Chapter 27: Stars and Galaxies

Observing the Night Sky

• Constellations are groups of stars named in antiquity.
• Ursa Major, the Great Bear, is a familiar constellation.
• The constellations visible in the night sky change monthly due to Earth's orbit around the Sun.
• Solar eclipses reveal constellations typically seen six months earlier or later.
• The Big Dipper is a well-known asterism (a pattern of stars that is not a constellation).
• The stars at the end of the Big Dipper's bowl point to Polaris, the North Star.
• The North Star is not the brightest star; Sirius is.
• The stars of the Big Dipper are at varying distances from Earth.
• Time-exposure photos show streaks of stars due to Earth's rotation.
• Watching stars in the northern sky at night is like observing a clock that runs four minutes fast daily.

The Brightness and Colors of Stars

• A star's color indicates its temperature; red stars are cooler than blue stars.
• A blue star is approximately twice as hot as a red star.
• Blue light has roughly twice the frequency of red light.
• Apparent brightness is how bright a star appears to us, while luminosity is its intrinsic brightness.
• The luminosity of stars is compared to the Sun's luminosity (L_{Sun}).
• Based on the inverse-square law, if a spaceship were twice as far from the Sun, the Sun's apparent brightness would be one-quarter of what it is now.

The Hertzsprung–Russell Diagram

• The Hertzsprung-Russell (H-R) diagram is a graph of intrinsic brightness versus surface temperature for stars.
• The H-R diagram plots stars based on their position in the main sequence, as well as the positions of exotic stars above or below the Main Sequence.
• The H-R diagram is to astronomy what the periodic table is to chemistry.
• On the H-R diagram, the Sun is considered an average star.
• A dying star that has collapsed to a small size and is cooling off (a white dwarf) would appear in the lower-left part of the H-R diagram.

The Life Cycles of Stars

• The life cycle of a star begins as a nebula, then progresses to a protostar.
• A star is born when fusion occurs in its core.
• Stars spend 90% of their lives on the main sequence.
• Depending on its mass, a star may:
  • Become a red giant and then burn out to leave a white dwarf behind.
  • Die in a supernova explosion, leaving a neutron star or black hole.
• White dwarfs cool for eons until they no longer emit light, sometimes pausing to crystallize.
• If a white dwarf is part of a binary system, it can pull matter from its partner, leading to a nuclear blast (nova).
• Massive stars undergo core collapse, resulting in a supernova.
• The Crab Nebula is a remnant of a supernova witnessed in 1054 A.D.
• The source of energy in stars is thermonuclear reactions.
• The fusion of hydrogen into helium releases energy, powering stars on the main sequence.
• Fusion occurs in the star's core, where temperatures are high enough.
• The proton-proton chain is the dominant process for fusion in lower-mass stars.

Black Holes

• A black hole can form from the collapse of a supergiant star's core.
• It is named because the gravitation at its surface is so intense that even light cannot escape.
• Black holes are spherical regions of space surrounding a collapsed object; the boundary is the event horizon.
• When a star collapses, gravitation at its surface increases due to the inverse-square law.
• If a star shrinks to half its radius, gravitation at its surface increases by a factor of 4.
• If a star collapses to one-tenth its size, gravitation at its surface becomes 100 times as much.
• Gravitation increases mainly at the surface of the collapsed star.
• If the Sun collapsed into a black hole, Earth's orbit would remain unchanged because the mass and distance would remain the same.
• F = G(m1 \\times m2)/d^2 illustrates that if mass and distance (d) remain constant, gravitational force (F) would not change.

Galaxies

• A galaxy is a vast collection of stars, interstellar gas, and dust.
• The Milky Way is the most familiar galaxy.
• Three types of galaxies are:
  • Elliptical
  • Irregular
  • Spiral
• Active galaxies emit a large amount of energy, significantly more than the Milky Way.
• Examples of active galaxies include:
  • Starburst galaxies
  • Galaxies with an active galactic nucleus (AGN)
• Starburst galaxies have a very high rate of star formation due to violent disturbances like galaxy collisions.
• Some active galaxies have supermassive black holes at their centers, causing jets extending thousands of light-years from the center (AGN).
• Individual stars in a galaxy typically move in elliptical orbits around the galaxy's center.

Clusters and Superclusters

• The Milky Way and its neighboring galaxies form the Local Group.
• The Local Group is located between the Virgo and Eridanus clusters, forming the Local Supercluster.
• The Local Supercluster is part of a network of superclusters.
• Galaxies are arranged in a foam-like structure with large voids.