Chapter 20 Galaxies Determining Distance
description
Transcript of Chapter 20 Galaxies Determining Distance
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