Sky & Telescope - March 2025 - 6

PATRICK MYERS/ NPS
FROM OUR READERS
Tour the Sky
Thank you so much for all the information
you provide to help us enjoy
the night sky. A special thanks to the
October episode of Kelly Beatty's Sky
Tour astronomy podcast (https://is.gd/
SKYpodcast), which outlined how
we could locate Comet TsuchinshanATLAS
(C/2023 A3). We were able to
see the comet both with the telescope
and our naked eyes. It was a very
exciting evening. Without the specifics
that Kelly Beatty provided, I'm not
sure we would have found it.
Mike Bliziotes and Iris Walling
Lake Oswego, Oregon
Comet Tsuchinshan-ATLAS cuts across the
sky above Great Sand Dunes National Park
on October 12th.
Monica Young replies: When you
measure the redshift of a relatively
nearby object, you measure that object's
physical velocity through space. One example
of this given in the October Beginner's
Space column is the motion of binary stars
in mutual orbit. Assuming their orbit isn't
exactly in the plane of the sky, then one star
will be physically moving in Earth's direction
by some amount as the other moves away,
and we can measure that motion by the
redshift in the stars' spectral lines.
However, when it comes to cosmologi "
cal
redshift, you're no longer measuring the
object's motion away from you in terms of
its velocity through space. Instead, you're
measuring the velocity an object appears to
be moving away from us due to the expansion
of space itself. That relationship is linear
for relatively nearby galaxies (as shown for
redshift-lookback time on page 76 of the
October issue). However, it quickly becomes
nonlinear at greater distances because the
universe hasn't expanded at a constant rate
over its history.
While I don't have a redshift-velocity
graph handy for you, I think you might find
this Physics Stack Exchange forum interesting:
https://is.gd/redshift_velocity.
Seeing Red
As a non-astronomer but curious lover
of astronomy, I would like to thank
Monica Young for the beautiful article
" What Is Redshift " (S&T: Oct. 2024,
p. 76). It was concise, easy to read, and
wonderfully explained.
However, her statement that " galaxies
aren't flying through space - it's
the space between us and them that's
expanding " is quite difficult to understand.
Could she clarify this statement?
Einar A. Berger
São Leopoldo County, Brazil
Monica Young replies: These
concepts are not intuitive and can
be quite difficult to wrap one's head around.
The typical analogy given for the expansion
of space is a rising loaf of raisin bread. The
raisins are the galaxies, and the dough is
the space between them. As the loaf rises,
the raisins appear to move away from one
"
6 MARCH 2025 * SKY & TELESCOPE
another. However, the raisins themselves
are not moving through the dough - it's the
dough between them that's expanding.
I was puzzled while reading the redshift
explanation in the October issue.
Young says " But really, redshift is a
measure of velocity. Yes, velocity . . . "
If I'm applying the redshift equation
correctly, a redshift of 1 is reached at
60% of the speed of light.
At much higher velocities, an
increase of 1 mph would also add 1
to the redshift. And even closer to the
speed of light, an additional inch-perhour
would add 1 to the redshift as
well, and so on.
Redshift must vary with velocity,
but not directly. Help readers like me
understand how and why the redshift
varies as it does.
Richard High
Via email
Defending Home Base
Emily Lakdawalla's " Asteroid Impact:
The Aftermath " (S&T: Oct. 2024, p. 14)
is both thrilling and informative. However,
there is mention of one matter in
passing, which has been the fly in the
ointment of many an otherwise excellent
account of planetary defense. " At
160 m, the moon Dimorphos was . . .
large enough to represent the class of
asteroid that poses the greatest impact
risk, because astronomers predict that
almost half of the objects in this size
range remain undiscovered. "
The quoted statement implies
that an object that could only wreak
regional damage presents a greater risk
than an object the size of the one that
wiped out most life on Earth 66 million
years ago. Why? Because we have
already discovered most of the latter
and know that none of them is on a
trajectory to do so in the next century.
But this perspective overlooks the
fact that risk is not just the probability
https://www.is.gd/SKYpodcast https://www.is.gd/SKYpodcast https://www.is.gd/redshift_velocity
Sky & Telescope - March 2025

Table of Contents for the Digital Edition of Sky & Telescope - March 2025
