How to determine antenna noise power?
A dipole is not very directional even if you pointed it upwards; 99% of the noise it will pick up is terrestrial hence the noise temperature is that of the earth. If you were using a dish and high frequencies and pointed it at the stars then the noise temperature would be a few tens of Kelvin assuming that your head antenna amplifier was very very good.

Wiki is short but sweet. Another interesting pdf article. Here's another. I quickly scanned thru this and it seems useful. Basically i searched "antenna noise temperature" and the first four links appear to be excellent.