PHGN324: Cosmic distance scalesFred Sarazin (fsarazin@mines.edu)Physics Department, Colorado School of Mines

Cosmic distance scalesWhen km simply isn’t going to cut it

PHGN324: Cosmic distance scalesFred Sarazin (fsarazin@mines.edu)Physics Department, Colorado School of Mines

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, ...

PHGN324: Cosmic distance scalesFred Sarazin (fsarazin@mines.edu)Physics Department, Colorado School of Mines

YouTube: Star Size Comparison 2

PHGN324: Cosmic distance scalesFred Sarazin (fsarazin@mines.edu)Physics Department, Colorado School of Mines

Earth – Moon distance

• Earth – Moon distance: ≈384,000 km

PHGN324: Cosmic distance scalesFred Sarazin (fsarazin@mines.edu)Physics Department, Colorado School of Mines

Sun – Earth distance

• Sun – Earth distance: ≈150,000,000 km

• New unit (appropriate within the solar system)

• 1 astronomical unit (AU) = 1.5x108km

PHGN324: Cosmic distance scalesFred Sarazin (fsarazin@mines.edu)Physics Department, Colorado School of Mines

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 5.20• Saturn 9.54• Uranus 19.18• Neptune 30.06

• Pluto* 39.53

* Dwarf planet (since 2006)

PHGN324: Cosmic distance scalesFred Sarazin (fsarazin@mines.edu)Physics Department, Colorado School of Mines

Plan(et) 9 from outer space?

Strange similarities of orbits of trans-Neptunian objects – existence of a 9th planet?

PHGN324: Cosmic distance scalesFred Sarazin (fsarazin@mines.edu)Physics Department, Colorado School of Mines

Nearby stars

• 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

17 ly

PHGN324: Cosmic distance scalesFred Sarazin (fsarazin@mines.edu)Physics Department, Colorado School of Mines

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

Measuring very large distances

PHGN324: Cosmic distance scalesFred Sarazin (fsarazin@mines.edu)Physics Department, Colorado School of Mines

d in parsec (pc) p in arc seconds à1 pc = 3.26 ly

Common in astronomy: 1kpc, 1Mpc…

Parsec (pc)

d = 1p

• 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

PHGN324: Cosmic distance scalesFred Sarazin (fsarazin@mines.edu)Physics Department, Colorado School of Mines

Inside the Milky Way

17 ly

~17

00 ly

• The solar system neighborhood

PHGN324: Cosmic distance scalesFred Sarazin (fsarazin@mines.edu)Physics Department, Colorado School of Mines

The Milky Way and its neighbors

7500

0 ly

Seve

ral M

illio

n ly

PHGN324: Cosmic distance scalesFred Sarazin (fsarazin@mines.edu)Physics Department, Colorado School of Mines

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).

PHGN324: Cosmic distance scalesFred Sarazin (fsarazin@mines.edu)Physics Department, Colorado School of Mines

10-3 1 pc kpc Mpc Gpc

Solar System Closest Star (1.3 pc)

Galaxy Diameter(25 kpc)

Closest Galaxy(700 kpc)

Visible Horizon~5000 Mpc

Size ofLocalSupercluster(50 Mpc)

1 parsec (pc) = 3.26 light-years ~3x1013 km

(Credit: X-ray: NASA/CXC/CfA/R.Kraft et al; Radio: NSF/VLA/Univ.Hertfordshire/M.Hardcastle; Optical: ESO/WFI/M.Rejkuba et al.)

Centaurus A (aka Cen-A) multi-wavelength

PHGN324: Cosmic distance scalesFred Sarazin (fsarazin@mines.edu)Physics Department, Colorado School of Mines

Centaurus A13 Million light years away from Earth