The Big Picture of Stars
I have not placed a page like this in any other part of the course, but felt
that it would be extremely helpful if you were to see the big picture of stars
before looking at the details. First, many of the students who are presently
enrolled in the online course and in the in-house course have a limited science
background, and are unfamiliar with some physics and chemistry. Since I attended
the High Energy Astrophysics Symposium in 2002, I have found my interest in
stars to be heightened as never before, and I have been feverishly trying to
place as much accurate information about stars into this class as possible.
Alas ... most of it will be over everyone's head, and suddenly I face the prospect
of failing as a teacher. While I do not wish to dummy down this course or this
unit, I must also be aware of the audience. This page here is to help you see
the big picture, get the basics, and then you can move forward and dive into
stars as far as you wish.
to the bottom of this page for the exact sequence of this unit for NSO students.****
All stars have lifecycles, and every lifecycle follows a path determined by
the mass. It is the mass that is the all important feature in helping
us understand the nature of a star, and it is the force of gravity acting upon
that mass that causes stars to either be exciting or boring. As a rule,
the more massive a star, the greater the gravitational compression in the core.
From this increased compression comes increased temperature. Increased temperature
energizes atomic particles in the core of stars to higher energy levels and
increases both the likelihood of collisions and successful fusion. The greater
the energy of the particles and the more compressed they are relative to each
other, the more particles will collide and fuse, and thus the faster the rate
of nuclear fusion or burning. High mass stars burn their internal fuel supplies
at tremendously rapid speeds, burn extremely bright, and live short, fast, and
furious lives. The low mass stars might intuitively seem to have shorter lives
owing to their small size and fuel supply, but the reduction in gravitational
pressure causes less frequent successful collisions and a slower rate of fusion.
The low mass stars burn their fuel slowly, shine with cool and dim temperatures,
and live tremendously long lives.
1.988 x 1030 kg ... the mass of our Sun. Every star in the night sky is compared to our Sun. This number is termed a "Solar Mass." It is the mass of the star when it first forms that determines the lifecycle of the star. Yes ... stars have "lives." They are "born," they "live," and they "die." Whether they life exceedingly long and boring lives, or short and spectacular lives is all based on the mass.
A generalized picture of the life of a typical star is this: 1) A large cloud
of dust and gas collapses under the influence of gravity. As the particles are
rushing inward due to gravitational pressure, these particles rub against each
other and emit heat due to friction. The gas/dust cloud begins to glow in the
Infrared part of the spectrum, and the object is called a PROTOSTAR. 2) When
the protostar shrinks enough so that internal pressure and temperatures rises
high enough, the core will spontaneously ignite the process of nuclear fusion
of 4 hydrogens --> 1 helium + the release of energy. The object is officially
a star. 3) When the core supply of hydrogen is converted into helium, the star
proceeds to its death. Low mass stars will "puff out" exterior gases
and form PLANETARY NEBULAE with a WHITE DWARF CORE. High mass stars will "blow
out" much more exterior gas and explode as SUPERNOVE, with tiny NEUTRON
STAR cores, or BLACK HOLES.
In every instance that we can theorize at present, based on actual evidence,
Star Evolution follows stages. As seen above, huge clouds of dust and gas are
in the sky (above and left). Gravitational collapse of parts of the cloud result
in clumps (above and middle). As gravity squeezes the ball of gas smaller and
smaller, it finally idniges (above and right).
Stars like our Sun (above and far left) will live a long time and die out
as planetary nebulae (above and second from left) with a white dwarf core. Eventually,
all of the gas will spread out and just the white dwarf core will remain (above
and second from right).
Larger stars like Betelgeuse (above and left) are destined to a different
fate. These stars are really huge (above center), and when they die, they explode
(above, right two images). The result can be a neutron star (below, left two
images) or a black hole (below, right two images).
Stars of average mass primarily generate energy from the fusion of Hydrogen
into Helium, but stars of increased mass can fuse Helium into Carbon, Carbon
into Nitrogen and Oxygen, and make even heavier elements like Magnesium, Neon,
Sulfur, Silicon, and Iron (below diagram). You will find that stars are element
factories, and the massive stars are responsible for making most of the elements
in the Periodic Table. Some contend that we owe our very existence to the massive
stars that manufactured the very heavy elements that not only adorn our ears,
necks, and banks, but also drive enzymatic pathways in our cells. When Crosby,
Stills, Nash, and Young sang "We are stardust," at the 1969 Woodstock
Rock Festival, they were singing about the Astronomy of stars.
In this unit, we will explore stars of various mass classes:
We have already studied our Sun. It is the standard to which we compare everything
else to. It is a main sequence G Class star presently fusing 4 Hydrogen nuclei
into 1 Helium nuclei with the release of 2 gamma rays. The mass of our Sun is
1.988 x 1030 kg. It is average in size, volume, temperature, and age. I want
to give every student a short introduction to the different kinds of stars,
and then pursue a more detailed look. Afterall, this is an Astronomy course
Brown Dwarfs may
be more planet that star, and have a mass less than 0.08 solar masses. The do not have enough mass to ignite the nuclear fires, and thus are not really stars.
