Lifetime on the Main Sequence

How long will a star remain on the main sequence? This is primarily a function of how fast it consumes the hydrogen in its core, and this in turn is primarily determined by the mass. The more massive the star, the shorter the period on the main sequence. The following table illustrates some important timescales for three different masses.

Timescales for Stellar Evolution
Mass (Solar Units)	Formation (years)	Main Sequence (years)	Giant Phase (years)
1	1x10⁸	9x10⁹	1x10⁹
5	5x10⁶	6x10⁷	1x10⁷
10	6x10⁵	1x10⁷	1x10⁶

Here is an calculator for the lifetime of a star on the main sequence. The following table, repeated from Chapter 18, summarizes some basic properties of main sequence stars for different spectral classes, including their main sequence lifetimes.

Main Sequence Spectral Class Properties
Spectral Class	Mass (Solar Units)	Luminosity (Solar Units)	Temperature (K)	Radius (Solar Units)	Time on Main Sequence (Million Years)
O5	40	400,000	40,000	13	1.0
B0	15	13,000	28,000	4.9	11
A0	3.5	80	10,000	3.0	440
F0	1.7	6.4	7,500	1.5	2,700
G0	1.1	1.4	6,000	1.1	8,000
K0	0.8	0.46	5,000	0.9	17,000
M0	0.5	0.08	3,500	0.8	56,000

The trends illustrated in these tables can be understood by comparing the luminosity and lifetime columns in the second table. Massive stars are so luminous that they must run through their core nuclear fuel in a very short period of time, while low-mass stars are much more frugal with their fuel because their luminosities are so low so they live for much longer periods.