1. Chapter 20 Galaxies Determining Distance

2. Measuring distances to galaxies:
We measure galaxy distances using a chain of interdependent techniques

3. Step 1: Radar
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

4. Step 2: Parallax Determine the parallax angle. Distance = 1/p
Use parallax to determine distance of nearby stars out to a few hundred light-years

5. Step 3: Main Sequence Fitting
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.

6. Step 3 – con’t
Calculate distance of more distant clusters by comparing their brightness to nearby clusters and use the distance- luminosity formula.

7. Visible UV
X-Ray
Supernova 2005ke
Image Credit: NASA
The remaining steps use standard candles. A standard candle is an object whose luminosity we can determine without measuring its distance.

8. Step 4: Cepheid Variables
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

9. Step 5: Type 1a Supernovae
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

10. Step 6 Tully-Fisher Relation
Use entire galaxies as standard candles because luminosity is related to rotation speed
(both depend on mass)

11. Hubble’s Law Lastly, use redshift.
Hubble measured the distance to the Andromeda Galaxy using Cepheid variables as standard candles
Before Hubble, some scientists argued that “spiral nebulae” were entire galaxies while others maintained they were smaller collections of stars within the MW
Edwin Hubble

12. He measured redshift & distance to other galaxies

13. Hubble found that redshift and distance are related in a special way
Hubble’s Law: velocity = H0 x distance

14. More distant galaxies have greater redshift.

15. Hubble’s constant tells us age of universe because it relates velocities and distances of all galaxies
Age =
~ 1 / H0
Distance
Velocity

16. But distances between faraway galaxies change while light travels
Astronomers think in terms of lookback time rather than distance
distance?

17. Expansion of the Universe