What happens to us when our Galaxy makes one revolution?
Another Asker reminded me of a hypothesis/theory I read a few years
ago, possibly in a New York Times science column.

It said that some scientists thought that the there was a beam of
gamma rays or other *immense* and *intense* source of energy that our
solar system was exposed to once every Galaxy orbit.  This beam was
from a source outside our galaxy but emitted from a galactic cluster
far far away.  When our Milky Way rotated into a certain position we
and other stars near us in our Galaxy would be blasted by this
radiation leading to mass extinctions.  I think the period was evey
500 000 000 of your earth years.

Is there such a thing?

I dont think that there is a Gamma gun some where firing in our
direction. if it was so we would have seen holes in our Galaxy...!!!

This appears to be a similar. 
http://answers.google.com/answers/threadview?id=420046
http://answers.google.com/answers/threadview?id=424260

It is unlikely the earth will encounter anything that would threaten
our civilisation as a result of our sun orbiting our galaxy. Any
energy beam associated with our Galaxy probably is emitted
perpendicular to the Galactic plane so our Solar system would always
be at a safe distance from it.

HOWEVER
Ever since the Cold War , Astronomers (originally the US and Soviet military)
have been observing intense Gamma Ray bursts(now called burstars).
Several of these Gamma ray burstars are observed each day although
each burst is observed for typically only a few secounds. Until
recently astronomers were not able to track down their optical
counterpart as a result of the brief time the burst lasted.

Recently however as the result of some very sophisticated satellite technology
Astronomers have now been able to identify the optical counterparts.
The understanding of these objects is now much better. Astronomers
have been able to determine that these objects fall into two very
distinct categories.
 
One type occurs as a result of a very powerful supernova (now called a
hypernova) which commonly occured in the early stage of the evolution
of our Universe. Astronomers were able to determine this by locating
the optical
counterpart of the Gamma ray burst and were able to detect a very faint galaxy
with a very high Red shift. The conclusion is therefore that during
ther early stage of the universe there were many hugh stars which went
supernova and the
Gamma Ray burstar is the result of this explosion

A second type of Gamma Ray Burstar, however have been observed to come
from WITHIN OUR GALAXY. These are as a result of objects known as a
magnetastars. This is a Pulsar with an unusually strong magentic
field. If the magnetastar is revolving around a normal star , then it
will steal ( or accrete) material from the companion star. When the
pulsar has accreted a certain amount of material it will in effect
explode. One result of the explosion is an intense beam of Gamma ray
radiation perpendicular to the plane of rotation of the Pulsar. If the
earth and sun are approximately in the direction of the beam, then the
Gamma ray burstar is observed.
    
Astronomers have calculated that should such a pulsar be close enough
to our sun and the beam goes in our direction , there could be enough
energy to destroy all major forms of life on our planet. The chances
of this having happened in the last few hundred million years are
quite high. Palaeontologists have now found evidence of massive
destruction of life in the last 500 million years  (please note this
long predated the extinction of the dinosaurs)

 Scientists now speculate that this massive destruction of life could
have been the result of the explosion of a magnetstar close to our sun
(i.e. within 50 light years). Furthermore there is no reason why this
will not happen sometime in the future.
(it could be tomorrow or sometime in the next 1000 million years)
It is also noteworthy that recently astronomers have observed a particulary 
violent Gamma Ray burstar which for a short time knocked out detection
instruments in some satellites.

Regards

DPRK007

Gamma ray burstars. Wow. Fascinating stuff dprk007.