Red Dwarfs are small,
dim, and naked eye invisible, and have a mass between 0.08 and 0.5 solar masses. Their low mass means slow rate of nuclear burning and very long lives.
whose final life stage masses lie between 0.5 and
1.44 solar masses will evolve first into Red Giants,
and then into tiny White Dwarfs
... some of the hottest stars in the Universe, and certainly among the prettiest.
They BEGIN their lives with between 0.5 and 8 solar masses, but lose a lot of
mass during the AGB
stage as a result of massive stellar winds that blow up to 7/8 of the mass
into space, and thus create Planetary Nebulae.
Stars that begin their lives with a lot more mass burn really hot and bright
in the O and the B range of the HR Diagram. These stars are among the brightest
stars in space, but they do not live very long, and when they start to die,
they evolve into Supergiants.
When supergiant stars end their lives, they go out with a bang, called a Supernova.
Those whose final life stage masses exceed 1.44
solar masses, but are less than 3 solar masses will end their lives in supernovae
explosions and tiny Neutron Star remnants.
These stars often spin rapidly and generate immense magnetic fields and sharp
energy beams. If our radio telescopes can pinpoint their location, these spinning
objects make audible pulses of energy that are so regular that the early discoverers
of them thought they were listening to signals from intelligent lifeforms in
space. Indeed, the first Pulsars were called "Little
Stars whose final life stage masses exceed 3
solar mass undergo sudden and violent collapse, spectacular supernovae detonation,
and crush themselves into volumes of 0 radius, infinite temperature and pressure,
and wink completely out of sight as Black
Holes. Yeah baby ... the Black Hole is no longer a monster to scare children
at the bedside, but real objects whose sizes range from that of a few solar
masses to billion solar mass behemoths that reside in the cores of many spiral
galaxies, including our own.
We will look at stars with interesting names like Asymptotic
Giant Branch (AGB), Luminous Blue Variable (LBV), Planetary Nebula, Wolf-Rayet,
OB Association, Giant, Supergiant, and Hypergiant, as well as Red, Brown, and
White Dwarfs. We will see stars that form double, triple, and even quaternary
systems. We will find stars that blow themselves out into space and enshroud
themselves with gas and dust coverings. We will find stars that suck material
from their companion and suddenly blow up. We will find Mira stars that are
naked eye invisible and then brighten by 8 magnitudes or more on a periodic
basis. There is tremendous diversity among the stars of the night sky! But one
thing we know about all of them ... they generate their own electromagnetic
energy via nuclear fusion pathways, and if we have telescopes tuned in to the
right frequency of that radiation, we can "see" all sorts of stars
doing all sorts of interesting things.
How do we know all of these things about stars that are so far away? The invention
of the stellar spectroscope has given us the ability to learn what stars are
manufacturing and discern subtle differences between different kinds of stars.
Indeed the spectral classes of groups of stars are very distinct. When we can
know for certain what stars close to us are doing, we can apply the same principles
of spectral relationship to dim stars that are a long way from here. We owe
a great deal to the work of Draper, Pickering, Fleming, Maury, Cannon, Hertzsprung,
and Russell for laying the foundation that brought us definitive relationships
between star spectra and their brightness and energy output (luminosity). Now,
with newer and larger telescopes, we can probe the depths of space and see stars
like our neighbors glowing in vastly distant galaxies and thus determine their
distances. Its just amazing.
I have highlighted the words final life stage
in reference to the star's mass. Recent evidence on planetary nebulae and their
white dwarf core stars indicate a tremendous outflow of stellar material during
the latter stages of a star's life. The result of this outflow means that stars
with significantly larger initial masses will blow enough material away during
their lifetimes that only a fraction of the original material remains at the
star's final life stage. Stars with initial masses of eight Suns or less will
blow away so much material that their outcome is for the core remnant to hold
less than 1.4 solar masses and collapse into a white dwarf. You will learn more
about this in the section, The Hottest Stars.
Stars whose initial mass exceeds 10 solar masses will evolve into Supergiants
and Hypergiants. They too will furiously blow much of their material into space
during their short lives, but the core remnant will exceed the 1.4 solar mass
limit and the star will explode in a spectacular supernova and collapse the
core into either a Neutron Star or Black Hole. We will learn about these monsters
in the section, The Largest Stars.
So, we move forward into the Star Unit, remembering the key concepts ...
star mass is the most important value of a star, and the balance between
thermonuclear fusion's outward pressure and the inward pressure of gravity are
the two forces that affect the lifecycle. Whether big, bright, and exciting,
or small, dim, and boring, I hope you will enjoy this unit because it represents
the best effort I can make to teach you about stars. Below is a sampling, but
I am not going to tell you what any of these images are ... you will learn about
them during this unit.
Once again ... here is the suggested sequence for this unit:
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