144
D
espite the vast difference
in scale, atoms, stars,
and galaxies share a
property in common: each in its
own size domain is a fundamental
construction unit of the universe.
Galaxies define the distribution
of matter in the universe on the
grandest scale; stars are a defining
constituent of those galaxies
(although galaxies may harbor
quantities of gas, dust, and
mysterious dark matter as well);
atoms are the units of matter that
make up the hot gas of stars (with
some simple molecules in cooler
stars). If we think of galaxies as
cities, stars are like individual
buildings within the city, and
atoms are the bricks.
In a mere 30-year period in
the first half of the 20th century,
astronomy took huge leaps in
understanding how the hierarchy
of matter in the universe is
organized. Underpinning these
developments was Einstein’s
general theory of relativity, in
which the concepts of mass
and energy are inseparable in
a unified fabric of space and time.Looking inside a star
Between 1916 and 1925, Briton
Arthur Eddington worked on the
physical nature of ordinary stars
such as the sun. He pieced together
a detailed physical description
of a sphere of hot gas, in which
energy makes its way from a central
source to the surface, from where it
then radiates into space. Eddington
also did much to convince
astronomers that stars are fueled
by subatomic processes—what
we would now call nuclear energy.In 1919, the New Zealand physicist
Ernest Rutherford was able to
transmute atoms of nitrogen
into oxygen by firing particles at
them from a radioactive element.
There was now ample evidence
that nuclear processes could
produce new elements and
release unimaginable quantities
of energy. For any remaining
doubters, Eddington reflected
on the experiments conducted at
Cambridge University by pointing
out that “what is possible in the
Cavendish Laboratory may not
be too difficult in the sun.”
When British astronomer Cecilia
Payne-Gaposchkin, working in
the US, concluded in 1925 that
stars are overwhelmingly made
of hydrogen atoms, astronomers
at last had a real grasp on the
true nature of “ordinary” stars.INTRODUCTION
1916
1917
1924
1926
1919
1920
Edwin Hubble finds a
relation between the redshift
and distance of nebulae,
showing that spiral
nebulae are galaxies.Vesto Slipher shows that
many nebulae show large
redshift, meaning that
they are moving away
from us rapidly.Observing a solar eclipse,
Arthur Eddington shows
that light from stars is
bent by the sun’s gravity,
just as relativity predicts.At the Smithsonian
museum, a “Great
Debate” takes place over
whether or not spiral
nebulae are galaxies.Albert Einstein
publishes his general
theory of relativity,
which explains gravity
as a warping of
spacetime.
Austrian physicist Erwin
Schrödinger formalizes
the equation describing
quantum mechanics, which
describes strange behavior
at the quantum level.