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Life and Death of Stars

The Big Bang
Early Universe
Life and Death of Stars
Galaxy Formation
The Solar System
Exotic objects
End of the Universe
Who created the universe?
What is Time?
Life beyond Earth
NASA Missions
Particle Map
Glossary
Sources
Timeline of the Universe
Between the Big Bang and before the creation of the first stars, the Universe was in the Primordial Era, which occurred between 10^-50 and 10^5 years after the Big Bang. During this era, the Big Bang, the inflation, and Big Bang nucleosynthesis cured. Toward the end of this age, the recombination of electrons with nuclei made the universe transparent for the first time.

Between 10^6 and 10^14 years after the Big Bang, the Universe is/will be in The Stelliferous Era, which is where we are now in the timeline of the Universe. In this era, the Universe is organized into space, stars, galaxies, galaxy clusters, dark matter, and dark energy. Most of the energy produced comes from stars. By the end of this era, bright stars as we know them will be gone, their nuclear fuel exhausted, and only white dwarfs, brown dwarfs, red dwarfs, and black holes will remain.

Between 10^15 and 10^39 years after the Big Bang, the Universe will be in the Degenerate Era. This is the era of brown dwarfs, white dwarfs, and black holes. White dwarfs will assimilate dark matter and continue with a nominal energy output. As this era continues, protons will begin to decay. If proton decay takes place, the sole survivors will be black holes.

Between 10^40 and 10^100 years after the Big Bang, the Universe will be in the Black Hole Era. In this era, organized matter will remain only in the form of black holes. Black holes themselves slowly "evaporate" away the matter contained in them, by the quantum mechanical process of Hawking radiation, gradually losing their mass. By the end of this era, only extremely low-energy photons, electrons, positrons, and neutrinos will remain.

After 10^101 years after the Big Bang, the Universe will be in the Dark Era. By this era, with only very diffuse matter remaining, activity in the universe will have trailed off dramatically, with very low energy levels and very large time scales. Electrons and positrons drifting through space will encounter one another and occasionally form positronium atoms. These structures are unstable, however, and their constituent particles must eventually annihilate. Other low-level annihilation events will also take place, albeit very slowly.

The Stars
At night when it is really dark and the sky is clear, we can see a few thousand stars with the naked eye. However, stars are everywhere throughout the universe and there are quadrillions of them. There are approximately 200 billion stars in our Milky Way galaxy alone. On top of that there are some 200 billion galaxies some very much larger than the Milky Way, each of them containing an average of 200 billion more stars. There are more stars than there are grains of sand on all the beaches of the Earth. Even with the world's largest telescopes, we can see only a fraction of all the stars there are. Stars are the main activity of the universe; they are the factories that produce all of the chemicals and substances that make up our universe, providing us with all that we need to live and survive and to sustain our technological world. Stars are really, really large and so is our Sun, but our Sun is only an ordinary, average sized star. It is 93 million miles from us, yet it appears to be the same size as the moon, which is only 250 thousand miles away. One million Earths could fit inside the Sun, yet it is only an average sized star.

Today's universe is full of stars and galaxies, all of which radiate electromagnetic energy of varying frequencies, such as radio, infrared, visible, ultraviolet, xrays and gamma rays. Heavy elements such as iron, copper, gold, platinum and uranium are manufactured by nuclear fusion in the most massive stars and are spread throughout the universe by exploding supernovae. The world we live in is a treasure trove of elements, which enhance our lives and make our modern, technological world possible and we owe it all to the stars.

When I look up at the nighttime sky, pondering the awesome size and power of the universe, the possibilities are endless. The stars are our birthplace and we are, very literally, made of stardust. The atoms that make up our bodies were forged in a star. The human story pales in comparison to the infinite size, power and age of the universe.

The earliest phase of the universe was characterized by intensely high temperatures of greater than 1 trillion degrees (that is 1,000,000,000,000 degrees) and was smooth, homogeneous, without form and opaque to light. Observations of distant galaxies have allowed scientists to look back in time and view the universe during the final days of the “cosmic dark ages”, just before stars began to form. Infinite energy filled the void. Matter did not yet exist and the four forces of nature were combined as one superforce. Under these conditions, it was not possible for atomic nuclei to form or for matter to exist. Some studies estimate that the first star illuminated the darkness around 100 million years after the Big Bang. Unmlike later stars, the first had only hydrogen available to them as their basic constituent. It is also believed that it took another 370 million years for the first galaxy as massive as our own Milky Way to form and a billion years for galaxies to proliferate. The first stars fundamentally changed the universe and its subsequent evolution.

