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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

29. 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