Chapter 20Galaxies

Determining Distance

Measuring distances to galaxies:

We measure galaxy distances using a chain of interdependent techniques

Light travels at a constant speed in space of 3x108 m/s).

We send a signal to a planet and measure the time for the reflected signal.

Distance = speed x time

Step 1: Radar

Determine the parallax angle.

Distance = 1/p

Use parallax to determine distance of nearby stars out to a few hundred light-years

Step 2: Parallax

Find the distance to a nearby star cluster with parallax. Use the distance and apparent magnitude to find the luminosity of each type of star it. Plot these stars on an HR diagram. Use the diagram as a reference diagram.

Step 3: Main Sequence Fitting

Step 3 – con’t

Calculate distance of more distant clusters by comparing their brightness to nearby clusters and use the distance- luminosity formula.

The remaining steps use standard candles. A standard candle is an object whose luminosity we can determine without measuring its distance.

Image Credit: NASA

Visible UV X-Ray

Supernova 2005ke

Cepheid variables are very luminous stars that have a period-luminosity relationship. We can use these stars as standard candles.Cepheid variable stars with longer periods have greater luminosities

Step 4: Cepheid Variables

All WD have the same maximum mass limit of 1.4 Ms, therefore they all have the same luminosity.

Their apparent brightness tells us distance to galaxies up to 10 billion light-years

Step 5: Type 1a Supernovae

Step 6 Tully-Fisher Relation

Use entire galaxies as standard candles because luminosity is related to rotation speed(both depend on mass)

Lastly, use redshift.Hubble measured the distance to the Andromeda Galaxy using Cepheid variables as standard candles

Edwin Hubble

Hubble’s Law

He measured redshift & distance to other galaxies

Hubble’s Law: velocity = H0 x distance

Hubble found that redshift and distance are related in a special way

More distant galaxies have greater redshift.

Hubble’s constant tells us age of universe because it relates velocities and distances of all galaxies

Age =

~ 1 / H0

Distance

Velocity

But distances between faraway galaxies change while light travels

Astronomers think in terms of lookback time rather than distance

distance?

Expansion of the Universe