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STARS
Ch. 16 – 21
I.
II.
What is a star?
a. Glowing ball of gas held together by its own gravity---Powered by
nuclear fusion at its center
b. Composed of mainly hydrogen and helium
a. Our sun is 91% hydrogen
b. 8.9 % helium
Active Regions (define each term)
http://apod.nasa.gov/apod/ap140506.html
a. Solar flares pg. 423
b. Solar prominences
c. Solar wind pg 417
d. Solar cycle
e. Sunspots pg 420
f. Spicules pg. 416
Class Work:
 How are solar flares related to the solar wind?
 How are solar prominences and solar flares related to sun spots?
2007 December 3
A Complete Solar Cycle from SOHO
Credit: SOHO - EIT Consortium, ESA, NASA
Explanation: Every eleven years, our Sun goes through a solar cycle. A complete solar cycle has
now been imaged by the sun-orbiting SOHO spacecraft, celebrating the 12th anniversary of its
launch yesterday. A solar cycle is caused by the changing magnetic field of the Sun, and varies
from solar maximum, when sunspot, coronal mass ejection, and flare phenomena are most
frequent, to solar minimum, when such activity is relatively infrequent. Solar minimums
occurred in 1996 and 2007, while the last solar maximum occurred in 2001. This picture is
composed of a SOHO image of the Sun in extreme ultraviolet light for each year of the last
solar cycle, with images picked to illustrate the relative activity of the Sun.
NASA Official: Phillip Newman Specific rights apply.
Class Work:
In the above APOD, the complete solar cycle shows the activity of the sun over a course of time. According to the
photographs, how often does the sun experience a solar maximum? A solar minimum?
III. Layers of our Sun (define & illustrate) pg 407
a. photosphere
b. chromosphere
c. corona
d. core
IV. Luminosity
a. This is different from a solar constant. (p. 432)
b. How does luminosity compare to apparent brightness? (p.
441)
V. Apparent brightness vs. absolute brightness
VI. Classification (ch. 17)
a. Magnitude scale pg.443
1. apparent brightness
2. Hipparchus –naked eye
3. 6 groups: 1-6
b. Modern Magnitude scale
1. -1to7 includes decimals
2. -30 to 30 see page 444
c. Color and Temperature**(flame test)
1. see page 446 – photometry
2. KNOW Table 17.1
What determines the color of a star?
What is the other name for our sun?
3. know table 17.2
d. HR Diagram (pgs. 450-454) define & illustrate
1. main sequence stars
a. blue super-giants
b. blue giants
c. white giants
d. yellow-white
e. yellow dwarfs
f. orange dwarfs
g. red dwarfs
2. red giant region
3. red supergiant region
4. white dwarf region
5. off the HR diagram is the black dwarf
region**(classify yourself as a type of star and
give the characteristics for the classification)
VII.
Evolutionary track http://www.enchantedlearning.com/subjects/astronomy/stars/lifecycle/starbirth.shtml
a. Interstellar medium
b. Interstellar cloud
c. Stages (ch. 19 p. 492)
a. Stage 1
Gravitational Collapse
The source of the energy for star formation is gravitational collapse - this collapse must
provide enough energy to heat the gas of the protostar to the ignition point of hydrogen
fusion, some 15 million Kelvins. Knowledge of the mass and distribution of the gas cloud
permits some fairly detailed modeling, because half of the energy from gravitational collapse
goes into kinetic energy according to the virial theorem.
The concept of the Jean's Mass as the critical mass for collapse into a star is an important
concept.
http://apod.nasa.gov/apod/ap110326.html/
Class work:
Write out the stages of star formation pages 493-497
Classwork: Illustrate and define the evolutionary track of our sun, a blue supergiant and a blue giant. Ch. 20
Stellar Evolution :
The Life and Death of Our Luminous Neighbors
by Arthur Holland and Mark Williams
Introduction and Why You Should Care
Observations of the stars with calculations of stellar models have give astronomers a comprehension of stellar evolution.
Stellar evolution is the process in which the forces of pressure (gravity)
alter the star. With these forces acting upon stars, their characteristics
change dramatically over the period of their existence.
Stellar evolution is inevitable as stars deplete their initial fuel sources.
The search for new fuel sources affects the properties of stars as they
evolve. This evolution is a process that consists of many different stages
with fuel consumption as the dominant life cycles of an evolving star.
Stellar evolution, in the form of these fuel consumption stages and their
finality, is important because it is responsible for the production of most
of the elements (all elements after H and He). Moreover, stages in the
life cycle of stars are a vital part in the formation of galaxies, new stars
and planetary systems.
