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STELLAR FORMATION AND EVOLUTION
To be able to understand the life cycle of a star, you need a good understanding of the
Hertzsprung-Russell Diagram. In 1905, two astronomers, Einar Hertzsprung and Henry Norris
Russell, independently plotted the temperature of a star versus the brightness of the star. They
did this with a large number of stars and noticed there were distinct groups. Hot stars inhabit
the left hand side of the diagram, cool stars the right hand side. Bright stars at the top, faint
stars at the bottom. Our Sun is a fairly average star and sits near the middle.
Most of the stars in the diagram fall on a curve that we call the main sequence. This is a
region of young stars. As time goes on, stars change or evolve as the physics in their cores
change. But for most of the lifetime of a star, it sits somewhere on the main sequence.
We will begin by looking at what happens to most stars in the universe. These are the
low mass stars. In space, there are enormous clouds of glowing gas called molecular clouds or
nebulae. These nebulae are millions of times bigger and brighter than any single star. They
are extremely cold and dark. To observe them, astronomers use IR and radio telescopes.
Pronounced dark areas can often be found in nebulae. For many years, astronomers
believed that these dark areas were holes or voids in space where there were no stars, gas or
anything. Today we realize that cool clouds of gas and dust frequently block background stars
and nebulosity.
The first stars in the universe began star formation as denser parts of the cloud
collapsing under their own weight and gravity. Today many astronomers believe that waves
from a supernova explosion rippling through a galaxy compress the interstellar gas and dust.
Lumps form in these locations where the waves have piled up the gas eventually resulting in
the formation of stars.
It is reasonable to suppose that the nebula is not perfectly smooth but contains some
lumps. In other words, there are places in the nebula where a little extra gas and dust has piled
up. A lump is a lump because it contains a little more gas and dust than the surrounding region.
But because the lump has a little extra matter, it must also possess a slightly higher than
average gravitational field. Due to the lump's greater gravity, it will attract some nearby gas
and dust in the nebula. As the lump gathers gas and dust, its mass and gravity increase. This in
turn allows the lump to attract still more gas and dust from the surrounding areas. The bigger it
grows, the more it attracts; the more it attracts, the bigger it grows. By this self-perpetuating
process, called gravitational accretion, a small lump can grow into a substantial object
containing many solar masses of interstellar material. These objects, called globules, are
thought to be the embryos of stars.
Globules are very hard to find. They are much smaller, darker, and denser than the dark
areas from which they form. Globules can only by seen if they happen to be situated in front of
a bright nebula. The smallest globules are only a trillion miles in diameter, roughly a hundred
times bigger than our entire solar system. Globules are the birthplaces of stars. Because
trillions upon trillions of tons of gas are pressing inward from all sides, the globule is unstable.
Unable to support the weight of layer upon layer of gas, the globule begins to contract fairly
rapidly as gravity pulls in material.
Soon pressures and densities at the globules center become so great that contraction at
the center is slowed. Once a globule has broken free from the other parts of the nebula and it
has created a fairly stable core, a protostar is created. As the gas is compressed at the core
under the influence of gravity, the temperature of the core increases.
Stellar Formation and Evolution - 1
The temperature inside a globule may start off at 50 degrees Kelvin, or even cooler. But
as the globule contracts to form a protostar, the temperature at the protostar's core may rise as
high as 150,000 degrees Kelvin. Quite naturally, as the gas is compressed under the influence
of gravity, the temperature increases. Momentum turns the irregular clump into a rotating disk.
The central region is denser and forms the protostar. The nebular disk forms slower to become
a planetary system.
As the temperature and pressure in the center begin to increase, the pressure from the
core stops the infalling of gas into the core and the object becomes stable as a protostar. If a
protostar forms with a mass less than 1/10 solar masses, its internal temperature never reaches
a value high enough for thermonuclear fusion to begin. This failed star is called a brown dwarf
(although it actually is red), halfway between a planet (like Jupiter) and a star.
Finally, the pressures, temperatures and densities at the center of a slowly contracting
protostar have become so great that the nuclei of hydrogen atoms can be fused. When the
temperature in the core has risen to about 3 million degrees Kelvin, hydrogen nuclei are packed
so tightly and are moving so rapidly that they can collide and stick together. As core
temperatures rise with gravitational pressure, the star must have a minimum mass: about 75
times the mass of Juptier of about 1/10 the mass of our sun.
The ignition of this thermonuclear reaction, called hydrogen burning, is the final event
in the birth of a star. The protostar no longer has to rely on gravitational contraction for its
source of thermal energy. Thermonuclear fusion now provides the energy. Contractions stops.
