Amateur Naturalist Star Communities

Star communities

It's possible - just possible - that space is not as lonely as it seems.
Stars, I mean, are not just solitary twinkles in the dark, but instead, they
actually travel in groups. Many, possibly most stars are small systems of two
or more stars circling around each other. Alpha Centauri, for example, is not
one but three stars connected by gravity. And there is a theory, based on
uncertainties in the math of its motion, that our own sun has a companion
that is too small and dim to be seen yet. If I could make an unscientific
comparison, it's as if stars gravitate together in systems the way people
gravitate to friendships and partnerships. Sometimes they settle into lifelong
mutual orbit, and other times they start to tear each other apart, but that's a
different topic.
Not only do they form individual gravitational relations, but many stars and
systems are traveling in the same general direction. The stars in the Pleiades
are flowing along together, and so is the Ursa Major cluster, whose center
lies about 75 light-years from us and includes many stars in Ursa Major and
a few in Corona Borealis. A hundred or so other stars in our neighborhood,
such as Sirius, might at one time have been part of the Ursa Major cluster
but apparently wandered off. Their paths are still similar to the Ursa Major
cluster's, though, and astronomers refer to this larger current as the Ursa
Major Stream.
The streams, clusters and mini-systems are all orbiting the center of the
galaxy. The galaxy is a spiral disk of somewhere between 400 billion and a
trillion stars 100,000 light-years from rim to rim, with a dense central bulge
of stars. It takes about 220 million years for the stars to make one circuit
around the central bulge - one galactic year, if you want to think of it that
way. Our sun flows along with everybody else about three-fifths of the way
to the edge of the disk.
Around the central bulge orbit 158 globular clusters. A globular cluster is a
ball-shaped group of a hundred thousand to a million stars living together in
extremely close quarters, about two stars per cubic light-year. (In our
neighborhood there is about one star per 300 cubic light-years.) Most
globular clusters are orbiting eccentrically in what astronomers call a galactic
"halo."
M13 in the constellation Hercules is one of the brightest and most
intriguing globular clusters to look at, not so much because of its appearance
- in your binoculars it's just another fuzzy (though unusually round) wash of
light - but because of a theory that goes with it: Globular clusters are
thought to contain the universe's most ancient stars. Astronomers believe
them to be 12 billion to 16 billion years old. (The sun is about 4.5 billion
years old.) When the back of your eye soaks in light from M13, it is
encountering thousands of beings born near the beginning of time.
It's hard to get a grip on what "the beginning of time" could actually mean,
sort of like imagining what life was like before World War I. And the
astronomers aren't really sure either. They're pretty confident that some stars
in globular clusters are up to 16 billion years old, but at the same time, they
generally agree the whole universe is only 13.7 billion years old.
I wonder what that means. For now I guess we have no choice but to go
with the flow.