Everything about The Main Sequence totally explained
The
main sequence is the name for a continuous sequence of stars that appear on a plot of
color versus brightness for groups of stars. These color-
magnitude plots are known as
Hertzsprung-Russell diagrams after their co-developers,
Ejnar Hertzsprung and
Henry Norris Russell. Stars on this band are known as
main-sequence stars or
dwarf stars.
After a star has formed, it generates energy at the hot, dense core region through the
nuclear fusion of
hydrogen atoms into
helium. During this stage of the star's lifetime, it's located along the main sequence at a position determined primarily by its mass, but also based upon its chemical composition and other factors. In general, the more massive the star the shorter its lifespan on the main sequence. After the hydrogen fuel at the core has been consumed, the star evolves away from the main sequence.
The main sequence is sometimes divided into upper and lower parts, based on the processes that stars use to generate energy. Stars below about 1.5 times the
mass of the Sun (or 1.5 solar masses) fuse hydrogen atoms together in a series of stages to form helium; a sequence called the
proton-proton chain. Above this mass, in the upper main sequence, the nuclear fusion process can instead use atoms of
carbon,
nitrogen and
oxygen as intermediaries in the production of helium from hydrogen atoms.
Because there's a temperature gradient between the core of a star and its surface, energy is steadily transported upward through the intervening layers until it's radiated away at the
photosphere. The two mechanisms used to carry this energy through the star are
radiation and
convection, with the type used depending on the local conditions. Convection tends to occur in regions with steeper temperature gradients, higher opacity or both. When convection occurs in the core region it acts to stir up the helium ashes, thus maintaining the proportion of fuel needed for fusion to occur.
History
In the early part of the
twentieth century, information about the types and distances of
stars became more readily available. The
spectra of stars were shown to have distinctive features, which allowed them to be categorized.
Annie Jump Cannon and
Edward C. Pickering at
Harvard College Observatory had developed a method of categorization that became known as the Harvard classification scheme. This scheme was published in the
Harvard Annals in 1901.
In
Potsdam in 1906, the Danish astronomer
Ejnar Hertzsprung noticed that the reddest stars—classified as K and M in the Harvard scheme—could be divided into two distinct groups. These stars are either much brighter than the Sun, or much fainter. To distinguish these groups, he called them "giant" and "dwarf" stars. The following year he began studying
star clusters; large groupings of stars that are co-located at approximately the same distance. He published the first plots of color versus luminosity for these stars. These plots showed a prominent and continuous sequence of stars, which he named the main sequence.
At
Princeton University,
Henry Norris Russell was following a similar course of research. He was studying the relationship between the spectral classification of stars and their actual brightness as corrected for distance—their
absolute magnitude. For this purpose he used a set of stars that had reliable parallaxes and many of which had been categorized at Harvard. When he plotted the spectral types of these stars against their absolute magnitude, he found that dwarf stars followed a distinct relationship. This allowed the real brightness of a dwarf star to be predicted with reasonable accuracy.
Of the red stars observed by Hertzsprung, the dwarf stars also followed the spectra-luminosity relationship discovered by Russell. However, the giant stars are much brighter than dwarfs and so don't follow the same relationship. Russell proposed that the "giant stars must have low density or great surface-brightness, and the reverse is true of dwarf stars". The same curve also showed that there were very few faint white stars. This name reflected the parallel development of this technique by both Hertzsprung and Russell earlier in the century.
A refined scheme for
stellar classification was published in 1943 by W. W. Morgan and P. C. Keenan. The MK classification assigned each star a spectral type—based on the Harvard classification—and a luminosity class. For historical reasons, the
spectral types of stars followed, in order of decreasing temperature with colors ranging from blue to red, the sequence O, B, A, F, G, K and M. (A popular
mnemonic for memorizing this sequence of stellar classes is "Oh Be A Fine Girl/Guy, Kiss Me".) The luminosity class ranged from I to V, in order of decreasing luminosity. Stars of luminosity class V belonged to the main sequence.
Characteristics
Main sequence stars have been extensively studied through stellar models, allowing their formation and evolutionary history to be relatively well understood. The position of the star on the main sequence provides information about its physical properties.
The
temperature of a star can be approximately determined by treating it as an idealized energy radiator known as a
black body. In this case, the luminosity
L and radius
R are related to the temperature
T by the
Stefan-Boltzmann Law:
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where
σ is the
Stefan–Boltzmann constant. The temperature and composition of a star's
photosphere determines the energy emission at different wavelengths. The
color index, or
B −
V, measures the difference in this energy emission by means of filters that capture the star's
magnitude in blue (
B) and green-yellow (
V) light. (By measuring the difference between these values, this eliminates the need to correct the magnitudes for distance.) Thus the position of a star on the HR diagram can be used to estimate its radius and temperature. By modifying the physical properties of the
plasma in the photosphere, the temperature of a star also determines its
spectral type.
