History of Astronomy Revision

History of Astronomy

• Early astronomers recognized five wanderers: Mercury, Venus, Mars, Jupiter, and Saturn.
• They understood lunar phenomena, such as the phases of the Moon and lunar eclipses.
• Aristotle provided evidence of a curved earth via its shadow during lunar eclipses.
• Aristarchus proposed a heliocentric model, asserting that the Sun is farther from Earth than the Moon.
• Eratosthenes calculated Earth’s circumference to be approximately 46,000 km.
• Hipparchus classified stars into six groups based on brightness.
• Ptolemy promoted the geocentric model, placing Earth at the center of the universe.
• Nicolaus Copernicus concluded from observations that the Sun is the universe's center.
• Johannes Kepler derived three laws of planetary motion using observations from Tycho Brahe.
• Galileo Galilei invented the first telescope, discovering Jupiter’s four largest moons.
• Sir Isaac Newton applied mathematics to astronomy, co-invented calculus, and made advancements in physics and light.
• William Herschel discovered Uranus using a telescope.
• Edwin Hubble revealed that some observed clouds were distant galaxies, proving galaxies are moving apart.

Cosmic Models

• Geocentric Model: Earth-centered view of the universe, dominant before heliocentric models.
• Heliocentric Model: Sun-centered cosmic model where planets, including Earth, orbit the Sun.

The Origins of the Universe

• Big Bang Theory: Universe originated 13.7 billion years ago from a singular point, expanding from high density and temperature.
• Proposed by Georges Lemaître; supported by Hubble's law and Cosmic Microwave Background Radiation.

Big Bang Timeline

• t=0: Birth of the universe.
• t=10^-43 seconds: Time and space began expanding.
• t=10^-34 seconds: Light matter forms; electrons and positrons arise.
• t=1/10000 seconds: Protons and neutrons form from particle collisions.
• t=1 second: Universe expands to over 10 trillion kilometers, cooling to about 10 billion degrees Celsius.
• t=5 minutes: Nuclei of hydrogen, helium, and lithium form.
• t=300,000 years: First atoms emerge as the universe cools to 3000 K.
• t=200 million years: Formation of first stars and galaxies.
• t=1 billion years: First planets develop.

Universe Theories

• Steady State Theory: Proposes continuous matter creation to maintain constant average density, suggesting no true beginning or end to the universe.

Possible Endings of the Universe

• Big Freeze: Universe experiences maximum entropy leading to heat death.
• Big Rip: Expansion overwhelms gravity, tearing galaxies apart and dissipating matter.
• Big Bounce: Universe oscillates between expansion and contraction, avoiding singularity.

Objects in the Universe

• Universe: Totality of all existing matter and energy.
• Star: Luminous ball primarily of hydrogen and helium, held by gravity.
• Planet: Celestial body orbiting a star.
• Moon: A celestial body orbiting a planet; Earth's only natural satellite.
• Galaxy: Massive collection of stars rotating around a central object.
• Black Hole: Region of space with gravitational pull so strong that nothing escapes.
• Constellation: Pattern formed by a group of stars.
• Comet: Ice and dust object that orbits the Sun.
• Asteroid: Small rocky object orbiting the Sun.
• Nebula: Cloud of gas and dust where stars are born.

Types of Galaxies

• Spiral Galaxies: Feature flat disks filled with stars, gas, and dust; includes normal and barred spirals; example: Milky Way.
• Elliptical Galaxies: Contain older stars, less gas; exist in various sizes as giant ellipticals.
• Irregular Galaxies: Lack a defined shape, rare in the universe today.

Star Classification

• Stars categorized into spectral classes based on temperature and color.
• Classification includes:
  • O: Blue (30,000 - 60,000 K), hot and luminous.
  • B: Blue (10,000 - 30,000 K), very luminous.
  • A: White (7,500 - 10,000 K), commonly occurring.
  • F: White-yellow (6,000 - 7,500 K), average size.
  • G: Yellow (5,000 - 6,000 K), like the Sun.
  • K: Orange (3,500 - 5,000 K), cooler stars.
  • M: Red (2,000 - 3,500 K), common stars.

Types of Stars

• Main Sequence Stars: Generate energy through nuclear fusion; brightness correlates with temperature.
• Dwarf Stars: Small in size; includes red and yellow dwarfs.
• Giant and Supergiant Stars: Large, old stars; supergiants explode as supernovae.
• White Dwarf: Small, dense star primarily made of carbon, remnants of larger stars.