Institute of Astronomy

 

Ask an Astronomer - Miscellaneous

Neutron Decay

Published on 14/03/2011 
Question: 

I understand that free neutrons pervade the universe and that they are relatively unstable. What do they decay into?

A neutron is not a fundamental particle, rather it is made of three particles known as quarks bound together. There are six types (or 'flavours') of quark and a neutron consists of one so-called 'up' quark and two 'down' quarks.

When not bound in a nucleus, a neutron is unstable and one of the down quarks undergoes a beta decay (like in some radioactive nuclei) in which it becomes an up quark - this remaining arrangement of 2 up quarks and one down quark is a proton. Therefore, a neutron decays into a proton and this process also emits an electron and an anti-neutrino (a very light, uncharged fundamental particle). The half-life of a free neutron (if you were to have a collection of them, the time it would take for half of them to decay) is around 10 minutes.

Nuclei of Atoms

Published on 14/03/2011 
Question: 

How are protons and neutrons combined to create the nucleus of an atom?

Protons and neutrons are not a fundamental particles, rather they made of three particles known as quarks bound together.

In addition to electrical forces from their charges, quarks also feel the strong force, another of the fundamental forces. It is this force that binds together quarks into neutrons and protons and also holds the protons and neutrons together in the nucleus of an atom. As the name suggests, this is a very strong force - it is able to hold together protons in a nucleus despite their like charges repelling each other and remarkably, the further apart you move the particles, the stronger the force gets between them!

The colour of the sky

Published on 28/02/2011 
Question: 

Why is the sky blue during the day then red at sunset?

Light from the Sun contains all colours of the rainbow. When we look directly at the Sun, we see all of the light from it, so it appears white or a faint yellow. Particles in the Earth's atmosphere scatter shorter wavelength (bluer) light more than the longer wavelength (redder) light due to a process called Rayleigh scattering. During the day, the entire sky looks blue, as we are seeing the light that has been scattered towards us from all directions. However, when the Sun's rays travel through more of the denser part of our atmosphere (at sunrise and sunset) it appears red, as more of the blue light is scattered.