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

Meteor sighting

Published on 24/09/2012 

Hi Again. It was not yet completely dark. Just dark enough for cars to have their headlights switched on at about 7:15 pm on 19 Sept 2012. We were travelling north on Airport Way, near Teversham heading towards Quy Waters roundabout. A bright light caught my eye. It was bluey green in colour and it was travelling at the speed of a meteorite but burned longer, maybe 3 seconds. It seemed to disappear perhaps behind a cloud or possibly burned itself out. It looked as if it were heading towards earth at about a 30 degree angle. It seemed significant enough to mention. I was wondering if other people saw it and what it might have been.

What you saw was most likely a relatively large meteor, as you can probably guess the larger a meteor is the longer it will last, and 3 seconds of visibility is not particularly unusual.  Meteors with a blue-green colour are less common, but not by no means unheard of, especially amongst brighter meteors since for fainter ones you won't be able to see the colour (the same reason almost all stars appear white).  The blue-green colour can be an indicator of a higher copper content in the meteor.

Addendum to original reply:
The BBC also picked up on this and Cambridge is far from the only place in the UK the meteors were seen.  One of the astronomers the BBC asked suggested the meteors might have been man-made space junk such as an old satellite burning up on re-entry.  This would otherwise look much the same as a natural rocky meteor, but could certainly explain a high copper content!

Observing the early universe

Published on 24/09/2012 

How do astronomers observe the beginning of the universe: what wavelengths do we use, and what instruments are best for these measurements?

The early universe is of great interest to astronomers and cosmologists. It is also a popular topic amongst the general public, as we like to know where we came from. Astronomy has typically relied on light, that is electromagnetic radiation, for observing the universe. Electromagnetic radiation comes in a range of wavelengths, or equivalently frequencies, of which familiar visible light is just a small part. The spectrum goes from gamma rays at the highest energies, smallest wavelengths and highest frequencies, through X-rays, ultraviolet light, visible light and infrared to radio waves at the lowest energies, longest wavelengths and lowest frequencies.

The most useful for observing the early universe are microwaves (part of the spectrum of radio waves). We can measure the cosmic microwave background (CMB), which is the oldest light still in existence. Before this was emitted, the universe was effectively opaque: light couldn't travel freely because it was so hot and dense. Our cosmological models predict that the light that makes up the CMB was emitted about 400,000 years after the Big Bang. So we can't quite see the beginning of the universe, but it is close!

The CMB has been observed using many instruments. It was first discovered using radio receivers on the ground by American astronomers Penzias and Wilson in 1964. They would win the 1978 Nobel Prize for this. Lots of exciting science has been done more recently following a number of space-based missions. There was COBE launched in 1989 (the team won the 2006 Nobel Prize), then WMAP launched in 2001. The most recent mission is Planck, from which the data is still to be released (due January 2013). This will produce the most detailed map of the CMB sky yet.

While the space based missions are best for entire sky surveys, it is possible to measure parts of the sky from Earth. There are a number of ground-based and even balloon-based detectors. One telescope, the Arcminute Microkelvin Imager (AMI) is based here in Cambridge.

The CMB is currently our best tool, but it may be possible in the future to see further back in time. For this we would need to shift from using electromagnetic waves to gravitational waves. Gravitational waves are predicted by Einstein's theory of general relativity, they are tiny ripples in spacetime. We know they exist from careful observations of binary pulsars (for which Hulse and Taylor won the 1993 Nobel Prize). However, we have still to detect them directly.

We have ground-based detectors (LIGO in America, and Virgo in Europe), that are currently being upgraded. Japan is also in the process of building a detector, KAGRA. It is hoped will make the first direct detection in the next few years. However, these are unlikely to see gravitational waves from the early universe - just like electromagnetic radiation, gravitational radiation comes in a range of frequencies. We might be able to detect the gravitational background using pulsar timing arrays. Radio telescopes watch the signals emitted from a network of pulsars across the sky and look for correlations in deviations from their regular signals. The difficulty with these measurements, apart from having to make very careful timings, is that the frequencies are so low, you have to wait for years to see an entire wave. It is hoped that the planned Square kilometre Array will make these measurements easier.

