seconds  per year ) The two-dimensional apparent change in the position of a star over the period of a year.  The components are: Right Ascension (a - alpha) measured in seconds of time per year and Declination  (d - delta) measured in seconds of arc per year. m ( mu) =  ( 225 mu 2a cos 2 d + mu d )^ 1/2.

Radial Velocity  (V_r ) - ( km/s - kilometers per second ) The movement of a star relative to the Sun directly towards (negative) or away (positive) which can be derived from the parallax and proper motion. Q is the angle between object star's celestial convergent point and the direction to the Sun. VR = 4.74 m / Parallax (tan Q).

Right Ascension - (a - alpha) Equivalent to Longitude. Instead of being divided into 360 degrees as on Earth, Right Ascension is noted in Hours, Minutes & Seconds. There are 24 Hours each equal to 15 degrees (24 hrs. x 15 deg. = 360 deg. - get it) starting at a 0 (zero) hour meridian to 23 hours. Each hour is divided into 60 minutes and each minute is divided into 60 seconds. For example the cooridinates (position) for a certain star can be expressed as a Right Ascension of 12h25m13s. This locates the position along the Celestial Equator and is used in conjuction with the Declination to define the exact location on the Celestial Sphere. An example of a complete set of coordinates (RA & Dec.) is 12h25m13s-42d14m37s. Also see Declination.

Space Velocity - A star's total velocity with respect to the local standard of rest. This is the combination of the star's U, V, and W velocities: space velocity = sqrt (U² + V² + W²). For example, the Sun (U = -9, V = +12, W = +7) has a space velocity of 17 kilometers per second.

Spectral Type - A system of stellar classification based on a star's surface temperature, color and size.  The familiar spectral classes of O, B, A, F, G, K, M have been expanded to include L & T.  These designate surface temperature (in kelvin) and thus the color.  These are each sub-divided into 10 classes (0 - 9) from hottest to coldest.   To both of these an additional term is added called the Luminosity Class ( Roman numerals I thru VII ) to designate the the size (based on the star's actual spectra) from largest to smallest.  The Sun is classified as a G2 V type star.  To better understand the Spectral Type designations you can visit the following website  http://www.answers.com/topic/stellar-classification .

Star Names - A complex topic and a Tower of Babel but it will suffice to say that some stars have dozens of different names and identifiers. For our nearby star research will only use the following identifier types which have historical or nearby star scientific research relavence. The list is in desending order of preference.

Bayer - ( alf Cen ) - The first scientific ordering of the brightest stars used on all star charts
Flamsteed - ( 61 Cyg) - The first scientific telescopic star study - also used on all star charts
GJ - (GJ 820) - The Gleise & Jahress Catalog of Nearby Stars
LHS - (LHS 62) - The Luyten Half-Second Catalog of high proper motion stars
HD - (HD 201091) - The Henry Draper Star Catalog
HIP - (HIP 104214) - The Hipparcos Star Catalog
NS - (NS 1757+0441) - The NStars Database at Northern Arizona State University
HR - (HR 8085) - The Harvard Revised Catalog
Giclas - (G 140-024) - The Giclas Star Catalog
SAO - (SAO 73743) - Smithonian Astrophysical Observatory Catalog
DM - (BD+043561a) - The Bonner Durchmusterung Catalog
Common Name - ( Sirius) Very few of the nearby stars have common names so its use is limited 

Transverse Velocity  (V_t ) - ( km/s - kilometers per second ) . Relative to the Sun and perpendicular to the observer's line of sight. Transverse velocity is related to proper motion by V_t = 4.74 m / parallax

Jerry Blackwell

NBSO.ORG