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Measuring the Hubble Constant. The Scale of the Universe I: The Solar System. Pluto is 5.913 billion km (39.53 AU) from the Sun. The Oort cloud extends much further than this: out to around 100,000 AU or about 2 ly . The next closest star, Alpha Centauri, is 4.2ly away. 1ly=9.46 x 10 12 km.

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the scale of the universe i the solar system
The Scale of the Universe I:The Solar System

Pluto is 5.913 billion km (39.53 AU) from the Sun

The Oort cloud extends much further than this: out to around 100,000 AU or about 2 ly

The next closest star, Alpha Centauri, is 4.2ly away.1ly=9.46 x 1012 km

the galaxy
The Galaxy

The Milky Way is full of stars, many with orbiting planets much like our solar system

the Milky Way is about 100,000 ly across

The Milky Way has two close satellite galaxies: The Large and Small Magellenic Clouds: each are about 50kpc across

1 kpc = 3261 ly

the local group
The Local Group

The Local Group consists of about 36 galaxies all loosely gravitationally bound together and including the Milky Way and Andromeda

The Local Group is about 3 million ly across

The Milky Way and Andromeda are the two largest (spiral galaxies) and each has several smaller 'satellite' galaxies

clusters and superclusters
Clusters and Superclusters

The Local Group is just one cluster of galaxies. There are many others 'nearby'

Clusters in today's lab which you should use:

Ursa Major I

Coma Cluster

Corona Borealis Cluster

The Local Group and several other clusters are members of the Virgo Supercluster which is about 100 million ly across

There are many Superclusters in the U.

  • The distance to nearby stars can be found using parallax which is watching the movement of an object across the sky
  • Farther away, we can use a relationship between oscillation period and luminosity in variable stars (Globular Clusters and even nearby Galaxies)
  • In more distant Galaxies we can use Supernovae explosions as 'standard candles'
  • For Galaxy Clusters we use the brightest galaxy in the cluster as a 'standard candle' (statistically the brightest galaxy in each cluster should have the same absolute magnitude)
  • Absolute magnitude is the magnitude an object would have if it were located 10pc or about 32.616 ly or 3×1014 km away from Earth
doppler effect
Doppler Effect
  • Object moving away have spectral lines shifted towards the red end of the spectrum: Redshift
  • Objects moving toward us have spectral lines shifted toward the blue end of the spectrum: Blueshift

All of our galaxies today have a redshift because they (and all galaxies in the Universe!) are moving away from us. We will use the Ca H and K lines to observe this.

edwin hubble
Edwin Hubble
  • First noticed that ALL galaxies are

receding from us by measuring

absorption line redshifts

  • 1929 noted that Galaxies with greater distance from us are moving away faster: Hubble's Law
  • Actually the space between galaxies is growing-this leads to the expansion of the Universe
  • We can measure how fast using Doppler shifts and calculating the distances by using galaxy magnitudes.
  • We will calculate the Hubble Constant
hubble constant h
Hubble Constant (H)
  • Units are kilometers per second per megaparsec
    • km/sMpc
    • 1 Mpc =1x106 pc
    • 1km = 3.24x1013 pc
  • To calculate the Hubble Constant, first determine the redshift (z) of the galaxy:
    • z = Δλ/λ where Δλ is the difference between the rest wavelength and the observed, redshifted wavelength and λ is the rest wavelength
    • λ H = 3969Å Ca H-line, λ K = 3934Å Ca K-line
hubble constant cont
Hubble Constant cont.
  • Next, use redshift (z) to find the velocity of recession: v = zc where z is redshift and c is the speed of light: c = 3x105 km/s
  • We will also find the apparent magnitude (m) of the galaxy. Using this, the absolute magnitude (M = -22) and an equation called the distance modulus, we can find the distance to the galaxy:
    • m-M = 5+5logd or logd = 1+(m-M)/5
    • d is in parsecs (pc) in this equation
  • Finally we can find the Hubble Constant H = v/d
the age of the universe
The Age of the Universe
  • The age of the Universe can be estimated using the Hubble Constant.
  • The units of the H can be converted into 1/s.

You will have to convert km to Mpc or vice versa...

  • 1/H is then the age of the Universe in seconds. Convert this to years and see what you get!