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!

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Fireball in the sky

Published on 07/01/2014 

I am currently living on Jeju Island off the coast of S.Korea.  On
Friday November 22nd I was on my way out with some college students
when we saw a bright burning light in the sky with a tail.  At first I
thought it was a plane on fire but realized it was not. I took a few
pictures as it moved across the sky.  I am curious to know what we

I suspect that what you saw was a meteor.  Bright meteors can be very
spectacular, and are fairly rare, so you are lucky to have seen it!  The
peak of the Leonid meteor shower, which can produce quite high numbers
of meteors, was around the 17th-18th, so it is possible to one you saw
was a Leonid.

Why is a black hole called a 'hole'?

Published on 07/01/2014 

At the greatest risk of appearing dumb....I really would love it if you could help me get my mind around a black hole...before I go any's the hole description I cannot conceive.

I am aware of what a black 'hole' is..I just don't understand why it is a black hole.

Let me expand:

A black 'hole' is formed when a star collapses on it self, which I understand, how ever it is the gravitational force that causes the black hole to appear which I don't understand.

Again let me expand:

If I was stood anywhere on earth, australia for example, would not the gravitational pull on me and everything be exactly the same as it would on me in england?

So when a neutron star collapses, would the gravitational  pull be equal in ALL directions? and if so..if this process continued to the creation of a black 'hole' surely the end result would be a 'SPHERE'  not a black 'hole'

And if..lets say a space craft approached a black 'hole' in earthly simulations it shows a swirling vortex which it is drawn into... you can only enter a black hole from one direction...yet with a sphere and in my mind..because of the way the black 'hole is created..the black 'sphere' would draw the same craft into it regardless of what direction it approached the 'hole' from?

Am I dumb or has my assumption any founding at all? this question has baffled me for years..I even asked a scientist on a radio programme who I think must of misinterpreted my question regarding this.

I would be grateful if you can point out where my theory fails and correct me.

That is not a dumb question at all, in fact I dare say you may have a better intuition than many film makers.

The 'hole' description is perhaps unfortunate in some ways, since as you rightly picked up on it naturally conjures up images of a swirling plug-hole which material can only fall into from one direction.  The idea of a 'black hole' was first suggested back in the 18th century by John Michell, though at the time he called them 'dark stars'.  If you increase mass of an object then the speed at which you have to travel to escape it's surface increases, so Jupiter has a larger escape velocity than Earth for example.  John Michell hypothesised that if a star could be massive enough then it might have an escape velocity greater than the speed of light, such that it would appear dark, hence 'dark star'.

The name 'black hole' dates from the 1960s when work on general relativity showed that massive enough objects will collapse down to an infinitesimal point (a singularity) due to the effects of gravity.  At some distance from this singularity the escape velocity is equal to the speed of light, and since relativity says that nothing can travel faster than light, once anything passes within this distance (called the event horizon), it cannot escape.  The 'hole' label is a partly result of this idea, that nothing can escape from it, it is simply sucked in and can never get out, like a bottomless pit.  The description of it as a 'hole' also references the existence of a singularity at the centre of the event horizon, since at the singularity the laws of physics break down and so it is in some ways a 'hole in space' that we can't describe.

However, as you have surmised while the name 'black hole' does make some sense, it is indeed a spherical hole.  It doesn't matter what direction material approaches the black hole from, once it passes the event horizon it will be unable to escape.

The 'swirling vortex' effect that is often used to indicate the presence of a black hole is partly the result of the fact that it is rather difficult to represent a spherical piece of absolute nothingness.  Particularly against the black background of space. Now that modern computer graphics are better we can show the presence of a black hole by the distorting effect it has on light which passes near to the event horizon, but does not cross it. Wikipedia in fact has an animation on their page about black holes showing just that.  The 'swirling vortex' effect is also partly a result of misunderstanding an accretion disc around a black hole.  Most material that approaches a black hole won't do so head on, but from an angle and will, at least initially, get trapped in orbit around it rather than falling straight in.  When there is a lot of material approaching the black hole the material will all collide with each other and find a mutual direction of rotation, forming a disc.  That disc, viewed face-on, looks rather like the swirling plug-hole, though material then falls in from the inner edge of the disc rather than from the 'top' or 'bottom'.

