How far can you see?

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How far can you see?

Post by joe »

Earlier this year, a student asked me what was the most distant star or the most distant thing we could see with the naked eye. I mentioned the 1980s supernova in one of the Magellenic Clouds as the most distant star that I had seen, and the Andromeda galaxy as the most distant naked-eye object.

Now read the story below about a much more distant object. It comes from the Physics News Update, to which you can subscribe at


FURTHEST SEEABLE THING. For the first time in history you could
have looked half way back to the origin of the universe with your
naked eye. On the night of March 19, 2008 a telescope mounted in
space observed a flash from a gamma ray burst, an extremely
explosive celestial object, which set several records. First, if
you’d been looking in that direction you would have been able to
see, with your own unaided eyes, something at a distance
further-seven billion light years---than anything a human being has
ever seen in history. Second, since looking out into space is
equivalent to looking back in time (it takes the light from distant
objects many years to reach the Earth), you would have been
witnessing the earliest thing ever seeable by the naked eye.
A new report describes observations made of the explosion by an
orbiting telescope called Swift and by some of ground-based
telescopes that got in on the action once they were notified by
Swift. Swift has three onboard detectors which look not at ordinary
visible light but at much more energetic light in the form of x rays
and gamma rays. One feature of Swift’s mission is that as soon as
it sees something interesting it alerts controllers on the ground so
that other telescopes can be turned in that direction. In this way
the explosive outburst, whose official name is GRB 080319B, could be
tracked by telescopes sensitive to other kinds of light, such as
infrared and even radio waves.

The March 19 event is an example of a gamma ray burst. This comes
about when certain heavy old stars have used up all their internal
fuel. When a star has no more fuel, the force of gravity causes it
to contract. If this process is violent enough, the star can blow
apart as a supernova. In some special cases, what is left behind is
a black hole, and outward going shock waves which, when they
criss-cross, can create a brilliant flash of light. For a short
time this light is more powerful than that coming from an entire
galaxy of stars.

The cone of energy flying away from the explosion can be quite
narrow, so to be observed from far away, as this object was, it had
to be aligned just right to be seen by Swift.

This gamma burst was not the furthest ever observed with a
telescope, but it was the brightest in terms of the energy
released. So bright, in fact, that it could have been seen unaided
in areas of North and South America the night of March 19, if only
for about 40 seconds. The splash of light arriving at Swift’s place
in orbit that two of Swift’s three detectors were temporarily

Fortunately several telescopes quickly maneuvered into position and
could study the stellar explosion as it unfolded. By then the gamma
rays, the most energetic part of the light blast, would have died
down. But other types of light continued to issue from the scene.
According to Swift scientist Judith Racusin, an astronomer at Penn
State, this has become the best-observed gamma ray burst, and the
observations have already changed the way we think about bursts

When you look out at the night sky about 3000 stars are visible.
Everything you can see at night is either a planet in our home solar
system or one of those stars, all of which are located in our home
galaxy, the Milky Way. The furthest thing you can normally see with
the naked eye, and with some difficulty, is the Andromeda Galaxy,
about 2.5 million light years away. Only about once a century is a
supernova visible from any further galaxy. And by now it’s been 400
years since we’ve seen one of those.

That makes GRB 080319B all the more impressive. It breaks the
record of most distant seeable-with-the-naked-eye thing by a factor
of a thousand. Located in the Bootes Constellation, the gamma burst
is at a distance of 7 billion light years, which means that it took
light seven billion years to come from the blast to Earth. That
means that a person seeing the visible portion of the blast would
have been looking halfway back toward the time of the big bang,
when, according to modern cosmology, the universe began. When the
blast occurred the sun hadn’t even appeared yet, much less the
Earth, much less the human species. (The results appeared Nature
magazine, 11 September 2008.)

And now we shall wonder whether anyone in North American that night actually noticed it.
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