Time scales of Stellar Fuel Consumption
The time scales of stellar evolution depend on the mass of the star. The
rule governing stellar evolution is the more mass present, the faster the
evolution for the star through the fuel consumption stages. Another
property directly linked to the mass and evolution of a star is its
luminosity. The bigger, the brighter the star then the shorter it lives.
The mass-luminosity relation demonstrates that the main sequence on an H-R diagram (a chart plotting the luminosities
of stars against their surface temperatures) is a progression in mass as well as in luminosity and surface temperature
(Kaufmann,1991, pg. 356). The Hertzsprung- Russell diagram is two possible plots of spectral types dependent on
absolute magnitude when compared to temperature and luminosity.
These H-R diagrams allow astronomers to conceptualize visually the mass-luminosity relationship as it pertains to the
fuel consumption evolution of stars. The H-R diagram allows astronomers to plot visually the different stages in the
evolving life cycle of stars.
A H-R diagram (the red line represents the Main
Sequence Stars).
Stages in the Life of an Evolving Star
Welcome to the life of a new born star. Main sequence stars are a range of stars based on size and surface temperature
starting from the hot, bright, bluish stars in the upper left corner of the H-R diagram to the cool, dim, reddish stars in the
lower right corner of the diagram. Life
for new stars begins in the Main Sequence. These
mature stars undergo a remarkable transformation after they consume all the
hydrogen in their core. With the hydrogen consumed, stars leave the main
sequence and expand to form red giants. With this new stage, the fusion of helium
begins to form heavier elements like Oxygen and Carbon. This process of
expansion- collapse-expansion of stars forms the light elements present in the
universe (up to Fe).
Life in the Suburb: a Main Sequence Star. Main sequence stars are stars whose
luminosity and surface temperature place it in sequence on a Hertzsprung- Russell diagram.
A fundamental property of all main sequence stars is thermal equilibrium. Thermal equilibrium is the liberation of
energy from the interior of the star balanced by the energy released at the surface of the star. The energy released by a
main sequence star is produced by hydrogen burning in its core (the fusion of 4H into 4He).
Another fundamental property of a main sequence star evolution is hydrostatic equilibrium. Hydrostatic equilibrium
reflects the required pressure in the core of a star to support the weight of the outer plasma layers. The heat produced
from hydrogen in the core burning supports this outward pressure upon the outer plasma layers.
As a main sequence star depletes the supply of hydrogen in the core, thermal equilibrium unbalances and the pressure
in the star’s core lessens. Thermal equilibrium unbalances because the fusion of four hydrogen atoms into one helium
atom decreases the number of particles present in the star� core. The star beings to collapse inward because the fewer
particles cannot maintain the pressure needed to support the star’s outer layers.
Without the necessary pressure, the star’s core contracts slightly under the weight of its outer layers. This collapse
increases the pressure and temperature of the core causing the luminosity of the star's surface to increase. This increase
in pressure on the layers just outside the core raises their temperature to the necessary point in which the outer layers
of hydrogen begin fusion.
This occurrence of hydrogen fusion in the outer layers of a collapsing star is called shell hydrogen burning. Shell
hydrogen burning allows the star to remain in the main sequence for another few million years. Despite this struggle to
remain on the main sequence, the supply of hydrogen for fusion into helium in a star’s inner most layers depletes, which
takes the star from the main sequence and to the next step of solar fuel consumption.
The next stage of solar fuel consumption starts when hydrogen burning in the core ceases and ignites hydrogen burning
in the star's outer layers. When hydrogen burning ceases in the star�core, it begins to collapse again. At this point, the
star converts gravitational energy into thermal energy because it must maintain thermal equilibrium.
Stars sustain thermal equilibrium within their interiors through the ignition of helium burning. The collapse of the outer
hydrogen burning shell upon the core raises the temperature and pressure in the core and begins helium burning.
Because the temperature and energy needed to ignite helium fusion is greater than that of hydrogen, the energy
released by helium fusion in the star�core is greater than needed to support the weight of the outer layer.
This excess energy expands the star�outer layers beyond its previous radius and star�volume increases. A star going
through this stage of fuel consumption (collapse and expansion) is a Red Giant. The following diagram shows the
dramatic expansion of a main sequence star as it begins helium fusion.
Our Sun today compared to its future
Red Giant self
1 AU is approximately 150 million kilometers or the distance from the sun to the Earth's orbit
The Life of a Red Giant : A Star in Old Age. In the final states of hydrogen fusion the hydrogen burning
shell adds to the mass of the star� helium core. This added mass and pressure increases the
star’s temperature. Temperature within the core of stars greater than three solar masses
soon exceeds the 100 million Kelvin degrees barrier, at which it reaches the temperature
needed for the onset of helium fusion.
