Originally Posted by

**mayflow**
Maybe this may help. This ain't mine, but I am learning on it. Distances and Sizes

One way to measure distances is if you know the size of an object. If you can then measure its apparent angular size, you can work out the distance:

Given the geometry above, and if D≫r, (almost always the case in astronomy), then

r=θD

as long as θ is measured in radians (a radian is 180π degrees).

Angles in astrophysics are often so small that even a degree is too large a unit to be convenient. We typically use arcminutes (one arcminute is 1/60 of a degree) and arcseconds (one arcsecond = 1/60 of an arcminute = 1/3600 of a degree).

Fluxes and luminosities

Luminosity L is the total amount of power put out by some object, and is measured in Watts. Flux f is the power we receive at our telescope, per unit collecting area, and is measured in Watts per square metre. They are related by the equation:

f=L4πD2

where D is the distance to the emitting object.

Spectra and the Doppler Effect

A spectrum is a graph of flux per unit wavelength plotted against wavelength. It will often show emission or absorption lines due to particular elements.

If an object is moving towards or away from you, these spectral lines will be moved in wavelength away from their normal wavelength λo. If you observe a line at wavelength λ, you can define a redshift z as:

z=λ−λ0λ0

If this shift is due to the doppler effect, and the velocity v<<c (velocity much less than the speed of light - nearly always true), then:

z=vc

To measure a redshift, you will need to know what lines to expect, and what their wavelengths are in the laboratory. The following graph shows you some of the typical lines you would see in a star or galaxy. Note that not all stars will show all these lines, and there are a variety of other lines that in certain stars can be strong. The C-H line is due to vibrations in the chemical bond linking carbon to hydrogen in molecules.

Hubble Law

Assuming that the brightest star in every galaxy had about the same luminosity (not a good assumption), Edwin Hubble calculated their relative distances. He found that the distances correlated with redshift. Everything was moving away from us and the speed correlated with how far things were from us.

The standard explanation is that space itself is expanding. Objects are not moving - they are just being carried apart by the expansion of space.

This means that unless more matter is created, the density of the universe must continuously go down (same amount of matter spread over more space). The alternative is that more matter is appearing out of nowhere - this is called the “steady state theory”.

The “Steady State Theory” predicts that the universe should always look the same. We actually observe, however, that the universe was different in the past (we can see the past by looking a distant objects). Quasars were more common and the microwave background emission comes from a time when space was opaque.