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.