Another possibility would be a space-based detector. This would be sensitive to frequencies between pulsar timing arrays and the ground-based detectors. Amongst many other sources, it could detect a gravitational wave background. Unfortunately, there is no funded space based detector mission at the moment. There may be eventually a detector based upon the design of the  Laser Interferometer Space Antenna (LISA). The European Space Agency is launching LISA Pathfinder in 2014 to test the technology needed, and hopefully a full mission shall follow that.

What happens when asteroids and planets collide?

Published on 10/06/2012 

Is asteroid 2003/Q0104 going to hit Earth in May 2031, and if not, then by how much will it miss us and what effect could the near miss have on us; could it hit our moon, and if so, what would the effects be for us?


This asteroid is no longer considered to be on a collision course with Earth (or the Moon)! It was removed from this risk category back in 2003 shortly after it was discovered (

If two large objects collide in space, there are a lot of different conditions which need to be accounted for when saying exactly what the effects would be e.g. the size of the objects, their speeds etc.. If the Earth and another object e.g. an asteroid did collide, there are a range of possibilities in terms of the kind of damage it would cause. If the asteroid hit an ocean it would create a mega tsunami while if it collided with land it would cause a large crater, examples of which can be found all over the Earth e.g. Meteor Crater in Arizona, USA. Material thrown up when the crater was formed would also be thrown out of the Earth's atmosphere and would be spread around the neighboring solar system. Depending if the asteroid hit directly or with a glancing blow, this would have a significant effect on the amount of material ejected.

So in summary - there is no reason to panic in 2031! And there are a range of possible outcomes when it comes to collisions of large bodies in the solar system.

Spinning Planets

Published on 15/02/2012 

How does rotation affect a planet?

Gravity affects all objects in the universe no matter how big or small they are making any two things with mass be attracted to each other. The way this is typically described is through Newton's Laws of Gravitation where the 'Gravitational Force' is increases with mass but decreases the further you move away from it. Under extreme conditions, Newton's Law fails to match what we observe out in space but then Einstein's Laws of General Relativity comes to the rescue to explain what we see!

Now on to spinning objects - if an object is spinning, it experiences an 'outward' force - you'll have experienced this when you've gone around a roundabout in a car and been pushed outwards. This centrifugal force depends how fast you're travelling around the point at the centre of the rotation and decreases the further you go away. All planets do rotate and as such have the effects of both gravity keeping them together but this centrifugal force pulling them apart. Fortunately for us, the gravitational force is much stronger - if we do the calculations for Jupiter, the centripetal force is about 8% of that created by Gravity while for Earth it's about 0.4%. As such, if Jupiter wasn't spinning, you would have feel a force 10% bigger keeping you on the surface while for Earth, the difference would be pretty much completely unnoticeable! The condition about this however - the speed with which a planet rotates doesn't have any relation to how massive it is! It depends on how it was formed and if it has experienced anything like asteroids collisions etc.

The only thing we notice about rotating planets is that they tend to bulge out at the minute (or the technical term being oblate). This is because as they spin, they want to flatten into a disk but again, the rotation speed of the planet limits the effects of this.

Falling into a Black Hole

Published on 15/02/2012 

Could a black hole swallow a star or galaxy?

Galaxies tend to me millions of times more massive than black holes though, so the separation and speed of all the stars within a galaxy means that a whole galaxy is not going to get pulled into a black hole!

Material from stars can, however, be pulled into a black  hole. We can observe this when a star is in orbit around a black hole companion. The gravity of the black hole pulls (or "accretes") material from the surface of the star into the black hole, releasing enormous amounts of energy as it does so