ASTR Lecture Notes 10/27

Structure of Our Galaxy

Three Populations of Stars

  • Star Distribution:

    • Young Stars:

    • Found mainly in the disk of the galaxy

    • Characterized by a flat distribution of stars, gas, and dust

    • Contains spiral arms, sites of new star formation with high metallicity

    • Old Stars:

    • Located in the bulge and halo

    • Bulge: Compact distribution of stars with some gas and dust, cigar-shaped bar structure containing both low and high metallicity stars.

    • Halo:

      • Spherical distribution of old stars, primarily found in globular clusters with low metallicity and no gas or dust present.

      • Stars here exist in a random orbit.

Formation of Our Galaxy

  • Galaxy Formation Process:

    • Likely initiated by the collapse of giant gas clouds approximately 10 billion years ago:

    • Stage 1: A significant gas cloud begins to collapse

    • Stage 2: Formation of halo stars first as gravity compresses the clouds

    • Stage 3: Gas clouds collide, flattening into a spinning disk where stars form

Stellar Orbits

  • Types of Stellar Orbits in the Galaxy:

    • Disk Stars:

    • Orbit in the same direction but exhibit oscillatory vertical motion, akin to a merry-go-round

    • Bulge Stars:

    • Mostly directed in the same direction but inclined in 3D orbits

    • Halo Stars:

    • Exhibit random orbits around the galactic center, occasionally passing through the disk

Stellar Life Cycle and Galaxy Dynamics

  • Life Cycle of Stars:

    • Stages:

    1. Stellar nursery (birth)

    2. Main sequence (adulthood)

    3. Red giant (old age)

    4. White dwarf (death)

    5. Planetary nebula leading to raw materials for new stars

  • Star-Gas-Star Cycle:

    • After stellar death, materials return to the interstellar medium (ISM), sustaining ongoing star formation in the disk

Spiral Arms Formation

  • Spiral Density Waves:

    • Proposed mechanism for spiral arms in galaxies where inner regions rotate faster than outer regions, leading to a winding effect.

    • Mechanism:

    1. Gas and dust approach the arm region

    2. Clouds compress, collapse, and form stars

    3. Newly formed blue stars burn out before leaving the arm

Galactic Center Dynamics

  • Sagittarius A*:

    • A compact radio and X-ray source at the center of the Milky Way, observed using advanced telescopes and techniques.

    • Required mass: M=4.3imes106MsunM = 4.3 imes 10^6 M_{sun} contained within a region comparable to our solar system, suggesting it is a supermassive black hole (SMBH).

Activity and Dormancy of SMBH

  • The Milky Way's SMBH is considered "dormant" due to low nearby gas presence, unlike active galaxies where SMBH undergo gas accretion resulting in energy emissions in the form of jets (quasars, blazars).

  • Historical activity indicated by gamma-ray bubbles detected by the Fermi space telescope suggests prior energetic activity.

Additional Resources

  • Recommended videos and links related to spiral density waves and galaxy dynamics are provided for further study.

Conclusion

  • As the course progresses, we will dive deeper into the complexities of our galaxy and others, exploring their formation, structure, and behavior across various phases and evolutionary stages.