04 Measuring and Classifying Stars Free Online FlashCards

What do astrometry, photometry, and spectroscopy measure?

Astrometry measures positions and distances, photometry measures brightness and color, and spectroscopy analyzes light by wavelength.

What is stellar parallax?

Stellar parallax is the apparent shift of a nearby star against distant background stars as Earth orbits the Sun. The parallax angle is half the apparent annual shift.

How is stellar distance related to parallax?

Distance in parsecs is the reciprocal of parallax in arcseconds: $d = \frac{1}{p}$. A parallax of $0.1$ arcsecond corresponds to $10$ parsecs.

Why is spectroscopic parallax not a geometric measurement?

Despite its name, spectroscopic parallax is not geometric. It estimates a star's type and luminosity from its spectrum, then compares intrinsic and apparent brightness to infer distance.

How do luminosity and apparent brightness differ?

Luminosity is the total energy a star emits per second; apparent brightness, or flux, is the energy received per unit area at Earth.

How does flux depend on luminosity and distance?

For an unobscured source, $F = \frac{L}{4\pi d^2}$. Flux decreases with the square of distance for a fixed luminosity.

What distinguishes apparent magnitude from absolute magnitude?

Smaller or more negative magnitudes indicate brighter objects. Apparent magnitude describes how bright a star looks; absolute magnitude describes how bright it would look from 10 parsecs.

How does a star's color indicate its surface temperature?

Hotter stars emit a greater fraction of their light at shorter, bluer wavelengths; cooler stars emit more at longer, redder wavelengths.

What is the stellar spectral sequence from hottest to coolest?

From hottest to coolest, the sequence is O, B, A, F, G, K, M. The Sun is type G2; cooler classes extend through L, T, and Y.

What does a star's spectral type primarily indicate?

A star's spectral type primarily classifies its temperature; it is not a direct measure of its size or luminosity.

How do spectral lines reveal a star's composition?

Atoms and ions produce spectral lines at characteristic wavelengths. Comparing observed lines with laboratory measurements and models helps infer the elements in a star's outer layers.

What is proper motion?

Proper motion is a star's apparent movement across the sky over time, separated from its annual parallax shift. With distance, it gives transverse speed.