Stars Sun data Local17 3D Local40 3D Bayer 3D Local Data Astronomy Constellation

Astronomic Constants & Formulae

This page lists units of measurement common in astronomy, common astronomic formulae, the spectral types of stars, details of the mass-luminosity relationship, a table of Greek letters & a glossary.

Units of Measurement

Astronomic Unit: the mean distance between the Earth and the Sun.
1 AU = 149,600,000 Km or 1.496*10¹¹ Metres

Light Year: the distance light travels in 1 year.
1 LY = 9.46 trillion Km or 9.46*10¹⁵ Metres

Parsec: the distance at which the semimajor axis of the Earth's orbit (1 AU) subtends one arc second.
1 Parsec = 3.26 LY or 3.084*10¹⁶ Metres

Solar Mass 1.98892x10³⁰ kg - a standard way to express mass in astronomy, used to describe the masses of other stars and galaxies

Formulae

Tan(Parallax) = Distance/AU ..also (from definition above)
Tan(1'') = Parsec/AU

M=(m+5)-5Log(d) ..where

M - Absolute magnitude (AKA M_v)

m - Apparent magnitude
(AKA Visual magnitude, abb. Vmag, m_v when measured through an astronomical V filter)

d - Distance in Parsecs

Spectral Types

Annie J. Cannon's spectral classes are a measure of surface temperature in the sequence O, B, A, F, G, K, M, R, N, S. This measurement is based partly on physicist Max Planck's formula, which gives the relative emissions of various colors from a hot body.

A cool star emits most of its light in the red; a hot star emits most of its light in the blue. A measurement of the ratio of blue to red light coming from a star (its color index) determines its temperature.

Each spectral class in turn is divided into 10 subcategories from 0 to 9. For example, the hottest B stars are B0, followed by B1, B2, B3, and so on. Spectral type B9 is followed by spectral type A0. The hot O, B, and A stars are blue, F and G stars are yellow, and K, M stars are red.

O stars are hot (surface temperature = 30,000 K)

A stars have surface temperature = 10,000 K

G stars, such as the Sun, have surface temperature = 6,000 K

M stars have surface temperature = 3,000 K

Mass-Luminosity Relation

For stars of about one solar mass, luminosity rises steeply with mass (L is approximately M to the power of 3); it rises even more steeply with mass for heavier stars (L is approximately M to the power of 4). The correlation means that the more massive a star, the more rapidly it will consume its nuclear fuel.

The mass-luminosity correlation is good only for main-sequence stars, that is, for those that shine by converting hydrogen into helium in their cores, as does the Sun. It fails for giant stars that have evolved away from the main sequence and that generate energy in shells surrounding an inert helium core. It fails most dramatically for white dwarf stars that have no nuclear energy sources.

Greek Letters

The letters of the Greek alphabet are used in Bayer names. Stars with a Bayer designation are bright stars that are usually part of a constellation. The 18th letter of the Greek alphabet, Final Sigma, does not appear in Bayer names; so I've left it out of the table..

#	Letter	Abb.	A	#	Letter	Abb.	A
1	Alpha	ALF	α	13	Nu	NU	ν
2	Beta	BET	β	14	Xi	XI	ξ
3	Gamma	GAM	γ	15	Omicron	OMI	ο
4	Delta	DEL	δ	16	Pi	PI	π
5	Epsilon	EPS	ε	17	Rho	RHO	ρ
6	Zeta	ZET	ζ	19	Sigma	SIG	σ
7	Eta	ETA	η	20	Tau	TAU	τ
8	Theta	THE	θ	21	Upsilon	UPS	υ
9	Iota	IOT	ι	22	Phi	PHI	φ
10	Kappa	KAP	κ	23	Chi	CHI	χ
11	Lambda	LAM	λ	24	Psi	PSI	ψ
12	Mu	MU	μ	25	Omega	OME	ω

Glossary

Abbrev.	Stands for	Description
RA	Right Ascension	The celestial equivalent of terrestrial longitude
Dec.	Declination	The celestial equivalent of terrestrial latitude
Vmag	Visual Magnitude	AKA apparent magnitude, a measure of a celestial object's brightness as seen by an observer on Earth
Mv	Absolute Magnitude	A celestial object's intrinsic brightness

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