Core helium burning reestablishes the thermal equilibrium needed to support the star’s outer layers preventing
further gravitational contraction. With gravitational contraction halted, the heat and energy from the core helium
burning again begins the expansion of the star's outer layers. The method by which a star begins core helium burning
depends upon the mass of the star.
In stars below three solar masses (low-mass stars), helium fusion begins in a more spectacular manner. Helium
burning begins explosively and abruptly. This process of quick ignition of helium fusion is a helium flash. The universe
does not limit helium flash to low-mass stars; Helium is present in red giants and red supergiants as well.
Red supergiants and red giants share the same physical characteristics and properties, with size and luminosity the
only major difference between them. To understand this size difference we use our sun as a reference point. Red
giants have solar radii up to 10 to 100 times larger than our sun, while supergiants boast a solar radii 1000 times
larger than that of our sun.
A Low-Mass Stars�ast gasp. Stellar evolution can end in several ways. After a long life, an aging star completes its red
supergiant stage of evolution and shell helium burning begins. Since this shell of helium burning is thin, the star
becomes unstable. This instability in the aging star increases the temperature and energy within the star, which
thickens the shell of burning helium surrounding the helium-depleted core.
The shell of burning helium thickens until it can support the pressures of the star�outer layers. As the shell helium
burning increases the temperature and energy of the surrounding outer layers of the star, it ignites the outer
hydrogen shell into fusion through thermonuclear reactions. This process of outer hydrogen shell fusion is a thermal
pulse.
During thermal pulses, the star�luminosity increases by a factor of ten. This final expansion and ignition of outer
layers is the star�final moments before death overcomes the star. In its final moments, a star ejects its outer layers
emitting ultraviolet radiation. This emission of ultraviolet radiation ionizes the ejected gases, giving them a glow
known as a planetary nebula.
A planetary nebula, the death of a low mass star
The ejection of stellar remnants is the low-mass star�supernova. On Earth, we measure the
effects of this supernova in the increased luminosity. After a low-mass star�death
(supernova), it often leaves behind material that forms new stellar bodies.
This is the end of stars with low masses (less than four solar masses) because they cannot
reach the appropriate t temperature levels to begin the fusion of carbon (C) into oxygen (O).
These stars burn off or eject their remaining outer layers leaving only the stellar core behind.
Stellar remnants, a star�afterlife: white dwarfs, neutron stars and pulsars. Due to the
enormous mass of this remaining stellar core, the core begins to collapse and fuse. As the
stellar core collapses, it electrons become degenerate (a phenomenon, due to the quantum
mechanical effects, whereby the pressure exerted by a gas does not depend on its
temperature) and stop the gravitation collapse. This contracted stellar core is a white dwarf.
White dwarf stars are approximately the size of our planet, but their mass and density is
much greater. White dwarfs are so dense, when compared to the Earth, that a teaspoon of
matter from a white dwarf would weigh 5.5 metric tons on the Earth�surface! The universe
places limits on the life of a white dwarf, familiar to the main sequence star from which it
originates.
The white dwarf glows for billions of year from the energy released from cooling thermal
radiation. Eventually all the radioactive matter of a white dwarf cools until it reaches the
temperature of surrounding space which is a few degrees above absolute zero.
A Neutron Star
Neutron Stars and Pulsars : Sometimes the core remnant of a low-mass star is too dense to
form a white dwarf. In these cases, the core�stellar collapse forms a neutron star.
Neutron stars are incredibly dense spheres of degenerate neutrons (a gas in which all the
allowed states for particles, electrons or neutrons, have been filled, thereby causing the gas
to behave differently from ordinary gases). Some neutron stars have massive magnetic fields
that sweep out beams of radiation into space, much like light beams from a lighthouse. Such
밵lsating�eutron stars are called pulsars. This measurable 밵lse�f radiation is where this
type of neutron star derives its name.
Astronomical models suggest that the properties of superconductivity and superfluity
dominate the cores of neutrons stars. These models also suggest there is an upper l limit in
the mass of neutron stars. These upper limits are in the range of the mass of the largest
possible white dwarfs.
The Little Bang : The death of high-mass stars. This death of a low mass star into a white
dwarf contrast with the final stages of high-mass, main sequence stars (i.e., greater than
four solar masses). Unlike low mass stars, high mass stars can extend their lives through the
fusion of elements heavier than carbon.
After the fusion of helium ends, high mass stars begin burning carbon as their next fuel
consumption stage. Carbon burning begins when the star's core reaches temperatures
greater than 600 million degrees Kelvin. The greater the mass of the original main sequence
star the longer the star continues to burn heavier elements. Incredibly massive stars
continue fusion from helium until they create iron (Fe) in their core.
