Protostars_Short - Proto-Stars Star or Gas? Gas Cloud...

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Unformatted text preview: Proto-Stars Star or Gas? Gas Cloud Contraction Explosion Star How Are Stars Born? Stellar Evolution The energy radiated by stars comes from thermonuclear reactions, which consume hydrogen each second and convert it into helium. The amount of hydrogen in a star is vast, but it is not infinite. Therefore, stars must not have always been shining, nor can they continue to shine forever. Thus, stars must have had a beginning as well as (someday) have an end. Variety of Stellar Ages Components of the ISM Interstellar Gas H II Regions H I Regions Cold Clouds Interstellar Dust Reflection Nebulae Dark Nebulae Reddening Giant Molecular Clouds H II Regions The process for producing clouds of glowing gas near hot stars is called fluorescence. The light emitted from regions of ionized gas consists largely of emission lines, so they are also called emission nebulae. Dark Nebulae While dust clouds are invisible in the optical region of the spectrum, they glow brightly in the infrared. Small dust grains absorb optical and UV radiation very efficiently. The grains are heated by the absorbed radiation, typically to between 20 and 500 K, and reradiate this heat at IR wavelengths. Giant Molecular Clouds In many cases, individual clouds have gathered into large complexes containing a dozen or more discrete clumps. Since the large molecular clouds and complexes are the sites where star formation occurs, most young stars are also to be found in spiral arms. Hydrostatic Equilibrium What conditions must exist for star formation to begin? Sir James Jeans (1877 - 1946) first investigated this problem in 1902 by considering the effects of small deviations from hydrostatic equilibrium. Hydrostatic Equilibrium – At each layer, the downward force of gravity is just balanced by the upward pressure of the material. Virial Theorem The Virial Theorem describes the condition of equilibrium for a stable, gravitationally-bound system. 2K + U = 0 2 (Kinetic Energy) + (Potential Energy) = 0 Jean’s Instability Conditions If 2 x Pressure > Gravity Then Gas Pressure dominates and no collapse If 2 x Pressure < Gravity Then Self gravity dominates and collapse occurs Example 1 HI Cloud T = 50 K n = 500 /cm3 All Hydrogen o = mH nH = 8.4 x 10-22 g/cm3 MJ = 1500 M which is much greater than the 1 - 100 M in an HI cloud Example 2 Giant Molecular Cloud T = 150 K n = 108 /cm3 All Hydrogen o = mH nH = 2 x 10-16 g/cm3 MJ ~ 17 M which is much less than the 100 - 1000 M in a GMC Tff = 4700 year Proto-Star Stages 1. The initial gravitational collapse from interstellar matter is relatively quick. Once the condensation is about 1000 AU in diameter, the time for it to reach hydrostatic equilibrium is measured in thousands of years. 2. Pre-main-sequence gravitational contraction is much more gradual. From the onset of hydrostatic equilibrium to the main sequence requires millions of years. For stars with masses just barely high enough to ignite hydrogen burning, this phase of evolution can take as long as 100 million years. Hayashi Limit Proto-Star Stages 3. Subsequent evolution on the main sequence is very slow, for a star changes only as thermonuclear reactions alter its chemical composition. For a star of 1 solar mass, this gradual process requires billions of years. All evolutionary stages are relatively faster in stars of high mass and slower in those of low mass. Evolutionary Tracks Function of Mass In general, the pre-main-sequence evolution of a star slows down as the star moves along its evolutionary track toward the main sequence. The time for the whole evolutionary process is highly mass-dependent. Stars of mass much higher than the Sun’s reach the main sequence in a few thousand to a million years. The Sun required millions of years; tens of millions of years are required for stars to evolve to the lower main sequence. PRS Question 1. The source of a protostar’s heat is a. Nuclear reactions converting H into He in its core b. Gravitational energy released as the protostar expands c. Gravitational energy released as the protostar contracts d. Nuclear reactions converting He to C and O in its core e. Radioactive decay of particular isotopes Not Understood 1. Lower mass limit Objects under 0.08 M are not able to generate thermonuclear reactions. These “brown dwarfs” are technically not stars nor are they planets, for Jupiter’s mass is only 0.001 M. Many brown dwarfs should exist, but only a few candidates have been identified. 2. Upper mass limit The upper mass limit is harder to calculate, but it is somewhere from 50 to 100 M. (Stars above 30 M are extremely rare.) The internal pressures are so much greater than the self-gravities that these stars are blown apart from within. Not Understood 3. Mass distribution The cluster of stars that is formed contains many more low mass stars than intermediate mass ones. Likewise, there are more intermediate mass objects than high mass stars. As stellar mass increases, the number of stars per that mass decreases. 4. Multiple stars About half of the protostars form gravitationallybound binary star systems. It is believed that single stars are the only ones that can be encircled with planets. ...
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This note was uploaded on 03/04/2012 for the course PHYS 2022 taught by Professor Jarrio during the Spring '12 term at Central GA Tech.

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