Fundamental Concepts in Astronomy and Astrophysics

Science & the Universe                                                                                                                                       

Astronomy = study of celestial objects and their interactions.                                    

Scientific method: relies on observation, testing, and revision.

Distances measured in light-years; light travels at ~300,000 km/s

Scientific notation helps handle large/small numbers.  γ

Observing the Sky

Constellations = regions in the sky (88 official).

Sky appears to move due to Earth’s rotation (24h) and orbit (365 days).        

Zenith = overhead; meridian = N to S through zenith.

Ecliptic = Sun's yearly path; celestial sphere = imaginary star backdrop.

Precession = slow wobble of Earth's axis (~26,000 yrs).

Retrograde motion = planet appears to move backward due to

Earth’s perspective.

Geocentric model: Earth-centered universe with epicycles.

Copernican (heliocentric) model: planets orbit the Sun. 

Gravity and Orbits

Kepler’s Laws:

Planets move in ellipses with the Sun at one focus.

Equal areas in equal time.

P2=a3P2=a3: period squared equals semi-major axis cubed.

Newton’s Laws: motion (F=ma), inertia, and action-reaction.

Universal gravitation: force ∝ mass product / distance².

Radiation and Spectra

Light behaves as both wave and particle (photon).

Speed: c=λfc=λf, where λλ is wavelength, ff is frequency.

Wien’s Law: hotter objects peak at shorter wavelengths (appear bluer).

Stefan-Boltzmann Law: energy ∝ T4T4, where TT is temperature.

Types of spectra:

Continuous (dense hot objects)

Emission (hot gas)

Absorption (cool gas in front of light)

Doppler effect: redshift = moving away; blueshift = moving closer.

The Sun

Composition: ~73% hydrogen, ~25% helium.

Structure: core, radiative zone, convective zone.

Surface layers: photosphere (visible), chromosphere, corona.

Corona emits solar wind (charged particles).

Energy produced by nuclear fusion: 4H → He + energy.

The Sun, A Nuclear Powerhouse

Fusion happens only in the hot, dense core (~10 million K).

E = mc²: mass lost in fusion is converted to energy.

Energy moves outward by radiation and convection.

Sun is stable due to hydrostatic equilibrium

 (gravity balanced by pressure).

Neutrinos confirm fusion activity in the core.

Analyzing Starlight

Luminosity = total energy emitted per second.

Apparent brightness depends on distance (inverse square law).

Magnitude system: lower number = brighter; scale is logarithmic.

Color → temperature: blue = hot, red = cool.

Spectral classes: OBAFGKM (hot → cool). Sun = G2.

Composition: ~73% H, 25% He, 2% heavier elements.

Celestial Distances

Parallax: apparent star shift due to Earth's orbit; measures distance.

d(pc)=1/p(arcsec)d(pc)=1/p(arcsec), 1 pc = 3.26 ly.

First accurate parallax in 1838.

Closest stars = >1 parsec away.