In this fusion process, these massive stars create neon (Ne), magnesium (Mg), oxygen (O),
sulfur (S), silicon (Si) and finally iron (Fe). Each stage of fuel consumption parallels a raise in
the star�core temperature. Neon fusion begins when the star�core temperature reaches one
billion degrees Kelvin followed by oxygen at 1.5 billion degrees Kelvin. Silicon fusion does
not begin until the star�core temperature reaches an amazing 3 billion degrees Kelvin.
As the star begins each new fuel consumption stage, its lifetime in each stage becomes
shorter. The following table illustrates the amount of time an aging star spends in each fuel
consumption stage.
A H-R diagram (The red line represents the Main Sequence stars and their
evolution toward the top right hand corner of the diagram).
These massive stars continue to exist by burning the heavier nuclides (magnesium, silicon, phosphorous et al) until
they reach the heaviest nuclide that fusion can produce, iron. The iron rich cores of these massive stars can reach
sizes equivalent to the Earth. The size of the surrounding shells burning the various lighter elements (H, He, C, O)
dwarf this iron core, as they reach sizes that would extend to the orbit of Jupiter.
When the core of a high-mass star consists completely of iron, fusion can no longer take place. At this point, the mass
of the daughter (one iron nuclide) is heavier than the total mass of the parent nuclides used to produce iron. With this
difference in mass, the process begins to absorb energy instead of releasing it back into the star, which unbalances
the thermal equilibrium of the star.
With fusion ceased and the thermal equilibrium unbalanced, a star� only source of energy is from the contraction of
the star�outer layers. This contraction raises the star�core temperature to 5 billion Kelvin and increases the pressure
to the point that the iron core collapses upon itself.
The incredible pressures that the collapse of the star�iron core force many of the existing (but not all) protons and
electrons to combine into neutrons. For a moment the star releases this abundance of neutrons as neutrinos. In this
brief escape, many neutrinos bombard the iron core and combine with iron nuclides to form the elements heavier
than iron. These escaping neutrinos and the electromagnetic forces that repel protons and neutrons force the star
into a final expansion.
A Star�Eulogy, a Supernova. This final expansion sends a shock wave through the star�outer layers until it reaches the
star�surface creating a supernova. This shock wave ejects all the material of the star, including the core into the
cosmos leaving behind only a nebula of cooling gasses.
A section of space before the supernova
The same section of space after the supernova
Stars too big for their britches : Black Holes. Sometimes high-mass stars are too massive to become white dwarfs or
neutron stars. A high-mass star this massive also has the gravitational forces to prevent the escape of stellar matter
through a supernova. Stars with this great of mass become black holes at the end of their stellar evolution.
Black holes are the result of the overpowering weight of the star� massive outer layers pressing inward from all sides
on the core causing the rapid contraction of the dying star. With this great mass pressing inward, the star�core can no
longer support the star�outer layer structure.
The matter within a high-mass star in this process is so condensed that the gravity produced is strong enough to
prevent the escape of light energy from the cooling radioactive material. Light cannot escape because the escape
velocity at the star�surface is greater than that of the speed of light.
Gravity of this magnitude has profound effects upon the star�shape and on the structure of the surrounding space.
With the star collapsing in upon itself, the star changes from a spherical shape to a broader plane in space. The gravity
from the center of this plane curves its surrounding space to create a whirlpool-like drain that absorbs the star itself
and all surrounding matter.
An Artist rendition of a black hole event horizon (notice the whirlpool like
shape that leads to the blackhole's singularity).
This plane at the mouth of the whirlpool-like drain is called an event horizon. The immense gravitational forces of the
black hole compacts all the matter within the star after it moves past the event horizon of the newly formed black
hole. The gravity of the black hole compresses the star�matter, as well as any other matter captured by the black
hole� gravitational forces through its existence, to infinite density. This infinitely dense matter within the black hole is
called a singularity. The gravitational pull of a singularity is so great that it pulls matter from all surrounding areas into
the event horizon of the black hole.
If light cannot escape black holes, how do we find them? Until recently black holes were only theoretical speculations,
but with today�technology we are now finding black holes by measuring x-ray emissions in space.
As the black holes immense gravity captures surrounding matter, this matter accelerates toward the event horizon
and singularity and releases high amounts of x-rays. These x-rays leave a trail through space as the black hole absorbs
the matter into its singularity. X-ray trails leave 먡los�round the black hole called accretion disks.
