BBC Knowledge Asia Edition

Are many visible

stars dead?

In Numbers

of the 150,000 tree species in the

Amazon could be at risk of extinction, if

deforestation continues at its current rate

Why do we see shapes

and colours when we

rub our eyes?

All the effects of

LSD without the

health concerns!

These shapes and colours, called
‘phosphenes’, were reported as long ago as
the time of the ancient Greeks. Rubbing your
eyes increases the pressure within the
eyeball and this pressure activates ganglion
cells in the retina in the same way as light
does. Your brain doesn’t know the difference
and so interprets the activation as though
you were seeing light from the world outside.
Most common phosphenes are diffuse blobs
of different colours that move with the
rubbing. Then there are scintillating and
rapidly moving grid-like patterns which
probably reflect the organisation of cells
higher up in the visual system. These
patterns are reminiscent of psychedelic
paintings because the major hallucinogens
also affect the visual system. Other effects
include an array of intense blue points of
light. If you want to experience these, be
careful and press gently for some time rather
than pressing too hard and risking damage
to the eye. SB

Probably not. All of the stars you can

see with the unaided eye lie within about

4,000 light-years of Earth. But the most

distant ones are intrinsically brighter, have

more mass and are therefore likely to die in

rare supernova explosions. We can only

see fainter (and hence less massive) stars

out to smaller distances and these stars

are more likely to end their lives in less

violent but more common deaths. This

complicates the estimate of the ‘death

rate’ for visible stars. But we can choose

an intermediate distance, say 1,000

light-years, to estimate this number. Using

our knowledge of the death rate in the

entire Milky Way, the death rate for visible

stars works out at about one star every

10,000 years or so. Given that all those

stars are closer than 4,000 light-years, it is

unlikely – though not i e – that any

of them are y G

Bacteria don’t have a fix p n

because they don’t grow old When

bacteria reproduce, they split into two

equal halves, and neither can be

regarded as the parent or the child. You

could say that so long as a single one of

its descendants survives, the original

bacterium does too. Individual bacteria

can also turn themselves into spores

with a tough coat to protect themselves

from dry conditions. Bacterial spores

have been successfully revived from

250-million-year-old salt crystals found

in New Mexico in 2000. But if we

assume that the global bacteria

population is stable, then it follows that

one bacterium must die for each new

one that is produced. Bacteria divide

somewhere between once every 12

minutes and once every 24 hours. So

the average lifespan of a bacterium is

around 12 hours or so. LV

Bacteria splitting

How long

does

a bacteriu

live?

xed lifespan

d. Whe

0 ligh

h not impossible

m are already dead. AG

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57%

PHOTO: GETTY X3, SCIENCE PHOTO LIBRARY