Philips Atlas of the Universe

THE STARS

they can be measured. The ‘speed record’ is held by a dim

red dwarf, Barnard’s Star, which is 5.8 light-years away

and is our nearest neighbour apart from the three members

of the Alpha Centauri group; the annual proper motion is

10.27 seconds of arc, so in about 190 years it will crawl

across the background by a distance equal to the apparent

diameter of the Full Moon. (It has only 0.0005 the luminos-

ity of the Sun, so is very feeble by stellar standards.)

Over sufficiently long periods, the shapes of the con-

stellation patterns will change. For example, in the Great

Bear (Ursa Major) we have the familiar seven-star pattern

often called the Plough or, in America, the Big Dipper.

Five of the stars are moving through space in much the

same direction at much the same rate, so that presumably

they had a common origin, but the other two – Alkaid and

Dubhe – are moving in the opposite direction, so that

in, say, 100,000 years’ time the Plough pattern will have

been distorted beyond all recognition. Neither are the

Plough stars equally distant. Of the two ‘end’ stars, Mizar

is 59 light-years away, Alkaid 108, so that Alkaid is very

nearly as far away from Mizar as we are. There is another

reminder that a constellation pattern is nothing more than

a line of sight effect, and has no real significance. If we

were observing from a different vantage point, Mizar and

Alkaid could well be on opposite sides of the sky.

61 Cygni, the first star to have its distance measured,

has an annual proper motion of over 4 seconds of arc, and

it is also a wide binary, made up of two components which

are gravitationally linked. That is why Bessel thought that

it must be relatively close to us – and he was right.

We also have to deal with the radial or towards-or-

away movements of the stars, which can be worked out

by using the spectroscope. As we have seen, the spectrum

of the Sun is made up of a rainbow background crossed

by the dark Fraunhofer lines, and this is also true of all

normal stars, though the details differ widely.

If the star is approaching us, all the lines in the spec-

trum will be shifted over to the blue or short-wave end of

the rainbow band, while if the star is receding the shift will

be to the red or long-wave end (this is the Doppler shift,

about which more will be said when we discuss galaxies).

By measuring the shifts of the lines we can find out

whether the star is approaching or receding; the apparent

proper motion of the star in the sky is a combination of

the transverse and radial motions. (Conveniently, radial

velocities are listed as negative if the star is approaching,

positive if it is receding.) At the moment Barnard’s Star

is coming towards us at 108 kilometres (67 miles) per

second, but it will not continue to do so indefinitely, and

I can assure you that there is absolutely no fear of an

eventual collision with our planet.

 Distances of the stars in

the Plough.The diagram

shows the seven chief stars

of the Plough, in Ursa Major,

at their correct relative

distances from the Earth.

Alkaid, at 108 light-years,

is the furthest away; Mizar,

at 59 light-years, is the

nearest. Therefore, Alkaid

is almost as far away from

Mizar as we are!

Distance from Earth (light-years)

0

20

40

60

80

100

120

·

·

‚

‚

Â

Â

‰

‰

Á

Á

Ë

Ë

108 59 62 65 75 62 75

̇

̇

▲Long term effect of

proper motion. The series

of diagrams shows the

movement of the seven main

stars in Ursa Major (the

Great Bear) that make up the

famous pattern known as the

Plough or Big Dipper. The

upper diagram shows the

arrangement of the stars as

they were 100,000 years ago,

the centre diagram gives the

present appearance, and the

bottom diagram shows the

Bear as it will appear in

100,000 years’ time.

Dubhe

Merak

Phekda

Megrez

Alioth

Mizar

Alkaid

Dubhe

Merak

Phekda

Megrez

Alioth

Mizar

Alkaid

Dubhe

Merak

Phekda

Megrez

Alioth

Mizar

Alkaid

Actual motionof a star in

space when observed from

Earth (A–B) is a combination

of radial and transverse

motion against the

background of distant stars.

Radial motion (A–C) is the

velocity towards the Earth

or away from the Earth;

it is positive if the star is

receding, negative if it

is approaching.

Proper motion (A–D) is

the transverse movement,

or the motion across the sky.

Barnard’s Star (10”.31 per

year) has the greatest proper

motion.

A

B

A

B

C

A

B

D

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