Star Lifecycles

Overview - Stars are born, live out their lives, and die

I want to give you a very brief overview of the rest of the Unit on Stars. In the lifetime of any star, it is the mass that determines its fate. The more massive the star, the shorter the lifespan, the more rapid the consumption of nuclear fuel, and the more exotic and spectacular the death. Large stars can evolve in their death stages and blow up, leaving behind tiny white dwarfs, tinier neutron stars, and impossibly tiny black holes. The less massive the star, the longer the lifespan, the slower the consumption of nuclear fuel, and the more boring the death. Very lightweight stars, in fact, never evolve into real stars at all, and simply exist as "Jupiters." We will look at the average star first, and compare all of the other star types to it. There is a lot of information packed into this section, and I do not expect anyone to get all the way through it. However, if you can learn the basics of star lives, I will be happy, and then if you want to learn more simply to satisfy your thirst for knowledge, then dig deeper into the links within this unit.

Mass, Pressure and Energy

A star's mass is the most important quantity that determines its lifetime. Paradoxically, the more massive a star the shorter its lifetime. Stars are extremely hot at their centers, where the energy is produced by thermonuclear fusion ... the same process that occurs when a hydrogen bomb explodes. Yet stars can remain stable. Why? Because the enormous outward pressure, produced by the thermonuclear fusion, is balanced by the inward force of gravity. It is gravity, as it were, that keeps the "lid" on the star and prevents it from exploding.
This balance is called hydrostatic equilibrium. If the force of gravity becomes stronger than the gas pressure from fusion, the star will contract; if the pressure from fusion becomes stronger the star will expand. Since gravity acts as a force toward the center, and fusion is an expanding force from the center, the stars assume a spherical shape, and the size of the star is determined by the interplay between these two forces.
Sooner or later, every star will exhaust the fuel at its center. When this happens the star will change, because it will no longer be in hydrostatic equilibrium.

Energy Transport

The energy produced in the star's core can reach the surface by: conduction, convection and radiation.
Conduction: energy moves from place to place as heat (motional energy), without the movement of material.
Convection: energy is transported from one place to another by the movement of material.
Radiation: energy is transported by light. To see this in our Sun, go to the website.

Stellar Models

We cannot actually follow the evolution of a star, because they live far too long compared to human timescales. Instead we see stars at different stages of their evolution, as represented on the H-R diagram. The only known way to follow the evolution of stars is to simulate the evolution of stars on a computer. A stellar (star) model is a complex calculation in which scientists try to describe the behavior and properties of stars using the best current understanding of the principles of physics. The models are used to make predictions about stellar characteristics that can be measured. That way we can check the validity of the stellar models.
With these models we can trace the evolution of stars and study how the mass, temperature, luminosity and chemical composition of stars change with time. When we plot, as a function of time, the luminosity versus the surface temperature of a stellar model we get an evolutionary track in the H-R diagram. Much of our understanding of how stars evolve is based on the study of stellar models whose predictions have been checked against observations.

Clusters Reveal Much


Important clues about stellar evolution can also be had by studying star clusters, such as M55 seen in the image to your left. The basic assumption made is that the stars in a star cluster were created at about the same time, and began with the same chemical composition. If one observes variations in the chemical composition of the stars in a star cluster this must be due to stars being at different stages of their evolution.

 

 


When stars are born as part of a star cluster, about the only variable that can vary across the cluster are the stellar masses: the cluster will have stars of differing mass and, therefore, right from the start, stars of differing luminosity. This means that a cluster that is very young will be composed of ALL of the star classes of the HR Diagram. Astronomers have looked at enough different clusters to discover that their appearance matches the prediction. Older clusters have no O or B class stars. The oldest clusters do not even have A, F, or G class stars. This is powerful evidence that the hot stars of the upper end of the main sequence of the HR Diagram die first.

The Pleiades, with its B stars, is about 100 million years old; the Hyades, with no B stars, is closer to a billion years. Open cluster ages range from near zero (those filled with O stars) to about 10 billion years (those filled with nothing by G,K, and M stars). Since open clusters make up much of our Galaxy's disk, we date the disk to be about the same age as the oldest open clusters.

