WWW.ASTRONOMY.COM 33This research led to a monumental
realization. In 1929, Hubble, with a
helpful assist from the Belgian astrono-
mer Georges Lemaître, suggested that
the new data he collected about galaxies
supported the theory that, if traced
backward in time, the paths of all the
galaxies led to a small, dense point at
which the whole universe began — a
“Big Bang” billions of years ago. This
Big Bang commenced the expansion
that is causing all galaxies to move
apart from one another more quickly
in space. The whole universe seems to
be f lying apart.
Hubble analyzed 46 galaxies and
proposed what came to be known as the
Hubble constant: the rate of expansion
of the cosmos. He fixed this number as
500 kilometers per second per megapar-
sec of space, a much higher value than
what we know is correct today.
Hubble and the
expanding universe
Hubble’s credibility skyrocketed follow-
ing the determination of an expanding
universe. This was big stuff: Hubble had
piled on lots of supporting evidence for
the ideas of the great physicist Albert
Einstein, who had proposed that time
and space are expanding and that the
cosmos is almost unimaginably large.
By the late 1930s, following Hubble’s
big discoveries, it was becoming clear
just how significant galaxies are to the
story of the cosmos. Astronomers knew
that most of the immensely large uni-
verse is filled with darkness. Little mat-
ter exists outside the island galaxies,
which contain all the bright stuff, the
normal matter — stars, gas, dust, and
planets. The universe is like a vast and
stormy sea, with little ships — galaxies
— f loating on a virtually limitless ocean
of utter darkness and with a foreboding
void between them.Classifying galaxies
By this time, Hubble understood the
broad types of galaxies, which he classi-
fied in a “tuning fork” diagram. There
are spiral galaxies, like Andromeda, and
barred spirals — similar to spiral galax-
ies but containing a rectangular “bar” of
material through their centers. There are
ellipticals: spherical masses of stars, gas,
and dust. There are lenticulars, whichappear lens-shaped; and there are irregu-
lars, relatively formless clouds of matter
lacking organized structure. In the late
1930s, astronomers discovered examples
of a new class, dwarf spheroidal galaxies,
and later found so-called peculiar galax-
ies, which seem highly distorted. By the
end of the 1950s, they had devised an
improved way to classify galaxies, based
on the research of the French astronomer
Gérard de Vaucouleurs of the University
o f Te x a s.
Examples of all these types of galaxies
can be viewed with a telescope from a
dark-sky site. They include:
Spiral galaxies: The Sunf lower
Galaxy (M63), IC 342, and NGC 1232
Barred spiral galaxies: NGC 1300,
NGC 1512, NGC 1530, NGC 4921, and
NGC 5701
Elliptical galaxies: M49, M87, and
NGC 1052
Lenticular galaxies: M84, NGC
2787, and NGC 4111
Irregular galaxies: NGC 1569, NGC
3239, and NGC 4214
Peculiar galaxies: Arp 81, Arp 220,
Centaurus A, Fornax A, M82, and
Perseus ANGC 1398: Another Milky Way analogue
The beautiful barred spiral NGC 1398 lies in the southern constellation
Fornax and provides another analogue to the Milky Way’s structure. This
galaxy is somewhat larger than ours, with a diameter of 135,000 light-
years, and sits some 65 million light-years from Earth. WARREN KELLERNGC 6744: A galaxy that looks like the Milky Way
The brilliant galaxy NGC 6744, which lies in the southern sky in Pavo, is a
larger version of the Milky Way. This barred spiral stretches 175,000 light-
years across — some 75 percent bigger than our home. Its structure,
however, is similar to ours, with a core, a strong bar through its center,
and radiating spiral arms filled with glowing stars and gas. DON GOLDMAN