What would a nearby quasar look like?

[cosmicalstorm]

Lets say one of the galaxies in the local group suddenly had a quasar erupt near its core.
(I assume thats where they typically occur)
I realize that this is really unlikely. But if it did happen, what would it look like on our sky? How bright would it shine?

Roughly speaking of course, lets say its no further away than 6 million ly from the Milky Way and that there are no great barriers inbetween. (like our own galactic core)

[Ryan Thunder]

Wouldn't it kill us all the moment the staggering amounts of radiation reached us?

[Kitsune]

I thought quasers were early universe events

[kinnison]

Kitsune, afaik that's right - at least that is the current, rather well-supported, theory. Basically, quasars are/were galactic core supermassive black holes with plenty to chew on.

A quasar outburst is extremely unlikely anywhere near us, as there just isn't enough matter to produce the energy required, in a place where it can be swallowed. The only thing likely to cause a quasar outburst is a glactic collision - and we will likely have rather a lot of warning of such an event!

For example, the collision of our galaxy with Andromeda somewhere around 3,000,000,000 AD will probably produce a quasar outburst. Either we won't be here, or we will be able to deal with it.

[starslayer]

Milkomeda could be an AGN for a while after the collision (if a gas cloud falls in), but it wouldn't last very long, and the energy would be concentrated in jets most likely pointed away from us, so we wouldn't have to worry at all. Quasars are entirely different beasts, and are a very special type of AGN.

Quasars are so bright that they can be easily seen across the universe. The brightest quasar is (apparent) magnitude 13, and about 13 billion light years away; the brightest regular galaxy at that distance is about magnitude 25 (or over 50,000 times fainter). Quasars emit radiation in jets, mostly, but also pour out an enormous flux in all directions. If it didn't kill us at Andromeda's distance, it would certainly be extremely bright, perhaps even as bright as the Sun. If the jets were pointed toward us, we'd fry at Local Group distances. Cosmic rays are no fun, nor are gamma rays.

[Surlethe]

Bright quasars emit energy at about 10^48 ergs/sec. That's 1E41 W. The enterprising reader (who has time, which I haven't right now) will figure out brightness and apparent luminosity formulas and apply them to hypothetical local quasars.

[Glom]

What happens to the super massive black holes? Do they just sit there forever?

I have a feeling I used to know this stuff, but time and alcohol has made it all go away.

[kinnison]

What happens to the super massive black holes? Do they just sit there forever?

I have a feeling I used to know this stuff, but time and alcohol has made it all go away.

That depends on the model of cosmic evolution that you favour. Either they just sit there until the Big Crunch, or they (along with everything else) are destroyed in the Big Rip, or (to my mind the most depressing) they sit there until the background radiation drops below their Hawking temperature and they simply evaporate by Hawking radiation. The last possibility is estimated to take some 1E65 years, IIRC.

[Samuel]

Bright quasars emit energy at about 10^48 ergs/sec. That's 1E41 W. The enterprising reader (who has time, which I haven't right now) will figure out brightness and apparent luminosity formulas and apply them to hypothetical local quasars.

How far away are we talking?

[starslayer]

How far away are we talking?

Local in this case means within a few million light years, or possibly all the way to the heart of the Virgo cluster (about 60 million light years away). Assuming a distance of 5 million light years, or about 5e22 meters, the average intensity (assuming the quasar emits its light evenly over a sphere, a patently absurd assumption, but let's roll with it for a second anyway) from a flux of 1E41 W is about 1E-6 W/m^2 (using I = P/[4πr^2]). For comparison, solar intensity is ~1000 W/m^2. It appears that I was wrong about their brightness.

Upon some further research, I found that even the brightest quasars would only approach solar apparent luminosity levels when they were very close to us (like, 10-20 parsecs close). They would still appear very bright from such a relatively short distance as 5 million light years. The cosmic ray and ionizing radiation flux may still be high enough to kill us at that distance if we happen to be in the path of the jets.

[Kuroneko]

Apparent magnitude matches absolute magnitude at 10pc distance, and increases by 5 for every factor of 10, all else being equal--which they typically aren't, so the following are slight underestimates. Considerations of redshift, dust, and radiation band (quasars have several peaks in the ultraviolet and shorter wavelengths) are neglected.

Code: Select all

Distance   Adjust   Examples      
8.0kpc     +14.5    Sagittarius A*
750kpc     +24.4    Andromeda. M32
850kpc     +24.6    M110, M33
1.0Mpc     +25.0
1.5Mpc     +25.9    ~local group lim. 
3.5Mpc     +27.7    M81 group (nearest)
20.Mpc     +31.5    ~local supercluster lim.

For example, the Sun has an apparent magnitude of about -26.7, and this value also happens to be on the very bright side of quasar absolute magnitudes. If such a quasar were to form at Sagittarius A*, its apparent magnitude would be -12.2, which is just a bit less bright than a full moon. A very faint quasar of magnitude -20 at Andromeda would double the galaxy's overall brightness, but otherwise not be significant.

[Ariphaos]

APM 08279+5255 is supposed to be -30.5. Could cast faint shadows from Andromeda on a clear night with a new moon and no other major lights around.