 

 

 

Globular clusters, however, ALL have stars missing above the cooler G subclasses, and therefore average about 13 billion years in age. They are the oldest known star system, and since they occupy a position in the Galaxy's halo, that too must be the age of the halo, and perhaps it means that the galactic halo came before the galactic disk!

 

The four images above show the difference between Open Clusters (M35 and M46 on the left and right atop respectively) and Globular Clusters (M13 and M4 left and right respectively below). The left and right top images are courtesy of Alson Wong, while the bottom left comes from Yuugi Kitahara, and the bottom right from the NOAO/AURA. M35 is guessed to be about 300 millions years old, M46 at 100 million years, and M4 and M13 at nearly 13 billion.

To see a recent press release where a cluster of sun-like stars is teaching us about the evolution of stars, go to the NGC 2420 star cluster site.

Energy Generation in a Sun-Like Star

Hydrogen Fusion
Stars like the Sun, with a core temperature of about 15 million K, generate energy by fusing four hydrogen nuclei to one helium nucleus. This is called the proton-proton (PP) reaction. At some point all the hydrogen in the star's core will be converted into helium. Without outward pressure from thermonuclear fusion, the core collapses under gravity. The gravitational collapse raises the internal pressure which, in turn, raises the internal heat.
Helium Fusion
If the star's internal temperature can reach about 100 million K, another reaction can happen: the triple-alpha reaction. In this reaction three helium nuclei fuse to form one carbon nucleus. It is this reaction that has produced the carbon in our bodies. At this point, a new type of thermonuclear fusion occurs and the outward pressure pushes against the inward gravitational pressure and the star may expand. Additionally, at the boundary between the helium-burning core and the next outer layer of hydrogen, thermonuclear fusion of hydrogen to helium may occur, creating further outward pressure. The star may expand in size. This expansion will spread out the photosphere and result in a cooler surface temperature. According to Wein's Law, the lowered temperature will cause the surface to appear more red. Voila! You may then see a Red Giant. These words are repeated later in this text, but serve here to help guide you through a star's life.
Carbon Fusion
Following the fusion of core helium into carbon, the core may collapse further, thus driving internal core pressures and temperatures to even greater extremes. If the star's internal temperature can reach about 600 million K the star can synthesize elements heavier than carbon. At higher and higher temperatures a star can create heavier and heavier elements. With each successive fusion/exhaustion/collapse/reignition/ sequence, layers of different forms of fusion occur and the star may get larger and produce more energy. At some point the star produces iron at which point this nucleosynthesis (creation of nuclei) ceases. Iron is the element in the periodic table which absorbs energy upon its fusion, and thus represents the endpoint in a star's life.


Main Sequence

This is a beautiful photograph of the interior of the Trifid Nebula. This giant cloud of dust and gas lies in the Sagittarius region of the Milky Way and is easily visible with a pair of binoculars on a dark summer night ... even in Minnesota. To me, part of the image looks like the head of a snail with two eye stalks. Now, I am quite partial to snails since my doctoral thesis was written about one particular species, so I see what I choose to see in this image while you may see something completely different. The purpose of this picture is to get you focused on these "eye" at the tips of the "stalks." Those are regions of active star formation, and the subject found briefly below.

 

This incredible NOAO/AURA photo from the southern Gemini telescope in Chile shows the Eagle Nebula as never before seen at this level of clarity. Earlier in the course, in the section "Tour of the Universe," you saw the famous Hubble Space Telescope photo of M16 ... the gaseous nebula at the heart of the Eagle Nebula where new stars are being formed. Below left is a look at the larger region of Aquila, and to the right below is a close-up of that same area. I put these pictures in here because they are so incredible. If you want, try clicking on the image to your lower left for a larger version of this tremendous picture, and a few more close-ups of the pillars.


Stars enter the main sequence on the HR Diagram when the nuclear fusion (often called nuclear burning) supplies enough energy, and therefore pressure, to stop further gravitational compression. The Sun is currently in its main sequence phase. Stellar models of the Sun predict that the Sun may have taken about 30 million years to reach this stage from its protostar stage (refer to image at your left). Main sequence stars convert hydrogen to helium. As noted above, how a star evolves depends upon its mass.

 

 

 

 

 

It is important for you to grasp from the image above is the realization that ALL stars begin their lives in the same fashion. A star is "born" from a giant cloud of gas and dust that somehow begins collapsing under the inward force of gravity. Funny, but gravity is the cause of the star's birth and ultimately the cause of the star's death.

