Astronomy

WWW.ASTRONOMY.COM 45

Measuring the motions of
the stars in a given globular
cluster is beyond the capabili-
ties of small telescopes. The
European Space Agency’s Gaia
observatory is currently mea-
suring the positions and veloci-
ties of thousands of star
clusters in our galaxy with
unprecedented accuracy. This
data, coupled with decades-
long observational campaigns
by the Hubble Space Telescope
and other surveys (e.g., the
Gaia-ESO survey, conducted
with the Very Large Telescope
in Chile), will soon enable us to
study the distribution of globu-
lar clusters’ individual stars in
position and velocity space.
Stay tuned for the many inter-
esting discoveries that will
surely emerge!
Anna Lisa Varri
Marie Skłodowska-Curie Research
Fellow, Institute for Astronomy,
University of Edinburgh, Scotland

Q: HOW FAR CAN A PAIR
OF STARS BE SEPARATED
AND STILL MAINTAIN
A STABLE ORBIT AROUND
EACH OTHER?
Robert Bobo
McKenzie, Tennessee

A: There are two issues here:
First, how far apart can two
stars form and remain bound?
And second, once formed, how
far apart can they survive as a
pair? There are a number of
ideas being debated for how
very wide binaries can form.
The Gaia satellite will, with its
precision measurements, iden-
tify many thousands of ultra-
wide binaries that we can
study, and their properties will
help astronomers to determine
the most likely formation
mechanism behind them.
The second issue is better
understood. If the two stars in
a very wide binary were the
only stars in the universe, the
pairing could survive forever.

But the universe is a busy

place, and our Milky Way

alone contains more than

100 billion stars moving

around its center. A very wide

binary has a very weak gravita-

tional bond, so if another star

passes near the binary, the pair

can break apart. Eighty years

ago, Armenian astronomer

Victor Ambartsumian calcu-

lated that a wide binary rarely

breaks apart as the result of a

single close encounter with

another star, but rather

through numerous distant pas-

sages that each gently tug on

the binary until it impercepti-

bly passes from being bound to

being unbound.

For a very long time, the two

stars will still travel together

through space until eventually

they part ways. An ultrawide

binary with a separation of 0.5

parsec (1.6 light-years) is statis-

tically likely to break up within

just 100 million years, while a

slightly less extreme binary

with separation around 0.1 pc

(0.3 light-years) may survive

for more than 1 billion years.

In summary, there is no

known fixed upper limit for

binary separations, but the

wider a binary is, the more

difficult it will be to find.

Bo Reipurth

Institute for Astronomy,

University of Hawaii

Q: HOW DO WE KNOW

WHAT STARS ARE BEHIND

DARK NEBULAE (IF THEY

ABSORB THE LIGHT FROM

BACKGROUND STARS)?

Margaret Lucero

Baldwinsville, New York

A: You’ve actually answered

your own question here. The

key word is “absorption.”

Interstellar space is not a

perfect vacuum. The mean

density in interstellar space in

our part of the galaxy is about

1 particle (mostly hydrogen

atoms) every 0.6 cubic inch

(10 cubic centimeters). Amid

this gas is a smattering of dust

grains; on average, 1 percent of

the interstellar mass is in the

form of solid silicate or carbon-

ate grains. Astronomer Robert

Trumpler showed in 1930 that

interstellar gas and dust

absorbs light at a typical rate of

2 magnitudes per kiloparsec

(3, 262 lig ht-yea rs).

All gas and dust in the inter-

stellar medium absorbs (or

scatters) light that passes

through it, resulting in the

extinction of light from back-

ground stars. Most of the

extinction at optical wave-

lengths is due to dust grains,

which have typical sizes of 0.01

to 1 micron. A photon with a

wavelength smaller than the

size of the dust grain can be

physically absorbed by the

grain, heating the grain up.

Longer wavelengths of light

can diffract (which causes the

light to bend or spread) around

the grain.

Optical light (about 400 to

700 nanometer wavelengths) is

strongly dimmed by these dust

grains. The amount of dim-

ming is proportional to the

cross sectional area of the

grains times the number of

grains in the line of sight. The

size of the grain affects the

wavelength of light that will be

dimmed. Extinction is stron-

gest at short wavelengths and

its effects decrease with

increasing wavelength.

William Herschel noted the

apparent lack of stars in certain

directions back in the 18th

century, and may be the dis-

coverer of dark nebulae

(a lt houg h he d id n’t u ndersta nd

what they were). These dark, or

absorption, nebulae are local-

ized enhancements in the den-

sity of the interstellar medium

by factors of 1,000 to 100,000.

A density enhancement of

10,000 means a 1-magnitude

attenuation occurs over a frac-

tion of a light-year.

To tell which stars are in the

background, look at their col-

ors. Stars whose light passes

through the absorption nebula

will be reddened — their blue

light will be preferentially

absorbed and scattered. Be care-

ful though — some stars are

naturally red. You really need to

have a spectrum (or spectral

type) that will tell you what the

colors of the star should be.

Frederick Walter

Professor of Physics and Astronomy,

Stony Brook University,

Stony Brook, New York

Send us your

questions

Send your astronomy

questions via email to

[email protected],

or write to Ask Astro,

P. O. Box 1612, Waukesha,

WI 53187. Be sure to tell us

your full name and where

you live. Unfortunately, we

cannot answer all questions

submitted.

Barnard 68 is a nearby molecular cloud with higher density than the

space around it. Left: The cloud absorbs light from the stars behind it at

optical (short) wavelengths, appearing dark. Right: At infrared (longer)

wavelengths, background stars become visible through the cloud. ESO