The Expanding Universe

Basic Properties of Stars Magnitude Measuring the Stars –One of the most basic observable properties of a star is how bright it appears. –The ancient Greeks established a classification system based on the brightness of stars. –The brightest stars were given a ranking of +1, the next brightest +2, and so on.

Basic Properties of Stars Apparent Magnitude Measuring the Stars –Apparent magnitude is based on the ancient Greek system of classification which rates how bright a star appears to be. –In this system, a difference of 5 magnitudes corresponds to a factor of 100 in brightness. –Negative numbers are assigned for objects brighter than magnitude +1.

Basic Properties of Stars Absolute Magnitude Measuring the Stars –Apparent magnitude does not actually indicate how bright a star is, because it does not take distance into account. –Absolute magnitude is the brightness an object would have if it was placed at a constant distance from Earth.

Spectra of Stars Wavelength Shift Measuring the Stars –Spectral lines are shifted in wavelength by motion between the source of light and the observer due to the Doppler effect. If a star is moving toward the observer, the spectral lines are shifted toward shorter wavelengths, or blueshifted. If the star is moving away, the wavelengths become longer, or redshifted.

Spectra of Stars Wavelength Shift Measuring the Stars –The higher the speed, the larger the shift, and thus spectral line wavelengths can be used to determine the speed of a star’s motion.

Expanding Universe In 1929 Edwin Hubble measured the redshifts of a number of distant galaxies. He also measured their relative distances by measuring the apparent brightness of a class of variable stars called Cepheids in each galaxy. When Hubble plotted redshift against relative distance, he found that the redshift of distant galaxies increased as their distance increased. The best explanation for this observation is that the universe is expanding.

Hubble’s Law