Galaxies
A galaxy is a vast group of stars held together
by gravity – it is thought that there could be
some 2 trillion in our Universe. They are
not scattered randomly but exist in clusters,
vast distances apart. All the galaxies together
take up just two millionths of space.131
(^1) SIZE
Galaxies are huge. The largest are more than a million
light-years across (one light-year is the distance that light
travels in a year). The smallest, called dwarf galaxies, are
a few thousand light-years wide. Andromeda measures
220,000 light-years from side to side.
(^2) SHAPE
A single galaxy is made of billions or trillions of stars
arranged in one of four basic shapes: spiral, barred
spiral, elliptical, or irregular. Spirals and barred spirals
are disc-shaped with arms of stars. In a spiral, such as
Andromeda, the arms wind out from a central bulge,
while in a barred spiral, they flow from the ends of a
central bar of stars. Elliptical galaxies are ball-shaped.
Irregular galaxies have no clear shape.
(^3) ORBITING STARS
Galaxies do not behave like solid objects. Each star
follows its own orbit around the centre of the galaxy.
Stars in a spiral galaxy typically take a few hundred
million years to make an orbit. Those further away
take longer than those closest to the core.
(^4) SPIRAL ARMS
Stars exist throughout a spiral galaxy’s disc.
The arms simply stand out because they are
full of very bright young stars.
(^5) CORE
The core of a spiral galaxy typically consists of
old red and yellow stars, with a supermassive
black hole in its centre. Andromeda’s black
hole is as massive as 30 million Suns.
(^6) DUST LANES
Dense clouds and lanes of dust within
the galaxy’s disc hide stars from view.
(^7) DWARF GALAXY
M110 is one of the dwarf elliptical galaxies
that orbit Andromeda. It is held in its orbit
by Andromeda’s gravity.
▲ ANDROMEDA GALAXY
Andromeda is one of the closest galaxies to our own,
the Milky Way. It is a spiral galaxy 2.5 million light-years
away from us – the most distant object that can be seen
by the naked eye from Earth.
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