Science

• Star


  A large ball of gas held together by gravity with a core so hot that nuclear fusion occurs
• Nuclear fusion


  1. Nuclei of several atoms combine into one larger nucleus

  2. Releases a large amount of energy
• A star shines because when energy leaves a star's core, it travels throughout the star and radiates into space
• Spectra


  Provide information about stellar composition
• Stellar


  • Science use: related to stars
  • Common use: outstanding, exemplary
• Interior layers of a typical star

  • Core
  • Radiative zone
  • Convection zone
• When first formed, all stars fuse hydrogen into helium in their cores
• Radiative zone


  A shell of cooler hydrogen around a star's core
• Convection zone


  Hot gases move toward the surface as cooler gases move down into the interior
• Layers of a star's atmosphere

  • Photosphere
  • Chromosphere
  • Corona
• Photosphere


  The apparent surface of a star, where light energy radiates into space
• Chromosphere


  The orange-red layer above the photosphere
• Corona


  The wide, outermost layer of a star's atmosphere
• Sunspots


  • Cooler regions of magnetic activity
  • Seem to move as the Sun rotates
  • Number varies on an 11-year cycle
• Coronal Mass Ejections (CMEs)


  • Huge gas bubbles ejected from the corona
  • Larger than flares
  • May reach Earth
  • Can cause radio blackouts
• Prominences


  • Clouds and jets of gases forming loops into the corona
• Flares


  • Sudden increases in brightness, often near sunspots or prominences
• Solar wind


  • Caused by charged particles streaming away from the Sun
  • Extends to the edge of the solar system
  • Causes auroras
• Star systems


  • Most stars exist in star systems bound by gravity
• Star clusters


  • Many stars exist in large groupings called clusters
  • Stars in a cluster all formed at about the same time and are the same distance from Earth
• Spectra


  Scientists classify stars according to their spectra
• Though there are exceptions, color in most stars is related to mass
• Star colors


  • Blue-white (most mass)
  • White
  • Yellow
  • Orange
  • Red (least mass)
• Hertzsprung-Russell diagram


  • A graph that plots luminosity against temperature of stars
  • Y-axis displays increasing luminosity
  • X-axis displays decreasing temperature
  • Most stars exist along the main sequence
  • Mass of a main-sequence star determines both its temperature and its luminosity
• Hot gas moves up and cool gas moves down in the Sun's convection zone
• Sunspots are relatively dark areas on the Sun that have strong magnetic activity
• Globular clusters contain hundreds of thousands of stars
• Nebula


  A cloud of gas and dust (from Latin nebula, means "mist" or "little cloud")
• Star formation


  1. Gravity causes the densest parts of a nebula to collapse, forming regions called protostars

  2. Over many thousands of years, the energy produced by protostars heats the gas and dust surrounding them

  3. A star becomes a main-sequence star as soon as it begins to fuse hydrogen into helium
• Low-mass stars stay on the main sequence for billions of years, and high-mass stars are there for only a few million years
• When a star's hydrogen supply is nearly gone, the star leaves the main sequence and begins the next stage of its life cycle
• All stars form in the same way, but stars die in different ways, depending on their masses
• Red supergiant


  The stage massive stars eventually become
• End of a lower-mass star

  1. After helium in the cores is gone, the stars cast off their gases, exposing their cores

  2. The core eventually becomes a white dwarf, a hot, dense, slowly cooling sphere of carbon
• The Sun will remain on the main sequence for 5 billion more years
• When the Sun becomes a red giant for the second time, it will probably absorb Earth and push Mars and Jupiter outward
• When the Sun becomes a white dwarf, the solar system will be a cold, dark place
• End of a very massive star

  1. Iron in the core does not fuse and the core collapses quickly under the force of gravity

  2. The normal space within atoms is eliminated, leaving a dense core of neutrons, or a neutron star

  3. For the most massive stars, atomic forces holding neutrons together are not strong enough to overcome so much mass in such a small volume, and gravity crushes the matter into a black hole
• Black hole


  An object whose gravity is so great that no light can escape
• Recycling of matter


  1. When a star becomes a white dwarf, it casts off hydrogen and helium gases in its outer layers, forming a planetary nebula

  2. During a supernova, a massive star comes apart sending a shock wave into space, forming a supernova remnant

  3. Gravity causes recycled gases and other matter to clump together in nebulae and form new stars and planets