A star is born.
In following theoretical stars along their travels, one important fact emerges. The more
massive the star, the more rapidly it evolves. High-mass stars very rapidly build up the
necessary temperatures and pressures to ignite hydrogen burning at their centers and therefore
take a short time to get to the main sequence. Low-mass stars take a much longer time.
A main sequence star’s mass determines all of its structural properties. The three
divisions in a star’s interior are the nuclear burning core, convective zone, and radiative zone.
The energy released from the stellar core heats the stellar interior producing the pressure that
holds a star up and keeps it from collapsing in on itself due to gravity.
Another important fact pertaining to the mass of a star on the main sequence is the
following generalization: the more massive the star, the greater its luminosity. A star equal to
the sun's mass will shine as brightly as the sun; a star with about four times the mass of the sun
will be about one hundred times as bright as the sun; a star with about half of the sun's mass
will be considerably dimmer.
Main sequence stars are stars that are burning hydrogen at their cores. Hydrogen
burning cannot go on forever at the center of a star. For example, the sun converts 600 million
tons of hydrogen into helium every second. If, in an imaginary experiment, you could weigh
the hydrogen nuclei before and the helium nucleus after fusion, you would discover that the
helium nucleus was lighter. This mass difference is converted to energy that produces the sun's
luminosity.
If stars were like cars, then they would burn their core hydrogen until they ran out and
the star would fade out. But fusion converts hydrogen into helium. The core does not become
empty. It fills with helium ash.
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A star on the main sequence is comfortably burning hydrogen at its center. Nothing
dramatic happens as this thermonuclear reaction gradually alters the chemical composition in
the core of the star. Over millions of years, the core contracts slightly and the outer layers of
the star expand slightly. The star gets a little brighter and its atmosphere gets a little cooler.
Stars begin their lives as 74% hydrogen, 25% helium, and 1% everything else on the
periodic table (by mass). Fusion has been ongoing in the core of the Sun for 5 billion years
and its core is now about 29% hydrogen, 70% helium and 1% everything else. Fusion alters
the chemical composition of stellar interiors.
As the helium ash in the center builds up, energy generation stops in the core. The
fusion process moves outward into a shell surrounding the hot helium core. Helium can also
undergo fusion but, since it is a larger atom, it requires over a 100 million degrees to fuse. For
small stars, this temperature is never reached and the helium core remains inert.
Equilibrium in the star's core is severely disrupted once the core is composed of helium.
For a short period of time, there is less pressure pushing out and under the overwhelming
influence of gravity, the star's core contracts and get hotter, causing greater pressure pushing
outward and the star's atmosphere begins to expand.
The hydrogen burning shell expands to consume more fuel in the star’s interior. The
hydrogen burning shell generates more energy than the core did. It has access to a much larger
volume of the star’s mass and the pressure inside of the star increases and causes the star to
expand.
As the energy from the contracting core and shell hydrogen-burning causes a star to
expand, the atmosphere of the star starts to cool. Regardless of where on the main sequence the
star originally came from, the star's surface temperature eventually gets as low as 4000 degrees
Kelvin. The star has become very large and very red. The star has become a red giant.
A red giant has increased luminosity (because the star gets bigger) and decreased
surface temperature (because the star's atmosphere expands and therefore cools).
While the outer layers of the star are expanding and cooling, the core continues to
become very compressed. When the core temperature reaches 100 million degrees Kelvin, a
new thermonuclear reaction is ignited. The temperature and pressure are now so high that
helium nuclei can be fused together to form carbon and oxygen. This is helium burning. The
star settles down to core helium burning surrounded by shell hydrogen burning.
A star probably moves back and forth from the red giant region toward the main
sequence several times before it enters the final stages of its life. Most stars probably change
from red giants to pulsating variable stars before they finally die. That is, they expand and
contract and grow bright and fade periodically.
Eventually, all the helium at the core of a red giant has been burned up and converted
into carbon and oxygen. Obviously, the helium burning shuts off and the inert core starts to
contract. What happens next critically depends on the mass of the star. New thermonuclear
reactions can be ignited if the temperature gets high enough. The star' s mass is the deciding
factor.
Stars that have burned all the helium in their cores are rapidly approaching old age.
They are the senior citizens of our galaxy. As the inert, carbon-rich core contracts,
temperatures above the core soon get high enough to ignite shell helium burning. A star at this
late stage in its life has thermonuclear reactions occurring in two shells: a hydrogen burning
shell surrounding a helium burning shell.
