>ky Reporter 
covered. Only twenty of the nearby 
stars are visible to the eye without 
a telescope since fully 90 percent of 
all nearby stars are very dim dwarfs. 
Of the twenty brightest nearby stars, 
we have excellent knowledge because 
their brightness permits application of 
all of the techniques at our disposal. 
For example, the angular diameter 
of Vega has been shown by means 
of interferometry measurement to 
have a value of 0.0037 arc seconds, 
about three times that which the sun 
would have if seen from the same dis- 
tance. Interferometry is a technique 
used for the measurement of wave- 
lengths of light in order to determine 
the distances between stars and the 
diameters of stars. Interferometry 
takes advantage of the wave nature 
of light. That is, light waves from two 
adjacent points, say, two stars or two 
points at opposite ends of a star’s di- 
ameter, will interfere with one an- 
other, causing light and dark fringes 
to appear in the image when an in- 
terferometer is properly adjusted. The 
presence or the absence of fringes as 
the instrument is adjusted provides 
a measurement of the diameter of the 
star or reveals the presence of a com- 
panion star. A stellar companion to 
Vega of similar brightness is easily 
excluded by such measurements, 
which also show that Vega has no 
fainter companions down to approxi- 
mately one-tenth of its brightness. 
The existence of a companion of 
less than this brightness would be re- 
vealed by a perturbation. A pertur- 
bation is a wobble in the motion of 
a star across the sky. Because binary 
stars revolve in orbits around their 
common center of mass, a faint com- 
panion of one-tenth the mass of Vega 
would cause Vega to wobble with a 
motion of one-tenth of the apparent 
distance between the two stars and 
with a period the same as their mutual 
orbital period. Because we can detect 
wobbles as small as 0.01 arc second 
by measuring photographs of bright 
stars taken over the years, a faint com- 
panion of one-tenth Vega’s mass would 
be detectable if its distance from Vega 
were larger than 0.1 arc second. That 
is equivalent to 0.8 astronomical units 
at Vega’s distance from the earth (one 
astronomical unit equals the distance 
between the sun and the earth). If 
the companion had a mass one-hun- 
dredth that of Vega’s, its separation 
would have to be larger than 1.0 arc 
second, or eight astronomical units, 
to be detectable, and its orbital period 
would be about fourteen years. An 
orbital period longer than this presents 
an observing problem because at least 
two orbital periods of observation are 
needed to confirm the perturbation. 
For longer periods the companion 
would at times be outside the fuzzy 
image of Vega, and direct detection 
would be possible by visual exami- 
nation or by photography. 
In addition to the wobbling motion 
caused by a stellar companion, there 
exists the changing orbital velocity of 
each member of the pair. The ratio 
of the velocities of the two members 
of the binary star, orbiting their com- 
mon center of mass, will be the inverse 
of the ratio of their respective masses, 
with the less massive star having the 
greater velocity. If the mass of one 
of the stars is one-tenth that of the 
other, then the less massive star will 
have an orbital velocity ten times that 
of the more massive one. If an as- 
tronomer obtains several spectra of 
r 
Share the Thrills . 
ol Exploring t A 
Outer space; 
& XI 
W "V 
\ s 359 95 
f.o.b. 
Hartford, 
Conn. 
$15 Crating 
Charge 
Now it’s easy to join the thousands of serious 
amateurs who have discovered the excitement 
of exploring our mysterious universe. Your en- 
joyment begins right from the start, yet the 
challenges and rewards go on for years! And 
it’s a hobby that can be shared at modest cost. 
Choose From Many DYNASCOPES® 
Picking a telescope to fit your needs and your 
pocketbook is simple when you select a 
DYNASCOPE — the same instruments used by 
more than 150 schools, colleges and observ- 
atories— and your satisfaction is guaranteed 
by a full-refund warranty. 
FASCINATING GUIDE 
YOURS FREE! 
Read these valuable facts be- 
fore buying any telescope. Mail 
coupon or postcard for your 
complimentary copy of this 
helpful guide. 
Criterion Manufacturing Co. 
620 Oakwood Ave , W Hartford. Conn 06110 
® TM Registered U.S. Pot. Office 
I 
CRITERION MANUFACTURING CO . Dept NH 104 
! 620 Oakwood Avenue , W Hartford. Conn 06110 
j Please send your free Telescope Guide 
| Name 
| Address 
I City State i 
I I 
Enter 
the world 
of Questers 
nature 
tours . 
When you travel with Questers, you have the 
advantage of our knowledge of the travel world 
And our experience of the natural world We are 
travel professionals And our only tour program is 
Worldwide Nature Tours 
Under the leadership of an accompanying 
naturalist, we search out the plants and animals 
birds and flowers rain forests, mountains, and 
tundra seashores, lakes, and swamps of the 
regions we explore At the same time, we include 
the more usual attractions in touring — the cities 
archaeological sites, and people 
The current Directory of Worldwide Nature 
Tours describes 29 tours varying from 9 to 36 days 
and going to virtually every part of the world In- 
cluded are The Amazon. Peru. Patagonia. Galapa- 
gos. Hawaii. Alaska. Death Valley. Everglades. 
Ladakh. Sri Lanka. Iceland, Scotland. Greece, 
Australia, and New Zealand Tour partiesare small 
the pace leisurely, and itineraries unusual 
Call or write Questers or see your Travel 
Agent today lor your free copy of the Directory of 
Worldwide Nature Tours 
QUESTERS 
Questers Tours & Travel, Inc. 
Dept NH 681, 257 Park Avenue South 
New York, N.Y. 10010 *(212) 673-3120 
7! 
