Star Formation in Galaxies

• Location of Star Formation

  • Star formation mainly occurs in the galaxy's disk.
  • The disk contains abundant gas and dust, necessary for forming new stars.
  • Halo and bulge regions lack sufficient material for significant star formation.
  • Centers often contain older stars.

• Characteristics of Spiral Arms

  • Spiral arms are regions of enhanced density where star formation is active.
  • Made up of dense clouds of gas and dust that collapse to form stars.
  • Milky Way's spiral arms host clusters of young stars.
  • Massive stars (O and B types) prevalent in these regions due to their short lifespans.

• Orbital Patterns of Stars

  • Disk stars orbit in the same direction and nearly the same plane, following organized circular paths.
  • Halo stars have more randomly oriented orbits with non-circular patterns.
  • Disk stars are varied in mass and color, while halo stars are predominantly low-mass and red.

The Galactic Disk and Halo

• Disk Composition

  • Rich in gas and dust; essential for star formation.
  • Young stars primarily located in the disk due to favorable conditions.

• Halo Composition

  • Mainly contains older stars.
  • Relatively empty in terms of gas and dust, resulting in fewer new stars.

• Orbit Characteristics

  • Stars in the disk follow nearly circular orbits around the center.
  • Halo stars exhibit random, non-circular orbits.

Spiral Arms and Density Waves

• Formation of Spiral Arms

  • Not fixed; they are formed by density waves that compress gas and dust as they propagate through the galaxy.
  • Dense and dusty gas clouds found in spiral arms are key to creating new stars.
  • Arms appear as bright areas rich with new stars and glowing gas, where star formation occurs.

• Mechanics of Spiral Arms

  • Stars move in and out of arms, giving the illusion of fixed structures.
  • Spiral arms can be compared to traffic jams, where congestion stays in place as stars travel through.

The Interstellar Medium

• Interstellar Medium Composition

  • Comprises gas and dust, filling the space between stars, crucial for star formation processes.
  • Approximately 2% of its mass consists of heavier elements created through stellar processes.

• Star-Gas-Star Cycle

  • Material returns to the interstellar medium post supernovae or stellar winds, leading to new star formation.

Cosmic Features

• Cosmic Rays

  • High-energy particles, mainly protons, from supernovae and active galactic nuclei, travel near the speed of light.

• Radio Signals

  • Radio telescopes detect specific wavelengths, such as the 21-centimeter wavelength from atomic hydrogen, crucial for mapping gas in the Milky Way.

Milky Way Structure and Measurements

• Milky Way Dimensions

  • The disk is about 100,000 light-years in diameter and 1,000 light-years thick.
  • The bulge is distinct and extends a few thousand light-years from the center.

• Location of the Sun

  • The Sun is positioned about 27,000 light-years from the center of the Milky Way.

• Sagittarius A*

  • The central supermassive black hole in the Milky Way, with a mass 4 million times that of the Sun, evidenced by observing rapid star orbits in the region.