Formation
When a
protostar is formed from the collapse of a
giant molecular cloud of gas and dust in the local
interstellar medium, the initial composition is homogeneous throughout, consisting of about 70% hydrogen, 28% helium and trace amounts of other elements, by mass. During the initial collapse, this
pre-main sequence star generates energy through gravitational contraction. Upon reaching a suitable density, energy generation is begun at the core using an exothermic
nuclear fusion process that converts hydrogen into helium. the star lies along a
curve on the
Hertzsprung-Russell diagram (or HR diagram) called the standard main sequence. Astronomers will sometimes refer to this stage as "zero age main sequence", or ZAMS. This is a line calculated by computer models of where a star will be when it begins hydrogen fusion; its brightness and surface temperature typically increase from this point with age.
The star remains near its initial position on the main sequence until a significant amount of hydrogen in the core has been consumed, then begins to evolve into a more luminous star. (On the HR diagram, the evolving star moves up and to the right of the main sequence.) Thus the main sequence represents the primary hydrogen-burning stage of a star's lifetime. These main-sequence (and therefore "normal") stars are called dwarf stars. This isn't because they're unusually small, but instead they've smaller radii and are less luminous than the other main type of stars, the
giant stars.
White dwarfs are a different kind of star which are smaller than main sequence stars—roughly the size of the
Earth. These represent the final evolutionary stage of many main sequence stars.
Energy generation
All main sequence stars have a core region where energy is generated by nuclear fusion. The temperature and density of this core are at the levels necessary to sustain the energy production needed to support the remainder of the star. A reduction of energy production would cause the overlaying mass to compress, increasing the temperature and pressure needed for fusion. Likewise an increase in energy production would cause the star to expand, lowering the pressure at the core. Thus the star forms a self-regulating system in
hydrostatic equilibrium that's stable over the course of its main sequence lifetime.
Astronomers divide the main sequence into upper and lower parts, based on the type of fusion process at the core. Stars in the upper main sequence have sufficient mass to use the
CNO cycle to fuse hydrogen into helium. This process uses atoms of
carbon,
nitrogen and
oxygen as intermediaries in the fusion process. In the lower main sequence, energy is generated as the result of the
proton-proton chain, which directly fuses hydrogen together in a series of stages to produce helium.
At a stellar core temperature of 18 million
kelvins, both fusion processes are equally efficient. This is the core temperature of a star with 1.5 solar masses. Hence the upper main sequence consists of stars above this mass. The apparent upper limit for a main sequence star is 120-200 solar masses. The lower limit for sustained nuclear fusion is about 0.08 solar masses. Medium-sized, low mass stars like the Sun have a core region that's stable against convection and a convection zone near the surface. This produces mixing of the outer layers, but a less efficient burning of the hydrogen in the star. The eventual result is the buildup of a helium-rich core, surrounded by a hydrogen-rich region. By contrast, cool, low-mass stars are convective throughout. The helium produced at the core is distributed across the star, producing a relatively uniform atmosphere. The luminosity increase of a star changes its position on the HR diagram; resulting in a broadening of the main sequence band because stars are observed at random stages in their lifetime.
The stars in the main sequence don't lie upon a narrow curve on the HR diagram. This is primarily because of the observational uncertainties that mainly affect the distance of the star in question, but also because of factoring in unresolved
binary stars. However, even perfect observations would lead to a fuzzy main sequence, because mass isn't a star's only parameter. In addition to variations in
chemical composition—both because of the initial abundances and the star's
evolutionary status, the presence of a
close companion,
rapid rotation, or a
magnetic field can also move a star slightly on the main sequence, to name just a few factors. For example, there are stars with a very low abundance of elements with higher atomic numbers than helium—known as
metal-poor stars—that lie just below the main sequence. Also known as
subdwarfs, these stars are also fusing hydrogen in their core and so they mark the lower edge of the main sequence's fuzziness due to chemical composition.
A nearly vertical region of the HR diagram is known as the
instability strip and is occupied by pulsating
variable stars. These stars vary in magnitude at regular intervals, giving them a pulsating appearance. The strip intersects the upper part of the main sequence in the region of class A and F stars; between one and two solar masses. However, main sequence stars in this region experience only small variations in magnitude and so are hard to detect.
Lifetime
The lifespan that a star spends on the main sequence is governed by two factors. The total amount of energy that can be generated through nuclear fusion of hydrogen is limited by the amount of available hydrogen fuel that can be consumed at the core. For a star in equilibrium, the energy generated at the core must be at least equal to the energy radiated at the surface. Since the luminosity gives the amount of energy radiated per unit time, the total life span can be estimated, to first approximation, as the total energy produced divided by the star's luminosity.
Our
Sun has been a main sequence star for about 4.5 billion years and will continue to be one for another 5.5 billion years, for a total main sequence lifetime of 10
10 years. After the hydrogen supply in the core is exhausted, it'll expand to become a
red giant and fuse
helium atoms to form
carbon. As the
energy output of the helium fusion process per unit mass is
only about a tenth the energy output of the hydrogen process, this stage will only last for about 10% of a star's total active lifetime. Thus, on average, about 90% of the observed stars will be on the main sequence.
On average, main sequence stars are known to follow an empirical mass-luminosity relationship. The luminosity (
L) of the star is approximately related to the total mass (
M) as the following
power law:
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