14 AUSTRALIAN SKY & TELESCOPE May | June 2018
FINDING OUR PLACE By Dave DickinsonT’S ONE OF THE TOP QUESTIONS
I get at public star parties. I’ll line up
the Andromeda Galaxy through the
eyepiece, then start the pitch.
“That’s the Great Andromeda
Galaxy, 2.5 million light-years distant.
Our galaxy, the Milky Way, is heading
toward it at 110 kilometres per second,
for a merger about four billion years
from now.”
“How do we know that?” comes the
inevitable refrain.
It’s a good question. Most lay
people have heard of the Big Bang, if
only because of the popular television
sitcom, and maybe a few have heard
of such cryptic terms as ‘supercluster’
and ‘the Great Attractor’. But how do
we know where all these things are and
how they’re moving?To the stars
To measure the distance to the nearest
stars, astronomers use a methodHow do astronomers know how big the universe is? The answer is in the stars.
cosmos
Mapping the
ESO / P. HORÁLEKaway only shifts one arcsecond, or
about 1/1800th the diameter of a full
Moon. No star is located that close to
our Solar System; the nearest, Proxima
Centauri, is 4.25 light-years, or more
than 260,000 a.u., away.
The first good stellar parallax
measurement was made by German
astronomer and mathematician
Friedrich Bessel in 1838, who calculated
that 61 Cygni was 10.3 light-years
distant. (The current measured value is
11.4 l i g ht-y e a r s .)
Stars can also help us determine
the location of our place in the galaxy.
German-English astronomer William
Herschel made the first serious
attempt at modelling the shape of our
galaxy in 1785, based on the apparent
distribution of stars in the sky. His
sketch looked like a Christmas tree
with two trunks, fallen on its side.
Since then, measurements of stellar
motions and the position of gas clouds,straight out of trigonometry class
known as parallax. This is simply the
apparent ‘jump’ an object makes against
the background when viewed from two
different observation points. If we know
both the distance between the two
points and how much the target object’s
position seems to shift, it’s easy to
calculate the distance to the object. You
can see a parallax shift by observing
your outstretched thumb through one
eye and then the other. In the 18th
and 19th centuries, astronomers used
parallax during rare transits of Venus to
measure the distance between the Sun
and Earth, called the astronomical unit
(a.u.), and to unlock the scale size of
the Solar System.
To measure the distance to the
nearest stars, astronomers must use
the 2-astronomical-unit-wide baseline
of Earth’s orbit. Even with such a
tremendously large baseline, however,
an object 3.26 light-years (one parsec)