The material in the collapsing cloud condenses as the radius of the cloud decreases, and the pressure of particles in the interior rises dramatically. With increased pressure comes increased temperature. Eventually, the pressure will become so great that temperatures soar to 7 million K, and this is the ignition temperature for Hydrogen fusion. At the point when fusion first begins and gamma rays first escape, the star is born, and it occupies a spot someplace on the main sequence of the HR Diagram, a spot whose location is dependent on the starting mass.

The two charts below depict the basic life of a star at its most stable stage. As you will be reminded repeatedly in this course, it is the mass of the star that determines its fate. The more massive the star, the greater the gravitational compression, the hotter the interior, and the more outward pressure. The less massive the star, the less gravitational compression, the cooler the interior, and the less the outward pressure. Throughout the life of all stars, it is this ceaseless interplay between gravity's crushing force and thermonuclear fusion's explosive force that cause the star to be spherical in shape and stable in a size dictated by the mass. The lower left diagram shows this balancing act in words called, "Hydrostatic Equilibrium." The HR Diagram to the left shows where stars on the main sequence are, and demonstrates the difference in spectral classes that is a direct result of the effect of mass on inward pressure and outward luminosity. Change the mass or change the thermonuclear fusion, and the star will change, and you will learn more about this later in this course.

Main Sequence Lifetime Formula


Every year, the Sun radiates energy equivalent to a mass of about 2 x 10e19 Kg. According to stellar models of the Sun (called solar models) it will leave the main sequence when it has used up about 1/10th of its total mass. Therefore, we can estimate how long a star, like the Sun, will remain as a stable main sequence star if we know how much fuel it has available (which is related to its mass) and the rate at which it uses that fuel (which is determined by its luminosity).
Let us assume that a star that is M times heavier than the sun will have M times as much fuel available. By definition, a star whose luminosity is L times that of the sun the star is using up its fuel L times faster.

 

 

 

If t is the length of time a star will remain on the main sequence then
t = Fuel Available/Rate of Fuel Consumption
= M x (1/10) x (2 x 10e30 kg) / L x 2 x 10e19 kg/year
= 10e10 x (M/L) years
For the Sun we have M = 1 and L = 1; so t = 10e10 years!


Mass-Luminosity Relation


The larger the mass of a star the greater the gravitational compression at its core and therefore the hotter it is. The hotter the core the greater the rate of fusion reactions. More fusion reactions means a larger energy release per second, that is, luminosity. Thus we expect the luminosity to increase with mass.
Indeed, that is what is observed. By measuring the mass and luminosity of a large number of stars it has been found that L and M (when measured relative to the luminosity and mass of the Sun) are approximately related as follows
L = M^3.5


Low-Mass and Medium-Mass Stars


Eventually, all the hydrogen in the core of a star will be transformed into helium. The enormous pressure of the surrounding material causes the helium core to shrink, thereby increasing the core's temperature and that of the material immediately surrounding the core.
In fact, the material surrounding the helium core becomes hot enough for hydrogen to start fusing into helium. This is called shell hydrogen fusion, because the hydrogen fusion reactions occur in a shell about the core. As more and more helium sinks into the core the latter continues to shrink and heat up.


Meanwhile, the hydrogen fusion shell expands away from the core causing the outer layers of the star to heat up and therefore expand. As the outer layers expand they cool, thus turning red. A RED GIANT is formed. Betelgeuse, the right shoulder of Orion is such a Red Giant, and is the first star whose disk has been photographed (left).

 

