The Milky Way Galaxy, our cosmic home, is a vast and complex structure. Its study has evolved from early star-counting efforts to modern multi-wavelength observations, revealing a rich tapestry of stars, gas, and dust arranged in a spiral pattern.

Mapping our galaxy is challenging due to our position within it and the obscuring effects of interstellar dust. Despite these obstacles, astronomers have identified key components: the disk, bulge, and halo, each playing a unique role in the Milky Way's structure and evolution.

The Milky Way Galaxy

Historical reasoning for Herschels' model

• William Herschel and sister Caroline conducted "star gauging" survey in late 18th century counted stars in 600+ sky regions to estimate Milky Way shape
• Assumed stars had similar intrinsic brightness, so fainter stars were farther away led to model of flattened disk with Sun near center
• Disk extended farther in Milky Way plane than perpendicular explained appearance of Milky Way band in night sky
• Model limited by assumption of uniform stellar brightness and lack of knowledge about interstellar dust absorption

Challenges of galactic mapping

• Earth located within Milky Way disk makes observing galaxy structure challenging like mapping a forest while standing inside it
• Interstellar dust absorbs and scatters visible light, obscuring distant galaxy parts causes dark bands in Milky Way (Coal Sack nebula) limits observing distant stars and structures in visible spectrum
• Vast distances within galaxy make determining precise locations and motions of celestial objects difficult parallax measurements only effective for nearby stars due to angular resolution limitations
• Sun's location within galactic disk provides edge-on view of galaxy structure complicates determining true shape and size
• Galactic coordinate system helps astronomers describe positions of objects relative to the Milky Way's center and plane

Primary components of Milky Way

• Galactic disk: flat, rotating component where most stars, gas, and dust reside
  • Thin disk: contains younger stars (Sun), ~1,000 light-years thick
  • Thick disk: contains older stars, ~3,000 light-years thick
  • Spiral arms: higher density gas, dust, and young star regions winding outward from center (Orion Arm, Perseus Arm)
  • Exhibits galactic rotation, with stars and gas orbiting the galactic center at different speeds
• Galactic bulge: central, spheroidal component
  • Contains mostly older stars, ~10,000 light-years diameter
  • Galactic center: innermost bulge region with supermassive black hole (Sagittarius A)
  • May have a bar-like structure, classifying the Milky Way as a barred spiral galaxy
• Galactic halo: spherical component surrounding disk and bulge
  • Extends up to 150,000 light-years from galactic center
  • Contains ancient stars, globular clusters (M13), and small amount of hot gas
  • Home to different stellar populations, reflecting the galaxy's formation history
• Dark matter halo: extended, spherical component of non-baryonic matter
  • Extends well beyond visible halo, responsible for galaxy's flat rotation curve
  • Plays crucial role in Milky Way formation and evolution

Galactic environment and structure

• Galactic magnetic field permeates the disk and halo, influencing cosmic ray propagation and star formation
• Milky Way is part of the Local Group, a collection of galaxies bound by gravity, including Andromeda and other smaller galaxies