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.