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:
REDSHIFT (Z) =
•
c = speed of light,
•
•
λ is the “at rest” wavelength
Δλ is wavelength shift
Δλ
v
=
λ
c
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