Institute of Astronomy


Ask an Astronomer - Black Holes

Radio emission from black holes

Published on 09/04/2014 

I've been reading about astronomical radio sources and I've noticed something that doesn't seem to make sense to me.
It is my understanding that gamma rays are the highest energy form of electromagnetic radiation, is it the shorter wavelength that gives it more energy?
If this is true then why do planets radiate mostly in the infrared and paradoxically, extreme energetic processes like those around supermassive black holes radiate in the radio spectrum even though infared wavelengths are shorter than radio wavelengths? 

You are quite correct that gamma rays are at the top end of the electromagnetic energy scale, and that a shorter wavelength corresponds to higher energy.

Part of the difference between the two scenarios you mention, radiation from planets and processes related to supermassive black holes, is what is generating the radiation.  For thermal radiation, that is light produced as a result of the heat of the object, the wavelength at which the emission peaks gets shorter as the object gets hotter.  This is why emission from Earth peaks in the infrared, while sunlight peaks in the visible.  The accretion disks around supermassive black holes are actually so hot that the radiation they produce peaks in X-rays.

Active supermassive black holes are also however often associated with strong emission in the radio, as in radio galaxies, like you've noticed.  This radio emission is much stronger than would be expected from thermal sources.  Instead, the radio waves produced by radio galaxies are the result of energetic electrons (accelerated by the black hole) spiralling around magnetic field lines.  We call this sort of radiation synchrotron radiation, so called because it was first associated with synchrotrons - devices that use powerful magnetic fields to bend beams of extremely energetic particles.  The spectrum of synchrotron radiation has a very different shape to thermal radiation. 

What happens to light falling into a black hole?

Published on 15/01/2014 

I can't find an explanation that my brain likes for my query.....
My logic thinks that light should 'pause' at the event horizon of a black hole, to an observer away from the outside of the event horizon.  I'm imagining firing a flare away from me into a black hole from a distance.  As the flare moves further away from me it becomes smaller, but once it reaches the event horizon it's size will remain the same from the distance where the observer remains stationary/fired the flare.  If observer moves closer to the light it will become larger as if the flare was coming back to me.
Now, going back to the initial query... being light has the fastest speed, at the point of the event horizon would the speed of light not be countered with the 'speed' of the gravitational pull of the black hole and just pause?

One of the things about relativity that can be a bit difficult to get your head around is that light always seems to move at the same speed, no matter how fast you are travelling, or in what direction. What happens instead is that the Doppler effect changes the wavelength of the light, similar to the way that the pitch of a siren on an ambulance or a police car seems higher as it is coming toward you, and then lower as it is moving away.

With light, if you are moving toward the light source then the light will seem bluer, whereas if you are moving away from it the light will seem redder.  Gravity has the same effect, so what happens to the flare as it approaches the event horizon is that it seems redder and redder, until at the moment that it actually crosses the event horizon the wavelength has been stretched so much that is undetectable.

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.

Black holes and companion stars

Published on 22/05/2013 

I was wondering what it's called when there is a star next to a black hole and you can watch the black hole pulling in particles of light? I know there is a word for it i just cant find it.

Your description could match a couple of things. I think you are looking for "accretion from a binary companion". Accretion is the term we use for an object growing by accumulating matter that falls onto it. Many stars a found in multiple systems like binaries (where there are two stars). The more massive star will evolve more quickly as it burns up its fuel quicker, and so may collapse down to a black hole while its companion is still a regular star. As the companion evolves it may become a giant, puffing up in size, so the outer layers can get stripped off by the black hole. The material swirls around the black hole forming an accretion disc before eventually spiralling in. I think this is what you had in mind. The accretion disc can get very hot, hot enough to emit X-rays, in which case the system is referred to as an X-ray binary. Studying X-ray binaries has given us our best understanding of stellar mass black holes.

The other effect you could be referring to is gravitational lensing. This is when the trajectory of light appears curved because of the gravity of a massive object (often a black hole, a galaxy or a cluster of galaxies). This might match "pulling in particles of light", but you can't normally see this (since if the light is being pulled in, it can't escape for us to see): I think you're actually thinking of the stream of hot plasma being accreted from a star.