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Asaf Pe’er Space Telescope Science Institute. The measure of Cosmological distances. August 2008. How far is Jupiter ??. I. Science at ancient times. Greece, c. 300BC What is the size of earth ?. Syene, Egipt. First measurement of earth’s radius: Eratosthenes of Cyrene (276BC- 196 BC). - PowerPoint PPT Presentation
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The measure of Cosmological distances
Asaf Pe’erSpace Telescope Science
Institute
How far is Jupiter??
Greece, c. 300BCWhat is the size of earth ?
I. Science at ancient times
Syene, EgiptFirst measurement of earth’s radius:
Eratosthenes of Cyrene (276BC- 196 BC)
Eratosthenes: Earth circumference =40000 km Earth radius = 6000 km
Measuring the size of the moon using lunar eclipse
Time to full eclipse~ moon radiusTotal eclipse time
~ earth radius
Moon radius ~ ¼ earth radius
Aristarchus of Samos, 310BC- 230BC
Distance to the moon ~380.000 km (240.000 mi)
Given moon radius, distance is simple geometry
Geocentric (=earth at the center) vs. Heliocentric (=sun at the center)
universeAristotle (384BC - 322BC) Aristarchus of Samos
(310BC - 230BC)
Why Geocentric ?1. "we see" 2. if the earth moves, where is the wind ?3. Gravity – everything is attracted to the center of the universe4. Parallax: stars don't move !
Parallax
Parsec = paralax-arcsecond =~ 3.3 l.y.
But some stars do move..
Retrograde motion of Mars
The universe according to Ptolemy
Ptolemy: 83-161 AD Circle - “Perfect” shape
Mars motion according to Ptolemy
Mars motion according to Heliocentric theory
Ptolemy model - consistent with observations!!!
Nicolaus Copernicus (1473 - 1543)
Advantages:1. Correct
2 .Simple
Disadvantage:1 .Less accurate than geocentric model
2. Copernicus was unknown
II. Rise of Heliocentric Cosmology
Tycho Brahe (1546 - 1601):Accurate measurements of planet orbits
Johannes Kepler (1571 - 1630):Planets move in ellipses (not circles) around the sun
Kepler’s laws of planetary motion helped Neuton to develop the theory of Gravity
Next breakthrough: the Telescope (1608)
Galileo Galilei (1564 - 1642):First astronomer to use a telescope
Moon has craters ! =(not perfect sphere)!
Jupiter has moons ! =(not everything rotates around earth)!
“Smoking gun”: Venus phases Cannot be explained by Geocentric model
Even the sun has spots ! =(sun is not perfect)!
Measuring the distance to the sun
Giovanni Cassini (1625 - 1712)
1672 -Cassini & Richer measure the distance to Mars
Using Kepler’s laws, Cassini deduce the distance to the sun:150.000.000 km = 1 Astronomical unit (1 A.U.)
William Herschel (1738 - 1822)
Herschel’s 20 foot reflector
III. Measuring distance to the stars
-Found new planet (Uranus)-Discover Infra-Red light-First map of the sky:
Idea: All the stars are the same. Therefore, bright stars are closer.
Herschel’s model of the milky way:
-Stars are ordered in space.
We are part of the Galaxy.
- But he could not scale the size of the galaxy
Friedrich Bessel (1784 - 1845)
1838 :First measurement of distance to a star
61 signi
Distance = 100,000,000,000,000 km =(11 light years)
Scaling the milky way: width = 10.000 l.y,.(today: 100.000 l.y ).Thickness = 1.000 l.y.
Charles Messier (1730 - 1817):deep sky catalogueof Nebulae
“The great debate”:Are nebulae part of the milky way galaxy
- or not?
M31
The great debate
John Goodricke (1764 - 1786):Discovery of variable stars & Cepheids
Mechanism: Envelope contains opaque He2+ - heated - pressure increases- expansion - radiation escape - cooling
Henrietta Leavitt (1868 - 1921):1908: Discovery of periodicity- Luminosity relation in Cepheids
25 Cepheids at the small Magellanic cloud All at same distance from us
1917 :Shapley & Hertzsprung measured the distance to a Cepheid - allow the use of Cepheids as “Standard candles”
1918 :Harlow Shapely measures the milky way
Size of the milky way: 100.000 l.y. ; Thickness = 1.000 l.y
Cepheids in Globular clusters
What about the nebulae?
Edwin Hubble (1889 - 1953):I. 1923 - Discovery of Cepheids in Andromeda galaxy
Distance to M31: 900.000 light years >> Milky
way!!
Spectroscopy: measuring the chemical elements in starsFraunhofer, Bunsen, Kirchhoff (1859)
IV. Measuring distance to the galaxies
Spectrum of the sun
1868 - Lockyer & Janssen discover a new element in the sun (He)1860’s - Huggins: stars contain the same elements as the earth.
1868 - William Huggins finds red shift of Sirius,determine its velocity: 45 km/s
Red Shift
1912 :Vesto Slipher measures red shift of galaxies
V ~ 300-1000 km/s
Strangley, most of the galaxies
are receding from us !
Edwin Hubble (1889 - 1953):II. 1929 - Discovery of distance - velocity relation in galaxies
Hubble’s law Velocity = Distance H0
H0 = Hubble’s constant = 70 (km/s) / Mpc
Mpc = Mega (Million)-parsec ;Parsec = paralax-arcsecond =~ 3.3 l.y.
Baade (1952) & Sandage (1954) corrected the value of H0
Back in time, all the matter was concentrated in a very small region
1915 :General Theory of relativity -- universe collapse (Gravity);
Cosmological constant 1922 :Alexander Friedman
Universe expands !
1927 :Georges Lemaitre
Albert Einstein (1879 - 1955)
V. The big bang theory and beyond
Further proofs for universe expansion & “Big bang”
Ralph Alpher (1921 - 2007):
1948 :Alpher, Bethe, Gamow - H, He production in big bang Alpher, Gamow & Herman - cosmic microwave background (CMB)
Universe: 90% H, 9% He
1964 :Penzias & Wilson discover the CMB
1991 -Fluctuations in the CMB (COBE satellite): “embryos” of galaxies
Mather & Smoot, 2006 Nobel prize
The future
1998 :A surprising twist
Astronomers led by Adam Riess (STScI), Saul Perlmutter (Berkeley)
-The universe accelerates!!!
The universe, 2008
Wmap