Lifecycle of the Sun



The lifecycle of any star is determined by the mass of the star. Our Sun is an average-mass object, and it lives an average life. Stars of lower mass live very boring lifecycles. Stars with a higher mass live very exciting lifecycles. On this page you will learn about the lifecycle of the Sun. The key events in our Sun's life are

1) A giant cloud of dust and gas begins to collapse under the influence of gravity. As the particles of gas move closer to the center, they rub against each other and heat up from the friction. The cloud will begin to glow in infrared, and is called a PROTOSTAR.

2) When gravity causes the center of this cloud to squeeze so tightly together that pressures and temperatures soar, thermonuclear fusion ignites Hydrogen in to Helium, and the object will generate light (gamma rays). A STAR IS BORN.

3) The outward pressure from nuclear fusion is balanced by the inward pressure of gravity, and the star will be round. This balance is called HYDROSTATIC EQUILIBRIUM.

4) When all of the Sun's core supply of Hydrogen has been converted into Helium, nuclear fusion will stop. With no outward pressure, gravity will crush the core smaller and smaller. The pressure and temperature becomes so great that Helium is converted into Carbon. Nearby this Helium-Carbon core, the temperture is hot enough for neary Hydrogen to fuse into Helium. The Hydrogen-Helium shell around the Helium-Carbon core adds up to create tremendous outward pressure. The Sun's outer photosphere expands to tremendous size, engulfing Mercury, Venus, and perhaps even Earth and Mars. The same amount of mass spread out over a larger area results in a lower surface temperture for the Sun's photosphere, and the color drops from yellow to red. The Sun is now a RED GIANT. The Sun is no longer considered to be living, but now is dying.

5) During this Red Giant phase, the shell of Hydrogen will fuse into Helium and collapse inward into the core. There is a brief time when the star shrinks and turns more orange in color. Then a new shell of Hydrogen will fuse into Helium and the star will expand again and turn red. The sudden burst of outward pressure will blow a bubble of stellar matter out into space, as if the star is shedding a layer of skin. The star does this shrinking and expanding event repeatedly. This is called the AGB STAGE (Asymptotic Giant Branch). Stars that have as much as 8 times the mass of our Sun might blow out 7/8 of its matter into space during this stage of death.

6) When all of the core supply of Helium is converted into Carbon, nuclear fusion stops and gravity goes to work again. The core collapses. Pressures soar. However, the mass is insufficient to generate the high temperatures necessary to fuse Carbon into something else. The core will shrink to an Earth-size ball with densities 100,000 to 1,000,000 times greater than the Sun's core at present. Most of the outer gas has been blown into space. The core is called a WHITE DWARF. The shells of gas around the White Dwarf creates a PLANETARY NEBULA.

7) Eventually, the nebular gases dissipate into space and all that remains is a "naked" White Dwarf. This will slowly cool off and die as a BLACK DWARF.


The lifecycle of a star is based entirely on its mass and the law of Gravity that operates on that value. Since gravity works to force all mass toward a center, its is concluded that within a mass of a star, the center of mass is the center of the star, and the place where that mass is condensed to the smallest possible space. Unusual properties of Physics govern the form of material in a stellar core, and we will look into those unusual properties later. For now, the simplest rule is that where gravitational pressure is greatest and thus the packing of material the most dense, the temperature will be the greatest. A star with much mass will have a greater gravitational force operating on its mass, generating greater internal pressures and thus higher temperatures. Higher temperatures means greater kinetic energy of the molecules and increased collision frequency, resulting in a greater release of energy. A star with less mass will have less gravitational force operating on it, resulting in lower internal pressures and lower temperatures. The low mass star will have interior particles at lower energy levels, reducing collision frequency and yielding a lower release of energy. To put it more simply, high mass stars burn hot and energetically, and low mass stars burn cool and less energetically. It may seem that high mass stars ought to live longer owing to their greater amount of material, but it is the low mass stars that live longest because they burn what little material they have more slowly.

Our Sun is an average star, and of spectral class G2V. No one on Earth can life long enough to watch the entire lifecycle of our Sun, so we turn to the stars in space to see those of similar mass and at different stages of their lifecycle to make a theoretical picture of what our Sun's life may have been like and will be. We turn to the HR Diagram for this help.


