Cosmology

• Key Facts about Earth

  Age - The earth is 4.5 billion years old.

  Distance from the sun – 150 million kms

  Orbit – It orbits around the sun in 365 days. One full orbit of the sun is called a year.

  Leap year – Normal year consists of 365 days. Every 4 years one extra day is added to the calendar to make 366 days.

  This is called a leap year.
• Why do we have leap years – Since the earth orbits around the sun in 365.25 days, but a calendar year is made up of

  365 days only, we add an extra day every 4 years. Then the calendar year reflects more accurately the

  time taken by earth to revolve around the sun.
• Seasons are when the earth spins on its axis, when the northern hemisphere is titled towards the sun, it is constant day in north pole.

  However, the south pole is then at constant nighttime

  Earth tilts while orbiting the sun. June is summer in the Northern Hemisphere as it leans towards the sun. December brings winter to the Northern Hemisphere when the Southern Hemisphere tilts towards the sun.
• The earth rotates around its axis and this causes day and night. The time the earth takes to complete one rotation

  around its axis is 24 hours and forms “one day”. Solar day is relative to the sun.

  Sidereal day - Sidereal day is the time taken by earth to complete one rotation about its axis with respect to distant

  stars. It is 23hours and 56 minutes.
• Tilted Earth in summer means longer days and shorter nights away from the equator, as the Northern Hemisphere gets more sunlight, and vice versa for the Southern Hemisphere.
• The moon orbits earth every 27 days

  tidally locked with the earth, that is the same side of the moon is always seen from earth (earth is NOT tidally locked with the sun)
• Tides: the rising and falling of sea levels caused by gravitational forces exerted by the moon on the earths oceans
• High tide: high tide occurs on the side of the earth's facing the moon

  Low tide: when the opposite side of the earth is facing away from the moon, on this side, gravitational forces are weaker
• High and low tides result from the moon's gravitational pull, creating tidal forces that cause water to bulge, forming tidal tides.
• Lunar eclipses happen when Earth's shadow falls on the Moon. Solar eclipses occur when the Moon comes between the Sun and Earth, casting a shadow on Earth.
• White dwarfs are small, dense stars leftover from when other stars die. They're made of dense material and cool down over a long time.
• The universe is expanding, and evidence points to a starting point called the "Big Bang," where everything was once compressed into a tiny point, an extremely hot and dense point called the singularity.
• Hubble's Law states that the farther a galaxy is from us, the faster it appears to be moving away.
• Penzias and Wilson stumbled upon the Cosmic Microwave Background Radiation (CMBR) a faint microwave signal coming from all directions in space. This discovery supports the Big Bang theory because the CMBR is considered leftover radiation from the early universe, validating the Big Bang's existence and helping us understand the universe's beginnings.
• Dark matter is invisible material in the universe that we can't see but can feel its gravitational pull.

  
  

  
  

  
  

  
  

  doesn't emit light or energy
• Dark energy is a mysterious force that causes the universe's expansion to accelerate, pushing galaxies farther apart from each other.
• • A galaxy is a vast system of stars, interstellar gas, dust, and dark matter bound together by gravity.
• 
  

  • furthest planet from the sun : Neptune
  • closest planet to the sun : Mercury
• • Universe:
  • The entirety of space, time, matter, energy, and all the galaxies, stars, planets, and other celestial objects within it.
  • Believed to be around 13.8 billion years old and expanding since the Big Bang.
  • Contains billions of galaxies, each containing billions of stars and planetary systems.
• Low-mass stars, such as the Sun, evolve through these stages:

  1. Main Sequence: Lasts billions of years, with hydrogen fusion in the core producing energy.
  2. Red Giant: Hydrogen fusion shifts to outer layers, causing the star to transform to a red giant. Helium fusion in the core forms heavier elements. The star cools as fusion slows.
  3. White Dwarf Formation: Fusion stops, and gravity collapses the red giant into a white dwarf—a dense, hot remnant. The white dwarf cools over time, eventually becoming a black dwarf, emitting no heat or light.
• 1. Main Sequence: Hydrogen fusion in the core creates energy.
  2. Red Supergiant Phase: Fusion moves outward, causing the star to transform into a red supergiant.
  3. Supernova: Fuel runs out, and the star collapses, triggering a powerful explosion.
  4. New Element Formation: The explosion creates heavy elements like gold and scatters them into space.
  5. Stellar Remnants: The core may become a neutron star (if around 3 solar masses) or a black hole (if greater), with neutron stars being dense cores of neutrons and black holes having immense gravity that even light can't escape.
• Types of galaxies – 1) Elliptical 2) Spiral – eg Milky way 3) Irregular 4) Special cases where they collide or interact.
• A planet is a large, rounded astronomical body which orbits a star, it is not a star.

  
  

  is massive enough to be nearly spherical in shape due to its own gravity, and has cleared its orbit of other debris or objects.
• a comet is a  cosmic snowballs of frozen gases, rock, and dust that orbit the Sun
• Planets in order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune
• The Milky Way is a flat, disk-shaped galaxy . It's about 100,000 light-years in diameter and contains hundreds of billions of stars, along with gas, dust, and other celestial objects.
• The property of a star that determines its ultimate fate at the end of its lifetime is its mass. Low to medium mass stars end as white dwarfs, while high mass stars can become neutron stars or black holes.
• if a core is LESS than 3 solar masses --> neutron stars

  if a core is MORE than 3 solar masses --> black hole
• the tools used to view a star spectrum are telescopes and spectrographs
• The milky way is a spiral galaxy
• We know the universe is expanding because distant galaxies appear to be moving away from us, as indicated by their redshifts. Edwin Hubble's observations and the discovery of cosmic microwave background radiation

  
  

  This means all objects came from one place
• Hubble's law: The expansion of the universe is accelerating.

  We can see this as the graph is proportional (Positive correlation)
• $Age of universe ​ =$$1/H0$

  Age of the universe equation Age of universe ​ = 1/H0

  Hubble's law: universe expansion velocity is constant!
• Age of universe 13.7 billion years old (approximately)
• Cosmic Microwave Background Radiation - electromagnetic radiation left over from the early stages of the Universe’s evolution
• The three tests for objects in the solar system to be classified as a planet

  1. Orbits the Sun.
  2. Sufficient gravity to be (nearly) spherical.
  3. Has 'cleared' its orbit of other debris.
• Our solar system formed 4.5 billion years ago from a collapsing cloud of gas and dust, forming a spinning disk called a solar nebula. In the center, the Sun formed due to fusion (accretion disk). Outer regions saw material clump into planets, dwarf planets, and moons by gravity. Remaining debris formed the asteroid belt.
• Accretion: An accretion disk is a disk of matter circling around a central object due to gravity.
• An AU is the average distance from the Earth to the Sun, 150 million km
• Be aware constellations are not of any scientific value