STELLAR EVOLUTION: A Condensed Version

I. EARLY FORMATION 1. The interstellar medium (ISM) is set in motion by a supernova shock wave, moving rocky body or star, interacting galaxy, or superwinds from a new star. 2. The nebula condenses in spots due to the force of gravity. These cool clouds of gas and dust are termed molecular cloud complexes (MCC). 3. The contraction causes internal heat and temperature to increase by adiabatic processes. A protostar forms. Protostars are hot enough to glow red. Dust “sacs” around them are termed EGGs (evaporating gaseous globules). These may be the beginnings of a solar system. 4. The star heats up and begins erratic behavior. It may flare, emit gusty and intense stellar winds (superwinds), or exhibit bipolar flow. The dust sac is blown away or thinned. The star is classified as a Pre-Main Sequence (PMS) star or Young Stellar Object (YSO). 5. Some protostars lack the mass necessary to create enough heat for nuclear fusion. They gradually cool and fade away, becoming brown dwarfs. Brown dwarfs are red in color while they still are hot enough to glow in visible light.

II. BIRTH 1. Temperature and pressure become high enough to begin nuclear fusion. 2. Fusion causes center gases to push outward. The outward push balances gravity’s pull. This state is known as hydrostatic equilibrium. The star stops contracting, and a star is born! A brand new adult star is termed a ZAMS (Zero-Age Main Sequence) star.

III. ADULT LIFE OF A STAR (XXX) A star’s mass predestines its personality and life span. Stable adult Main Sequence stars are related by mass-heat-color- lifespan-burn rate and size (The Color Rule): Spectral class M stars (most abundant): Small, cool, red and dim. Will burn the slowest and live the longest (about 50 billion years).

Spectral class A, F, G, K stars (like the Sun): Middle of the road habits. Orange, yellow or white in color. Typically will live for 1-20 billion years.

Spectral class O, B stars (rare, but interesting): Giant, hot, bright, blue stars burn up quickly and die violently. Lifetime is only 1-10 million years. IV. ONE FOOT IN THE GRAVE 1. Stars smaller than 8 solar masses die when hydrogen fusion can no longer proceed. They collapse into a white dwarf. 2. For all but the smallest stars, (less than .4 solar masses) as hydrogen fusion

falters, the core begins to collapse. This creates an outward-moving shock wave that causes the outer layers of the star to expand, becoming a giant. This occurs in about 10 seconds. The star turns red or orange because the expanding gas cools. 3. The temperature of the core increases due to collapse. This now permits helium to be fused into carbon. 4. A superwind begins, causing the star to rapidly lose mass. The larger the star, the more intensely the superwind blows. The largest stars end up with the greatest mass loss. 5. The star may blow off an outer layer once or repeatedly, forming a planetary nebula. At the center of a planetary nebula, a white dwarf star is formed. 6. In stars of 5 solar masses or higher, new nuclear fusion reactions ignite as the Helium core begins to lose the ability to sustain fusion. The most common reactions involve: He ---> C 1 x 108 K C ---> N, Si & O 6 x 108 K O & Si ---> Fe & a small amount of Ni 1 x 109 K

Each new nuclear reaction yields less energy than its predecessor, and requires higher temperatures. 7. These new nuclear reactions begin at the center, pushing the previous still- ongoing nuclear reactions in bursts or pulses further away from the core, causing the star to shrink and swell, creating a pulsating variable star. Larger stars pulsate slowly, smaller stars pulsate more quickly. These stars make up the instability strip and supergiant zones of an H-R Diagram. 8. The cores of massive pulsating stars contain matter that is compressed beyond the normal physical limits of electron pressure. This kind of ultra- dense matter is known as degenerate matter. 9. Stars reach different endpoints of nuclear fusion, depending on their mass: -small stars (up to 4 solar masses): C builds up in core -larger stars: Fe builds up in core

VI. THE END Star death occurs when nuclear fusion ceases. Stars die differently depending on their final mass: 1. White dwarfs - White dwarfs give off remnant light and heat, but are dead stars. They are dense, compact objects made of carbon degenerate matter. Typically, they are smaller than the Earth and have densities of 15 tons/ teaspoon. White dwarfs have shallow, layered atmospheres, and in some cases, crystalline crusts. A. The smallest stars (up to 0.4 solar masses) die quietly. After fusion ceases, they collapse to form a white dwarf. B. Stars between 0.4 - 8.0 solar masses become white dwarfs after going through the planetary nebula stage.

C. If the star is in a close binary system, white dwarfs often nova if they accrete material from the partner star onto their surface. A nova is a rapid brightening of a star, due to a flare off of the surface of the white dwarf. It may happen repeatedly. D. If a white dwarf star is in a close binary system, and accumulates so much material that it reaches 1.4 solar masses, it will explode as a Type Ia supernova. A small, weak neutron star forms.

2. Supernovae - Stars larger than 8 solar masses supernova (blow up) after the supergiant stage. The iron core collapses, but cannot ignite any new fusion reactions. The explosion happens within seconds. Supernovae create new nebulae and elements heavier than iron. A. Extreme main sequence giant stars don’t bother becoming supergiants before dying. They just blow up, creating the most energetic types of explosions known as a hypernova. B. If the supernova leaves behind a mass of 1 or 2 suns, the remnant collapses to form a neutron star. Neutron stars are even more compact and bizarre than white dwarfs, having sizes of 10 km and densities of 100 million tons/teaspoon. Their smaller size is due to the fact that electrons are destroyed by being forced at extreme pressure into the nuclei of atoms. This annihilates both the electrons and the protons, creating much energy and forming a “neutron” star. C. Pulsars are spinning neutron stars that give off radio pulses in a regular pattern with a frequency of 1 to .000000001 seconds. Most pulsars slow down as they age. Most neutron stars are pulsars, but they are not all oriented properly for radio telescopes to detect the radio pulses emanating from the poles of the pulsar, D. Neutron stars with extremely intense magnetic fields are termed magnetars. E. If a supernova leaves behind more than 3 solar masses, its gravitational collapse cannot be halted and it turns into a black hole.