Star formation
As the temperature of the expanding universe cooled, gravity became an independent force in the universe and, through collisions and gravitational attraction, density fluctuations began to occur, giving gravity a foothold in the smooth clouds of gas and dust. The eventual formation of the first stars would never have happened without this asymmetry of density. As the growing objects got larger, their mass increased and they heated up. Eventually, the heat reached the point where nuclear fusion of hydrogen molecules began to create helium. Under its own weight, gravity caused the star embryo to collapse into a star and the space surrounding the new star was illuminated.

How elements are formed

The elements that exist here on Earth are forged in the stars. The elements beyond helium, the heavier elements, are made in dying stars where the temperatures reach high enough for the stages of fusion beyond hydrogen fusion to be possible. The substances and the elements in nature (and in technology) are constantly being recycled into new substances. We did not invent the concept of ecological recycling, it is one of the Earth's native processes. The atoms in my body were once part of a rock, an earthworm, a tree or another human's body that lived long ago in history. At any given point in history, there is a finite number of atoms and molecules of each natural element, which is the sum total of all of each element that has been produced in all the stars that had existed up to that moment.



Why are the elements rarely found in nature in their native form? It is because the elements are mostly highly reactive. Each element has a tendency to combine with other elements forming complex substances. There are many more complex substances than there are elements. Some examples are water (H2O) which is the compound formed by hydrogen (The "H2" in H2O) and oxygen (the "O" in H2O), salt (NaCl) sodium chloride (sodium is th "Na" in NaCl and Chlorine is the "Cl" in NaCl) and limestone is comprised of calcium, oxygen and carbon. It is interesting to mention the properties of the elements that comprise salt. Salt is highly reactive, in fact it is explosive and flammable when it comes in contact with water. Chlorine is a deadly poisonous gas, so deadly that it is a major component in the manufacture of chemical weapons. Together, as the substance salt, all of those properties disappear making one of the most useful and safe substances we know of. After all, we eat salt every day.

All stars share a common life cycle and proceed through a series of stages. The least massive stars do not even reach stage 2 and their life ends when stage 1 is complete. The more massive stars reach the later stages but can only progress to the next stage if certain conditions of mass, pressure and heat are met. The most massive only reach stage 4, where a violent end to their life, a supernova explosion, awaits them. As they move from one stage to the next, stars use a predetermined series of various fuels, one heavier than the next, until the last of their fuel supplies is depleted, until they fail to meet the conditions to enter the next stage or until iron is created. Only the most massive stars ever reach stage 4, where iron is created. All stars, as they progress through their succeeding stages of fusion, create increasingly heavy and complex elements, and leave a shell of each element behind in a concentric shell. Each shell encircles the previous shell and an outwardly growing series of concentric shells around the core of the star accumulates.

The light from each star has encoded in it the complete chemical composition of the star. The light is a band of all of the wavelengths (frequencies) of the electromagnetic spectrum. The portion of the spectrum visible to humans is called visible light, with the various colors representing the elements in nature. In a particular star only certain elements may exist. For each element that does exist in that star, there is a black line within the color for that element that is called an absorption line. The array of absorption lines within the visible light radiated by the star represents that star's elementary composition.



Stage 1
Depending upon how massive a star is and its core temperature, nuclear fusion will occur at the core of the star. A temperature of 15 million degrees must be reached in order for fusion to take place at all. When hydrogen fusion begins, the star enters stage 1, in which it is fusing hydrogen into helium. When the star uses up all of its hydrogen fuel, its heat and energy output are diminished and are no longer able to offset the force of gravity. The star is squeezed, converting gravitational potential energy into thermal energy. Consequently, gravity takes over and causes the star to begin to collapse in on itself. As it collapses, the pressure, and therefore, the temperature of the star's core rebounds, energy output increases and if adequate heat and pressure are produced, the collapse ceases and the star progresses to the next stage of its life cycle.

