Space^25
A black hole is a region of space with such strong gravity that
it swallows up everything that comes near it, even light. A black
hole may form when a very massive star blasts itself apart as
a supernova. The core of the star collapses so violently that all
its matter is crushed into almost no space at all, leaving behind
a region of intense gravity – a black hole.
WHAT ARE SUPERMASSIVE BLACK HOLES?
Ordinary black holes are formed when massive stars
die, and they typically have the mass of about 5–10
Suns. A supermassive black hole, however, has a mass
millions of times greater, and is formed when huge gas
clouds collapse. Supermassive black holes seem to be
the power source of high-energy active galaxies, such
as quasars. Astronomers believe that a supermassive
black hole lurks at the centre of our own galaxy.
BLACK HOLE RADIATION 3
A computer simulation shows the
radiation in space around a black
hole. As matter spirals into the
hole, it is accelerated and heated to
temperatures up to 100 million°C
(180 million°F). It gives off high-energy
radiation, such as X-rays, in pathways
that are distorted by the intense gravity.
BLACK HOLES
FIND OUT MORE. Galaxies 27 • Matter 156 • Solar System 14 • Sun 15 • Telecommunications 192 • Universe 26
HOW DO WE MEASURE A STAR’S BRIGHTNESS?
Astronomers measure star brightness in magnitudes.
The lower the magnitude, the brighter the star. Most
stars we can see with our eyes are magnitude 1–6, but
the faintest stars visible with telescopes are magnitude
- Exceptionally bright stars have negative
magnitudes, such as –1.44 for Sirius.
4 DOUBLE STAR
The large glowing light at the centre of this
X-ray image is not a single star – it is actually
made up of two stars circling around each other.
It is a double-star system called a binary.
A star is born when a dense region of a nebula collapses under its
own gravity. It shrinks into a denser, hotter ball, called a protostar.
When the protostar gets hot enough, nuclear fusion begins inside
it, and it starts to shine as a new star. After billions of years, the
star runs out of fuel and starts to die, expanding hugely and
becoming cooler and redder. Small dying stars swell into red
giants, and massive dying stars swell into supergiants. A red giant
blows out its outer layers to become a planetary nebula, and
finally becomes a dense dwarf star. A supergiant explodes as a
supernova. It may end up as a neutron star or a^. BLACK HOLE.
STAR FORMATION
HOW STARS LIVE AND DIE
Star gives off
light and heat
produced by
nuclear fusion
in its core
Dense region
shrinks to form
protostar, with
temperatures up
to 15 million°C
(27 million°F)
SMALL STARS (INCLUDING THE SUN)
100 MILLION KM
(60 MILLION MILES)
RED GIANT
Core shrinks
and becomes
incredibly
dense before
disappearing
100 MILLION KM
( 60 MILLION MILES)
PROTOSTAR
10,000 BILLION KM
(6,000 BILLION MILES)
PLANETARY NEBULA
1 MILLION KM
(600,000 MILES)
MAIN-SEQUENCE STAR
1,000 MILLION KM
( 600 MILLION MILES)
RED SUPERGIANT
200,000 BILLION KM
(125,000 BILLION MILES)
NEBULA
MASSIVE STARS
Outer layers of
star blown away
in explosion
10,000 KM
(6,000 MILES)
COOLING
WHITE
DWARF
Dense region
in nebula
begins to
contract
Star expands,
cools, and
reddens
Star greatly
expands, and
gets redder as
it cools
10,000 KM
(6,000 MILES)
BLACK
DWARF
10,000 KM
(6,000 MILES)
WHITE
DWARF
50 KM
(30 MILES)
BLACK HOLE
Core turns
red as it
cools
15 KM ( 9 MILES)
NEUTRON STAR
Core stops
glowing
SUPERNOVA
Expanding
gas shell
Intensely
hot core
stars