Philips Atlas of the Universe

Variable Stars

V

ariable stars are very common in the Galaxy – and, for

that matter, in other galaxies too. They are of many

types. Some behave in a completely predictable manner,

while others are always liable to take us by surprise.

First there are the eclipsing binaries, which are not

truly variable at all. The prototype is Algol or Beta Persei,

which, appropriately enough, lies in the head of the

mythological Gorgon, and has long been nicknamed the

‘Demon Star’. Normally it shines at magnitude 2.1; but

every 2.9 days it begins to fade, dropping to magnitude 3.4

in just over four hours. It remains at minimum for a mere

20 minutes, after which it brightens up again.

Algol is in fact a binary system. The main component

(Algol A) is of type B, and is a white star 100 times as

luminous as the Sun; the secondary (Algol B) is a G-type

subgiant, larger than A but less massive. When B passes

in front of A, part of the light is blocked out, and the mag-

nitude falls; when A passes in front of B there is a much

shallower minimum not detectable with the naked eye.

The eclipses are not total, and if Algol could be observed

from a different vantage point in the Galaxy there would

be no variation at all.

Incidentally, we have here a good example of what is

called mass transfer. The G-type component was originally

the more massive of the two, so that it left the Main

Sequence earlier and swelled out. As it did so, its gravita-

tional grip on its outer layers was weakened, and material

was ‘captured’ by the companion, which is now the senior

partner. The process is still going on, and radio observa-

tions show material streaming its way from B to A.

Other naked-eye Algol stars are Lambda Tauri (Map

17) and Delta Librae (Map 6). Beta Lyrae, near Vega

(Map 8) is of different type. The period is almost 13 days,

and there are alternate deep and shallow minima; the com-

ponents are much less unequal than with Algol, and varia-

tions are always going on. Apparently the two components

are almost touching each other, and each must be drawn

out into the shape of an egg. Different again is Epsilon

Aurigae, close to Capella (Map 18). The primary is a

particularly luminous supergiant; the eclipsing secondary

has never been seen, but is probably a smallish, hot star

surrounded by a cloud of more or less opaque material.

Eclipses occur only every 27 years; the next is due in 2011.

Close beside Epsilon is Zeta Aurigae, also an eclipsing

binary with a period of 972 days. Here we have a red

supergiant together with a smaller, hot companion; it is

when the hot star is eclipsed that we see a drop in bright-

ness, but the amplitude is small (magnitude 3.7 to 4.2).

Pulsating stars are intrinsically variable. Much the

most important are the Cepheids, named after the proto-

type star Delta Cephei (Map 3) in the far north of the sky.

They are yellow supergiants in a fairly advanced stage of

evolution, so that they have exhausted their available

hydrogen and helium ‘fuel’ and have become unstable,

swelling and shrinking. The period of pulsation is the time

needed for a vibration to travel from the star’s surface to

the centre and back again, so that large luminous stars

have longer periods than stars which are smaller and less

powerful. There is a definite link between a Cepheid’s

period and its real luminosity – which in turn gives a clue

to the distance, so that Cepheids are useful as ‘standard

candles’, particularly since they are powerful enough to

be seen over an immense range. W Virginis stars are

not unlike Cepheids, but are less luminous; the brightest

example is Kappa Pavonis in the southern hemisphere

(Map 21). We also have RR Lyrae stars, which have short

periods and small amplitude; all seem to be about 90 times

as powerful as the Sun.

ATLAS OF THE UNIVERSE

 RR Lyrae variables. All

have short periods and there

are three main groups. In

the first, the periods are

about 0.5 days and the

rise to maximum is sharp,

followed by a slower decline;

RR Lyrae is of this kind (see

light-curve). Variables of the

second class are similar, but

 Cepheid variableshave

periods of from three days

to over 50 days. Their light-

curves are regular and are

related to their absolute

magnitudes. The light-curve

shown here is for Delta

Cephei, the prototype star.

Delta Cephei belongs to

Population I: there are also

 Long-period variables.

With long-period variables,

of which the prototype is

Mira Ceti (light-curve shown

here), neither the periods nor

the maximum and minimum

magnitudes are constant.

For instance, Mira may

at some maxima attain

magnitude 2: at other

 RV Tauri variables.

The RV Tauri variables

are very luminous and

are characterized by light-

curves which show alternate

deep and shallow minima.

There are considerable

irregularities: two deep

minima may occur in

succession, and at times

 SS Cygni or U

Geminorum variables.

The so-called ‘dwarf novae’

are characterized by

periodical outbursts; for

most of the time they remain

at minimum brightness.

Outbursts may occur at fairly

regular intervals or may be

unpredictable. The prototype

 R Coronae Borealis

variables. The most striking

feature of the light-curve

of the typical R Coronae

Borealis variable shown

here is that its magnitude

remains more or less

constant, but then at

unpredictable intervals

the magnitude plunges

sharply to a deep, but brief,

minimum. R Coronae, for

example, is normally of

the amplitudes are smaller

and the rise to maximum

is slower. Stars of the third

class have symmetrical

light-curves and periods

of some 0.3 days.

Cepheid variables of

Population II, which are

of similar kind but are less

luminous; these are now

known as W Virginis

variables.

maxima the magnitude never

exceeds 4. Most long-period

variables are red giants of

spectral type M or later: and

there is no Cepheid-type

period–luminosity law.

there is no semblance of

regularity. RV Tauri stars

are rare; the light-curve of

a well-known member of the

class, AC Herculis, is shown

in the diagram here.

stars are U Geminorum and

SS Cygni (light-curve shown

here) which has outbursts

approximately every 40

days, when its magnitude

increases from 12 to 8.25.

around magnitude 6, but

it may drop to below even

magnitude 14; for long

periods it may remain at

its maximum. R Coronae

variables are rare. Only six

of them so far observed – R

Coronae itself, UW Centauri,

RY Sagittarii, SU Tauri and

RS Telescopii – can exceed

the 10th magnitude at their

maximum brightness.

Magnitude

Hours

7.0

7.8

024 681012 14 16 18

7.2

7.4

7.6

Magnitude

Months

12

021 4 6810

9

10

11

Magnitude

Days

2.9

2.5

012 34 5 67

2.8

2.7

2.6

2.1

2.4

2.3

2.2

Magnitude

Days

8.4

02040 60 80 100 120 140 160 180 200

7.2

7.6

8.0

Magnitude

Days

3 5 7911

12

14

01020 30 40 50 60 70 80 90 100110120130140

6

8

10

Magnitude

Days

3

050100 150 200 250300 350 400450500550600650700

5

7

9

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