WWW.ASTRONOMY.COM 67between the particles, the
angular momentum is redis-
tributed and they settle into a
less collision-prone orientation
— a disk.
This mathematical truth is
the inspiration for the artist
drawings. However, as you so
aptly point out, the picture
does not capture the reality.
Sgr A and other supermassive
black holes lie at the centers of
galaxies and are not accreting
matter from a single source.
The influx of material is lumpy
and can be affected by colli-
sions with dense matter like
stars, asteroids, or interstellar
gas. In fact, when we’re making
calculations of the accretion
rate of Sgr A, we use both disk
and spherical accretion models
(which is more like you’re sug-
gesting, where accretion hap-
pens on multiple planes) and
compare them to observations.
Given that we can tell Sgr A is
surrounded by its fuel source
but not uniformly enveloped by
it, the truth lies between the
two models. However, Sgr A is
currently in a low-luminosity
state: It’s not accreting much,
so it’s not giving us much light
to work with.
The stars orbiting Sgr A do
orbit on a variety of planes.
Their motion (and the motion
of all stars in the galaxy) is not
dominated by the black hole,
but rather affected by their
neighboring stars. They also
may retain some of the motion
of the gas clouds that formed
them. These stars blast right
through the accretion disk in
the near part of their orbit, and
when they do, part of their gas
shells interact with accreting
material. We’re actually using
the stars orbiting Sgr A to
better understand how accre-
tion works in that system.
Valerie Mikles
National Oceanic and Atmospheric
Administration Contractor,
Quality Assurance, I.M. Systems Group,
College Park, Maryland
Q: WHAT FREQUENCIES
DOES NASA USE TO
COMMUNICATE WITH
SPACECRAFT? ARE THESE
THE SAME FREQUENCIES
USED TO SEND BACK TV
SIGNALS FROM THE MOON?
HOW ARE THE SIGNALS
AFFECTED BY LOSS AND
DEGRADATION?
AnonymousA: The Deep Space Network,
or DSN, is NASA’s interna-
tional array of giant radio
antennas that supports inter-
planetary spacecraft missions,
plus a few that orbit Earth. The
DSN also provides radar and
radio astronomy observations
that improve our understand-
ing of the solar system and the
larger universe.
The DSN is operated by
NASA’s Jet Propulsion
Laboratory, which also oper-
ates many of the agency’s inter-
planetary robotic space
missions. The DSN consists of
three facilities spaced equidis-
tant from each other —
approximately 120° apart in
longitude — around the world.
These sites are near Barstow,
California; near Madrid, Spain;
and near Canberra, Australia.
The strategic placement of
these sites permits constant
communication with space-
craft as our planet rotates.
Before a distant spacecraft
sinks below the horizon at one
DSN site, another site can pick
up the signal and carry on
communicating.
One of the frequencies we
use is the same NASA used to
beam TV signals from the
Moon. That’s called S band
(2–4 GHz), but we also use X
band (8–12 GHz) and the Ka
band (27–40 GHz). The earliest
widely used DSN frequency
was S band. Later added were
X band and Ka band, which
can send much more data
per second. In the future, the
DSN will support opticalcommunication in the infrared
frequency band.
These communications sig-
nals can be extremely difficult
to receive, due to the relatively
low power used by spacecraft
and the great distances the sig-
nals must travel. The received
power drops by the square of the
distance traveled. The signal can
also become degraded by many
sources, such as solar system
background noise or interfer-
ence, interference from passing
through either Earth’s or
another planet’s atmosphere,
and by noise introduced by the
receiving system. To compen-
sate, we use very large antennas
— the largest is 230 feet
(70 meters) in diameter —
with precise antenna pointing,
cryogenically cooled low-noise
amplifiers, sensitive receivers,
and computer code designed for
error detection and correction.
Michael Levesque
Deep Space Network,
Jet Propulsion Laboratory,
Pasadena, CaliforniaQ: “BREAKFAST TIME” ON
P. 9 O F T H E N O V E MB E R 2 018
ISSUE SAYS OUR GALAXY
COLLIDED WITH ANOTHER
8 TO 10 BILLION YEARS AGO.
BUT I’VE ALWAYS HEARD
WE ARE ONLY 4.6 BILLION
YEARS OLD.
John Bauernhuber
Whiting, New JerseyA: Our solar system — includ-
ing the Sun and the planets
— is roughly 4.6 billion years
old, which is why you’ve heard
that number. But the Sun isjust one star in the Milky Way
Galaxy. There are several ways
to calculate the age of the
Milky Way, but one of the most
common is determining the
ages of its oldest stars. Many
of these are found in globular
clusters, which are the galaxy’s
oldest star clusters; some con-
tain stars more than 13 billion
years old. This already gives a
clue to the age of the galaxy:
at least 13 billion years.
By combining precise mea-
surements with models of how
stars fuse and create elements,
astronomers have determined
that the first stars in the Milky
Way likely began forming by
about 200 million years after
the Big Bang. Based on the
age of the universe — about
13.7 billion years — that makes
our galaxy roughly 13.6 billion
years old. Given this age, it’s
completely possible for colli-
sions with other galaxies to
have occurred 8 billion to
10 billion years ago, before
our 4.6 billion-year-old solar
system had begun forming.
Alison Klesman
Associate EditorSend 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.Stars such as these, located in the 13.4 billion-year-old globular cluster
NGC 6397, have helped astronomers measure more precisely the age of
the Milky Way. ESOA228 A2111