sequence, in mathematics, ordered set of mathematical quantities called terms. A sequence is said to be known if a formula can be given for any particular term using the preceding terms or using its position in the sequence. For example, the sequence 1, 1, 2, 3, 5, 8, 13, … (the Fibonacci sequence) is formed by adding any two consecutive terms to obtain the next term. The sequence -1/2, 1, 7/2, 7, 23/2, 17, … is formed according to the formula (n² - 2)/2 for the nth, or general, term. A sequence may be either finite, e.g., 1, 2, 3, … 50, a sequence of 50 terms, or infinite, e.g., 1, 2, 3, … , which has no final term and thus continues indefinitely. Special types of sequences are commonly called progressions. The terms of a sequence, when written as an indicated sum, form a series; e.g., the sum of the sequence 1, 2, 3, … 50 is the series 1 + 2 + 3 + … + 50.

main-sequence star: see Hertzsprung-Russell diagram.

Class	Color	Surface Temperature	Strong Lines
O	blue-white	35,000°C;	ionized helium
B	blue-white	21,000°C;	helium
A	white	10,000°C;	hydrogen
F	creamy	7,000°C;	ionized calcium
G	yellow	6,000°C;	calcium
K	orange	4,500°C;	titanium oxide
M	red	3,000°C;	titanium oxide