We have seen how important it is to know the distances to the celestial
bodies, in order to be able to interpret what we see.
To try to understand our universe, we must first be able to measure it. Galaxies appear as basic blocks of this universe, so it becomes essential to measure the distance between them and the Earth.
As it is impossible to directly measure the distance to the furthest galaxies, we will use a set of measurements which are supported together, from the nearest distances to the furthest ones.
There are three kinds of methods, the first one, the parallax gives us an absolute measurement, while the two others give only relative data.
As the Earth moves along its orbit, you don't see a nearby star
at the same place on the sky at two different times. This effect is called the
parallax, and considering the distances to the stars - even the nearest ones
- it is always very small. That's why it was not before 1838 that Friedrich
Bessel was able to measure the parallax of the star 61 Cygni, which is only
11 light-years away.
This method is in fact the only absolute method that we have at our disposal to know the distance of a star. So, it is essential to calibrate the other methods.
The Earth moves along its orbit,
hence a nearby star seems
to move against the deep sky.
The method is only subject to measurement errors, but can only
give distances less than 1000 Parsec.
It is with this process that the Hipparcos satellite was able to map more than 120.000 stars.
If the use of interferometry
in optics is quite restricted, on the contrary we are able to build interferometers
working in the radio waves range, with a base quite as large as we want. The
VLBA - Very Large Base Array - is an array of radiotelescopes whose antennas
are spread across the whole US territory, from Puerto Rico to Hawaï.
As the base is very large, one can separate sources with an excellent resolution. So we are using this resolution to apply the parallax method to much greater distances than with visible light. With this method, the VLBA was able to measure the distance of the galaxy NGC 6264 with precision. This galaxy is located at 450 Mal (about 140 Mpc), with an accuracy of 9%.
Of course, this method can only be used with galaxies which are radiowave emitters. But the correlation of this direct measurement method with other methods (that we are going to discover) will allow to increase the accuracy of these methods and to widely ameliorate our current measurements.
Photometric measurements are based upon the visual magnitude of an object, whose absolute magnitude is supposed to be known.
This is the use of the distribution of frequencies of the light received from an object.
So, the broadening of the spectral lines is an indicator of the mass of
The heavier the galaxy is, the brighter it is. And so, we can find the intrinsic luminosity of the galaxy from the broadening of its spectrum, with the help of the Tully-Fisher relationship (for the spiral galaxies), or the Faber-Jackson relationship (for the elliptical galaxies).
This method is available as far as 3000 Mpc.
Even if it seems easy, it is in fact a very tricky measurement : first, the X-ray detectors are difficult to gauge, and the electronic cloud is surely far from a perfect uniform and homogeneous sphere -it is at least denser at its center- and this fact makes difficult the interpretation of measurements.
This method is very new. It is currently being developed by NASA, by means of the Chandra observing satellite.
Having established the distances to some galaxies, Hubble noticed
an important fact : every galaxy seems to move away from us.
A more precise study reveals that the recessional velocity of an object increases in proportion to its distance.
As it seems quite presumptuous to think that the Earth is at the very center of the Universe, we must deduce that the Universe is expanding.
If H is known, we can deduce the distance of a galaxy from its redshift (usualy denoted z). Because of the uncertain value of this constant, this method is only meaningful and interesting for the furthest objects.