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Galaxy NGC4414 from HST Astronomy 162:
Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30

Lecture 35: The Cosmic Distance Scale

Readings: Ch 26, section 26-4

Key Ideas

Measuring the Hubble Parameter, H0

Distance Methods:
Trigonometric Parallaxes
Spectroscopic Parallaxes
Cepheid Period-Luminosity Relation
Galaxy Standard Candles
Galaxy Luminosities
Redshift Distances

The Distance Problem (again!)

Cepheid P-L relation is good but limited:

This is only next-door in cosmic terms, so we need to seek other methods to estimate very large cosmic distances.

Hubble's Law & its Discontents

Ideally, we could just use the Hubble Law:
Hubble Distance (or Redshift-Distance) Formula for nearby galaxies

At least nearby, all you need to measure is the cosmological redshift, z, in order to get the distance.

The problem is, what is H0?

Steps to the Hubble Parameter

No single distance method is universal.

Must work towards large distances from nearby.

Bootstrap Process:

Step 1: The Astronomical Unit

1 AU = Mean Earth-Sun Distance

Method: Geometric Triangulation

Permits measurement of:

Step 2: Trigonometric Parallaxes

Calibrated by the AU (size of Earth's orbit).

Method: Measure stellar parallax angles

Permits measurements of:

Step 3: Spectroscopic Parallaxes

Calibrated by Trigonometric Parallaxes.


Permits measurements of:

This is enough to reach out to the Large Magellanic Cloud (LMC).

Step 4: Cepheids

Calibrated by cluster H-R diagrams.


Cepheids give distances to:

Only works for Spirals because you need young (Population I) star clusters for Cepheids, and Ellipticals have only old Population II stars and no young stars.

Step 5: Galaxy Standard Candles

Look for new, very bright standard candles that will be found in both Spiral and Elliptical galaxies

Calibrated by:

Step 5 (cont'd): The Bottom Line

Variety of techniques get used:

Bottom Line:

Step 6: Galaxy Luminosities

Calibrated by the distance to the Virgo Cluster


Seek a refined estimate of H0 out to greater distances that gets you beyond the regime where the random (aka "peculiar") velocities of galaxies due to their orbits around each other or within groups are important.

Step 6 (cont'd): Specific Techniques

Tully-Fisher Relation for Spirals:

Fundamental Plane Relation for Ellipticals:

Step 7: Redshift Distances

Calibrated against all previous steps.


Allows us to probe the Universe on the largest scales observable.

Current Status

Current critical areas:

Best Estimate: H0 = 70 +/- 7 km/sec/Mpc

Why do we care?

Measuring accurate cosmic distances is essential for answering these questions:

What is the Hubble Parameter (H0)?

What is the expansion history of the Universe?

Return to [ Unit 5 Index | Astronomy 162 Main Page ]
Updated: 2006 February 18
Copyright Richard W. Pogge, All Rights Reserved.