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
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
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
BASIC GOVERNING PHYSICAL PROPERTY OF ALL STARS
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.
FROM GAS CLOUD TO PROTOSTAR
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
A STAR IS BORN
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.
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
THE SUN BECOMES A RED GIANT
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.
THE SUN BECOMES A WHITE DWARF
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,
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
*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
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.
BLACK DWARF AND FINAL DEATH
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.
A HUMOROUS LOOK AT A WHITE DWARF
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.