Australian Sky & Telescope - May 2018

http://www.skyandtelescope.com.au 17

fartherawayobjectsare.
Thinkofaballoonwithpoints
drawnonitssurface.Allthepoints
expand away from each other as the
balloon inflates. Likewise, the universe
itself — the very fabric of space — is
expanding. As the universe expands,
thegalaxies(thepointsonthe‘surface’
of the expanding space) all move away
from one another. There’s no true
‘centre’ for the universe, just as there’s
no stationary point on the surface of
the balloon.

How big is the universe? Based on

the history of its expansion and its age,

we think the observable universe is

93 billion light-years across, or 6,

trilliontimesthedistancebetweenthe

Sun and Earth. But our observations of

the residual radiation from the Big Bang

indicate that the universe is only 13.

billionyearsold.So,howcanwesee

parts of the cosmos whose light should

take longer to reach us than the actual

ageoftheuniverse?

Well,althoughEinsteindecreedthat

Local Group

Sextans A

Sextans Dwarf

Large Magellanic Cloud

Small Magellanic Cloud

Carina Dwarf

Sculptor Dwarf

Fornax Dwarf

IC 10

M

M

NGC 185 NGC 147

Boötes Dwarf

Leo A

Leo I

Leo II

Ursa Major I

Andromeda I

Phoenix Dwarf

WLM

IC 1613

Pisces Dwarf

Aquarius Dwarf

Pegasus Dwarf

Cetus Dwarf

Tucana Dwarf

Andromeda II

Andromeda III

M

Andromeda Galaxy (M31)

NGC 6822

Ursa Major II

Milky Way Galaxy

Ursa Minor Dwarf

Draco Dwarf

Sagittarius dSph

Canes Venatici Dwarf

NGC 3109

Antlia Dwarf

Sextans B

Sagittarius DIG

HOW TO FIND US The Milky Way and the Andromeda Galaxy dominate the Local Group, shown
here with its brightest members. At least 54 galaxies belong to the Local Group, which stretches
across 10 million light-years. Our galaxy and Andromeda, both of which are large enough to
sport their own satellite galaxies, combine to contain the large majority of the Local Group’s total
stellar mass. The Local Group is part of the larger Virgo Supercluster, which in turn is a small
part of the Laniakea Supercluster that spans some 510 million light-years and includes some
100,000 galaxies.

nothing in the universe can travel faster

than the speed of light, this prohibition

actually only applies to objects within

the universe, not the universe itself.

Based on the incredible homogeneity

and uniform pattern of cosmic

structure, astronomers think that the

fabric of space itself exceeded that limit

during a brief moment right after the

Big Bang, called inflation.

Try explaining that one at a late-

night star party.

■ DAVID DICKINSON is a freelance

science writer, high-school science

teacher, science iction author, retired

enlisted air force veteran, and avid

stargazer. He shares the universe and

more on his website astroguyz.com and

@astroguyz on Twitter.