Accretion disc temperatures

Published on 07/01/2014 

Hi, I've been looking everywhere online trying to find out what the roughly average temperature you would expect to find in the acceleration disk of a black hole. Nearly every scientific document published that I can find doesn't go into anymore detail than "hot enough to produce x-rays". Thanks for your time.

As you may know the colour something hot appears changes depending on how hot it is, starting from a very dull red, to a brighter red then orange, yellow and white as it gets hotter.  This is because as an object gets hotter the wavelength at which it emits light most strongly moves to shorter wavelengths and higher energies.  For very high temperatures scientists tend to think in terms of what this peak wavelength is rather than the temperature itself, this is why everything that you have found just says 'hot enough to produce X-rays'.  To be hot enough for the peak of emission to be in the X-ray range the material would have a temperature of around 300,000-300,000,000K.

Earth-like Moons part 2

Published on 21/11/2013 

I am a writer and need more info on the question : Earth-like moons

Since the tides are higher would sailing be possible? Also, the seasons; would winter be longer on the dark side of the gas giant?

In and of themselves the much higher tides that one would expect on an Earth-like moon of a gas giant would not be a big problem for sailing.  There would be a lot more small islands, sandbanks, etc. that appear at low tide and disappear at high tide, so it would be necessary to be more careful around the coast and have accurate charts in order to avoid grounding your ship, but out in the middle of the ocean there wouldn't be any issues at all.

The biggest issue would probably not be any direct impact of the tides on the act of sailing, but rather on the ports and shore facilities that are necessary to support fleets of sailing vessels. As an example take what would be a relatively modest mid-ocean tidal range of 5m, roughly ten times that we experience today. Translating that directly into tidal ranges at the coast is not totally straight forward because geographic features can interact with the tide making the local range higher or lower.  For example in the Bristol channel the present day tidal range is up to 9m. Let's assume that here in Cambridgeshire and East Anglia the tidal range is about 10m.  In Cambridgeshire that would mean that at high tide the waters edge would be in the city centre in both Cambridge and Peterborough.  At low tide on the other hand the Wash would be high and dry and the sea would be well over 100km from Cambridge, even Skegness would several 10s of kilometres inland.  This is partly because of the Fens are low lying and the Wash is shallow, but even if we moved elsewhere the effects would still be significant.  If we went to Bristol for example even if we only increased the tidal range to 20m, a much smaller amplification than the Bristol channel experiences today, the Severn would be tidal all the way to Kidderminster and at high tide Cardiff, Newport and Weston-super-mare would all be under water.  At low tide there would be probably no more than 100 metres or so of water separating England and Wales as far down as Exmoor.

The issue of very large tidal plains could certainly be overcome, ships have always generally entered and left harbour at high tide, but it would make things more difficult.

As to the seasons there would not be any substantial effect (assuming that you maintain the same tilt of Earth's axis of rotation).  There would be regular eclipses whenever Earth passed behind the gas giant, but these would only be a few hours every month, not sufficient to have a dramatic effect on climate.

Seeing back in time

Published on 08/11/2013 

I was wondering. Is it possible to see back into the past?
I was debating with a friend, about this. And I believe that, it is possible, if you, eg. Travel 3.5 billion light years away from earth in an instant, and if you are able to locate the earth, from where you are, that if you take a REALLY powerful pair of binoculars, and look back at the earth, then you would be able see back into time by 3.5 billion years.
Or, do you just see the present? I would be able to see my house and school, and so on?

I mean, the light reflected, does not come any closer/faster to you, just because you zoom in, right?  It would take 3.5 billion years for you to be able to see, the present?
Or am I completely wrong?

It is indeed possible to 'see into the past' because of the finite speed of light.  In fact it is impossible to see distant objects as they appear in the present.  As an example the Andromeda galaxy, the nearest large galaxy to the Milky Way, is about 2 million light years away, and so when we look at Andromeda (which you can see with the naked eye or a pair of binoculars) you are actually seeing it as it was 2 million years ago.  Now galaxies don't change much in 2 million years so that doesn't make too much difference, but Andromeda is very nearby, if we look at more distant galaxies we see further back in time and then we do see differences with nearby, older, galaxies.

If you imagined an alien 3.5 billion light years away with a powerful enough telescope to see Earth, then they would indeed see Earth as it was 3.5 billion years ago, when it was only a billion years old and the first life was just appearing.