3.3 - The Stars

Star Types

• Protostar: A stage before a star forms, consisting of gas collapsing from a giant molecular cloud.

  • Duration: Around 100,000 years.

  • Energy release from gravitational energy, no nuclear fusion.

  • Size: Can be up to 40 astronomical units (au).

• Brown Dwarf: Objects between giant planets and small stars.

  • Mass: 15 to 75 times the mass of Jupiter.

  • Classified as "failed stars"; unable to sustain hydrogen fusion.

• Main Sequence: Stars in hydrostatic equilibrium.

  • Inward gravitational pull balanced by outward light pressure from fusion.

  • Size dependent on mass; minimum mass ~0.08 solar masses.

  • Can grow to >100 solar masses.

Main Sequence Stars Characteristics

• Spectral Types and Properties:

  • O: >40,000K, Radius ~10x Sun, Mass ~50x Sun, Luminosity ~100,000x Sun.

  • B: 20,000K, Radius ~5x Sun, Mass ~10x Sun, Luminosity ~1,000x Sun.

  • A: 8,500K, Radius ~1.7x Sun, Mass ~2x Sun, Lifespan ~10 million years.

  • F: 6,500K, Radius ~1.3x Sun, Mass ~1.5x Sun.

  • G: 5,700K, Radius ~1x Sun, Mass ~1x Sun, Lifespan ~10 billion years.

  • K: 4,500K, Radius ~0.8x Sun.

  • M: 3,200K, Radius ~0.3x Sun, Lifespan ~200 billion years, most abundant type.

• Giant Stars: Low mass stars nearing end of life.

  • Mainly G, K, M spectral types, larger and brighter, mass 1-5 solar masses.

• White Dwarf: Remnants of dying stars.

• Supergiant Stars: Massive stars nearing end of life, consuming hydrogen rapidly.

Red Dwarf

• Most common stars in the Universe.

• Mass: 0.075 to 0.5 solar masses.

• Cooler than the Sun, conserve hydrogen fuel, potentially burning for up to 10 trillion years.

Supergiant Stars

• Largest stars in the Universe, mass dozens of times that of the Sun.

• Rapid consumption of hydrogen leading to short lifespans ending in supernova explosions.

Variable Stars

• Change in brightness from Earth’s perspective.

  • Changes can range significantly, including our Sun's output, which varies by ~0.1% over 11-year cycles.

Binary Stars

• More than 80% of stars are part of binary systems (two stars orbiting together).

• Important for star classification and potential impact on life development.

Red Giant Phase

• Occurs after hydrogen in the core is exhausted.

  • Inward pressure dominates, causing expansion of outer layers.

  • Size can increase up to 100 times its former size, lasting a few hundred million years.

White Dwarf

• Formed from stars with insufficient mass to undergo further fusion.

  • Collapses under gravity, no further fusion, will cool down over hundreds of billions of years.

Black Dwarf

• Theoretical end stage of evolution, when a white dwarf cools down completely.

  • Takes quadrillions of years to form; universe still too young for black dwarfs.

Neutron Star

• Formed from the core collapse of massive stars.

  • Extremely dense, with neutron composition.

  • Size = ~20 kilometers, with immense mass; 1 sugar cube weighs about 1 trillion kg.

Pulsar

• A rapidly spinning neutron star that emits twin beams of light.

  • Appears to blink from Earth due to its rotation.

  • Can spin hundreds to thousands of times per second.

Black Hole

• Region in space with gravity so strong that light cannot escape.

• Formed from dying stars; can’t be seen directly; detected through gravitational effects.

  • Types include stellar black holes (up to 20 solar masses) and supermassive black holes (mass >1 million solar masses).

  • Example: Sagittarius A* in the Milky Way, mass ~4 million solar masses.