Our Sun, like all other stars in the Universe begins its life as a cloud of dust and gas. The exact materials in the dust and gas will vary depending on the galaxy type, the location of the cloud within the galaxy, the source of the cloud. Typically, these clouds of material are immense, spanning light years, but are also very sparse, with densities of less than several hundred atoms per cubic centimeter. In the case of our Sun, the cloud of dust and gas must have come from a much older star which blew up and scattered its remains outward (you will learn about this later). Somehow, that cloud, or at least a portion of it, was affected in such a way that the material began to collapse inwardly. With the collapse now under the relentless force of gravity, the dust particles and gaseous elements began to move toward the center of mass and on their journey encountered other particles. The physical bouncing of these particles into each other generated frictional heat. This heat is akin to rubbing your hands together. While this form of heat is invisible to the naked eye, it is visible to an Infrared Camera or telescope. The heat may only be several hundreds of Kelvins, but the widespread scattering of the particles results in a reduction of energy per unit area. The kinetic energy of the particles will be increasing, but they are simply too spread out to be visible. However, the very broad spreading of these particles means an infrared glow over a large area and thus resulting in a higher Absolute Magnitude. This glowing cloud of collapsing material is called a "Protostar" and is indicated within the HR Diagram image, but not on the Diagram itself. This is how our Sun began it life so very long ago.


As the dust and gas matter collapsed further and further toward the center of mass, the collisions became more frequent and the energy release was greater. However, with the reduction in size of the cloud, the energy output was reduced too. Eventually, the material in the center of the cloud mass was squeezed so strongly by gravity that pressures exceeded billions of atmospheres and temperatures soared up to 7 million K. This is the critical temperature, for at this temperature, Hydrogen nuclei can collide with such speed that they fuse. The result of this Hydrogen fusion is the release of Gamma radiation and the star is now generating light of its own. Astronomers define the moment of Hydrogen fusion ignition as the moment a star is born. Ed McMahon may think his show is special, but this stellar show is absolutely fantastic. Our Sun has shrunk to a ball about the size of 1 million earths. The interior core temperature is 7 million K. Hydrogen is being fused into Helium and huge amounts of energy are being released.


Gravity is NOT resting. This force never stops, and if left to its own would crush the Sun into nothing. However, the fusion of Hydrogen into Helium is the same as the events of a hydrogen bomb. The sheer energy of the fusion is making every effort to literally blow up the Sun. The ever-present crushing force of gravity is trying to collapse the Sun into the smallest possible ball. The two forces reach an point where balance is achieved ... a term called Hydrostatic Equilibrium. Inward gravitation pressure is balanced by outward fusion pressure in all directions and the Sun is shaped nice as nice round ball of glowing gas. It looks so tame up there in the sky, but it is anything but tame. In a future part, we will look at the wild events that are happening at the surface of the Sun and what those events portend for life here on Earth, but for now, just look up there at the exploding and collapsing ball of gas and enjoy its warm light (unless it is the dead of winter when you are reading this).


Our Sun has sufficient mass and internal pressures to fuse Hydrogen into Helium for about 10 billion years. The rate of this fusion is 700 million tons of Hydrogen into 695 million tons of Helium per second ... for 10 billion years. Eventually, the entire core of our Sun will be fused Helium. Sure, the majority of the Sun's volume is Hydrogen gas, but only in the core is the pressure and temperature sufficient to fuse that Hydrogen into Helium. When all of the core is Helium, there is no longer any Hydrogen fuel for further nuclear fusion. With no more outward fusion pressure, the core begins to shrink under the patiently waiting force of gravity. However, as the core shrinks, the pressure increases and so too does temperature. At a core temperature of 100 million K, the Helium in the core suddenly ignites a sets of fusion reactions that make Carbon. A great deal more gamma radiation is generated from this fusion than simple Hydrogen fusion, and the outward pressure is very great. In fact, the heat of the interior is so great, that a layer of Hydrogen gas around the Helium-fusing core begins to fuse into Helium, generating even more outward pressure. The gas of the Sun is blown outward, and the Sun grows larger and larger. Gravity eventually balances this expansion, but only after the Sun has enlarged to an object perhaps with a diameter someplace between Venus and Mars. Earth may very well be swallowed up. The Sun still has the same amount of material, but is spread over a much larger surface area, resulting in a more cool "surface." This makes the spectral class of the Sun in the "M" range owing to the cooler Red color. However, the large size of the Sun brings it to the Giant class (III) and its Luminosity and Absolute Magnitude are increased due to the large size. The Sun has become a "Red Giant."

