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Cosmic distance scales
When km simply isn’t going to cut it
Fred Sarazin ([email protected])
Physics Department, Colorado School of Mines
PHGN324: Cosmic distance scales
The Universe is a really big place
• Planets – from the greek word meaning “wanderers”
• Stars – our sun is a very typical star
• Classified as a function of luminosity / surface temperature
• At various stages of life: giant, supergiant...
• Dying or dead: white / brown dwarves, neutron stars, pulsars,...
• Exploding: supernovae, novae, X-ray / gamma-ray bursts...
• Nebulae – clouds of glowing gas / star birthplaces
• Galaxies
• Milky Way (our galaxy) contains 2x1011 stars. There are 1011 galaxies in the
observable Universe
• Various shapes: spiral, elliptic, irregular…
• Some are very active – Active Galactic Nuclei (AGN) powered by giant black
holes?
• Quasars, Blazars...
• Large scale structures: galaxy (super)clusters, filament, voids, walls, ...
Fred Sarazin ([email protected])
Physics Department, Colorado School of Mines
PHGN324: Cosmic distance scales
Earth – Moon distance
• Earth – Moon distance: ≈384,000 km
Fred Sarazin ([email protected])
Physics Department, Colorado School of Mines
PHGN324: Cosmic distance scales
Sun – Earth distance
• Sun – Earth distance: ≈150,000,000 km
• New unit (appropriate within the solar
system)
• 1 astronomical unit (AU) = 1.5x108km
Fred Sarazin ([email protected])
Physics Department, Colorado School of Mines
PHGN324: Cosmic distance scales
Solar system and a little beyond
Distance to the Sun in AU:
• Mercury 0.39
• Venus
0.72
• Earth
1.00
• Mars
1.52
•
•
•
•
Jupiter
Saturn
Uranus
Neptune
5.20
9.54
19.18
30.06
• Pluto*
39.53
* Dwarf planet (since 2006)
Fred Sarazin ([email protected])
Physics Department, Colorado School of Mines
PHGN324: Cosmic distance scales
Plan(et) 9 from outer space?
Strange similarities of orbits of trans-Neptunian objects – existence of a 9th planet?
Fred Sarazin ([email protected])
Physics Department, Colorado School of Mines
PHGN324: Cosmic distance scales
Nearby stars
17 ly
• Nearest star (Proxima Centauri): 39,900,000,000,000 km or 271,000 AU
• Need a new unit:
• 1 light-year (ly) = distance travelled by light in 1 year
= 9.46 x 1012 km
à Distance to Proxima Centauri: 4.2 ly
Fred Sarazin ([email protected])
Physics Department, Colorado School of Mines
PHGN324: Cosmic distance scales
Measuring very large distances
Trigonometric Parallax
• A star appears slightly shifted from different positions of the Earth on its orbit.
• The further away the star is (larger d), the smaller the parallax angle p.
• Past a certain distance, the parallax angle becomes too small to be measurable
Fred Sarazin ([email protected])
Physics Department, Colorado School of Mines
PHGN324: Cosmic distance scales
Parsec (pc)
• 1 “minute of arc” = 1/60 of
one degree = 1 arcmin =1’
• 1 “second of arc” = 1/60
arcmin = 1/3600 degree = 1
arcsec =1”
• Apparent displacement of a
star over 6 months:
– d = 1 / tan(p) [AU]
– p is a small angle à tan(p) ~ p
1
d=
p
d in parsec (pc)
p in arc seconds
Fred Sarazin ([email protected])
Physics Department, Colorado School of Mines
à 1 pc = 3.26 ly
Common in astronomy: 1kpc, 1Mpc…
PHGN324: Cosmic distance scales
Inside the Milky Way
Fred Sarazin ([email protected])
Physics Department, Colorado School of Mines
17 ly
~1700 ly
• The solar system neighborhood
PHGN324: Cosmic distance scales
Fred Sarazin ([email protected])
Physics Department, Colorado School of Mines
75000 ly
Several Million ly
The Milky Way and its neighbors
PHGN324: Cosmic distance scales
The large scale structure of the Universe
• Clusters of galaxies are grouped into superclusters. Superclusters form
filaments and walls around voids. Those large scale structures are
important to understand the origin of the Universe (Cosmology).
Fred Sarazin ([email protected])
Physics Department, Colorado School of Mines
PHGN324: Cosmic distance scales
Centaurus A (aka Cen-A) multi-wavelength
(Credit: X-ray: NASA/CXC/CfA/R.Kraft et al; Radio: NSF/VLA/Univ.Hertfordshire/M.Hardcastle;
Optical: ESO/WFI/M.Rejkuba et al.)
Solar System
Closest Star
(1.3 pc)
10-3
1 pc
Size of
Galaxy
Closest Local
Diameter Galaxy
Supercluster Visible Horizon
~5000 Mpc
(25 kpc) (700 kpc) (50 Mpc)
kpc
Mpc
Gpc
Fred Sarazin ([email protected])
1 parsec (pc) = 3.26 light-years ~3x1013 km
PHGN324:
Physics Department, Colorado School of Mines
Cosmic distance scales
Centaurus A
13 Million light years away from Earth
Fred Sarazin ([email protected])
Physics Department, Colorado School of Mines
PHGN324: Cosmic distance scales