16 AUSTRALIAN SKY & TELESCOPE May | June 2018
a = 1 a.u.Apparent
shift = 2pDistance
to starParallax
angle (p)universe due to gravity. We’re part of
this flow. As it turns out, galaxies are
like drops of water trickling through
streamsinavastcosmicwatershed.
Besides revealing the existence of
Laniakea, our local river in this
watershed, this work (again using shifts
in spectral lines) showed that the Local
Group is moving toward two cluster
structures called the Great Attractor
and the Shapley Supercluster; the latter
is some 650 million light-years away.
Wherearethesesuperclusters
located in the sky? As bad luck would
have it, both lie right along the
diskplaneoftheMilkyWay,inthe
direction of the southern constellation
Norma. This poses an observational
problem similar to the one encountered
by earlier astronomers when they
attemptedtomodelthestructureof
thegalaxy,theclassic‘youcan’tseethe
forest (of galaxies) for the trees’. In this
case,thetreesarestarsanddustygasin
the plane of our own galaxy, blocking
ourearthboundviewofthemany
galaxies beyond.Avoiding our home galaxy
TheMilkyWay’sbandontheskyis
sometimes calledthezoneofavoidance,
becausegalaxiesandancientstar
clusters seem to avoid that region on the
sky.Todayweknowthat’snotbecausethe objects aren’t there but because
ourgalaxy’sdiskblocksourviewof
anything that’s beyond it. But though
opaque in optical light, the plane of the
Milky Way is largely transparent at radio
wavelengths. Because of this, projects
suchastheParkesHIZoneofAvoidance
(HIZOA) survey observe at the classic
21-cm radio wavelength for neutral400 500 600
Wavelength (nanometers)700400 500 600 700400 500 600 700The spectra of galaxies show that everything is speeding away from everything
else at the largest scale of the universe. In these example spectra, the dark
absorption lines show where light has been absorbed by hydrogen (the absorbing
hydrogen could belong to the outer layers of the star producing the light or to
interstellar clouds of gas and dust). If a light-producing body is moving away
from us, the absorption lines shift toward the red end of the spectrum. If the
body is approaching, the lines shift toward the blue end. The top example shows
the hydrogen absorption lines for a body at rest. The middle spectrum shows a
radiating body moving away from us at 3,000 km/s. The bottom spectrum shows
the same body moving away from us at 30,000 km/s.
LEAH TISCIONE /SKYWATCHFROM HERE TO THERE Using a
method called parallax, astronomers can
measure the distance to a nearby star
using trigonometry. First, they measure
the position of the star in relation to more
distant background stars. When Earth is
at the opposite point in its orbit around
the Sun (six months later), they again
measure the stars’ positions. If the target
star is close enough to us, its position
will have shifted just a bit — less than an
arcsecond — against the background
stars. (There are 60 arcminutes per degree
and 60 arcseconds per arcminute, so one
arcsecond =^1 / 3600 of one degree.)hydrogen (the ‘HI’ in ‘HIZOA’), looking
through the dust and gas along the
plane of the Milky Way to find ‘missing’
objects beyond.
HIZOA and other surveys have
turned up 957 galaxies, mapping out
the suburbs of the Norma Supercluster,
a massive collection of galaxy clusters
that forms a ‘wall’ 250 million light-
years across, covering about 50° of sky.
But beyond even this, the local cosmic
watershed seems to be moving towards
the massive Shapley Supercluster. Future
surveys will peer through the veil and
tell us more about where we’re headed.
Maybe future discoveries will add to our
star party repertoire of how we know
what we know about our place and
motion through the cosmos.To infinity, and beyond
The redshift of galaxies’ spectra already
tells us something else important
about the universe: On the largest
scales, everything is rushing away
from everything else. An observer at
any given point in the universe sees all
other points receding away from them,
and this recession speed gets greater theExpanding at speed
FINDING OUR PLACE