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William's sister Caroline became an astronomer too, through
childhood misfortune. Stricken successively with smallpox and
typhus as a child, scarred, stunted and deemed unmarriageable, she
became a family servant. William rescued her from this drudgery
(their mother did not believe in education for women). At his house-
hold in England, he taught her music (she became an accomplished
singer) and mathematics. She began to help him in his astronomical
work, came to be as skilled as her brother, and helped educate another
Herschel, William's son John, who grew to become a celebrated
astronomer in his own right.
Caroline Herschel discovered several comets, which we now know
to be space-borne dirty snowballs. It is apt, therefore, that the Her-
schel Observatory searched for water in deep space. Earth-bound
observatories are hindered by having to peer through an atmosphere
that itself is charged with water vapour—but not so the Herschel. Its
detectors were set to the infrared, to observe the cool regions of deep
space. It found water vapour in the spectra of colliding galaxies, and
(perhaps the feedstock for comets) in the disc of a young star, while
VY Canis Major, a massive dying star, revealed an envelope of steam
at 1,000 degrees Celsius. Cold water vapour was observed erupting
from the dwarf planet Ceres in the asteroid belt of our own solar sys-
tem, and in Jupiter's upper atmosphere. Herschel anatomized cosmic
water, near and far.
It is now clear that much of the water in outer space takes the form
of thin icy coatings on grains of interstellar dust. At temperatures
below −170 degrees Celsius (and a lot of outer space is yet colder than
that) water takes the form of ice. As the temperature warms above
this level, the ice converts directly into a gas, without—because of the
very low pressures in space—passing through a liquid stage. Even at
cold temperatures, though, molecules of water can be dislodged from
the ice layers by the energetic photons of starlight, to drift off into
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