Evening star., 1939-04-24

spectrum OF gas, DR.. MOhler eX-
planned, varies according to the
proportion of free electrons. Hence
the spectrum of hydrogen from
star would be expected to reveal
the degree of ionization of Its at
mosphere, Of the number of free
electrons per cubic centimeter. Five
times this, If the star happened
to be in the right temperature class,
would be the total density per cubic
centimeter at theatmospheric height
of the measurement. The density
of the atmosphere depends upon
the gravitational pull of the star.
This in turn depends upon the
size and weight of the star. If one
is known, or can be assumed, the
other can be calculated.
Astronomers have realized this
theoretically for years. 1t has been
impossible, however, to measure on
earth the actual differences in the
spectrum of hydrogen with electron
pressure under conditions simu-
lating those which would be found
in the atmosphere Of star. While
he himself could not make his
tests with hydrogen, DR.. Mohler
was able to do so with cesium,
one of the rare alkali metals, where
the effects. it could be determined
theoretically, would be quite similar
to those with the most abundant
substance in creation. He then
made comparisons with the hydro
gen spectra obtained from different
stars-especially Alpha Cygmi,
large, light, diffuse star about a
thousand times the size Of the
star and on several of the mys-
terious white dwarfs.
In the hydrogen spectra, said DR..
Mohler, there are two prominent
bands of lines. One Of them extends
downward from the near ultra-vio-
let into the edge Of the visible violet.
The other band. which can be used
as a check, is at the red end Of the
spectrum. The less the lines extend
into the range of visible light from
the ultra violet, he explained, the
greater the electron pressure and
consequently the greater the den-
sity, which can be calculated quite
precisely.
Actually, DR.. Mohler explains, the
method probably will be found of
more value In measuring the sizes,
rather than the weights, of stars.
Star masses are about the same
throughout the universe within a
narrow range of about the mass Of
the sun to 10-fold it's mass. Star
sizes. however, vary from the dimen-
sions of the earth to 10000 times
the dimensions of the sun.
Hence, as a rule it will be safe to
assume a weight and work out the
equation for dimensions. With the
white dwarfs, however, the opposite
may be true. Their sizes are fairly
well mountaineering range from
not more than the size of the earth
to a few times as big. It is valid
to assign to any new ono which may
be discovered a size within this
range. The interesting feature about
them Is their weight, several tons to
the cubic inch representing a state
of matter inconceivable on earth.
Hitherto the only means of
weighing them was when they
were parts of double stars, when the
calculations could be made on the
basis of their gravitational attrac-
ton for the other and larger celes-
tial body. This was the way the
weird objects were discovered. One
of them happened to be companion
star to Sirius, the brightest star in
the heavens.
Far Removed in Space.
But, DR.. Mohler explained. some,
and perhaps most, of them are lone
stars, far removed in the vast
reaches Of space from any other
bodies upon which they can exert
a gravitational pull. In these cases
both size and weight have been as
sumptions.
Already, DR.. Mohler explains, he
has studied the hydrogen spectrum
of one dwarf which looks like the
heaviest body yet known in the uni