Institute of Astronomy

 

Ask an Astronomer - Miscellaneous

Gravity

Published on 15/12/2011 
Question: 
If gravity is the force that attracts bodies with mass together, how does this force of attraction work? Why should two bodies be attracted to each other?

Gravity is a familiar force: it is what keeps us on the surface of the Earth, and what keep planets in orbit about the Sun. Despite being such an everyday force, it has taken scientists a long time to unravel how it works, and there still remain some unanswered questions.

Our best theory of gravitation is Einstein's theory of general relativity (GR for short). In GR space is bent by the presence of mass (or energy, as Einstein's earlier theory of special relativity showed they are equivalent, yielding the infamous E = mc2 which allows you to convert between the two). The bending of space is commonly explained using the analogy of a rubber sheet. The sheet is flat when there is no mass, but if you were to place something heavy, say a bowling ball, in the middle, the sheet would stretch. Now imagine rolling a table tennis ball across the sheet. If the sheet were flat the table tennis ball would move in a straight line, but with the bowling ball there the table tennis ball moves on a curved path. If you couldn't see the rubber sheet it would appear as there was some force that is pulling the table tennis ball towards the bowling ball. This is the force of gravity in our analogy.

The real universe works in a similar way. An objects want to travel in a straight line, but when mass bends space, this line gets bent too, so the object follows a deflected trajectory. We say that this deflection is due to gravity. The physicist John Archibald Wheeler summarised this as "Space acts on matter, telling it how to move. In turn, matter reacts back on space, telling it how to curve".

Two bodies are attracted to each other as they curve space in a way that makes them want to move together. Imagine putting two bowling balls on our rubber sheet: they will fall towards each other. That is how the force of attraction works.

J1950 and J2000 Epochs

Published on 23/08/2011 
Question: 
What are the epochs J1950 and J2000 when looking at objects in the sky? And what do I have to put in my telescope program?

Astronomers use different epochs to give coordinates of objects in the sky due to changes in motion due to primarily the precession of the Earth on its rotation axis. Much like a spinning top, as the Earth rotates, it's rotation axis gradually rotates as well although much slower than the daily rotation we see. Because of this precession, the positions of the stars change over time with a small motion every day. On small timescales this motion isn't noticeable however over decades it is. For this reason, astronomers update their coordinates every 50 years to make it simpler when finding objects.

Although for an object you can find coordinates in either J1950 or J2000 (the two most recent epochs), this won't actually be the correct location in the sky today. However, what your telescope program will do will take the coordinates from those epochs and then calculate what they should be today i.e. J2011 and then move your telescope there. This saves people doing things by hand and so means you can find objects with the standard coordinates quickly and easily.

As such, all you should need to do is find the coordinates of the object in either J2000 or J1950 and can then put those into your program. The program will then do the rest.

Stars on the Moon

Published on 02/05/2011 
Question: 
Is it possible to see stars from the surface of the moon? I thought it was but in an interview in 1970 (http://bbc.in/fRz0Mh) Neil Armstrong said you can only see the Sun and Earth.

When US astronauts visited the moon as part of the apollo program, people often comment in images that you don't see the stars. This is the same phenomenon Neil Armstrong described in the interview on the BBC in 1970.

The stars are actually visible from the surface of the moon but they are particularly difficult to observe. If on Earth you compare a night where there is no moon with a night with a full moon, it is much easier to see stars on the dark night with no moon. The same phenomena happens on the moon with the Earth reflecting the Sun's light as well as the astronauts only working in sunlight. And as you know from experience, during the day time we don't see any stars (unless you know exactly where to look using a telescope!) even though they're still shining.

So although the stars are there, the light conditions when the astronauts were working meant that they were unable to see them.

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!