Chapter 17 Measuring the Stars Acknowledgements: These lecture notes have been adapted from lecture materials developed by Dr. Bruton of the Dept. of Physics and Astronomy at S.F.A. Measuring the Stars

Properties of Stars Distance Speed Radius Brightness Temperature

Measuring a Star’s Distance Parallax - the apparent change in the position of a star due to the motion of the Earth Nearby objects exhibit more parallax that remote ones.

Stellar Distances Parsec - the distance from our Sun at which the angle between the Earth and the Sun subtends an angle of one arc second 1 parsec = 3.26 light years 1 arc second = 1/3600 degrees Light-year - the distance that light travels in one year Proxima Centauri is 4.2 light years from Earth (24 trillion miles).

Measuring A Star’s Brightness Inverse-Square Law - the apparent brightness of a star decreases with increasing distance from Earth

Power = (Area)x(Intensity) Luminosity - the amount of Energy per unit time (power) a star radiates Stefan-Boltzmann Law - a star of temperature T radiates an amount of energy each second equal to sT4 per square meter - This is the intensity. Power = (Area)x(Intensity)

The Magnitude System Apparent Magnitude - logarithmic scale of brightness for stars (e.g. the size of the dots on star charts) Absolute Magnitude - the apparent magnitude that a star would have if it were 10 parsecs away from Earth

Measuring A Star’s Temperature A star’s surface temperature can be determined from its color using Wien’s Law. Reddish  coolest star Orange-ish Yellowish White Bluish  hottest star

Stellar Spectroscopy Stellar Spectroscopy - the study of the properties of stars by measuring absorption line strengths Spectral Class - classification of star according to the appearance of their spectra O B A F G K M

Compare these spectra. What do these spectra tell us about the star? Spectrum of Hydrogen in Lab Spectrum a Star What do these spectra tell us about the star?

Compare these spectra. What do these spectra tell us about the star? Spectrum of Hydrogen in Lab Spectrum a Star What do these spectra tell us about the star?

Compare these spectra. What do these spectra tell us about the star? Spectrum of Hydrogen in Lab Spectrum a Star…..Day 1 Spectrum a Star…..Day 2 Spectrum a Star…..Day 3 Spectrum a Star…..Day 4 What do these spectra tell us about the star?

Mystery Star Properties 1. The star appears as a point of light through a telescope. 2. The absorption lines appear split and move over a 4 day period. 3. The brightness of the star also varies over 4 days. Question: Why do you think the brightness of the star is varying? Answer: This could be an eclipsing binary star system that cannot be resolved by a telescope!

Binary Stars Optical Double - two stars that just happen to lie in the same direction as seen from Earth Visual Binary - two stars that are orbiting one another and can both be seen from Earth

Binary Stars Spectroscopic Binary - two stars that are found to orbit one another through observations of the Doppler effect in their spectral lines Eclipsing Binary - two stars that regularly eclipse one another causing a periodic variation in brightness Light Curve - a plot of a variable star's apparent magnitude versus time

Variable Stars Stars that have a change in brightness over time are called variable stars. Examples: eclipsing binary stars Cepheid variables Lyra variables

Matching Questions (a) composition. (b) parallax shift. (c) motion. 1. The temperature of a star can be determined from its_____________. 2. The pattern of the absorption spectral lines for a star contains information about a star’s________________. 3. The Doppler shift of a star's spectral lines tells us something about the star’s_______________. 4. The distance of a star from Earth can be determined from the star’s_______________. 5. The radius of a star can be determined from its ________________. (a) composition. (b) parallax shift. (c) motion. (d) luminosity and temperature. (e) color.

The H-R Diagram Hertzsprung-Russell Diagrams - plots of luminosity versus temperature of known stars Most stars on the H-R diagram lie along a diagonal curve called the main sequence.

Stellar Luminosity Classes CLASS DESCRIPTION Ia Bright supergiants Ib Supergiants II Bright giants III Giants IV Subgiants V Main-sequence stars/dwarfs

End of Chapter 17...