The Universe and Its Stars takes us on a cosmic journey from the Big Bang to the present day. We explore the birth of stars, their life cycles, and the vast structures they form, like our Milky Way galaxy.

Understanding astronomical distances is key to grasping the universe's scale. Light-years help us measure these vast spaces and realize that looking at distant objects means peering into the past, offering glimpses of cosmic history.

The Big Bang Theory

Origin and Early Universe

• The Big Bang theory proposes that the universe began as an extremely hot, dense point approximately 13.8 billion years ago and has been expanding and cooling ever since
• According to the theory, the early universe was composed of high-energy radiation and subatomic particles, which gradually cooled and formed the first atoms, primarily hydrogen and helium (protons, neutrons, and electrons)
• The expansion of the universe is supported by the observed redshift of distant galaxies, which indicates that they are moving away from us, with more distant galaxies receding faster than nearby ones (Hubble's law)

Evidence and Ongoing Research

• The Cosmic Microwave Background (CMB) radiation, discovered in 1965, provides strong evidence for the Big Bang theory, as it is the remnant heat from the early stages of the universe
  • The CMB is a nearly uniform background of microwave radiation that fills the entire sky, with a temperature of about 2.7 Kelvin (-270.45°C or -454.81°F)
  • The CMB's uniformity and slight fluctuations in temperature support the idea of an initially hot, dense universe that has been expanding and cooling over time
• The Big Bang theory does not explain what caused the initial expansion or what existed before the Big Bang, leading to ongoing research and speculation in cosmology
  • Some theories propose a cyclical model of the universe, with repeated Big Bangs and contractions (Big Crunch)
  • Others suggest the existence of a multiverse, where our universe is one of many that may have originated from quantum fluctuations or other mechanisms

Stellar Life Cycles

Star Formation and Main Sequence

• Stars form from collapsing clouds of gas and dust called nebulae, which are composed primarily of hydrogen and helium
• As the cloud contracts under its own gravity, it heats up and forms a protostar. When the core temperature reaches about 10 million Kelvin, nuclear fusion begins, and the star enters the main sequence
• Main sequence stars are classified by their surface temperature and luminosity using the Hertzsprung-Russell (H-R) diagram, with categories ranging from hot, bright, blue stars (O and B types) to cool, dim, red stars (M type)
  • The Sun is a G-type main sequence star, with a surface temperature of about 5,800 Kelvin and a lifespan of approximately 10 billion years
  • More massive stars have shorter lifespans, as they burn through their fuel more quickly, while less massive stars have longer lifespans

Post-Main Sequence and Stellar Remnants

• When a main sequence star exhausts its hydrogen fuel, it expands into a red giant. The ultimate fate of a star depends on its initial mass:
  • Low-mass stars (less than 8 solar masses) shed their outer layers to form a planetary nebula and end their lives as white dwarfs
    • White dwarfs are dense, compact objects supported by electron degeneracy pressure, with no further nuclear fusion
  • High-mass stars (more than 8 solar masses) explode as supernovae, leaving behind a neutron star or, if massive enough, a black hole
    • Neutron stars are extremely dense objects supported by neutron degeneracy pressure, often observed as pulsars
    • Black holes are regions of spacetime with such strong gravitational fields that nothing, not even light, can escape from within the event horizon

Milky Way Components

Galactic Structure

• The Milky Way is a barred spiral galaxy, consisting of a central bulge, spiral arms, and a disk, surrounded by a spherical halo
• The central bulge is a dense region of older stars and contains a supermassive black hole, Sagittarius A, at its center
  • Sagittarius A has a mass of about 4 million solar masses and is responsible for the high velocities of stars orbiting near the galactic center
• The disk is composed of gas, dust, and stars, and is divided into the thin disk (younger stars) and the thick disk (older stars)

Spiral Arms and Halo

• The spiral arms are regions of active star formation, containing younger, hotter stars and emission nebulae
  • The Sun is located in the Orion Arm, a minor spiral arm of the Milky Way, approximately 26,000 light-years from the galactic center
  • Other prominent spiral arms include the Perseus Arm and the Sagittarius Arm
• The halo surrounds the disk and bulge and contains ancient, metal-poor stars and globular clusters
  • Globular clusters are dense, spherical collections of old stars that orbit the galactic center in the halo
  • The halo also contains a significant amount of dark matter, which is inferred from its gravitational effects on the rotation of the galaxy

Light-Years and Astronomical Distances

Definition and Scale

• A light-year is the distance light travels in one year, which is approximately 9.46 trillion kilometers or 5.88 trillion miles
• Light-years are used to measure the vast distances between astronomical objects, as conventional units like kilometers or miles are too small to be practical
  • The nearest star to the Sun, Proxima Centauri, is about 4.24 light-years away, meaning that light from this star takes 4.24 years to reach Earth
  • The Milky Way galaxy is approximately 100,000 light-years in diameter, and the Andromeda galaxy, our nearest large galactic neighbor, is about 2.5 million light-years away

Observing the Past

• Understanding the concept of light-years is essential for grasping the immense scale of the universe and the time it takes for light from distant objects to reach us, which means that we observe these objects as they appeared in the past
  • When we observe a galaxy 10 million light-years away, we see it as it appeared 10 million years ago, as the light has taken that long to travel to Earth
  • This means that studying distant objects allows astronomers to look back in time and observe the universe at different stages of its evolution, providing insights into the history and development of the cosmos