Evening star., 1934-12-09
SCIENTISTS CHART
SUMS "NE|GHB0R"
"Sky Surveyors" to Explain
Methods to Carnegie
Trustees.
BY THOMAS r. HENRY.

Direct measurements of the dis
tances CT almost 1000 stars closest to
the sun have been made by astrono-
mers of the Carnegie Institution of
Washington. These sky surveyors"
use about the same methods as a civil
engineer would use in determining the
boundaries of plot Of land.
The mechanisms and accomplish
ments of the Photographic parallax
method being used by the staff of the
institutions Mount Wilson observatory
will be shown here otter the annual
trustees meeting on December IA. Is
and l6 at the administration building,
Sixteenth and P streets.
BY patiently measuring the distance
Of one after another of the nearer
stars the cosmic engineers are gradu-
ally reducing to order the fields of
space within the solar systems i'm
mediate neighborhood, extending for
few trillion miles in all directions
They have found the suns family has
on neighbor much closer than the
average distance between stars in this
part of the heavens. The star Alpha
Centuri in the southern heavens is
only about 2600O 900 100000 miles
away. 1t is about the size and lumin-
osity of the sun and its light, moving
through space at the rate of about
10000 miles second requires only
about four years and three months to
reach the earth.
Unusually CLOSE Neighbor."
If there were uniform distribution
OF population" in this corner of the
universe the sun would have no
nelghbor within approximately twice
the distance, or about $4,000, ,000,-
100000 miles, distance which light
would traverse in about seven years
and six months
Looking at the sky on Winter
night the stars appear closely packed
together, because most of them are so
distant. But there is reason to be-
lleve that the heavens around the sun
are fairly representative of the cosmos
es a whole and there the Carnegie
surveyors find one heavenly body for
approximately each 440 cubic light
years
The distance Of star is found by
measuring its position on two photo
graphs taken six months apart. Dur
ing this interval the earth moves
from one side of its orbit to the
other. thus causing the star to shift
It's positltion slightly with respect to
fainter and more distant stars used
as reference points. If the dates of
the two observations are chosen COR
rectly, the change in the direction in
which star is seen will be twice the
angle subtended by the radius of the
earths orbit as seen from the star.
A parallax of one second of arc is
the angle made at the eye by length
OF three-tenths of an inch a mile
away. This corresponds to a distance
of one parsec. or 1926 light years.
which in turn is about 200 600 times
the radius of the earth's orbit, of
100000, 100000 miles.
BY this method fairly accurate
measurements can be made down to
parallaxes of hundreth of a second
OF arc. Considerable dependence can
be placed even on measurements
which show a displacement of little
more than a thousandth of a second
OF arc. This represents a distance of
approximately 1900, 900 900 900. 900
miles. or pit light years On the larg-
est scale photographs which can be
taken of the heavens the displace-
ment which must be measured is less
than one -thouSandth of an inch.
The parallax method Is the Oldest |
known for surveying the heavens and !
was used with the earliest telescopes,
but became practicable for large scale |
measurements only with the perfec- |
ton of Photographic apparatus. 1t
has little value for stars outside of the
suns immediate neighborhood be-
cause. even at small distances, stars
OF low luminosity become SO faint
that they cannot be distinguished
from the multitude of other faint
stars Faint stars predominant, the
survey shows, in the region close to
the sun and probably in the heavens
as whole.
Great Distances ApprOximated.
Once beyond the local system of
stars, distances can be approximated |
but not measured with much accur-
acy, by the spectroscopic method the |
mechanism OF which also will be ex-
hibited. When the apparent bright |
ness of star and its intrinsic bright
ness are known the distance can be |
calculated
Studies OF stellar spectra prove that !
the intensities of certain spectral lines |
vary progressively with the intrinsic |
brightness of the stars themselves. !
This method. under favorable condi- !
tions, can be used to measure dis |
tances up to about 100000 OOO,-
100000 miles, or 5000. light years, |
more than 10 times the distance over
which the trigonometric method can |
be applied. More than 1000 spec !
troscopic determinations of star dis |
fannp hQNa honfa Ia L,,rt thn