Andromeda galaxy

• Obviously not!• have the same apparent brightness, but

very different luminosities

• If you knew the luminosity of…• a standard lighthouse beacon• or a standard candle

• you could measure the distance toeach given their apparent brightness

Standard Candles

• Astronomers call any astronomical object whose luminosity can bedetermined without knowledge of its distance a standard candle.

• Calculate distance to any standard candle by…• measuring its apparent brightness and using the luminosity/distance formula

Are these lights at the same distance?

Brightness ~ Luminosity/ Distance2

�

B =L

4!D2

�

D =L

4!B

standard candle

observe & measure

What Makes a Good Standard Candle?• We know luminosity of Sun, as well as stars on main sequence.• know this by using the parallax method for nearby stars• can measure distance to stars of same type that are far away

What Makes a Good Standard Candle?• For a cluster of stars, we can compare

the entire main sequence.• we know distance to Hyades from parallax• by measuring how much fainter Pleiades’

MS is, we can calculate its distance

• This is called main-sequence fitting.

• But main sequence stars are too faint to observe in other galaxies!• So we need a more luminous standard candle to measure distances

to galaxies: Cepheid variable stars

Cepheid Variables

• Cepheid variables make good standard candles because:• they follow a well-defined period-luminosity relationship• they are bright giants…luminous enough to see at great distances

Edwin Hubble(1889-1953)

• Discovered Cepheid variables inthe Andromeda galaxy.

• Calculated distance to Andromeda• 2 million light years• not in the Milky Way

• Developed classification schemefor galaxies.

• Has space telescope named afterhim!

Milton Humason(1891-1972)

• Former mule-team driver whodropped out of 8th grade.

• Took spectra and measuredredshifts of many galaxies.

• Worked with Hubble, whomeasured the distances tothose same galaxies.

• They plotted distance vs.velocity and formulated :

Hubble’s Law: V = Hod

Hubble’s Law• Hubble supplied the distance to a galaxy.

• using Cepheid or “brightest star” standard candles• Humason measured the shift to longer wavelengths

of absorption lines in the galaxy’s spectrum.• used Doppler formula to calculate velocity

• Plot resulted in a straight line.• the farther away a galaxy was, the

faster it was moving away from us• velocity increased linearly with

distance• V = Ho d

• Ho, the slope of the line, is calledHubble’s constant [km/s per Mpc]

© 2010 Pearson Education, Inc.

What is Hubble’s law?

© 2010 Pearson Education, Inc.

The spectral features of virtually all galaxies are redshifted, which means that they’re all moving away from us.

© 2010 Pearson Education, Inc.

By measuring distances to galaxies, Hubble found that redshift and distance are related in a special way.

© 2010 Pearson Education, Inc.

Hubble’s law: Velocity = H0 × distance

© 2010 Pearson Education, Inc.

Redshift of a galaxy tells us its distance through Hubble’s law:

Distance = velocity H0

© 2010 Pearson Education, Inc.

Distances of the farthest galaxies are measured from their redshifts.

© 2010 Pearson Education, Inc.

How do distance measurements tell us the age of the universe?

© 2010 Pearson Education, Inc.

Thought Question

Your friend leaves your house. She later calls you on her cell phone, saying that she’s been driving at 60 miles an hour directly away from you the whole time and is now 60 miles away. How long has she been gone?

A. 1 minute B. 30 minutes C. 60 minutes D. 120 minutes

© 2010 Pearson Education, Inc.

Thought Question

Your friend leaves your house. She later calls you on her cell phone, saying that she’s been driving at 60 miles an hour directly away from you the whole time and is now 60 miles away. How long has she been gone?

A. 1 minute B. 30 minutes C. 60 minutes D. 120 minutes