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Physical Science 1421 Department of Physics and Geology Star Cycle
Background
“ Star Cycle ” or “ life cycle of a star ” refers to the different stages a star goes through during its existence. A star is born from the collapse of an interstellar cloud. A star matures, grows old, and either explodes or slowly dies out. Matter left behind by dying stars will become part of the interstellar medium again, and in turn, will later form new stars. In this manner the abundance of “ heavy elements ” in the interstellar medium is increased (note that in Astronomy “ heavy elements ” refers to all elements other than hydrogen and helium).
This increase in the abundance of heavy elements due to successive stellar cycles is important in that in our current understanding of the Universe , shortly after the Big Bang, the matter of the Universe was composed of hydrogen, helium, and just traces amounts of lithium. The heavy elements were created later through nuclear fusion during the stellar cycles. Thus Earth, and ourselves, are largely made up of recycled stellar material.
Stage 1: Collapse of Interstellar Cloud
The life of a star starts out as a cold and dense interstellar cloud or interstellar nebula (the term “ nebula ” means a cloud in space). The cloud needs to be dense enough and cold enough so that gravity will overcome pressure. Interstellar clouds are made up of mostly of hydrogen and helium, with heavy elements present in significantly smaller amounts.
When gravity overcomes pressure, the cloud will begin to shrink in size (collapses). As it shrinks it not only increases its density, but it also loses gravitational energy. This gravitational energy in turn is converted into thermal energy , increasing the temperature of the cloud. Thus as the interstellar cloud collapses it becomes denser and hotter.
As it collapses, due to conservation of angular momentum , the cloud will also begin to spin faster.
Stage 2: Protostar
When the interstellar cloud becomes dense enough so that light cannot escape directly from its interior, but rather has to scatter (“bounce its way out”), the interstellar cloud becomes a “ protostar ”. When the collapsing interstellar cloud becomes a protostar , it will tend to heat up faster as it continues collapsing since energy can only be radiated away at its surface.
Due to conservation of angular momentum , a spinning disk of gas and dust will be formed around the protostar. This spinning disk in turn could give birth to planets.
When the core of the protostar reaches sufficiently high temperatures to initiate the nuclear fusion of hydrogen to helium, it becomes a star.
End Stage 3B - Brown Dwarf
If a protostar does not have sufficient mass to generate high enough temperatures to initiate nuclear fusion , the protostar will become a brown dwarf or “failed star”. That is, since the brown dwarf does
not generate energy through nuclear fusion , it is not a star. To become a star a mass between 0. Msun and 150 Msun is needed. If the mass of a protostar is less than 0.08 Msun it will become a brown dwarf.
Brown dwarfs radiate energy due to their temperatures, and as they do so they cool down. A brown dwarf does not shrink because of the balance between electron degeneracy pressure and gravity. Electron degeneracy pressure does not depend on temperature and thus the size of the brown dwarf will stay the same as it cools down. Electron degeneracy pressure is a quantum mechanical effect that comes into play at high densities. Just as electrons in an atom can only have specific energy levels, quantum mechanics places constraints on how closely together electrons can be packed in a gas.
Stage 3LH - Main Sequence Star
When the core of a protostar becomes hot enough to initiate nuclear fusion of hydrogen to helium, it becomes a star. A star at this stage (that of fusing hydrogen to helium in its core to produce energy) is called a “ main sequence ” star. As already stated above, to become a star, a protostar has to have a mass greater than about 0.08Msun.
Also note that if the mass of the protostar is too large (greater than about 150 Msun), it will produce so much energy that it will literally blow itself apart. Evidence for this comes from mathematical/theoretical models and from the fact that to date a star of mass greater than 150 Msun has not been detected. Thus to become a star , the protostar must have a mass somewhere between 0. Msun and 150 Msun.
The time it takes for an interstellar nebula to become a star depends on the mass of the star. A star of mass 30 Msun (thirty times the mass of our Sun) will go from nebula to star in a few tens of thousands of years. A star with a mass similar to our Sun will have taken a few tens of million years to go from cloud to main sequence star. A star with less mass than our Sun can take up to a billion years or more to reach main sequence stage from the initial nebula.
A main sequence star will have a stable size since gravitational equilibrium is achieved; that is once a star reaches the main sequence the pressure generated by the nuclear fusion reactions in its core is balanced by gravity, just as in our Sun.
The size of a main sequence star will depend on its mass. The larger the mass the larger the size of a main sequence star. Also, the temperature (and therefore spectral type and color) of a main sequence star will also depend on its mass, the larger the mass the higher the surface temperature.
Note that a higher mass implying a larger size and a higher temperature will necessarily hold true only if the stars considered are main sequence. For example, a white dwarf (which is not a main sequence star ) can have a mass lower than our Sun but have a higher surface temperature.
Once a main sequence star has consumed all its hydrogen fuel in its core, it will leave the main sequence stage. The time a star remains in the main sequence stage depends on its mass. The larger the mass, the more luminous the star , the more rapid the rate of conversion of hydrogen to helium, the shorter the life time on the main sequence. For example a star of 30 Msun will remain in the main sequence for about 5 million years, and a star with a mass similar to our Sun will remain in the main sequence for about 9 billion years.
