Naturalist nebulae

The Nebulae and the Island Universes

Before telescopes were invented in the early 1600s, most of what
we now know exists in outer space was completely unimagined and
largely unimaginable by ordinary human beings (like us). Galileo
observed bizarre and shocking things - spots on the sun, planetlings
orbiting Jupiter, countless stars never seen before - that exploded
ideas in the old cosmological philosophies.
Certain fuzzy washes of light - a handful of which (like the great
galaxy in the constellation Andromeda) can be seen by the unaided
eye - were revealed in telescopes to exist by the dozens, and more.
For decades it was generally assumed that the washes of light were
all of the same ilk, although what that ilk was, was unknown. By the
mid-1700s telescopes had improved, more starlight was charted, and
new ideas about the structure of the cosmos were formulating out of
the old. Many wild notions about the fuzzy patches eventually
condensed into two basic theories: the "nebular hypothesis" and the
"island universe" hypothesis.
The Latin word "nebula" means "cloud," and the nebular
hypothesis was that the fuzzy patches were distant clouds of gas
slowly coalescing into planets. The French mathematician
Pierre-Simon LaPlace suggested that our own sun and planets had
formed this way. Even more spectacular was the idea proposed by
the mighty Enlightenment philosopher Immanuel Kant, that the
clouds were extremely distant island universes - collections of stars
beyond our own collection, the Milky Way galaxy.
It turned out both, overall, were right. There are extragalactic
galaxies, and there are several kinds of nebulae. Two hundred and
some years later it's a generally accepted fact of astronomy that so
many galaxies exist, so far away, that their number is essentially
unknowable. Galaxy M31 in Andromeda is an island in outer space
of possibly a trillion stars about 2.2 million light-years away.
The nebulae seen in telescopes are inside our own galaxy. A nebula,
as astronomers use the word, is a cloud of gas and dust floating near
stars. Four basic kinds are detected in telescopes: reflection,
emission, dark and planetary. A reflection nebula reflects visible
light from nearby stars strongly enough for telescopes to pick up. An
emission nebula glows with heat, or energy, detected by telescopes
that pick up light in wavelengths the human eye can't see (which is
most of the light there is).
A dark, or absorption, nebula is a cloud of gas and dust that neither
reflects light nor emits heat, but instead is seen as a silhouette
against a background of star or nebula light. A dark nebula familiar
in Hubble Space Telescope pictures is the Horsehead Nebula.
A planetary nebula is a shell of gas surrounding an old star. It does
not actually involve planets - the phrase is a linguistic fragment left
over from the ideas of philosophers like LaPlace and Kant.
It is theorized that stars are created when dust and gas condense
over immensely long periods of time, and that planets coalesce from
the disks of material left over from star formation. At the end of
their lives, some stars explode into gas, dust and energy. New stars
reformulate out of the material from the old exploded stars.
What a star explosion or a star birth is actually like, is unimaginable
by ordinary beings like us. There are more things in heaven and
Earth than are verified in our astronomy.