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Astronomy Study Guide: Galaxies and the Universe

1. Types of Galaxies

Galaxies are massive systems composed of stars, gas, dust, and dark matter, bound together by gravity. They are classified based on their shape and structural features.

Main Galaxy Types:

• Spiral Galaxies:

  • Have a flat, rotating disk with a central bulge surrounded by spiral arms.

  • Contain large amounts of gas and dust, leading to active star formation.

  • Typically have both young (in the arms) and old (in the bulge) stars.

  • Example: Milky Way Galaxy.

• Elliptical Galaxies:

  • Range from nearly spherical to elongated (oval) shapes.

  • Contain very little gas and dust, so minimal star formation occurs.

  • Dominated by older, redder stars.

  • Can be very small (dwarf ellipticals) or enormous (giant ellipticals).

• Irregular Galaxies:

  • No definite shape or structure.

  • Often the result of gravitational interactions or collisions.

  • Contain young stars and active star-forming regions.

  • Examples include the Large and Small Magellanic Clouds.

Classification Systems:

• The Hubble Tuning Fork Diagram classifies galaxies into spirals, barred spirals, ellipticals, and irregulars.

2. Our Place in the Universe

The Milky Way Galaxy:

• A barred spiral galaxy.

• Contains 100–400 billion stars.

• About 100,000 light-years in diameter.

Structure:

• Spiral Arms: Regions of active star formation with bright, young stars.

• Central Bulge: Dense area at the center with older stars.

• Galactic Halo: Spherical region containing old stars and globular clusters.

• The Solar System is located in the Orion Arm, about 27,000 light-years from the galactic center.

3. Light and Distance in Astronomy

Using Light to Study Galaxies:

• Spectroscopy: Light from galaxies is split into a spectrum to analyze elements, temperature, and motion.

• Different elements absorb and emit light at specific wavelengths, producing spectral lines.

Redshift and Blueshift:

• Redshift: When a galaxy moves away, its light shifts to longer (redder) wavelengths.

• Blueshift: When a galaxy moves closer, its light shifts to shorter (bluer) wavelengths.

• Redshift is evidence for the expanding universe.

Looking Back in Time:

• Light travels at a finite speed (about 300,000 km/s).

• When we observe distant galaxies, we see them as they were in the past.

• Example: A galaxy 1 million light-years away is seen as it was 1 million years ago.

4. The Expanding Universe

Hubble’s Law:

• States that the farther away a galaxy is, the faster it is moving away from us.

• Expressed as: Velocity = H₀ × Distance (H₀ is the Hubble constant).

Edwin Hubble's Contributions:

• Discovered that "nebulae" were actually other galaxies.

• First to observe redshifts increasing with distance.

• Provided key evidence for an expanding universe.

Evidence for Expansion:

• Redshift data from galaxies.

• Hubble's Law measurements.

• Cosmic Microwave Background Radiation (CMB).

5. Big Bang Theory

Overview:

• The universe began about 13.8 billion years ago in a hot, dense state.

• Has been expanding and cooling ever since.

Cosmic Microwave Background (CMB):

• Faint radiation left over from the Big Bang.

• Discovered in 1965 by Penzias and Wilson.

• Uniform across the sky, supports the idea of a hot, dense early universe.

Other Supporting Evidence:

• Abundance of light elements (hydrogen, helium).

• Expansion of space as seen in redshift.

6. Dark Matter and Dark Energy

Dark Matter:

• Invisible matter that does not emit light or energy.

• Detected through its gravitational effects on visible matter.

• Helps explain galaxy rotation curves (outer stars move faster than expected).

• May make up about 27% of the universe.

Dark Energy:

• A mysterious force causing the accelerated expansion of the universe.

• Discovered through observations of distant supernovae.

• Makes up about 68% of the universe.

Why Important:

• Although invisible, both dark matter and dark energy shape the structure and future of the universe.

7. Cosmological Concepts

Age of the Universe:

• Estimated at ~13.8 billion years.

• Determined using Hubble's Law, CMB data, and models of stellar evolution.

A Dynamic Universe:

• The universe is not static; it evolves over time.

• Galaxies form, merge, and change.

• The expansion rate is increasing due to dark energy.

Gravity’s Role:

• Gravity pulls gas and dust together to form stars and galaxies.

• Determines galaxy shapes and motion.

• Competes with expansion on local scales.