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.

****Go to the bottom of this page for the exact sequence of this unit for NSO students.****

STARS

All stars have lifecycles, and every lifecycle follows a path determined by the mass. It is the mass that is the all important single variable that controls the birth, life, and death of every 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 pressure and 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, symbolized by MO. 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 live 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 (about 7 million K), 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 all of 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 these 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 ignites (above and right).

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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 than star, and have a mass less than 0.08 solar masses. The do not have enough mass to raise the internal core pressure and temperature to 7 million K, and this ignite the nuclear fires. They never become stars, and are more like wannabees.

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.

Sun-class Stars 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 can 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 Green Men."

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:

Brown Dwarfs

Red Dwarfs

Sun-Class Dwarfs

Red Giants

White Dwarfs

Supergiants

Supernovae

Neutron Stars

Black Holes

You may now cautiously proceed to the Spectral Classes of Stars, or return to the Star Introduction, but do not go to the Syllabus yet.


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