1.6 A Tour of the Universe

Our solar system is a cosmic neighborhood centered around the Sun. Planets, moons, asteroids, and comets orbit this fiery star, each with unique characteristics. Earth, our home, sits in the "Goldilocks zone," perfect for life as we know it.

The Milky Way, our galactic home, is a vast spiral of stars, gas, and dust. Our Sun is just one of billions of stars in this cosmic pinwheel, orbiting the galactic center where a supermassive black hole lurks.

The Solar System

Structure of the solar system

• Sun at the center with planets, dwarf planets, moons, asteroids, and comets orbiting it
  • Terrestrial (rocky) planets: Mercury, Venus, Earth, Mars
  • Jovian (gas giant) planets: Jupiter, Saturn, Uranus, Neptune
  • Dwarf planets orbit the Sun but have not cleared their orbital path (Pluto, Eris, Ceres)
  • Moons are natural satellites that orbit planets or dwarf planets
  • Asteroids are rocky objects mostly found in the asteroid belt between Mars and Jupiter
  • Comets are icy objects with long, elliptical orbits around the Sun
• Sun is the largest object in the solar system with a diameter of approximately 1.39 million km (109 times Earth's diameter)
• Earth, the third planet from the Sun, has a diameter of about 12,742 km
  • Orbits the Sun at an average distance of 149.6 million km (1 astronomical unit or AU)
• Moon, Earth's only natural satellite, has a diameter of approximately 3,474 km (about 1/4 of Earth's diameter)
  • Orbits Earth at an average distance of 384,400 km (about 30 times Earth's diameter)
• Gravity plays a crucial role in maintaining the structure and dynamics of the solar system

The Milky Way Galaxy

Characteristics of Milky Way galaxy

• Barred spiral galaxy consisting of a flat disk with spiral arms, a central bulge, and a halo
  • Disk is approximately 100,000 light-years in diameter and about 1,000 light-years thick
  • Central bulge is a densely packed region of older stars with a diameter of about 10,000 light-years
  • Halo is a spherical region surrounding the disk and bulge containing older stars and globular clusters
• Contains an estimated 100-400 billion stars of various sizes, colors, and ages
  • Sun is located in the Orion Arm about 26,500 light-years from the galactic center
• Composed of stars, gas (mostly hydrogen and helium), and dust
  • Interstellar gas and dust make up about 10-15% of the galaxy's total mass
  • Dust is composed of small solid particles such as silicates and carbon compounds
• Has a supermassive black hole at its center known as Sagittarius A* with a mass of about 4 million solar masses
• Contains regions of active star formation in nebulae, where new stars are born from collapsing clouds of gas and dust

Stars

Features and life cycles of stars

• Form from the gravitational collapse of dense regions within molecular clouds of gas and dust
  • Protostars undergo further collapse until nuclear fusion begins in their cores marking the birth of a main-sequence star
• Main-sequence stars fuse hydrogen into helium in their cores generating energy that counteracts gravitational collapse
  • A star's mass determines its size, color, luminosity, and lifetime on the main sequence
• When a star exhausts its hydrogen fuel, it evolves off the main sequence and its ultimate fate depends on its initial mass:
  1. Low-mass stars (< 8 solar masses) become red giants, shed outer layers to form planetary nebulae, leaving behind white dwarf remnants
  2. High-mass stars (> 8 solar masses) become red supergiants, explode as supernovae, leaving behind neutron stars or black holes
• Many stars exist in multiple star systems where two or more stars orbit a common center of mass
  • Binary stars are the most common type of multiple star system
• Play a crucial role in producing elements found on Earth through nuclear fusion and supernova explosions
  • Main-sequence stars fuse hydrogen into helium, later helium into carbon and oxygen
  • Massive stars can fuse elements up to iron in their cores
  • Supernova explosions create and disperse elements heavier than iron enriching the interstellar medium

The Universe and Beyond

Cosmic structure and components

• The universe is the entirety of space, time, matter, and energy
• Galaxies are the building blocks of the universe, containing billions of stars, gas, and dust
• The cosmic microwave background radiation provides evidence for the Big Bang theory
• Telescopes are essential tools for observing and studying celestial objects across various wavelengths
• Dark matter, though invisible, plays a significant role in the structure and evolution of the universe