Institute of Astronomy


Ask an Astronomer at the IoA

Ever had a question about astronomy you've want answered? Have a look through the previous questions which we've been asked and if you can't find find your answer, ask us!

Ask a question

The half-moon terminator

Published on 04/04/2011 

At half moon when the sun is in the sky, why does the terminator of the moon appear not to be perpendicular to a line between the moon and the sun (which it must be)
A perpendicular to the terminator always passes considerably above the sun.

The terminator on the Moon is the line that divides day from night on the moon - the daylight side is that half of the Moon that is closer to, and illluminated by the Sun. The effect you mention is most apparent when the Sun and the Moon are widely separated in the Sky, eg when there is a half-moon. I think the solution to your question lies in the way that we perceive straight lines in a 3-dimensional situation.  For example, if you were to stand on a infinitely long rail track, the parallel rails converge in the distance to either side of you, and thus must appear to curve around you.

We know the Sun, Moon and Earth lie in the flat disc of our solar system, which we see edge-on, so it should appear as a straight line - but this is projected on the sky as an arc. The Sun rises in the east, sets in the west, and its 'straight path' is seen as an arced path when projected onto the sky. The Moon follows the same route across the sky, and the further away it is along that arc from the sun, the more you’ll be off if you’re imagining the path of the light as a straight line across the sky.

Cosmic Expansion Beating the Speed of Light?

Published on 02/04/2011 

When the Universe starts to expand from a single point (singularity) and expands to a size of lightyears in just 10 minutes, then it seems to me that there must have been speeds exceeding the speed of light. Surely that cannot happen?!

Einstein's theory of Special Relativity states that information cannot travel from one place to another faster than the speed of light. When the Universe expands, it is the entire of space and time that is expanding. If you imagine two points in the Universe, as the Universe expands, it is the space between them that expands and everything moves away from everything else, so you will not be able to transmit anything (whether radiation or matter) between them faster than the speed of light (in fact the Universe may be expanding so fast that light travelling from one place to the second may not ever be able to catch up with it, so one place will not be visible to the other!)

Black Hole Formation

Published on 02/04/2011 

Are all black holes formed after the death of a massive star? If not, how are these non stellar black holes formed?

Black holes fall broadly into two categories. Firstly there are galactic, stellar mass black holes, which as the name suggests are found throughout galaxies and have masses similar to that of stars (of order 10 times the mass of the Sun). These form at the end of the lives of stars that are too massive to become white dwarfs or neutron stars. This occurs when they are still greater than two or three times the mass of the Sun by the time they have shed material through stellar winds or supernova explosions during their death. The gravitational force causing them to collapse is too strong for the pressure of electrons or neutrons to support the star as in the case of white dwarfs and neutron stars, so it keeps collapsing down to a black hole.

Secondly, there are so-called supermassive black holes, which tend to be around 100 million times as massive as the Sun. These are believed to be found at the centre of most galaxies about which the stars in that galaxy orbit (in fact our own galaxy, the Milky Way, has a black hole 4 million times the mass of the Sun at its centre). While the exact mechanism of their formation is still unknown, it is believed they form when a large gas cloud collapses to the centre of the large gravitational 'well' in the centre of a galaxy as it forms.

In addition there may be 'intermediate mass black holes' which could explain a number of observed phenomena such as 'ultraluminous X-ray sources.' When matter falls into a black hole, it emits radiation, part of which is seen as X-rays. These appear much brighter than stellar mass black holes, implying their mass is around 1000 times that of the Sun, but they are not at the centre of galaxies. Their formation is a mystery. They are too massive to form from a single star, but they could be formed when multiple stars or stellar mass black holes are pulled together by gravity and merge. Alternatively, they could be the central black hole from a smaller galaxy that has merged with the galaxy in which they are found and in the process, were thrown out from the centre.

Neutron Decay

Published on 14/03/2011 

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 

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!