Stellar Physics: Light, Telescopes, and Star Properties

Light and Telescopes

What distinguishes the various types of light (radio, infrared, visible, etc)? How does the apparent brightness of light vary with distance?

Answer: Different wavelengths distinguish the various types of light. Apparent brightness becomes fainter if a star is farther away.

Atomic Structure and Spectra

What is the simple structure of an atom? How are continuous, emission, & absorption spectra formed?

Answer:

• Atom Structure: The nucleus contains nearly all of the atom's mass, consisting of protons and neutrons (though the original text mentioned 'photon and nucleus', the standard model involves protons and neutrons in the nucleus). Electrons are smeared out around the nucleus.
• Kirchhoff's Laws:
  • A hot, dense glowing object emits a continuous spectrum.
  • A hot, low-density gas emits light of only certain wavelengths: emission line spectrum.
  • When light having a continuous spectrum passes through a cool gas, dark lines appear in the continuous spectrum: this is called absorption line spectrum.

Thermal Radiation and Telescopes

What is a Thermal (Blackbody Radiator)? How do energy & color relate to temperature for objects radiating this way? Why are hotter stars bluer & brighter than cooler stars of the same size?

Why are bigger telescopes better?

Answer: 'Bigger is better' for telescopes because the light-gathering power of a telescope is proportional to the size (area) of its aperture (the diameter of the objective lens). In other words, the bigger the diameter of the objective lens, the higher the light-gathering power.

Atmospheric Transparency and Imaging

What wavelengths of light get through our atmosphere—what types must be observed from space?

Answer: Only visible, radio, and certain UV and IR light make it through to the ground. Light outside these ranges must be observed from space.

Why do astronomers combine images at different wavelengths into one picture?

Answer: Because they can observe the emission at a certain wavelength of a star without the distraction of other colors.

Properties of Stars

Understand the basic properties of stars & how astronomers obtain this information.

Stellar Motion and Temperature

• Motions of Stars: Radial velocities and proper motion.
• Temperature: Determined by color, spectral types, and spectral lines.

Brightness Measurement

Brightness: Difference between apparent brightness & luminosity, as measured by apparent or absolute magnitudes.

• Every 5 magnitudes represents a 100x difference in brightness. Example: Which is brighter in our sky, a star with mag=5 or a star with mag=10? A star with magnitude 5 is 100 times brighter.
• Example: Which has greater luminosity, a star with M_abs= 8 or a star with M_abs= -2? A star with M_abs= -2 is 100² (10,000) times more luminous.

Mass and Distance Determination

• Masses: Measured from different types of binary stars, using Kepler’s Law.
• Distances: Definition of light-year & parsec.
• Methods for measuring distances to stars: Trigonometric parallax, spectroscopic parallax.