The stages of the stellar life cycle that follow after the main sequence depend on the mass of a star. Stars are typically classified into two types depending on their mass: low-mass stars and
The luminosity of the star begins to increase again in this stage, its photosphere expands, and it surface temperature begins to decrease again.
Stage 7L – Planetary Nebulae
A low-mass star does not have sufficient mass to generate the temperature necessary to burn carbon. When the carbon core of the star stops shrinking due to degeneracy pressure , energy sources for the star will have been consumed, and the star will begin to eject or “blow away” its outer layers.
The stage where the outer layers of the low-mass star are ejected is called “ Planetary Nebulae ”. The name of stage comes from the fact that the outer layers of star begin to form a cloud that looks like a planet with the hot core at its center.
The outer layers ejected from the star will continue moving outwards until eventually they become part of the interstellar medium.
End Stage 8L – White Dwarf
Once the ejected outer layers of the low-mass star become part of the interstellar medium , its hot core is left behind and the low-mass star has entered in the White Dwarf stage. A white dwarf star is thus the leftover core of a low-mass star.
A white dwarf star is similar to a brown dwarf star in that its size is sustained by electron degeneracy pressure and its radiation is directly due to its surface temperature. That is, as the white dwarf emits radiation it’s cooling down.
However since the white dwarf star was originally the core of a low-mass star , its surface temperature will tend to be much higher than that of a brown dwarf.
When a white dwarf cools down to a point where it no longer emits significant amount of light it’s called a “ black dwarf ”.
High Mass star
A high-mass star is star of about 8 Msun or greater (equal to or greater than eight solar masses). Nuclear fusion of hydrogen into helium in its core occurs in a high-mass star through the CNO cycle (rather than the proton-proton chain as in low-mass stars). The CNO cycle has the same net effect as the proton-proton chain (that of converting four protons into a single helium nucleus and energy), however the CNO cycle requires a higher temperature and thus does not occur in low-mass stars at main sequence stage.
An important difference between the CNO cycle and the proton-proton chain is that the CNO cycle burns hydrogen in a star’s core at rate far higher than the proton-proton chain could. As a consequence of this the luminosity of high-mass stars are much greater than those of low-mass stars.
When the hydrogen fuel is consumed in the star’s core, the high-mass star leaves the main- sequence stage and enters the following stages (that we discuss below): Supergiant Stage, Mutliple Burning Shell Stage, Supernova Stage, and depending on the amount of remaining mass, the high- mass star will enter either the Neutron Star Stage or the Black Hole Stage.
Stage 4H– Supergiant
When the hydrogen at the core of a high-mass main sequence star is consumed, the star is no longer at gravitational equilibrium , and the core of the star begins to shrink and its photosphere begins to expand. The material remaining in the core is helium which requires a higher temperature to burn.
In this stage nuclear fusion is not occurring in the high-mass star’s core, but just as in the low- mass Red Giant stage, nuclear burning occurs in a hydrogen shell around the helium core. In this stage the total luminosity of the star increases that is the energy rate production of the hydrogen burning shell is higher than that of the hydrogen burning core when the star was in the main sequence. The radius of the photosphere increases in this stage, however the surface temperature decreases.
Note that in the Supergiant stage the luminosity increases even though the surface temperature decreases. Also note that in this stage the core temperature increases, and that it’s the surface temperature that decreases (just as in the low-mass Red Giant stage).
Stage 5H – Multiple Burning Shell
When the core of a supergiant becomes hot enough to burn helium, the star leaves the supergiant stage. However, unlike low-mass stars that do not have further nuclear burning in the core after the helium is consumed, high-mass stars have sufficient mass to generate temperatures capable of igniting the nuclear burning of carbon into oxygen. The nuclear carbon burning core will have helium burning shell above it and a hydrogen burning shell above that.
Once the carbon is consumed in the core, the star’s core collapses until nuclear burning of oxygen into neon occurs in the core. At this point the oxygen burning core has a carbon burning shell around it, that has a helium shell around it, that in turn has a hydrogen burning shell around it; thus the name “ Multiple Burning Shell ” for this stage.
The process continues as heavier and heavier elements are produced in the core and as more layers of burning shells are added around the core, until an inert iron core is produced. Iron has the property of being the element with lowest mass per particle in its nucleus, and as such cannot produce energy through nuclear fusion nor through nuclear fission.
Stage 6H– Supernova (Type II) Once the Multiple Burning Shell stage of a high-mass star produces an inert iron core, nuclear fusion in the core ceases, and the core collapses from gravitational forces. The gravitational forces at this point are so strong that electron degeneracy pressure cannot stop the shrinking, and the collapse continues until neutron degeneracy pressure balances gravity. At this point the electron and protons have come together to form neutrons and the size of the core is a few kilometers in size with densities comparable to the density of an atomic nucleus. The collapse of the stellar core to a size of just a few kilometers liberates an enormous amount of energy that drives the outer layer of the star into space through a titanic explosion called a supernova ( Type II). For about a week a supernova can emit at an energy rate equal to 10 billion Suns (about the same as a moderate size galaxy). The ejected outer layers may remain visible for thousands of years and are called “ supernova remnant ”. These outer layers of the star ejected into space contain heavy elements and will eventually become part of the interstellar medium. Depending on the amount of mass in the star’s core, once the outer layers of the high-mass star have been ejected, the leftover core will become either a neutron star or a black hole.
