PHY 1060 Lecture Notes - Lecture 7: Hayashi Track, Hydrostatic Equilibrium, Main Sequence
Module 7
Chapter 15: Star Formation and the Interstellar Medium
• The interstellar medium: gas and dust between the stars
o Most is gas; 1 percent is interstellar dust
o The gas is very tenuous: about 1 atom per cubic centimeter
o It emits various kinds of light, depending on its
• Dust is in the form of solid grains
o Interstellar soot (iron, silicon, carbon, and more)
o Dust blocks visible radiation from stars, galaxies, etc.: interstellar extinction
• Size of dust particles: large molecules up to 300 nanometers
o Therefore, dust blocks short wavelengths more efficiently
o More red light is let through: interstellar reddening
o Love wavelengths (infrared and radio) penetrate
• A star emits radiation at all wavelengths
o Interstellar dust blocks short-wavelength UV and blue light but lets most long-
wavelength infrared and radio radiation pass
o As a result, a star looks redder and fainter when viewed through interstellar dust
• Most gas and dust is concentrated in relatively dense interstellar clouds
o The material found between clouds is called intercloud gas
• Some regions can be very hot (10^6 K)
o X-rays emitted, but extremely tenuous
o Most intercloud gas is 8000 K
• H II regions (about 10^4 K)
o Hydrogen heated and ionized by ultraviolet light from hot, luminous stars (O and
the hottest B)
o Ionized: stripped of one or more electrons
• At lower temperatures, hydrogen is in single, neutral atoms
o This gas emits radio waves with wavelength of 21 cm
o Light of this wavelength penetrates the dust
o Good for mapping the Milky Way
• The low temperature of dust means that it glows in the infrared
• Many clouds are cold enough for hydrogen to be in the H2 molecule
o These are called molecular clouds
o Dense and cold, and appear dark
• Temperatures are around 10 K, with densities as high as 10^10 molecules/cm^3
o Emit radio waves
o Many other molecules are in the mix
• Some clouds can have masses as large as 10 million times that of the Sun: giant
molecular clouds
o On average, 120, light-years in size
o Stars form in these molecular
• Molecular clouds are cold and dense
o Some places in the cloud are denser than average
o Self-gravity will make these regions collapse
• Rate of collapse is slowed by magnetic fields, turbulence, and angular momentum (spin)
o Collapse and fragmentation lead to dense star-forming molecular-cloud cores
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