An interesting article appeared in the October, 2002 issue of "Sky and Telescope Magazine" that warned of the demise of the Earth well BEFORE the Sun becomes a Red Giant. Due to more of the core of the Sun being fused into Helium, the core itself will have less atomic nuclei inside, and gravitational pressure will increase. The remaining Hydrogen nuclei will reach high levels of kinetic energy and collide successfully more frequently. Nuclear fusion will happen faster and the Sun will thus burn hotter. The Earth will lose its oceans to massive greenhouse gas effects and then lose those greenhouse gases to the rocks itself and be a barren, seething hot desert. The article, "The Fate of the Earth" has been posted to this course for your reading pleasure and to increase your dread of a hotter summer.


After an estimated billion years or so, the rapid fusion of Helium into Carbon ceases because all of the core of the Red Giant Sun is now Carbon. Sure, there are other lighter elements in the core, but the majority is Carbon. With no more nuclear fuel for fusion, patient gravity again takes over and squeezes the core in a smaller and smaller ball. Pressure and temperature rise, but never enough to achieve 600 million K, at which Carbon can ignite into heavier element fusion. The core cannot reignite nuclear fusion, and the outward pressure is lost. Eventually, the Red Giant Sun will lose its outer envelope, leaving behind a tiny ball of tightly squeezed Carbon. The object is very hot, so its spectral class moves toward the left on the HR Diagram. But the object is very small, a diameter equal to that of the Earth, so the Luminosity and Absolute Magnitudes are significantly low. The Sun has become a "White Dwarf." Our Sun will be very, very small, and yet very hot, with an expanding shell of released gas flying out away from it. The expanding shell of gas and inner White Dwarf comprise a "Planetary Nebula."

*There is more to this story than I am placing into this account, and you will be able to see more details, including the Sun's second rise up the HR Diagram ... an event not shown here. You can see more in the Cool Red Giant page, or the AGB page.

To the right is a Hubble Telescope image of the famous Ring Nebula. This faint ring is seen directly overhead in summer as part of the constellation Lyra. In the center of the dust and gas cloud is a White Dwarf. This is the presumed fate of a star like our Sun. It sure is pretty to look at, but who cares anyway. All of humanity is long gone in the death throes of our future Sun. The inner planets are absorbed and vaporized by the expanding Red Giant. The outer planets and comets are blow clear out of here by the fierce stellar winds from the Red Giant that give rise to the nebulosity. No one here will be around to watch the fine site you see at your left, but remember that you need not fear. The Sun is expected to live for a long time yet (another 5 billion years) before disaster sets in.







Finally, the White Dwarf cools over a long period until it is nothing but a black stellar corpse, called a "Black Dwarf." The entire life of the Sun has been portrayed in the HR Diagram. Stars that are on the Main Sequence line are those that burn Hydrogen into Helium. All of the other dots correspond to stars in one or another stage of their lives.





Now ... what about this black dwarf corpse? Imagine an Earth-sized ball of hot Carbon. If you have never taken a Paleontology course, you might not grasp the significance of this event. When Carbon is squeezed under great pressure and heated to a high temperature, it changes its form into something VERY hard and crystalline ... Diamond. The White Dwarf is literally a form of a giant diamond in the sky. Yes, it has impurities from some of the lighter elements on the Periodic Table, but it is still a pretty big diamond.


Please move to "Diamonds are a Girl's Best Friend."

This complete our study of the lifecycle of the Sun. Before you run into your basement and await the catastrophe of our Sun's death, you still have a little time left in your measly existence to go ahead to Features and Events on the Sun, or return to the Introduction to the Sun, or to the Syllabus.

| Home | Course Information | Assignments | Teacher Bio | Course Units | Syllabus | Links |