Another method of detecting black holes in space is their effects on binary star systems. When a black hole occurs
close to two stars, its gravity impacts their spatial relationships. Each star�gravity has a calculable effect upon the
other�orbit, however the invisible black hole affects these orbits.
An Artist rendition of a blackhole syphoning plasma gas from a
nearby star (this acceleration of matter emits x-rays that form a accretion disk and make the blackhole detectable [xray emissions represented by the arrows]).
To contrast the idea of black holes being places in the universe with infinite lifetimes where no matter (including light)
ever escapes, the universe holds a few exceptions. The British physicist Stephen W. Hawking has proven that black
holes do in fact have definite lifetimes.
As black holes reach the end of their life, they begin to evaporate. In the final stages of this evaporation, the black
hole reverses itself and pours matter back out into the universe. When a black hole begins to eject its matter, it is a
white hole. With this transformation in the life of a black hole, the universe appears to maintain the fundamental
universal energy-matter law with this process. While the black hole, to physics' laws, is an unbalancing factor in
universal laws, the white hole exists to restore this matter and balance.
Our Two-cents--Please, no change
Stellar evolution is the necessary fundamental building block and distributive method of most common elements in
the universe. Within the interior of stars, fusion creates new elements from the basic elements (H, He). While this
process takes billions of years as measured by human standards, the life of a star is minor in comparison to the age of
the universe.
Much like its life span, the outcome of one star�life is insignificant. When we add the efforts of one star, however, to
the production of the billions of stars that have existed, exist and will exist again they are invaluable to the creation of
life. Without the elements produced by stellar evolution (the processes within the interiors of stars), Carbon-based
life as we know it would not be possible. The next time you are star gazing, remember, you are a product of the stars.
Reference
1. Universe (fourth edition). Kaufmann, William J. III. W. H. Freeman and Company New York, 1994.
2. http://www.astro.washington.edu/strobel/evolution.notes/evolutio n.notes.html
Glossary of bold face items
absolute zero - Temperature of -273 degrees Celsius where all
molecular motion stops; the lowest possible temperature
accretion disk - A disk of gas orbiting a star or black hole
binary star system - Two stars revolving around each other
black hole - An object that is so strong that its escape velocity
exceeds the speed of light
core helium burning - The thermonuclear fusion of helium at the
center of a star
degenerate - The phenomenon, due to quantum mechanical
effects, whereby the pressure exerted by a gas does not depend
on its temperature
event horizon - The location around a black hole where the
escape velocity equals the speed of light; the surface of a black
hole
fusion - combination of lower mass nuclides into higher mass
nuclides
helium flash - The nearly explosive beginning of helium burning
in the dense core of a red giant star
hydrostatic equilibrium - The balance between the weight of a
layer in a star and the pressure that supports it
luminosity - The rate at which electromagnetic radiation is
emitted from a star or other object
main sequence star - A star whose luminosity and surface
temperature place it on the main sequence on an H-R diagram; a
star that derives its energy from core hydrogen burning
neutron star - A very compact, dense star composed entirely of
neutrons
planetary nebula - A luminous shell of gas ejected from an old,
low- mass star
plasma - A hot ionized gas
pulsar - A pulsating radio source believed to be associated with a
rapidly rotating neutron star
shell helium burning - The thermonuclear fusion of helium in a
shell surrounding a star�core
shell hydrogen burning - The thermonuclear fusion of hydrogen
in a shell surrounding a star�core
singularity - A place of infinite space-time curvature; the center
of a black hole
solar mass - The mass of our sun; reference point for the masses
of other stars
solar radii - The distance from a star�core to the surface of its
outermost plasma layer
spectral type - A classification of stars according to the
appearance of their spectra
supernova - A stellar outburst during which a star suddenly
increases its brightness roughly a millionfold
thermal pulse - Brief bursts of energy output from the heliumburning shell of an aging low-mass star
thermonuclear reaction - A reaction resulting from the high
speed collision of nuclear particles that are moving rapidly
because they are at a high temperature
white dwarf - A low-mass star that has exhausted all of its
thermonuclear fuel and contracted to a size roughly equal to the
size of Earth
white hole - A black hole from which matter and radiation
emerge
X-Ray - Electromagnetic radiation whose wavelength is between
that of ultraviolet light and gamma rays
 Define and compare types of nebulae: Planetary,
reflection and emission Pg. 504, 525, 530
 Compare nova to supernova pg 548 & pg 561
 Compare neutron stars to pulsars pg 568-569
 Explain what a black hole is and its event horizon
 Explain escape speed. What will it take for NASA to
launch a spaceship into outer space to orbit Earth?
What will it take for them to launch the spaceship to
travel beyond our solar system? Is it the same or not?