The Resolution: Edwin Hubble (1889-1953) Measured Distance to Andromeda Galaxy (M31) • Noticed individual bright stars in Andromeda • Calculated the distance using the Period-Luminosity Relation for Cepheid Variable Stars The Resolution: Edwin Hubble (1889-1953) Measured Distance to Andromeda Galaxy (M31) • Noticed individual bright stars in Andromeda • Calculated the distance using the Period-Luminosity Relation for Cepheid Variable Stars 1. MEASURED magnitude & Period The Resolution: Edwin Hubble (1889-1953) Measured Distance to Andromeda Galaxy (M31) • Noticed individual bright stars in Andromeda • Calculated the distance using the Period-Luminosity Relation for Cepheid Variable Stars 1. MEASURED magnitude & Period 2. CALCULATED Luminosity The Resolution: Edwin Hubble (1889-1953) Measured Distance to Andromeda Galaxy (M31) • Noticed individual bright stars in Andromeda • Calculated the distance using the Period-Luminosity Relation for Cepheid Variable Stars 1. MEASURED magnitude & Period 2. CALCULATED Luminosity 3. CALCULATED Distance The Resolution: Edwin Hubble (1889-1953) Measured Distance to Andromeda Galaxy (M31) • Noticed individual bright stars in Andromeda • Calculated the distance using the Period-Luminosity Relation for Cepheid Variable Stars 1. MEASURED magnitude & Period 2. CALCULATED Luminosity 3. CALCULATED Distance • Result was MUCH farther than expected... • (2.3 Million Ly - well beyond Milky Way) Henrietta Leavitt & Period-Luminosity Relation Cepheid Variables: Bright stars whose Luminosity (energy output) varies every few days Henrietta Leavitt & Period-Luminosity Relation Cepheid Variables: Bright stars whose Luminosity (energy output) varies every few days Period-Luminosity Relation The Resolution: Vesto Slipher (1875-1969) •Obtained ____ Spectra of Galaxies The Resolution: Vesto Slipher (1875-1969) •Obtained ____ Spectra of Galaxies •Found that Galaxy spectra are REDSHIFTED The Resolution: Vesto Slipher (1875-1969) •Obtained ____ Spectra of Galaxies •Found that Galaxy spectra are REDSHIFTED •Used redshift to calculate ________? Redshift: Calculating Velocity • Relationship between red/blue shift and the velocity: Δλ v REDSHIFT (Z) = = λ c • c = speed of light, • λ is the “at rest” wavelength • Δλ is wavelength shift REDSHIFT Redshift shows up as shifted absorption or emission lines in galaxy spectra REDSHIFT Redshift shows up as shifted absorption or emission lines in galaxy spectra REDSHIFT Redshift shows up as shifted absorption or emission lines in galaxy spectra What part of the EM spectrum would you expect to see high z galaxies? Redshift vs. distance Blue Yellow Red IR The most distant galaxies are detected only in the infrared! Expansion of the Universe: Cosmological Redshift Distant galaxy redshifts caused by the “stretching” of light waves as space itself is expanding! Cosmological Redshift is slightly different from Doppler shift (why?) Hubble’s Law & Hubble Diagram Hubble found that recession velocity of galaxies is proportional to their distance away from us... (km/s) v = H0 * d (Mly) Hubble’s Law & Hubble Diagram Hubble found that recession velocity of galaxies is proportional to their distance away from us... (km/s) v = H0 * d (Mly) ...implying that the Universe is expanding!! Hubble’s Law: The Age of the Universe (km/s) v = H0 * d (Mly) Hubble’s law also implies that the Universe has a finite age! Age = 1/H0.