Stage 2
If the star is massive enough, the temperature at the core of the star may reach 100 million degrees after stage 1 ends. If that happens, the star begins fusing helium into carbon and oxygen. When the helium is used up, the same sequence of events occurs as at the end of stage 1. Energy output decreases when the helium runs out, gravity causes the beginning of the star's collapse in on itself, pressure and heat rebound, the collapse ceases and, if the core temperature reaches high enough, the star enters stage 3.

Stage 3
If the core temperature is high enough and the star is massive enough, nuclear fusion will proceed to begin fusing carbon and oxygen into magnesium, sodium, aluminum and neon. When the carbon and oxygen is used up, energy output decreases, gravity causes the beginning of the star's collapse in on itself, pressure and heat rebound, the collapse ceases and, if the core temperature is high enough, the star enters stage 4, the beginning of the end for the star.

Stage 4
If the core temperature is high enough and the star is massive enough, nuclear fusion will proceed to begin fusing magnesium, sodium, aluminum and neon into iron, a deadly metal for any star. Once iron is produced, the star rapidly collapses in a matter of days to a sphere maybe the size of the Earth. This time there will be no temperature rebound when fusion ceases and gravity wins. Depending upon the star's original mass, two things may happen. *** detail *** In one second, the exploding star produces as much energy as the Sun will produce in its entire lifetime.

The most famous diagram in astronomy is the Hertzsprung-Russell diagram. The Hertzsprung Russell Diagram shown above classifies stars into five main classes of luminosity and size and describes where stars are relative to the main sequence. This diagram is a plot of luminosity (absolute magnitude) against the color of the stars ranging from the high-temperature blue-white stars on the left side of the diagram to the low temperature red stars on the right side.

This diagram is a plot of 22000 stars from the Hipparcos Catalogue together with 1000 low-luminosity stars (red and white dwarfs) from the Gliese Catalogue of Nearby Stars. The ordinary hydrogen-burning dwarf stars like the Sun are found in a band running from top-left to bottom-right called the Main Sequence. Giant stars form their own clump on the upper-right side of the diagram. Above them lie the much rarer bright giants and supergiants. At the lower-left is the band of white dwarfs - these are the dead cores of old stars which have no internal energy source and over billions of years slowly cool down towards the bottom-right of the diagram.

The Luminosity Classes of Stars

Supergiants
Very massive and luminous stars near the end of their lives. They are sub classified as Ia or Ib, with Ia representing the brightest of these stars. These stars are very rare as only 1 in a million stars become supergiants. The nearest supergiant is Canopus 310 light years away. Others are Betelgeuse, Antares and Rigel.

Bright Giants
These stars have a luminosity between the giant and supergiant stars. Some examples are Sargas and Alphard.

Normal Giants
These are mainly low-mass stars at the end of their lives that have swelled to become a giant star. This category also includes some high mass stars evolving on their way to supergiant status. Some examples are Arcturus, Hadar and Aldebaran.

Subgiants
Stars which have begun evolving to giant or supergiant status. Some examples are Alnair and Muphrid.

Dwarfs
All normal hydrogen-burning stars. Stars spend most of their lives in this category before evolving up the scale. Class O and B stars in this category are actually very bright and luminous and generally brighter than most Giant stars. Some examples are the Sun, Sirius, and Vega.

Massive stars live fast and die young. They burn brightly throughout their short lives and use their fuel quickly just like a car that is driven at a high rate of speed does. The first stars lived in the fast lane. They were exceptionally massive and bright but were metal poor, since there had yet been no prior stars to create the first heavier elements and metals. As the early massive stars began dying, the cycle of star birth and death became the predominant activity of the growing universe. When the most massive stars die, the largest of them explode in a supernova, leaving behind a smattering of heavy elements which are scattered throughout the Universe. This scattering of heavy elements includes the gold in your wedding band, the silver from which your eating utensils were manufactured and the carbon that is the diamond in your engagement ring. The uranium that is used to manufacture nuclear weapons and to drive our power generating stations also originate in this scattering of material. This ongoing activity is directly responsible for creating the conditions that enabled our solar system to form and for us to eventually evolve. Gradually, succeeding generations of stars had more metals in their cores and produced more as they died. The death of one star creates the conditions for the birth of the next.