Sky & Telescope - December 2023 - 4

SPECTRUM by Peter Tyson
Great Lengths
IN HIS COVER STORY on page 12, Ken Croswell mentions in
passing the Andromeda Galaxy's proximity to us - " it's only 2.5
million light-years from Earth, " he writes. Only 2.5 million lightyears?!
One light-year is about 9.5 million million kilometers (6
trillion miles). Multiply that by 2,500,000 and you get, well, brain
fog. In what universe could 2.5 million light-years possibly be considered close?
Ours. Apropos of that, it's worth reminding ourselves now and then just how
staggering the distances to even " nearby " celestial objects truly are. And how
quickly our ability to envisage those gaps with any verisimilitude falters.
To get some sense of the distance to Andromeda, let's start small. The distance
to the nearest star beyond the Sun, Proxima Centauri, is about 4.2 lightyears.
(Or about 1.3 parsecs - see Beginner's Space on page 74 for more on that
term and why professional astronomers prefer it.) Sounds reasonably nearby,
right? Actually, that's nearly 40,000,000,000,000 km away, or close to 267,000
times the average Earth-Sun distance. If the Voyager 1 spacecraft, which is
speeding away from our solar system at about 17
km/s, were heading to Proxima instead of just out
into interstellar space, it would take over 74,000
years to reach the star. We'd have to multiply that
eon of time by 2.5 million to get the years Voyager
would need to attain just the next galaxy over.
Given how far away Andromeda is, it's amazing
Our closest major neighbor:
the Andromeda Galaxy
that we can see it with the naked eye from a darksky
site. But with large telescopes, we can observe
galaxies that lie over a billion light-years away. In
his article on page 18, amateur Jimi Lowrey describes the 12 most distant galaxies
in the NGC and IC catalogs that were discovered visually (most, remarkably
enough, in the 1800s). Observe these yourself, as Lowrey has, and you join an
elite bunch - the billion-light-year club. Forget trying to grasp the stretch that
photons, zipping along at roughly 300,000 km/s, cover in a billion years.
Then there are the galaxies the James Webb Space Telescope is detecting.
Light left some of those well over 13 billion years ago, within a few hundred
million years of the Big Bang. With the universe's expansion, it's hard to say
just how far such early galaxies lie, but it's as mind-boggling as the notion that
in all that insanely vast distance, nothing blocks that light from reaching JWST.
Contemplating such temporal and spatial scales can be draining. Better to
just accept that the light we see from Andromeda, for instance, left that galaxy
2.5 million years ago and traveled a ginormous distance to
reach us. Only then can one say, with equanimity, that it's
" only 2.5 million light-years from Earth. "
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Sky & Telescope - December 2023

Table of Contents for the Digital Edition of Sky & Telescope - December 2023

Contents
Sky & Telescope - December 2023 - Cover1
Sky & Telescope - December 2023 - Cover2
Sky & Telescope - December 2023 - 1
Sky & Telescope - December 2023 - Contents
Sky & Telescope - December 2023 - 3
Sky & Telescope - December 2023 - 4
Sky & Telescope - December 2023 - 5
Sky & Telescope - December 2023 - 6
Sky & Telescope - December 2023 - 7
Sky & Telescope - December 2023 - 8
Sky & Telescope - December 2023 - 9
Sky & Telescope - December 2023 - 10
Sky & Telescope - December 2023 - 11
Sky & Telescope - December 2023 - 12
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Sky & Telescope - December 2023 - Cover3
Sky & Telescope - December 2023 - Cover4
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