Chapter Sixteen: The Sun and the Stars

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Chapter Sixteen The Sun and the Stars

• 16.1 The Rest of the Universe!

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16.1 Stars

• A star is essentially an enormous, hot ball of
  gas held together by gravity.
• The density at the suns core is about 158.0
  g/cm3.

This is about 18 times the density of solid
copper!
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16.1 Why stars shine

• Due to the high density, nuclear fusion occurs,
  releasing tremendous amounts of energy.
• Fusion reactions in the sun combine hydrogen to
  make helium.

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16.1 Types of stars

• Stars come in a range of sizes and temperatures.
• Blue giant stars are hot and much more massive
  than the sun.
• Stars that are smaller than the sun come in two
  main categories, dwarfs and neutron stars.

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16.1 Distance to the nearest stars

• One light year is the distance that light travels
  through space in one year.
• A light year is a unit of distance, not time.

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16.1 Temperature and color

• Red stars are cooler than white stars, and blue
  stars are the hottest.
• White light is a mixture of all colors at equal
  brightness.

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16.1 Brightness and luminosity

• From experience, you know that as you move away
  from a source of light, the brightness decreases.

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16.1 Temperature and luminosity

• There is a diagram for classifying stars the
  Hertzsprung-Russell, or H-R diagram.
• H-R diagrams are useful because they help
  astronomers categorize stars into groups.

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16.2 Nebulae, birth and life span of stars

• A star, regardless of its size, begins its life
  inside a huge cloud of gas (mostly hydrogen) and
  dust called a nebula.
• The Eagle Nebula is the birthplace of many stars.

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16.2 Red giant formation

• Eventually the core of a star runs out of
  hydrogen.
• The core contracts, raising the temperature.

• The hotter core radiates more energy, pushing the
  outer layers of the star away.
• The star expands into a red giant as the outer
  layers cool.

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16.2 White dwarves

• Once the helium is used up, the nuclear reactions
  essentially stop.

• With no more energy flowing outward, nothing
  prevents gravity from crushing the matter in the
  core together as close as possible.
• At this stage, the core glows brightly and is
  called a white dwarf.

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16.2 Supernovae

• Stars of more than 12 times the mass of the sun
  have a violent end.
• The result is a spectacular explosion called a
  supernova.

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17.1 Galaxies

• A galaxy is a huge group of stars, dust, gas, and
  other objects bound together by gravitational
  forces.
• The sun, along with an estimated 200 billion
  other stars, belongs to the Milky Way galaxy.

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17.1 Galaxies

• Astronomers classify galaxies according to their
  shape.
• Spiral galaxies like the Milky Way consist of a
  central, dense area surrounded by spiraling arms.
  
• Barred spiral galaxies have a bar-shaped
  structure in the center.
• Elliptical galaxies look like the central portion
  of a spiral galaxy without the arms.
• Lenticular galaxies are lens-shaped.

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17.1 Distances between galaxies

• The distances between stars are 10,000 times
  greater than the distances between planets.
• The distances between galaxies are a million
  times greater than the distances between stars.