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Our sun cannot develop sufficient temperatures at its core to go beyond burning
hydrogen and helium shells surrounding a carbon-oxygen core. Once this occurs, instabilities
within a low-mass star develop. The low-mass star begins to pulsate causing its atmosphere to
expand briefly. Eventually, the pulses become so powerful that the outer layers of the star
completely separate from the core, thus exposing the core.
The expanding envelopes of ejected stellar layers are called planetary nebulae. Each
nebula is clearly concentric about one particular star. This 'star' is actually the burned out
carbon-oxygen core that has finally been exposed to view. Planetary nebulae have a very
temporary existence. After a very short period of time - perhaps 50,000 years - the expanding
envelope of gas has become so thin and dispersed that it no longer glows. The gases simply
merge with the interstellar medium. All the planetary nebulae that we see are therefore very
recent phenomena; they must be less than 50,000 years old.
As the expanding gases of a planetary nebula begin to spread out, the hot central star
starts to cool off. The star's core becomes more and more compressed. Trillions upon trillions
of tons of matter pressing inward from all sides cause the densities inside the star to become
higher and higher. By this time, the surface temperature of the star has fallen below 50,000°K.
The star is dead - there are no internal sources of energy; fusion has stopped. The star is
small - typically, an entire solar mass of burned-out stellar matter has been compressed into a
sphere no larger than the earth. The star is hot. Although it may start with a surface
temperature of 50,000 degrees Kelvin, the star cools very slowly. Consequently, as seen
through a telescope, one of these tiny stars appears to be white hot. It has become a white
dwarf.
As the years pass by, white dwarfs simply cool off. And as their temperatures drop,
their luminosities also decline. In principle, white dwarfs eventually cool off completely. But
this cooling process is very slow and it would take trillions of years for a star to approach
absolute zero. Since this is much longer than the age of the universe, even the most ancient
white dwarfs are still fairly warm.
Since the vast majority of stars have low masses, most stars must become white dwarfs.
Up until the mid-1960's, it was generally supposed that all dying stars eventually became white
dwarfs. After all, there seemed to be enough white dwarfs in the sky to account for the corpses
of all the stars that should have died by now. But what about the most massive stars?
We need to return to the red giant stage. After core helium burning with shell hydrogen
burning, if a star contains more than four solar masses, then densities and temperatures at its
center get high enough to ignite carbon burning. The thermonuclear fusion of carbon nuclei
produces oxygen, neon, sodium, and magnesium. Three independent thermonuclear reactions
are therefore occurring in the star: core carbon burning surrounded by shell helium burning
surrounded by shell hydrogen burning.
Eventually, of course, all of the carbon at the center of one of these stars is burned up.
Carbon burning shuts off, the star's core contracts, and shell carbon burning is ignited. If the
star is more massive than about 6 solar masses, temperatures at the core climb above 1 billion
degrees. At these extraordinary temperatures, oxygen burning commences. This reaction
produces silicon and sulfur. The ending of core oxygen burning is followed by yet another core
contraction and the ignition of shell oxygen burning. And if the star is sufficiently massive
enough to push the central temperature above 2 billion degrees, silicon burning is ignited.
Stellar Formation and Evolution - 4
Nickel and iron are the ashes of silicon burning. Unlike the lighter elements, iron does
not 'burn.' The buildup of iron at the core of a very massive star (for example, greater than
about 10 solar masses) signals the impending, violent death of the star. Immediately before its
death, a massive star can burn silicon in a shell surrounding the iron-rich core. The core is
about the size of the earth.
Since no new thermonuclear reactions are possible, the burned out core simply
contracts and gets hotter and hotter. The infalling layers collapse so fast that they bounce off
the iron core at close to the speed of light. The nuclei begin breaking up. The negatively
charged electrons get squeezed into the positively charged protons, thereby creating many
neutrons. A collection of neutrons occupies a much smaller volume than an equivalent amount
of iron nuclei. Consequently, as the electrons and protons combine to form neutrons, the star's
core rapidly implodes.
The sudden, catastrophic collapse of a star's core releases an incredible amount of
energy. In fact, the amount of energy released during core collapse can be as great as all the
energy radiated by the star over its entire preceding life. As the energy from core collapse
surges outward, the star is completely ripped apart. In one of nature's most violent cataclysms,
the star's luminosity suddenly increases a billion-fold. For a few days, this single star can be as
bright as the entire galaxy. The star has become a supernova.
