🪐Intro to Astronomy Unit 25 – The Milky Way Galaxy

What's the Milky Way?

• The Milky Way is the galaxy that contains our Solar System
• It is a barred spiral galaxy with a diameter of about 100,000 light-years
• The Milky Way is part of the Local Group, a collection of more than 50 galaxies
• It is estimated to contain between 100 and 400 billion stars
• The name "Milky Way" comes from its appearance as a faint band of light in the night sky
  • This band is the result of viewing the galaxy edge-on from within the disk
• The Milky Way is composed of stars, planets, dust, gas, and dark matter
• It is believed to have a supermassive black hole at its center (Sagittarius A*)

Structure and Components

• The Milky Way has a flattened disk structure with a central bulge and spiral arms
• The disk is approximately 1,000 light-years thick and contains most of the galaxy's stars
• The central bulge is a dense region of older stars and extends about 10,000 light-years from the center
• The Milky Way has four major spiral arms (Perseus, Sagittarius, Centaurus, and Cygnus) and several minor arms
  • These arms are regions of active star formation and contain many young, hot stars
• The halo surrounds the disk and bulge and contains older stars and globular clusters
• The Milky Way also has a bar-shaped structure across its center, composed primarily of older stars
• The galaxy's components include the interstellar medium (gas and dust) and dark matter

Our Cosmic Address

• Earth is located in the Solar System, which is part of the Milky Way galaxy
• The Solar System is located in the Orion Arm, a minor spiral arm of the Milky Way
  • The Orion Arm is between the Perseus and Sagittarius arms
• Our Solar System is approximately 26,000 light-years from the galactic center
• The Sun orbits the center of the Milky Way at a speed of about 220 km/s
• It takes the Solar System about 225-250 million years to complete one orbit around the galactic center (a galactic year)
• The Milky Way is part of the Local Group, which is a member of the Virgo Supercluster
• The Virgo Supercluster is part of the larger Laniakea Supercluster

Stellar Populations

• The Milky Way contains two main stellar populations: Population I and Population II stars
• Population I stars are younger, more metal-rich stars found primarily in the disk and spiral arms
  • These stars form from gas and dust enriched by previous generations of stars
  • Examples include the Sun and most visible stars in the night sky
• Population II stars are older, metal-poor stars found in the halo and bulge of the galaxy
  • These stars formed early in the galaxy's history when the interstellar medium had fewer heavy elements
  • Examples include stars in globular clusters and the galactic halo
• The stellar populations provide insight into the formation and evolution of the Milky Way
• The difference in metallicity between the populations suggests a gradual enrichment of the interstellar medium over time

The Galactic Center

• The center of the Milky Way is located in the direction of the constellation Sagittarius
• It is hidden from view by dust and gas in the disk, but can be observed using infrared, radio, and X-ray telescopes
• The galactic center is marked by a compact radio source called Sagittarius A*
  • Sagittarius A* is believed to be a supermassive black hole with a mass of about 4 million solar masses
• The region around the galactic center is densely packed with stars, gas, and dust
• It is a site of intense star formation, with many young, massive stars and stellar clusters
• The galactic center also contains numerous stellar remnants, such as neutron stars and black holes
• Observations of stars orbiting Sagittarius A* have provided strong evidence for the existence of the supermassive black hole

Dark Matter and Mass

• The Milky Way's mass is estimated to be about 1.5 trillion solar masses
• However, the visible matter (stars, gas, and dust) accounts for only a small fraction of this mass
• The majority of the Milky Way's mass is believed to be in the form of dark matter
  • Dark matter is a hypothetical form of matter that does not interact with electromagnetic radiation (light)
  • Its presence is inferred from its gravitational effects on visible matter
• Dark matter is thought to form a halo surrounding the Milky Way, extending far beyond the visible disk and bulge
• The dark matter halo plays a crucial role in the galaxy's structure and evolution
  • It helps to explain the observed rotation curve of the Milky Way
  • It also influences the formation and motion of the galaxy's visible components
• Understanding the nature and distribution of dark matter is an active area of research in astronomy and cosmology

Milky Way's Evolution

• The Milky Way formed from a cloud of gas and dust about 13.6 billion years ago
• The first stars (Population III) formed from the primordial gas, consisting mainly of hydrogen and helium
  • These stars were massive, short-lived, and contributed to the initial enrichment of the interstellar medium with heavier elements
• As the galaxy evolved, Population II stars formed from the enriched gas, creating the halo and globular clusters
• The disk and spiral arms formed later, as the interstellar medium became increasingly enriched with metals
  • This led to the formation of Population I stars, including the Sun
• The Milky Way has undergone several episodes of star formation throughout its history
• It has also experienced mergers and interactions with other galaxies, such as the Sagittarius Dwarf galaxy
• These events have shaped the galaxy's structure and composition over billions of years
• The Milky Way continues to evolve, with ongoing star formation and the potential for future galactic mergers

Observing Our Galaxy

• Observing the Milky Way is challenging because we are located within the galaxy
• The best views of the Milky Way are from dark, clear skies away from light pollution
  • The band of the Milky Way appears as a faint, glowing strip across the night sky
• Different wavelengths of light reveal different aspects of the galaxy
  • Optical telescopes show the distribution of stars and dust in the disk and bulge
  • Infrared telescopes penetrate through the dust, revealing the structure of the galaxy's center and hidden star-forming regions
  • Radio telescopes map the distribution of gas and the location of the galactic center (Sagittarius A*)
  • X-ray telescopes detect high-energy phenomena, such as binary star systems and supernova remnants
• Surveys and mapping projects, such as the Gaia mission, provide detailed information about the positions, motions, and properties of stars in the Milky Way
• Studying the Milky Way helps astronomers understand the structure, evolution, and composition of galaxies in general
• Comparing the Milky Way to other galaxies in the Local Group and beyond provides insights into the diversity and evolution of galaxies in the universe