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Cosmic distance scales
When km simply isn’t going to cut it
Fred Sarazin (fsarazin@mines.edu)
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 (fsarazin@mines.edu)
Physics Department, Colorado School of Mines
PHGN324: Cosmic distance scales
Earth – Moon distance
• Earth – Moon distance: ≈384,000 km
Fred Sarazin (fsarazin@mines.edu)
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 (fsarazin@mines.edu)
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 (fsarazin@mines.edu)
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 (fsarazin@mines.edu)
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 (fsarazin@mines.edu)
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 (fsarazin@mines.edu)
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 (fsarazin@mines.edu)
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 (fsarazin@mines.edu)
Physics Department, Colorado School of Mines
17 ly
~1700 ly
• The solar system neighborhood
PHGN324: Cosmic distance scales
Fred Sarazin (fsarazin@mines.edu)
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 (fsarazin@mines.edu)
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 (fsarazin@mines.edu)
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 (fsarazin@mines.edu)
Physics Department, Colorado School of Mines
PHGN324: Cosmic distance scales