JH

Astronomy Lecture Review: Stars, Galaxies, and the Universe

Milky Way and Galactic Scale

  • The Milky Way is a spiral galaxy, approximately 100,000 light-years wide and 10,000 light-years thick.

  • The Local Group contains about 40 galaxies, including the Milky Way.

  • Galaxies are classified into four main types:

    • Elliptical galaxies (about 60%)

    • Barred spiral galaxies (about 20%)

    • Irregular galaxies (about 10%)

    • Spiral galaxies (about 10%)

  • Galaxies can interact through close passes and collisions.

  • The universe is expanding, indicated by redshift (distant galaxies moving away).

  • Blueshift implies objects are moving toward us.

  • The observable universe exhibits background radiation consistent with a Big Bang origin, with an estimated age of about 13.7\times 10^9\ \text{years}.

Binary Stars and Stellar Balance

  • Binary stars are two stars gravitationally bound, affecting each other

  • The center of balance (fulcrum) in a binary system lies closer to the more massive star.

  • If two stars are equal in size, the center of mass is midway between them.

Stellar Birth and Early Life: Protostar to Main Sequence

  • Stars originate from collapsing gas clouds, seeking a balance between outward pressure and inward gravity.

  • Protostar phase: The object is formed but not yet undergoing sustained hydrogen fusion, representing approximately 10% of a star
    H-R diagram.

  • Main Sequence phase: Steady hydrogen fusion in the core sustains luminosity and structure, comprising about 90% of a star

Stellar Mass Classes and Evolutionary Outcomes

  • Stellar mass dictates a star's evolutionary path and final remnant:

    • Low mass stars (M \lesssim 0.5-0.8\ M_\odot):

    • Do not fuse helium in their core during later stages.

    • Evolve from protostar to main sequence, then to a red giant, and finally become a white dwarf. A theoretical black dwarf is a white dwarf cooled to emit negligible radiation, not yet observed.

    • Medium mass stars (roughly 0.5 \lesssim M \lesssim 8\ M_\odot):

    • Follow a path from protostar to main sequence, then to a red giant, shed outer layers to form a planetary nebula, and end as a white dwarf.

    • High mass stars (M \gtrsim 8\ M_\odot):

    • Evolve from protostar to main sequence, then to a red supergiant. They end in a core-collapse supernova, leaving behind either a neutron star or a black hole, depending on the remnant mass.

    • Stars with remnant masses in the range of 8-20\ M_\odot typically result in a neutron star.

    • Very massive stars (>20\ M_\odot) usually result in a black hole.

End States of Stars and Compact Objects

  • White Dwarf: A dense stellar core remnant, supported by electron degeneracy pressure.

  • Black Dwarf: A theoretical, non-luminous white dwarf that has cooled completely, not yet observed in the universe.

  • Neutron Star: Formed after a core-collapse supernova if the remnant mass is below the maximum limit for a neutron star.

    • Extremely compact: diameter about 12\ \text{to}\ 18\ \text{miles}\ \approx 20\text{–}30\ \text{km}.

    • Strong magnetic fields can power pulsars, which emit beams of radiation as they spin.

  • Black Hole: Formed when a core's mass exceeds the neutron star limit; nothing, not even light, escapes its event horizon.

  • Primordial Black Holes: Theorized to have formed in the early universe, distinct from stellar-mass black holes.

  • Supernovae: Explosive end states of stars.

    • Core-collapse supernovae (Type II, Type Ib/c) are associated with massive stars.

    • Type Ia supernovae involve white dwarfs in binary systems.

Stars, Color, Temperature, and the Hertzsprung–Russell Context

  • Stellar color indicates temperature:

    • Blue stars are the hottest.

    • White stars are hot.

    • Yellow/orange stars are cooler.

    • Red stars are the coolest among visible stars.

  • Stars on the main sequence are defined by hydrogen fusion in the core, placing them along a diagonal band on the Hertzsprung–Russell (H-R) diagram.

  • The daytime sky appears blue due to atmospheric scattering; sunrises and sunsets show red/orange hues due to light passing through a longer atmospheric path.

Planets, Dwarfs, Moons, and the Solar System

  • Venus: The brightest object in the nighttime sky (the "morning star"), reflects sunlight, does not produce its own energy.

  • Planets: Celestial bodies that reflect light, unlike stars which produce energy.

  • Miranda: A moon of Uranus, notable for a large surface scar.

  • Charon: Classified as a satellite of Pluto within the Pluto–Charon system, with ongoing discussion about its classification relative to dwarf planets.

  • Dwarf Planets: Include Ceres (in the asteroid belt) and several Kuiper Belt objects like Pluto, Makemake, Haumea, and Eris. QR is a dwarf planet mentioned as having rings.

  • Ganymede: The largest moon of Jupiter, larger than Mercury but still classified as a satellite.

  • Rings: Most ice giants have rings, and at least one dwarf planet (QR) is noted to have rings.

  • Belts and Clouds: The asteroid belt, Kuiper Belt, and Oort Cloud are reservoirs of solar system objects.

    • Short-period comets originate from the inner Solar System, specifically the Kuiper Belt.

    • Long-period comets originate from the Oort Cloud.