When this happens to the Sun it will probably engulf Mercury and perhaps Venus. The Earth will, for a while, orbit through the Sun's atmosphere at its present velocity. But gradually, due to the friction between the Earth and the Sun's atmosphere, the Earth will spiral in towards the Sun and be vaporized.
Degenerate Gas. After about 1 billion years the star shrinks until the temperature of the core is hot enough (about 100 million kelvin) to cause helium to fuse to carbon. But to reach such enormous temperatures the gas within the core must become highly compressed; so compressed in fact that the electrons are forced to fill all the available energy states up to some maximum level.
When all the lowest available energy states are occupied the gas is said to be degenerate. The electrons are so compressed that they are unable to change their speeds: they can't slow down because all the lower energy states are occupied; they cannot speed up because to get to the higher unoccupied energy levels would require the input of an enormous amount of energy.
A degenerate gas is one in which the heating of the gas does not raise the pressure of the gas. The pressure cannot increase because of the difficulty in speeding up the electrons.
Because the pressure does not rise, even as the core is heated, no expansion occurs. Within a matter of seconds the core grows hotter and hotter and fuses helium to create heavier elements. This explosive destruction of helium is called the Helium Flash.
The star becomes a variable star as the outer layers are periodically re-heated and cooled. This causes the outer layers to pulsate. The star pulsates with a period of between 200 and 600 days and loses matter from its surface in the form of powerful winds that seed space with the newly formed carbon and other elements, thereby setting the stage for the birth of the next generation of stars and planets. See the Ring and Dumbbell Nebulae in the summer sky to witness the results of this form of star death.


After the helium flash the star becomes again a red giant and then cools to a WHITE DWARF. Such a white dwarf occupies the center of the Ring Nebula (M51) which is visible with a small telescope in the constellation Lyra, or visible above these lines in this stunning Hubble Space Telescope image. The expanding gas from the dead star is seen in this beautiful image to your left.

High Mass Stars

Stars whose mass is greater than the theoretical Chandrasekhar Limit of 1.44 Suns end their lives in a different manner than White Dwarf types. When the hydrogen in the core of a more massive star is used up the helium core shrinks, the temperature of the core increases and shell hydrogen fusion occurs. The luminosity increases, the star expands thereby cooling its outer layers. The star becomes a red giant. (The shell of fusing hydrogen is called a a shell source.) Meanwhile, the core contracts, but usually does not become degenerate.
Because the star is so massive helium, in the core, begins to fuse with only a little extra compression, to form carbon. If the core does, however, become degenerate before carbon fusion a runaway increase in temperature can occur, similar to what happens in the helium flash of low to medium mass stars, causing an explosive destruction of carbon in what one could call a "carbon flash", but which is usually referred to as a carbon detonation. This is a much more violent explosion than that of the helium flash. Carbon fusion creates oxygen and neon.


The core burns ever more furiously creating heavier and heavier elements (magnesium, sulfur, silicon) eventually reaching iron (Fe) created by silicon fusion. When that stage is reached the massive star is doomed. The nuclear reactions that produce iron are endothermic, that is, the reactions absorb heat and thus cool the core. The cooling reduces the pressure in the core, which pressure is insufficient to hold off the crushing force of gravity. In a split second the core collapses and triggers the most powerful explosions in the universe, called supernovae. Such a supernova event was witnessed and recorded by the Chinese in 1054. Today that object looks like the image to your left ... the Crab Nebula. Within these explosions all the elements heavier than iron are created. The shock waves from these explosions can trigger star formation as the shock compresses nearby molecular clouds. This is really cool! Imagine my wedding ring. It is made of gold. You have just learned that stars cannot manufacture elements heavier than iron in their cores because fusion of iron is endothermic. Yet there is gold in them hills, which was mined and molded into my ring. Gold, silver, platinum, copper, lead, uranium are all heavier elements than iron, yet are not made in a star's lifetime. ONLY when a massive star goes supernova and the energy of the explosion reaches trillions of Kelvins are the heavier elements forged, despite the endothermic nature of iron fusion. Shortly thereafter, the supernova's expansion cools and all which remains is debris. If that remaining debris cloud recollapses into a new star, and if some of the material happens to collapse into local bodies of planet size, then some of those heavy metals will be locked in the rocks of those newly formed planets. Gold, silver, platinum, copper, uranium and others are all so rare because the forces of nature which create them are short-lived, and supernovae are not all that common. Indeed, my gold ring may be made of elements forged in a massive star which exploded a very long time ago, but whose remains recollapsed to form our sun and this planet. STAR LIVES

From this reading, you should now be aware that stars lives their lives according to the physical laws governed by their masses. The more massive a star, the hotter the nuclear burning and the shorter the lifespan. The less massive a star, the cooler and also slower the nuclear burning and the longer the lifespan. You can take a break and return to Introduction to the Stars, or to the Spectral Classes of Stars. Or you can move forward to a discussion on Main Sequence Dwarf Stars. Just do not go back to the Syllabus as a time like this.


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