Beyond Solar System

• Apparent group of stars originally named for mythical characters.

  Constellation
• The sky contains 88 constellations.
• Clue to a star’s temperature.

  Color
• One of two stars revolving around a common center of mass under their mutual gravitational attraction.

  Binary star
• Binary stars are used to determine the star property most difficult to calculate:

  Mass
• Slight shifting of the apparent position of a star due to the orbital motion of Earth.

  Parallax
• Distance light travels in a year, about 9.5 trillion kilometers.

  Light-year
• Brightness of a star when viewed from Earth.

  Apparent magnitude
• Apparent brightness of a star if it were viewed from a distance of 32.6

  light-years.

  Absolute magnitude
• Shows the relationship between the absolute magnitude and temperature of stars.

  Hertzsprung–Russell diagram
• Star that falls into the main sequence category on the H–R diagram.

  Main-sequence star
• Large, cool star of high luminosity;

  Red giant
• Very large, very bright red giant star.

  Supergiant
• Star whose brightness varies periodically because it expands and contracts; it is a type of pulsating star.

  Cepheid variable
• Star that explosively increases in brightness.

  Nova
• Cloud of gas and/or dust in space.

  Nebula
• Two major types of nebulae:

  Bright nebula

  Dark nebula
• Bright nebula consists:

  Emission nebula

  Reflection nebula
• Collapsing cloud of gas and dust destined to become a star—a developing star not yet hot enough to engage in nuclear fusion.

  Protostar
• Main-Sequence Stage

  • Stars age at different rates.

  Massive stars use fuel faster and exist for only a few million years.

  • Small stars use fuel slowly and exist for perhaps hundreds of billions of years.

  • A star spends 90 percent of its life in the main-sequence stage.
• Red-Giant Stage

  • Hydrogen burning migrates outward. The star’s outer envelope expands.

  • The core collapses as helium is converted to carbon. Eventually all nuclear fuel is used and gravity squeezes the star.

  • Its surface cools and becomes red.
• Burnout and Death

  • All stars, regardless of their size, eventually run out of fuel and collapse due to gravity.

  Death of Low-Mass Stars

  • Stars less than one-half the mass of the sun never evolve to the red giant stage but remain in the stable main-sequence stage until they consume all their hydrogen fuel and collapse into a white dwarf.
• ◆ Death of Medium-Mass Stars

  • Stars with masses similar to the sun evolve in essentially the same way as low-mass stars.

  • During their collapse from red giants to white dwarfs, medium-mass stars are thought to cast off their bloated outer layer, creating an

  expanding round cloud of gas called planetary nebula.
• Exploding massive star that increases in brightness many thousands of times.

  Supernova
• The massive star’s interior condenses and may produce a hot, dense object

  Neutron star or a black hole
• Burnout and Death

  ◆ H–R Diagrams and Stellar Evolution

  • Hertzsprung–Russell diagrams have been helpful in formulating and testing models of stellar evolution.

  • They are also useful for illustrating the changes that take place in an individual star during its life span.
• Star that has exhausted most or all of its nuclear fuel and has collapsed to a very small size, believed to be near its final stage of evolution.

  White dwarf
• The sun begins as a nebula, spends much of its life as a main-sequence star, and then becomes a red giant, a planetary nebula, a white dwarf, and, finally, a black dwarf.
• Star of extremely high density composed entirely of neutrons.

  Neutron star
• Source that radiates short bursts or pulses of radio energy in very regular periods.

  Pulsar
• Massive star that has collapsed to such a small volume that its gravity prevents the escape of everything, including light.

  Black hole
• Group of stars, dust, and gases held together by gravity.

  Galaxy
• Three distinct spiral arms, with some splintering.

  Milky Way
• About 30 percent of all galaxies are spiral galaxies.
• About 60 percent of galaxies are classified as elliptical galaxies.
• Only 10 percent of the known galaxies have irregular shapes and are classified as irregular galaxies. Irregular galaxies contain young

  stars.
• System of galaxies containing several to thousands of member galaxies.

  Galaxy cluster
• Toward the red end of the spectrum, occurs because the light waves

  are “stretched,” which shows that Earth and the source are moving away from each other.

  Red shift or a Doppler shift
• Law that states that the galaxies are retreating from the Milky Way at a

  speed that is proportional to their distance.

  Hubble’s law
• States that at one time, the entire universe was confined to a

  dense, hot, supermassive ball. (13.7 billion years ago)

  Big bang theory