BBC_Knowledge_2014-06_Asia_100p

Each of your eyes sees a
nearby object in a different
position relative to the back-
ground. Similarly, nearby stars
undergo tiny shifts in position
when viewed on each side of the
Earth’s orbit (every six months).
Armed with the Sun-Earth
distance, which is measured by
looking at the positions of the
inner planets, these shifts in
position reveal the star’s distance.
For stars too far away to use
this ‘parallax’ technique,
astronomers rely on a simple
principle – if you know exactly

how intrinsically bright a star is,

then measuring how bright it

appears to us on Earth will reveal

its distance. Some stars vary in

brightness and the time taken for

these variations is directly related

to their actual luminosity. The

intrinsic brightness of some

exploding stars can also be

worked out by looking at their

evolution. Objects of this kind,

with known brightness, are called

‘standard candles’ and are the

main tool astronomers use to

determine distance to other stars.

AG

How do scientists determine

the distance to a star?

Why does hair get

darker when wet?

When light strikes dry hair, some of it

is absorbed by the pigment in the hair and

some reflects back to your eyes. Water is

transparent and a thin film absorbs very little

light by itself. But when light hits wet hair,

some of the light reflecting off the surface

of the hair strikes the inside surface of the

water film at the right angle to be reflected

or refracted back onto the hair again. This

creates a second chance for the light to be

absorbed, making the hair appear darker. LV

It appears that the usual process of moon

formation, by ‘accretion’, is not efficient

enough to produce moons more than 0.025

times the mass of Earth. This explains why

Jupiter’s moon Ganymede, the largest

moon in the Solar System, is only 2 per cent

of Earth’s mass. But there are other ways

planets can gain moons. A large planet could

disrupt a binary system of two Earth-sized

planets, ejecting one but capturing the other

as a moon. AG

Could an Earth-sized

moon exist?

Ganymede is the largest moon we know of, but it

only has 2 per cent of Earth’s mass

Thanks to some

clever maths, we

now know just how

far away those

points of light are

Your hair likes to absorb light when wet