Dividing the Main Sequence by Evolutionary Paradigm.
Stars can be classed three ways in terms of their evolution: Those
that fall on the lower main sequence, those that fall on the mid main sequence,
and those that fall on the upper main sequence. Each of these divisions
has unique characteristics in terms of stellar evolution:
- The Lower Main Sequence.
This contains stars of about 0.65 solar masses and down, comprising
types from early K on down through the M dwarfs and the L/T "brown dwarves".
Stars of less than about 0.65 (Some sources suggest 0.8, and at least
one suggests values of only 0.45 to 0.5) solar masses are generally believed
to be too small to ever evolve into any type of giant. The reason
suggested is that they lack sufficient mass to ever achieve the high internal
temperatures and pressures required to initiate any type of helium fusion
in their cores, let alone in those regions immediately surrounding those cores.
Further, a number of sources indicate that these stars are completely
convective, that is, the energy transport mechanism used by the stars, both
in their cores and in their outer layers is convection, rather than radiation
or conduction or a combination thereof.
It seems likely that these stars will last longer than their more massive
brethren simply by virtue of the fact that the convection within them effectively
homogenizes their entire hydrogen fuel supply. Helium "ash" is never
given a chance to accumulate in the core as it is carried off and circulated
throughout the star, in the process being replaced by fresh hydrogen. This,
in combination with relatively cool, slow-burning cores (perhaps 10 to 12
million degrees at the upper end of the K range, and as low as 7 million
degrees at the lower end of the M range) seems to present a logical
explanation for the surprising longevity of these stars. It is thought that
between 75 and 85 percent of the stars in the Universe belong in this category,
the majority of those being small, cool red dwarves of about type M5V.
- The Intermediate Main Sequence.
Stars ranging from about 0.65 to between 8 and 10 solar masses
comprise the intermediate main sequence. These stars range in type
from early K upward through early- to mid-B. Our sun falls in this
range, as does MIra, the subject of this pictoral essay. As we move
upward along the main sequence, core temperatures rise. At about
two solar masses, the core temperature reaches the neighborhood of 17 to
20 million degrees kelvin. At this point, the prevalent process by
which nuclear fusion takes place changes from the proton-proton chain to
the carbon cycle. The carbon cycle is much more sensitive to temperature
than is the proton-proton chain, thus, as the temperature goes up, the rate
of fusion increases rapidly. Beyond about 2 solar masses, the life-times
of stars begin to shorten dramatically. Thus, for example, a star
of about 3 times the mass of the Sun will last only about 600 million years,
while the Sun is expected to last over 10 billion years, and the most common
type of star, the M5Ve red dwarf can expect to last some 671 billion years,
probably all of that on the main sequence. (Recall that stars smaller
than about 0.65 solar masses do not really evolve off of the main sequence.)
These stars comprise about 10 to 15 percent of the stellar population
of the Universe.
- The Upper Main Sequence.
These stars are rare. One reason for this is that smaller stars
are more frequently created than are larger ones. Another is that
these stars are fleeting. They just don't last long enough for very
many of them to accumulate. Indeed, some of the more massive stars
only last a million years or so, and it is quite possible that the most
massive ones, e.g. those over 60 times the sun's mass may not even last
that long. These stars can be true giants. While some of the
largest of the mid-main sequence stars may be three to five times the sun's
diameter, the largest of the upper main sequence giants can be as much
as 10 to 15 times the sun's diameter, perhaps even larger. Interestingly,
the most massive of these do not evolve into giants quite so large as do
the less massive stars. In the cases of stars between 40 and 60 solar
masses, they move back and forth across the HR diagram from red (or yellow)
supergiant to blue and back again as they evolve, eventually to explode
as supernovae. Stars larger than this do not really evolve into giants
at all, as they are so massive and their stellar winds so prodigious that,
over their lifetimes, they literally erode away their outer layers, ultimately
to explode as supernovae, or they explode as supernovae before they get a
chance to evolve significantly toward gianthood. Only about one percent
of the stars in the Universe fall into this category.