Star Stellar System

• Main sequence star


  Most of the stars in our galaxy are main sequence stars. They are converting hydrogen into helium in their cores, releasing a tremendous amount of energy
• Types of main sequence stars

  • Small (red, cool 3000C-4000C)
  • Medium (yellow-white, fairly hot 6000C-10,000C)
  • Large (blue, very hot 30,000C)
• Main sequence star


  • In a state of equilibrium, gravity pulling inward and light pressure from fusion pushing outward, maintaining a spherical shape. Size depends on mass which determines gravity
• Yellow dwarf (Sun-sized) star


  Medium-small main sequence star, yellow hot (6000°C) at the surface. Our sun is an example
• A star is a hot ball of mostly hydrogen gas, gravity keeps the gas from evaporating and pressure from the high temperature and density sustains nuclear fusion reactions
• Stellar evolution


  Building up heavier elements from lighter ones by nuclear reactions, and adjusting the internal structure to balance gravity and pressure
• Blue giants


  • Huge, very hot (blue hot) stars that burn through their fuel quickly and have a short lifespan
• Supergiants


  • The largest stars in the universe, form when large, blue main sequence stars run out of fuel and start to collapse, dozens of times the mass of the Sun
• Nuclear burning


  The nuclei of atoms fusing into nuclei of heavier atoms, not chemical burning
• How stars evolve


  Start with mostly hydrogen, helium and small amounts of heavier elements, generate energy by converting hydrogen to helium in their cores (fusion)
• The energy produced by nuclear burning heats the star's interior to millions or billions of degrees Fahrenheit
• Nebula


  Interstellar clouds of dust and gas, some several light years in diameter and 100 times bigger than our solar system
• Emission nebula


  Clouds of high temperature gas and dust, usually red because hydrogen emits red light
• Main sequence star


  A star that burns hydrogen in its core, the Sun is an example
• The Sun's brightness is equivalent to four trillion trillion 100-watt light bulbs, all generated by hydrogen burning in the core
• Life cycle of a star

  • Main sequence stars
  • Giants and supergiant stars
  • Faint, virtually dead stars
• Red giant


  • 100 times bigger than originally, cooler, frequently orange in color, 20 times as massive as the Sun and 14,000 times brighter
• Red giant formation


  When a small to medium (Sun-sized) main sequence star has used up most of the hydrogen in its core, fusion stops and the core contracts, causing the outer shell of hydrogen to ignite and balloon out dramatically
• Supergiants


  • Largest known type of star, some almost as large as our entire solar system, consume hydrogen fuel at an enormous rate and will explode as supernovas within a few million years
• Supernova


  When a blue main-sequence/supergiant runs out of fuel, gravity causes a final collapse that creates huge pressures and temperatures (8 billion C) leading to an explosion
• Dwarf stars


  • Small, very dense, made of carbon, about the size of Earth but much heavier, formed from red giants losing their outer layers
• White dwarf


  When a medium-size main sequence/red giant star has run out of hydrogen fuel and lacks the mass for higher fusion, it collapses inward under its own gravity
• White dwarf


  • Only about the size of Earth but with most of the Sun's mass, core temperatures soar to 200 million C and the helium fuses to carbon
• Black dwarf


  When a white dwarf has turned most of its helium core into carbon and cooled to the background temperature of the universe, it becomes a cold, dark carbon cinder
• Neutron star


  • Very small, super dense, tightly packed neutrons, has a thin hydrogen atmosphere and a diameter of 5-10 miles, can be a rapidly spinning pulsar
• Black hole


  A region of space containing a huge amount of mass compacted into an extremely small volume, with gravitational influence so strong that nothing can escape
• Stars less than 8 solar masses stop nuclear burning at core helium burning, while more massive stars continue burning heavier elements up to iron
• The evolution from main sequence to red giant occurs at different times for different stars, with hotter, more massive stars becoming red giants in 10 million years compared to 10 billion years for the Sun
• Star formation


  The process by which dense regions within molecular clouds in interstellar space collapse and form stars, inside a dust cloud called a nebula
• Nebula


  An interstellar cloud of dust, hydrogen, helium and other ionized gases, considered the birthplace of new stars
• Types of nebulae


  • Planetary nebula
  • H II regions
  • Supernova remnant
  • Dark nebula
• Planetary nebula


  A region of cosmic gas and dust formed from the cast-off outer layers of a dying star
• Supernova remnant


  The remains of a supernova explosion, extremely important for understanding the galaxy
• Diffuse nebula


  Extended nebulae with no well-defined boundaries, can be further classified as emission, reflection or dark nebulae
• Emission nebula


  Nebulae that emit spectral line radiation from excited or ionized gas, mainly ionized hydrogen, also called H II regions
• Dark nebula


  Interstellar clouds that contain a very high concentration of dust, allowing them to scatter and absorb all incident optical light
• Star formation process


  Relatively dense concentrations of interstellar gas and dust (molecular clouds) collapse under their own gravity, fragmenting into clumps that form protostars
• Protostar


  A collection of gas and dust that has collapsed down from a giant molecular cloud, before nuclear fusion reactions have started
• Molecular clouds


  Relatively dense concentrations of interstellar gas and dust where stars form
• Molecular clouds are extremely cold (temperature about 10 to 20K, just above absolute zero)