Sky & Telescope - May 2020 - 4

SPECTRUM

by Peter Tyson

A Vote for Motes

The Essential Guide to Astronomy
Founded in 1941 by Charles A. Federer, Jr.
and Helen Spence Federer

IN SKY & TELESCOPE and in the community we serve, we often

Editor in Chief

Editorial Correspondence
(including permissions, partnerships, and content
licensing): Sky & Telescope, One Alewife Center,
Cambridge, MA 02140, USA. Phone: 617-5006793. E-mail: editors@skyandtelescope.org.
Website: skyandtelescope.org. Unsolicited
proposals, manuscripts, photographs, and
electronic images are welcome, but a stamped,
self-addressed envelope must be provided to
guarantee their return; see our guidelines for
contributors at skyandtelescope.org.

M AY 2 0 2 0 * S K Y & T E L E S C O P E

Advertising Information:
Tim Allen: 773-551-0397
E-mail: tallen@skyandtelescope.org
Web: skyandtelescope.org/advertising
Customer Service: Magazine customer
service and change-of-address notices:
skyandtelescope@emailcustomerservice.com
Phone toll-free U.S. and Canada: 800-253-0245
Outside the U.S. and Canada: 386-597-4387

EDITORIAL
Editor in Chief Peter Tyson
Senior Editors J. Kelly Beatty, Alan M. MacRobert
Science Editor Camille M. Carlisle
News Editor Monica Young
Associate Editor Sean Walker
Observing Editor Diana Hannikainen
Consulting Editor Gary Seronik
Editorial Assistant Sabrina Garvin
Senior Contributing Editors
Dennis di Cicco, Robert Naeye, Roger W. Sinnott
Contributing Editors
Howard Banich, Jim Bell, Trudy Bell, John E. Bortle,
Greg Bryant, Thomas A. Dobbins, Alan Dyer,
Tom Field, Tony Flanders, Ted Forte, Sue French,
Steve Gottlieb, David Grinspoon, Shannon Hall,
Ken Hewitt-White, Johnny Horne, Bob King,
Emily Lakdawalla, Rod Mollise, James Mullaney,
Donald W. Olson, Jerry Oltion, Joe Rao, Dean Regas,
Fred Schaaf, Govert Schilling, William Sheehan,
Mike Simmons, Mathew Wedel, Alan Whitman,
Charles A. Wood
Contributing Photographers
P. K. Chen, Akira Fujii, Robert Gendler,
Babak Tafreshi

A R T, D E S I G N & D I G I T A L
Art Director Terri Dubé
Illustration Director Gregg Dinderman
Illustrator Leah Tiscione
Webmaster Scilla Bennett
ADVERTISING
Advertising Sales Director Tim Allen

N ASA / ESA / HUBBLE HERITAG E TE A M (STSCI / AUR A)

toss around a certain astronomical term without thinking much
about it: dust. "Interstellar clouds of gas and dust," we'll say, or
"Check out that galaxy's dust lane, will you?" But in light of several articles in this issue, it's worth pausing to consider just what
astronomers mean by dust and how much we owe to it.
Cosmic dust comes in many sizes and ﬂavors. It all starts with those chemical building blocks, elements such as carbon or silicon. When interstellar particles with such elements at their core grow too large to be called molecules -
when, through collision, aggregation, and other processes, they reach umpteen
millions of atoms - we start calling them dust. We're still talking tiny: only up
to micron-size, or one-millionth of a meter (0.000039 inch). By comparison, a
droplet of water in fog is about 10 microns in diameter; a strand of spider silk is
3 to 8 microns across.
Bits of cosmic dust stick together, of course. When a particle reaches a size
you could see if it were perched on your ﬁngertip, we often label it a pebble.
After all, these centimeter-size grains are, in essence,
minuscule rocks. That's what you see shooting out
of the asteroid Bennu in the photo on page 20. In
her article on protoplanetary disks (page 34), Megan
Ansdell describes how astronomers use the ALMA
radio-telescope array to study pebble-size dust and
the SPHERE instrument to analyze micron-size dust,
which she likens to smoke.
Hubble close-up of the
Then there's the dust that grows so large you could
Horsehead Nebula
stand on it (see cover). It's arresting to think that our
very Earth, as all planets, arose from motes: particles to pebbles to planetesimal
to planet. Did our vast world really begin as mere specks? It had to start somewhere, and just as we might follow a mighty river back to its source in a hillside
spring, could we, if we had a time machine, trace our globe back to grains?
Back to Earth: As all astronomers know, dust enables us to see and learn
from wonders of the night sky that might otherwise remain invisible to us.
Supernova remnants. Meteor showers. The zodiacal light. Star-blocking clouds
like the Coalsack. Star-forming regions like the Orion Nebula. Dust storms on
Mars. The tails of comets (page 41). Want to add your own examples?
Then there's the fact that we're all stardust, as Carl Sagan reminded us. Every
atom in our bodies, save for atoms of hydrogen and one or two of the other
lightest elements, was forged in stars billions of years ago.
So let's give a nod to cosmic dust. It's us.

AMERICAN ASTRONOMICAL
SOCIETY
Executive Ofﬁcer / CEO, AAS Sky Publishing, LLC
Kevin B. Marvel
President Megan Donahue, Michigan State University
President Elect Paula Szkody, University of Washington
Senior Vice-President Michael Strauss, Princeton
University
Second Vice-President Joan Schmelz, Universities
Space Research Association
Third Vice-President Geoffrey C. Clayton, Louisiana
State University
Treasurer Nancy D. Morrison, University of Toledo (ret.)
Secretary Alice K. B. Monet, U.S. Naval Observatory (ret.)
Trustees Marcel Agüeros, Columbia University;
Kelle Cruz, CUNY Hunter College & AMNH;
Kelsey Johnson, University of Virginia
At-Large Trustees Tereasa Brainerd, Boston University;
Hannah Jang-Condell, University of Wyoming

Visit shopatsky.com
Shop at Sky customer service:
shopatsky.com/help

Newsstand and Retail Distribution:
Marisa Wojcik, mwojcik@i-cmg.com
Comag Marketing Group

Subscription Rates:
U.S. and possessions: $54.95 per year (12 issues)
Canada: $69.95 (including GST)
All other countries: $84.95, by expedited delivery
All prices are in U.S. dollars.

The following are registered trademarks of
AAS Sky Publishing, LLC: Sky & Telescope and
logo, Sky and Telescope, The Essential Guide to
Astronomy, Skyline, Sky Publications, skyandtelescope.org, skypub.org, SkyWatch, Scanning the
Skies, Night Sky, SkyWeek, and ESSCO.

http://www.shopatsky.com http://www.shopatsky.com/help http://www.skyandtelescope.org/advertising http://www.skyandtelescope.org http://www.skyandtelescope.org http://www.lescope.org http://www.skypub.org http://www.skyandtelescope.org

Sky & Telescope - May 2020

Table of Contents for the Digital Edition of Sky & Telescope - May 2020