Lecture 2

PHYC 2452: Intro to Stellar & Galactic Astrophysics

Lecture 2: The Stars as Distant Suns

  • Course focuses on the understanding of stars and their characteristics.

  • Location: Dalhousie University, on Mi’kma’ki territory.

The Distance to the Stars

Historical Approaches

  • Isaac Newton's Method: Compared the apparent brightness of Sirius and the Sun, noting similarity in brightness with Saturn.

  • Simplified the problem by determining how much light Saturn reflects, which relies on its known distance of ~9 AU.

Light Reflection and Distance Calculations

  • Saturn’s albedo (reflection coefficient) assumed to be around 0.25 (actual value is 0.47).

  • Calculated that Saturn reflects approximately 2.38 × 10^{-10} of the Sun’s light.

  • If Sirius has comparable brightness to the Sun, the estimated distance to Sirius is calculated as:

    • Distance to Sirius = 520,000 AU or approximately 2.5 parsecs (pc).

  • Correct value for the distance to Sirius is about 2.6 pc.

Stellar Parallax Measurement

Friedrich Bessel's Contribution

  • First successful parallax measurement in 1838 for 61 Cygni:

    • Measured parallax π = 0.314”, yielding a distance of 1/π = 3.18 pc; modern value is 3.50 pc.

  • Explained previous failures in measuring stellar parallaxes due to their vast distances.

Visual Representation

  • Diagram showing parallax angle of 1 arc second and its relation to distance.

Challenges in Parallax Measurement

  • The small angular shift for parallax is hard to measure due to atmospheric turbulence.

  • Turbulence causes stars to appear as a blob rather than a point source, complicating measurements.

Cataloguing the Stars

Early Stellar Mapping

  • Before the mid-19th century: focus on mapping night skies visually.

  • Main task involved determining right ascension (RA) and declination (Dec), analogous to longitude and latitude on Earth.

RA & Dec System

  • Right Ascension and Declination rotate with the observer; thus, they provide a fixed coordinate grid for stellar mapping.

  • Example: Bright star Vega position given as α = 18h 36m 56s, δ = +38° 47’ 01”.

Progress by the Late 19th Century

  • Transition from mapping to measuring and compiling stellar distances through paralaxes.

  • Introduction of photographic plates for permanent records, leading to the study of proper motions.

Completion of Atlases

  • By the end of the 19th century, atlases compiled included positions and brightness of ~325,000 stars through the Bonner Durchmusterung catalogue.

  • Move towards understanding intrinsic nature of stars, establishing foundations for modern astrophysics.

Basic Astronomical Concepts

Magnitude Scale

  • Astronomers measure stellar brightness on a logarithmic scale originating from ancient Greek classifications.

  • Magnitude scale defined quantitatively—1st mag star is 100 times brighter than a 6th mag star.

Distance Measurements

  • Distances measured in Astronomical Units (AU) or parsecs (pc); further distances use kiloparsecs (kpc) or megaparsecs (Mpc) and redshifts.

Key Points about Stellar Spectra

Discovery of Spectral Lines

  • Light through a prism reveals colors; spectra show characteristic absorption/emission features of star compositions.

  • Fraunhofer studied solar spectra (1814–1823), naming prominent absorption lines, contributing to modern spectroscopy.

Kirchhoff's Contributions

  • Kirchhoff and Bunsen established relations between laboratory emissions and stellar spectra, identifying elements such as Na and Fe.

  • The introduction of spectral line analysis facilitated composition and temperature identifications of stars.

Conclusion

  • From early cataloging and parallax measurements to sophisticated spectral analysis, the understanding of stars evolved significantly.

  • The insights gained paved the way for advanced concepts in stellar and galactic astrophysics.