Peculiar velocities

Measuring distances

Distance measurement is difficult in space. We have to rely on the equivalent of rods of different kind depending on how far the object is.

Parallax distance

This method is available only to the most nearby objects. We rely on our possibility to see a same object (like a star) from two different point of view. The direction the star is seen on the sky will vary as a function of its distance and the distance between the two different view points. This method is clearly explained here. This distance is also called the angular diameter distance in cosmology.

I give here an illustration of this phenomenon:

Luminosity distance

The second class of methods relies on knowing what is the intrinsic lumininosity of the considered object. As we can always measure the apparent luminosity, i.e. its brightness in the sky, the ratio between the two luminosities gives us the luminosity distance. This distance can be converted into an angular diameter distance, which for a non-flat metric is different from this one.

Cepheid stars

The first object for which we know how to compute intrinsic luminosities from other observable properties are Cepheid stars. Their brightness is related to the periodicity of the variations of their brightness.


Spiral galaxies

Elliptical (E-S0) galaxies

Maser distances

Reconstructing velocities

(C) Guilhem Lavaux - 2014,
Last updated: November 21, 2014