Lecture 31 Flashcards
What are the five steps of the cosmic ladder?
- solar system
- nearby stars
- Milky Way stars
- nearby galaxies
- distant galaxies
How is ladder step 1 (the solar system) used?
- radar ranging is the technique used
- by sending a radar signal and measuring the time it takes to return distance can be calculated
- a signal travels for twice distance and radio waves propagate at the speed of light
- d= 1/2*(time lag * v)
How is ladder step 2 (nearby stars) used?
- parallax is the technique used
- the position of nearby stars relative to the background changes as the Earth orbits the sun
- parallax distance: parsec = star with parallax of one arcsec = 3.26 light years
How is ladder step 3 (distant stars) used?
- the main sequence fit idea is the technique used
- temperature/colour gives stellar type and in turn mass and absolute luminosity.
- from the brightness it is possible to infer distance
- limit= nearest galaxies
How is ladder step 4 (nearby galaxies) used?
- the relationship between period of oscillation and absolute luminosity of Cepheids is the technique used
- by measuring period and observed brightness distance can be calculated
- limit: nearby galaxies (individual stars in non-crowded environments must be able to be identified)
How is ladder step 5 (distant galaxies) used?
- the Tully-Fisher relationship is the technique used.
- the Tully-Fisher relationship relates rotational velocity and absolute luminosity- this allows us to calculate distance
- limit: galaxies up to a few billion light years
- white-dwarf binary supernova can also be used to calculate distance as the explosion almost always gives the same energy- this is indicative of the distance
What happens to uncertainty as distance increases?
Since each step is calibrated on the previous one uncertainty increases with distance
What was Edwin Hubble’s first big discovery?
Hubble’s first big discovery was that he discovered Cepheids in Andromeda and showed that they were far outside the Milky Way
What did Vesto Slipher discover?
He discovered that nearly all galaxies are redshifted.
What is Hubble’s Law?
The farther away a galaxy is, the faster it is moving away from us.
v= H * d
What is the Hubble Constant?
H= 70km/s/Mpc
This is calculated from the slope of the Hubble Diagram which plots velocity against distance
What does the Hubble Law assume?
The Hubble Law is not a “standard candle” for measurement- this assumes that the Hubble Law is linear
What is a standard candle?
Astronomical objects that are used to calculate distance; these objects are on the cosmic ladder. They include Cepheid variables.
Why is the Hubble Law useful for measuring distance?
It is applicable to any distant galaxy. By measuring the redshift we can approximate the distance
What is a key concept in the structure of the universe?
Clustering
