Philips Atlas of the Universe

THE SUN

correctly, and so laid the foundations of modern astro-

physics.

An incandescent solid, liquid, or gas at high pressure,

will yield a continuous spectrum, from red to violet. An

incandescent gas at low pressure will produce a different

spectrum, made up of isolated bright lines, each of which

is characteristic of one particular element or group of

elements; this is known as an emission spectrum. For

example, incandescent sodium will produce a spectrum

which includes two bright yellow lines; if these are seen,

then sodium must be responsible, because nothing else can

produce them. Many elements, such as iron, have spectra

so complex that they include many thousands of lines in

their unique fingerprints.

The Sun’s photosphere yields a continuous spectrum.

Above the photosphere lies the chromosphere, which is

made up of low-pressure gas and produces an emission

spectrum. Normally these lines would be bright; because

they are silhouetted against the rainbow background they

appear dark, but their positions and intensities are un-

altered, so that there is no problem in identifying them.

Two prominent dark lines in the yellow part of the band

correspond exactly to the two famous lines of sodium, and

therefore we can prove that there is sodium in the Sun.

It has been found that the most plentiful element in

the Sun is hydrogen, which accounts for 71 per cent of the

total mass; any other result would have been surprising,

since in the universe as a whole the numbers of hydrogen

atoms outnumber those of all the other elements combined.

In the Sun, the next most plentiful element is helium, with

27 per cent. This does not leave much room for anything

else, but by now most of the 92 elements known to occur

in Nature have been identified in smaller quantities.

Helium was actually identified in the solar spectrum

before it was known on Earth; it was found by Lockyer

in 1868, who named it after the Greek helios(Sun). Not

until 1894 was it tracked down on our own world.

Many instruments of various kinds are based on the

principle of the spectroscope. One such is the spectrohelio-

graph, where two slits are used and it is possible to build

up an image of the Sun in the light of one selected element

only (the visual equivalent of the spectroheliograph is

the spectrohelioscope). Similar results can be obtained by

using special filters, which block out all the wavelengths

except those which have been selected. Today, equipment

of this sort is used by many amateur observers as well as

professionals – and solar observation is always fascinat-

ing, if only because there is always something new to see;

the Sun is always changing, and one can never tell what

will happen next.

Letter Wavelength, Å Identification

C (H-alpha) 6563 Hydrogen

D 1 5896 Sodium

D 2 5890

b 1 5183 Magnesium

b 2 5173

b 3 5169

b 4 5167

F (H-beta) 4861 Hydrogen

G 4308 Iron

g 4227 Calcium

h (H-delta) 4102 Hydrogen

H 3967 Ionized calcium

K 3933

(One Ångström (Å), named in honour of the Swedish

scientist Anders Ångström, is equal to one hundred-

millionth part of a centimetre. The diameter of a human

hair is roughly 500,000 Å. Another often-used unit is

the nanometre. To convert Ångstroms into nanometres,

divide by 10, so that, for instance, the wavelength of

the H-alpha line is 656.3 nm.)

SOME IMPORTANT FRAUNHOFER

LINES IN THE SOLAR SPECTRUM

▲ The McMath-Pierce Solar

Telescope on Kitt Peak in

Arizona. The Sun’s light is

collected by the heliostat, a

mirror at the top of the

structure, and is directed

down the slanted tunnel on to

a curved mirror at the bottom;

this in turn reflects the rays

back up the tunnel to a flat

mirror, which sends the rays

down through a hole to the

lab below.

▼ The visible spectrum

of the Sun is very complex;

more than 70 elements

have been identified. The

photosphere produces a

rainbow or continuous

spectrum. The rarefied

gases in the chromosphere

would yield bright lines if

seen on their own but

against the photosphere are

‘reversed’ and appear dark,

although their positions and

intensities are unaffected.

K H4,000Å

H
(C) 6,000Å 6,000Å D 1 D 2 5,500Å

4,000Å 5,000Å bbb

Hydrogen Hydrogen Hydrogen Magnesium

Hydrogen Sodium

Calcium Iron Calcium

H g H H (F)

E152- 191 UNIVERSE UK 2003mb 7/4/03 5:41 pm Page 159