The ejected material is extremely rich in the elements beyond hydrogen and helium; not
only is the original light element shell ejected, but more elements are formed in the explosion even elements as heavy as plutonium and uranium. Most of the heavy elements in the universe
are made in supernova explosions, scattered across space to condense and form new stars. Any
element that has not been produced during earlier thermonuclear, processes is now created in a
few moments as the star rips itself apart.
During the first few days, the envelope continues to expand. The giant hot envelope is
the source of the visible supernova. At maximum, it is about ten billion times brighter than the
star was before detonation. In a week, the shell of ejected material is as large as the solar
system. The brightness of the supernova begins to decline.
Some 10,000 years later, this envelope will be a cloud still spreading into space. The
core - now a neutron star and pulsar - will continue to spin for millions of years, emitting
pulses of radio energy, cooling and slowing, until it too becomes invisible.
A supernova explosion therefore spews large amounts of heavy elements out into the
universe. Interstellar clouds of gas become enriched with these heavy elements. Stars and
planets that condense from this enriched interstellar medium will contain all these elements.
The sun is a late-generation star. Ten to fifteen billion years had passed before it even began to
form. During that time, countless millions of millions of massive stars lived out their lives and
supplied our region of space with the full array of chemical elements. Our world and ourselves
- every atom heavier than hydrogen and helium in our bodies, every atom we touch and eat and
breathe - were created long ago at the centers of doomed stars. Quite literally, we are made of
the dust of stars.
A supernova explosion will generate a gravity wave that will pass through the universe
at the speed of light compressing areas in a nebula resulting in new stellar formation.
There are different outcomes for massive stars after a supernova; possibly a neutron
star, pulsar star, or black hole.
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The idea of a neutron star was developed in 1939. If the mass of a normal star were
squeezed into a small enough volume, the protons and electrons would be forced to combine to
form neutrons. A star the size of our sun would produce a neutron star that was only 15 km in
radius. Even if the object had a surface temperature of 50,000°K, it has such a small radius
that its luminosity would be a million times fainter than the Sun.
Neutron stars have hotspots at their magnetic poles emitting tremendous amount of
radiation. The energy from the hotspots sweep out into space light a lighthouse. Only when
the Earth lies along the axis of the neutron star is the energy detected as a series of pulses and
the object is called a pulsar.
The best chance of observing neutron stars is to see their influence on nearby objects.
Gas from a nearby star flowing towards a neutron star forms a thick disk of orbiting material
called an accretion disk. If the gas is dumped in vast amounts from the accretion disk to the
neutron star, then the energy can not be released fast enough and tremendous pressures build
up. The pressure can only be relieved if the gas is ejected. It is easier for the material to be
ejected at the poles where two powerful jets of high velocity hot gases form perpendicular to
the accretion disk.
A star more massive than a neutron star could wind up with quite a different scenario.
After exploding in a supernova, the star's matter is squeezed into a smaller and smaller volume
and the intensity of gravity above the dead star becomes stronger.
Einstein predicted that gravity curves space. The greatest curvature (and thus the most
intense part of the star's gravitational field) is found immediately above the star's surface. Far
from the star, where gravity is very weak, space is almost perfectly flat. Light rays moving
through curved space must be deflected from their usual straight paths. Einstein therefore
predicted that stars seen near the sun at the time of a total solar eclipse would be shifted
slightly from their usual positions. This deflection was first observed during an eclipse in 1919.
As a massive dead star collapses, the intensity of gravity above the star gets stronger
and stronger. This means that, as the collapse proceeds, light rays leaving the star's surface are
bent through increasingly larger angles. In a massive star just before the onset of collapse,
space around the star is only slightly curved and light rays leave the star along nearly straight
lines. During collapse, the curvature of space across the star increases and light rays become
increasingly deflected from their usual straight paths. This deflection eventually becomes so
severe that all light rays are bent back down to the collapsing star's surface. The star has
formed a black hole.
Since light cannot get out, nothing else can get out. The star has literally vanished from
the universe. A black hole of 10 solar masses is 60 km (37 miles) in diameter. Thus, as soon as
the star collapses to a size less than 60 km in diameter, a black hole is formed. There is still
nothing to hold up all the dead stellar matter. The star continues to get smaller and smaller until
its mass is crushed into a single point called the singularity where the density is infinite.
Since black holes do not emit light, it is impossible to see them. The only hope we
have of finding a black hole comes from the possibility of observing the influence of the hole's
enormous gravitational field on something that we can see. For example, we might stand a
chance of seeing matter or gas falling into a black hole. Just before the material takes the final
plunge down the drain in space, radiation might be emitted that announces the existence of the
hole. This would amount to an indirect discovery since the black hole itself is never observed.
Stellar Formation and Evolution - 6