Apollo 17 Crew Debriefing for Science, 1973

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ll9
HENRY
gravitationally holding it together°
We though it might be
(CONT D)
in the form of ionized hydrogen.
We looked for Lyman-alpha
radiation, red shifted from the ionized hydrogen, and we
didn't see any.
We set a lower limit, which
certainly ex-
cludes the possibility that the Coma cluster is held together
J
by this ionized hydrogen.
I think that may leave a real
mystery
as to what
is holding the thing together.
The fourth point may turn out to be the most interesting
thing of all.
When you look in the Milky Way, you see a
lot of UV coming from the stars, but the question is, what
do you see when you look up to the North Galactic Pole or
down to the South Galactic Pole.
One of the most exciting
results of X-ray astronomy was the fact that an X-ray back-
ground was observed over the sky that nobody had expected,
and part of this is the gamma-ray background that Dr. Trombka
talked
about.
In the UV, nobody knows, but you never know
until you look.
You do have to deal with this background
of stars that we know is there.
So we
did look at a large
number of different points at high galactic latitudes, both
.
north and south.
The spectrum that we
see is above this
dark count.
In other words, this abnormally high dark
current did not, in fact, interfere with that experiment.
The spectrum that we
see looks like the spectrum of the hot
'