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34 Scientific American, April 2020 Illustration by Elena Hartley

that far surpasses that of any other current telescope at any
wavelength. Researchers must observe simultaneously with all
the telescopes and synchronize the data recorded on computer
disks at each site with the best atomic clocks. They then ship the
recorded data to a special computer that cross-correlates the sig-
nals among the telescopes. After some calibrations, the result is
a digital image of what we would see if our eyes were sensitive to
radio waves and separated by almost the entire width of the
planet. Such imagery represents an incredible angular resolu-
tion of better than 0.001 second of arc (there are 3,600 seconds
of arc in one degree, and the entire celestial sphere is 360 de -
grees). By comparison, the human eye can resolve structures
separated by at best about 40 seconds of arc, and even the Hub-

ble Space Telescope can achieve a resolu-

tion of only about 0.04 second of arc.

With VLBI, we can measure the position

of a radio-bright star relative to back-

ground quasars (bright active black holes at

the centers of distant galaxies) with an ac-

curacy approaching 0.00001 second of arc.

Making this comparison allows us to survey

very great distances by observing the paral-

lax effect, whereby a nearby object seen

against a distant background will appear at

different positions when viewed from dif-

ferent vantage points. You can simulate this

effect by looking at your thumb at arm’s

length and alternately closing your left eye

and your right eye. Our eyes are separated

by several centimeters, so a thumb at an

arm’s length will appear to shift by an angle

of about six degrees when viewed through

one eye and then the other. If one knows

the separation of the vantage points and

the observed angular shifts, it is easy to cal-

culate the distance. This is the same princi-

ple that surveyors use to map cities.

Ideally, to map spiral structure, astron-

omers should observe young massive stars.

These short-lived stars are often associat-

ed with intense bouts of stellar formation

within spiral arms and are so hot that they

ionize the gas around them, causing it to

glow in blue light and creating a spiral-

arm-tracing beacon visible across the cos-

mos. But trapped within the Milky Way’s

dusty disk, we cannot easily observe such

stars throughout our own galaxy. Fortu-

nately, molecules of water and methyl al-

cohol just outside the regions ionized by

these hot stars can be very bright radio

sources because they emit natural “maser”

emission that is barely attenuated by ga-

lactic dust. The word “maser” is an acro-

nym for “microwave amplification by

stimulated emission of radiation,” and

this radiation is the radio analogue of an

optical-light laser. In astrophysical set-

tings, maser emission comes from solar

system–scale clouds of gas whose mass is comparable to that of

Jupiter. What we see in radio images are extremely bright “spots”

that are nearly ideal targets for parallax measurements.

THE UPDATED PICTURE

Between the Bessel survey and the VERA project, astronomers

have amassed about 200 parallax-based distance measurements

for young hot stars across large regions of the Milky Way. These

data, which give us good coverage of about one third of the Milky

Way, reveal four arms that are continuous over great distances.

The map also shows that the sun is very close to a fifth feature

called the Local arm, which seems to be an isolated fragment of a

spiral arm. Previously this fragment had been called the Orion or

1 arcsecond

Hubble

resolution

0.04

arcsecond

VLBI

resolution

0.001

arcsecond

1 degree

Circle

(360 degrees)

Area

enlarged

below

1 degree

1 arcsecond

Eagle Eyes on the Sky

Measuring the minuscule parallax angle for star-forming regions on the other side

of the galaxy requires extreme angular resolution currently achievable only through

precisely combining simultaneous observations from multiple radio telescopes

across the globe. This illustration reveals the power of the

technique, known as Very Long Baseline Interferometry,

which can reach resolutions about 40 times better than

the sharpest images from the Hubble Space Telescope.

© 2020 Scientific American