Time to Collapse
to the Main Sequence
|
Mass
(Solar
Units) |
Time
(Million
Years)
|
| 0.5 |
100
|
| 1 |
30
|
| 2 |
8
|
| 5 |
0.7
|
| 15 |
0.16 |
|
|
|
|
|
Kelvin-Helmholtz Timescale
We have seen that the time for a protostar to collapse to the main sequence varies
substantially with the mass of the protostar.
The characteristic
time for this collapse
to occur is called the Kelvin-Helmholtz timescale for the star in
question. It is a function of the competition between gravity and pressure that governs all of
a star's life, and depends primarily on how fast the collapsing protostar can radiate its excess
heat released by the gravitational contraction into space. This is a very strong function of
mass.
Simple estimates
indicate that this timescale for the Sun to collapse to the
main sequence was of order 10 million years. Detailed calculations give a more precise number
of about 30 million years.
The collapse timescales for several different masses are given in
the preceding table and the following figure (repeated from an earlier page)
gives the elapsed times on
protostar collapse tracks for a range of masses.