Scientific American - USA (2020-04)

32 Scientific American, April 2020

Galactic centerGalactic center

SunSun

Quadrant 2

Quadrant 1

Quadrant 3

Quadrant 4

3 kpc^ a

rm

3 kpc^ a

rm

Perseu

s^ arm

Perseu

s^ arm

Norma

• Outer arm

Norma

• Outer arm
  Scu
  tum


• Ce
  ntaur
  us–OSC ar
  m

Scu

tum

• Ce
  ntaur
  us–OSC ar
  m

Sag

itta

riu

s–C

arin

a (^) arm
Sag
itta
riu
s–C
arin
a (^) arm
Loc
al^ ar
m
Loc
al^ ar
m
Distance from the sun
to galaxy’s center:
8,150 ± 150 parsecs
(or 26,600 light-years)
Each dot represents
a star-forming region
measured with the
trigonometric parallax
method: Measurement
error ranges are low for
the larger dots and higher
for the smaller dots,
Star-forming regions
are color-coded according
to the spiral arm with
which they were
previously associated.
Each dot represents
a star-forming region
measured with the
trigonometric parallax
method: Measurement
error ranges are low for
the larger dots and higher
for the smaller dots.
Star-forming regions
are color-coded according
to the spiral arm with
which they were
previously associated.
Galactic rotation
Most distant direct
measurement to a
star-forming region
×
Illustration by Elena Hartley (parallax diagram)
TRIGONOMETRIC
PARALLAX
Astronomers measure
interstellar distances by
watching for the offset, or
parallax angle, of a star’s
position when viewed from
opposing sides of Earth’s
orbit. The closer a star is to
Earth, the larger its parallax.
Paired with the known
Earth-sun distance, a star’s
parallax lets astronomers
use basic trigonometry to
calculate that star’s distance
from Earth.
A Whirlpool of Stars
Based on distance measurements gathered using thousands of hours of radio-telescope observations
( below ), this is the best bird’s-eye view of our galaxy’s structure ever assembled ( right ). The data reveal
four major spiral arms around a central, barred bulge of stars. Our sun—which astronomers treat as
the nexus of a quadrant mapping system—lies in a 212-million-year orbit around the galactic center,
near a smaller spiral-arm fragment ( in blue ). Future studies using radio telescopes in Earth’s Southern
Hemisphere could unveil additional structures in the mostly unmapped fourth quadrant.
Parallax angle
Position of Earth
in December (blue) Position in sky
Star of interest
Background stars
Position of Earth
in June (purple)
Sun
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