As we scurry around the surface of our planet doing all the things that we think are so
important, we rarely think how lucky we are to living not only in a fairly benign and
stable point in the earth’s lifetime but also in the life of our nearest star, the
sun but as we look farther out to our own galaxy and others things are not so quite
rosy, far from it, in fact, they are some downright scary as hell with monsters out
there that make our problems here on earth pale in to utter insignificance, so what are
the scary cosmic monsters that could wipe us out in the blink of an eye.
Up until the last 60 years or so we have been living in what could be considered an ignorant
bliss about the space, stars and galaxies we saw around us, from our perspective they
seem elegant and unchanging but as our technologies for probing the universe got better, the more
we found things aren’t as stable as we used to think.
During in 1960s as the partial nuclear test ban treaty that came into effect between the
US and the USSR, the US didn’t trust the Soviets to stick to the plan and thought they
might be doing secret testing. So in 1963 they launched the Vela satellite system into
a very high orbit almost 1/3 of the way the moon to look for the tell tail gamma rays
bursts from nuclear tests, infact it was high enough so that it could even detect blasts
behind the moon. On the 2nd July 1967, two of the Vela satellites
detected a burst of gamma radiation but it wasn’t from the Soviets, infact is was unlike
anything they had seen before and didn’t match that of the characteristic double flash
of a nuclear weapon and such it was seen as not warranting immediate action but instead
it was filed away for further investigation. Over time as more vela satellites were launched
more and more of these mysterious gamma bursts were detected. Eventually, after using the
timing difference between different satellites to try work out the origin of 16 bursts, it
was found that they didn’t originate from earth, our solar system or even our galaxy
but much farther away. What the satellites had picked was a nuclear
explosion but something far bigger than anything we could imagine at the time and at an extreme
distance from earth. It took many years of observing similar gamma bursts and tracking
them down with new network linked telescopes that allowed their observation within minutes
of their detection. After comparing the images and gamma bust
signatures, astronomers were able to conclude that these most probably caused by the core
collapse of the very biggest stars, bigger even than supernovas, so they gave them the
name Hypenova’s. Their extreme distance meant that they occurred
billions of years ago, in fact one of the farthest known objects is GRB 090429B, a gamma
burst that was detected in April of 2009 and has a distance estimate of 13.14 billion light
years. Considering that the latest estimate for the total age of the universe is 13.8
billion years, these would have been amongst the early massive stars and hence the equally
massive explosions when they died. And this is one of the issues we have with
things like supernova’s, they are so big and release so much energy its difficult to
equate them to the things we see normally like our own sun.
If we look at this image taken by the Hubble telescope of a supernova SN 1994D in the outer
edges of the galaxy wwww some 50 million light-years away, it's shining as bright as the galaxy
itself and did so for about two weeks.
Our sun is 1 million times the size of the earth and is a giant nuclear reactor converting
4.26 million metric tons of hydrogen in the helium per second. This process releases the
equivalent to about 380 million, billion, billion watts of power every second, enough
to power the world for about 650,000 years at current consumption rates.
But a supernova is the explosive core collapse of a giant star maybe 8 to 15 times the mass
of the sun. To give you some small idea of just how bright
these are, here is a little thought experiment. Which do you think would be the brightest,
a supernova at the distance of the sun to the earth, about 148 million Km, or a hydrogen
bomb 1 mm from your eye, if you said the supernova your correct and it would be about 1 billion
times brighter than the hydrogen bomb. But the gamma-ray bursts that the Vela satellites
detected were about 10 times brighter again than a typical supernova.
The stars that go one to create hypernovas are also astronomical in size and or mass.
The most massive star yet found is R136a1 which has about 300 times the mass of the
sun but is only 30 times the physical size of the sun.
Its also the most luminous star known at around 8.7 million times more luminous than the sun,
radiating more energy in 4 seconds than the sun does in an entire year. If it were at
the same distance as our nearest star, Proxima Centauri, 4.2 light-years away, it would be
as bright as the full moon. However, its thought that the earliest population
3 stars, that formed about 200 million years after the big bang and that were made from
just hydrogen and helium could have been over 1000 time the mass of the sun but it thought
that because these are so massive that when they collapsed the gravitation forces are
so strong they wouldn’t have exploded in a supernova but just collapsed completely
and become black holes. From what we know now, the heavier elements
that were created in the first supernovas and that ultimately makes up us, limits the
size that stars can grow to so there are none of these monster-sized stars around now.
The largest star yet found is UY Scuti, a hypergiant with a radius of about 1700 times
that of the sun. If that was to be placed where our sun is in the solar system, it’s
surface as we define it would be just beyond the orbit of Jupiter but for all its size
is quite a lightweight and comes in at about 30 solar masses.
But Gamma-ray bursts aren’t just created by exploding stars, they can also be created
by what’s left over after the supernova explosions.
Far from just exploding and scattering all the material out into space, the gravitational
collapse of a supernova is so great that the electrons and protons of the remaining iron
core of the star are fused together to become neutrons that are so dense that it shrinks
to about 20km in diameter but still has about the mass of our sun, a matchbox-sized piece
of it would weigh about 3 billion tons. If the star prior to its collapse had a very
strong magnetic field and it collapses in just the right way, then the spin and temperature
is converted into a magnetic field, adding to the already extremely powerful one that
a neutron star has and it becomes one of the most powerful magnetic objects in the universe,
a Magnetar. Large magnets are normally measured in Tesla’s,
with the Earths magnetic field of coming in at lowly 30 -60 micro Telsa’s. The strongest
magnet we have created so far is 45 Telsa and the requires 30MW of power to run and
has to be cooled to just 1.8 degrees above absolute zero.
The strength of the magnetic field in a magnetar is around 1 billion to 100 billion Tesla.
At these magnetic field densities, strange things happen to matter, atoms of hydrogen
are stretched out into spindles 200 times narrower than their normal diameter and the
vacuum of space itself becomes polarized like a polarizing filter.
This magnetic field also produces huge amounts of X-ray and Gamma rays which can be radiated
in narrow jets from the neutron stars poles and as they also rotate very quicky some become
pulsars that act like a cosmic clock with extremely regular pulses that can be heard
by radio telescopes. However, when a disturbance in the neutron
stars crust occurs, also known as a star quake, a truly astronomical burst of gamma rays is
produced and in 1979 one of these was picked up by both soviet and US spacecraft around
the earth and venus as the strongest pulse of cosmic gamma rays ever detected swept thru
the solar system as the speed of light. The source was tracked down to what is believed
to be a Magnetar in the Large Magellanic Cloud about 165,000 light years away.
Luckily for us these Gamma ray Bursts are very far away, often occurring long before
our solar system formed so they have virtually no effect on the earth because if one were
to be close it could be devastating, from damaging the atmosphere to irradiating half
the planet. But there aren’t that many massive stars
in the milky which greatly limits the chances of a GRB being close enough to cause us any
real harm. So again we have to thank the time that we
are living in for allowing us to evolve to where we are now without being zapped by a
nearby exploding star. Because of the huge distances involved with
gamma ray bursts, neutron stars, Magnetars and the massive stars from which they come
from, it makes it difficult to study these directly so a lot of what we know comes from
our knowledge of our sun and then applying what we have learned to these phenomena.
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