Philips Atlas of the Universe

ATLAS OF THE UNIVERSE

Nebulae

W

hen Messier published his catalogue, in 1781, he

included nebulae of two types – those which looked

as though they were gaseous, and those which gave every

impression of being made up of stars. William Herschel

was among the first to recognize a definite difference

between the two classes, and in 1791 he said of the Orion

Nebula: ‘Our judgement, I venture to say, will be that the

nebulosity is not of a starry nature.’

Proof came in 1864, when Sir William Huggins, the

pioneer English astronomical spectroscopist, found that

the spectra of bright nebulae were of the emission type,

while the starry objects, such as M31 in Andromeda,

showed the familiar absorption lines. But there are also

two other classes of objects which have to be considered.

In particular M1, the first entry in Messier’s list, was

proved to be a supernova remnant – the wreck of the star

seen by the Chinese in 1054; its nickname of the Crab was

bestowed on it by the Earl of Rosse when he looked at it

with his great 183-centimetre (72-inch) telescope in the

mid-19th century. Another supernova remnant is the Gum

Nebula in Vela (Map 19), named after the Australian

astronomer Colin Gum; in this case the supernova blazed

forth in prehistoric times, and must have been exceptionally

brilliant back then.

Planetary nebulae have nothing to do with planets, and

are not true nebulae; they are old, highly evolved stars

which have thrown off their outer layers. The discarded

shells shine because of the ultra-violet radiation emitted by

the central star, which is extremely hot (with a surface

temperature which may reach 400,000 degrees C) and is

well on its way to becoming a white dwarf. All planetary

nebulae are expanding, and on the cosmic timescale the

planetary nebula stage is very brief. The best-known mem-

ber of the class is M57, the Ring Nebula in Lyra (Map 8),

which is easy to locate between the naked-eye stars

Beta and Gamma Lyrae; telescopically it looks like a

tiny, luminous cycle tyre, with a dim central star (although

very recent research suggests that it may really be in the

form of a double lobe rather than a ring). Other planetary

nebulae are less regular; M27, the Dumbbell Nebula in

Vulpecula (Map 8), earns its nickname, while the rather

faint M97, in Ursa Major (Map 1), is called the Owl

because the positions of two embedded stars do give a

slight impression of the eyes in an owl’s face.

True nebulae consist mainly of hydrogen, together

with what may be termed ‘dust’. They shine because of

stars in or very near them. Sometimes their light is due to

pure reflection, as with the nebulosity in the Pleiades, but

in other cases very hot stars make the nebulosity emit a

certain amount of luminosity on its own account, making

up what are termed HII regions. Such is the Great Nebula

M42, in Orion’s Sword (Map 16), where the illuminating

▲ M17, the Omega Nebula in

Sagittarius: John Fletcher,

25-cm (10-inch) reflector.

The Nebula is almost 6000

light-years away, but is

an easy telescopic object. It

has also been nicknamed the

Swan or Horseshoe Nebula.

 M42, the Orion Nebula:

Commander H. R. Hatfield,

30-cm (12-inch) reflector.

 Hen 1357,as imaged by

the Hubble Space Telescope.

It is about 18,000 light-years

away, and lies in Ara (it was

the 1357th object in a list

of unusual stars compiled

by Karl Henize). Previous

ground-based observations

indicate that over the past

few decades Hen 1357 has

changed from being an

ordinary hot star to an object

with the characteristics of a

planetary nebula. Only the

HST can show it in detail.

The image also shows a

companion star (at about 10

o’clock) within the nebula.

The Trapezium (Theta

Orionis) appears near the

centre of the picture, and

the bright and dark nebulosity

is well shown. Exposure,

10 minutes.

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