Philips Atlas of the Universe

Next come the longer-period Mira stars, named after

Mira or Omicron Ceti, the first-discovered and brightest

member of the class (Map 15). Unlike the Cepheids, the

Mira stars – all of which are red giants or supergiants – are

not perfectly regular; their periods vary somewhat from

the mean value (332 days in the case of Mira itself) and so

do the amplitudes. At some maxima Mira never becomes

brighter than the fourth magnitude, while at others it has

been known to reach 1.7. In general the amplitudes are

large – over 10 magnitudes in the case of Chi Cygni, in

the Swan (Map 8). Several Mira stars reach naked-eye

visibility at maximum, but at minimum all of them fade

below binocular range.

Semi-regular variables are not unlike the Mira stars,

but have smaller amplitudes, and their periods are very

rough. Betelgeux in Orion is the best-known example

(Map 16). At times it may almost equal Rigel, while at

others it is little brighter than Aldebaran in Taurus. RV

Tauri stars have alternate deep and shallow minima, inter-

spersed with spells of complete irregularity; R Scuti, in the

Shield (Map 8) is the only brightish example.

Eruptive variables are unpredictable. Some, such as

Gamma Cassiopeiae (Map 3), occasionally throw off

shells of material; T Tauri stars are very young and have

not yet joined the Main Sequence, so that they are varying

irregularly. R Coronae Borealis stars remain at maximum

for most of the time, but undergo sudden drops to mini-

mum – because they accumulate clouds of soot in their

atmospheres, and fade until the soot is blown away. These

stars are very rare, and R Coronae itself (Map 4) is much

the brightest member of the class.

Cataclysmic variables remain at minimum when at

their normal brightness, but show outbursts which may

be roughly periodical – as with the SS Cygni or U

Geminorum stars – or else quite unexpected, as with

classical novae. All these are binary systems. One com-

ponent is a white dwarf, which pulls material away from

its Main Sequence companion; when enough material has

accumulated the situation becomes unstable, and a short-

lived outburst results.

Supernovae, the most colossal outbursts known in

nature, are best described separately (see pages 182–3).

Mention should also be made of the unique Eta Carinae,

in the Keel of the Ship (Map 19). For a time during the

19th century it was the brightest star in the sky apart

from Sirius, but for over a hundred years now it has

been just below naked-eye visibility; it is associated with

nebulosity, and when seen through a telescope looks quite

unlike a normal star. At its peak it must have been six

million times as powerful as the Sun, making it the most

powerful star known to us. It is highly unstable, and in

the near future – cosmically speaking – it will probably

explode as a supernova.

There are so many variable stars in the sky that profes-

sional astronomers cannot hope to keep track of them all,

so that amateurs can do very valuable work. Accurate

measurements can be made with sophisticated equipment

such as photoelectric photometers, but a great deal can

be done by making eye-estimates through an ordinary

telescope. The procedure is to compare the variable with

nearby stars of constant brightness. At least two com-

parison stars are needed. For example, if star A is known

to be of magnitude 6.8 and star B of 7.2, and in brightness

the variable is midway between them, its magnitude must

be 7.0. With practice, estimates can be made to an accuracy

of a tenth of a magnitude, and variable star work has now

become one of the most important branches of modern

amateur astronomy.

THE STARS

BASIC CLASSIFICATION OF VARIABLE STARS

Symbol Type Example Notes

EA Algol Algol Periods 0.2d–27y. Maximum for most

of the time.

EB Beta Lyrae Beta Lyrae Periods over 1 day. Less unequal components.

Continuous variation.

EW W Ursae Majoris W UMa Dwarfs; periods usually less than 1 day.

PULSATING

M Mira Mira Long-period red giants. Periods 80–1000 days.

Periods and amplitudes vary from

cycle to cycle.

SR Semi-regular ËGeminorum Red giants. Periods and amplitudes very rough.

RV RV Tauri R Scuti Red supergiants; alternate deep and shallow

minima. Marked irregularities.

CEP Cepheids ‰Cephei Regular; periods 1–135 days; spectra F to K.

CW W Virginis ÎPavonis Population II Cepheids.

RR RR Lyrae RR Lyrae Regular; short periods, 0.2–1.2 days;

all of equal luminosity.

ERUPTIVE

GCAS ÁCassiopeiae ÁCassiopeiae Shell stars: rapid rotators; small amplitudes.

IT T Tauri T Tauri Very young, irregularly varying stars.

RCB R Coronae RCrB Unpredictable deep minima. Large

Borealis amplitude. Highly luminous.

SDOR S Doradûs S Doradûs Very luminous supergiants with

expanding shells.

CATACLYSMIC

UG U Geminorum SS Cygni Dwarf novae

or SS Cygni.

UG2 Z Camelopardalis Z Cam. Dwarf novae with occasional standstill.

N Novae DQ Herculis Violent outburst.

SN Supernovae B Cassiopeiae Violent outburst. Type I; destruction of the

white dwarf component of a binary system.

Type II; collapse of a supergiant star.

▲Eta Carinae photographed

with the Wide Field and

Planetary Camera of the

Hubble Space Telescope in

1996. Eta Carinae has a
      mass about 150 times that
      of the Sun, and may be the
      most luminous star known;
      it will eventually explode as
      a supernova. The image
      shows some of the material
      ejected during the
      19th-century outburst, when
      for a time Eta Carinae
      outshone every star apart
      from Sirius.

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