Sky & Telescope - USA (2019-11)

skyandtelescope.com • NOVEMBER 2019 27

retirement years (and with yet another

pattern of spots). The location is fi tting

— next to the center of the array sits

the Hooker telescope, where Michel-

son used the same principles to size up

Betelgeuse nearly a century ago.

A day’s drive from Mount Wilson,

in the pine forests of northern Ari-

zona, sits the Naval Precision Optical

Interferometer, the largest such facility,

with telescope separations as large as

432 meters. Keep heading east and just

outside Socorro, New Mexico, you’ll

run into the construction site for the

Magdalena Ridge Observatory Inter-

ferometer, where scientists are assembling an array of 10

linked telescopes. Finally, in Chile’s high-altitude Atacama

Desert lies the European Southern Observatory’s Very Large

Telescope Interferometer, which can link as many as four

telescopes up to 202 meters apart.

These observatories, with newfound abilities to simulta-

neously combine the light from all of their telescopes, are

in the vanguard of the optical and infrared interferometry

revolution. “We’re starting to really get into the astrophys-

ics of what’s happening on the surfaces of stars,” says Gail

Schaefer (CHARA).

tPI^1 GRUIS Each of the convection cells on

the surface of this aging star spans about 120

million km, grown to gigantic proportions due

to the bloated star’s weakened surface gravity.

Telescope 1 Baseline Telescope 2

2. Geometric delay
   Thhe light has to travel farther to
   reach one telescope than the re
   other. The distance depends on
   the star’s location in the sky.


3. Star
   Light wavess
   travel from
   the star to thhe
   telescopes.


4. Wavelengths aligned
   Having now traveled the same distance,
   the starlight is combined, creating
   interference fringes. The fringes’ amplitude
   encodes information about the star’s size,
   shape, and surface patterns.


5. Adjustable delay lines
   Astronomers send the starlight
   through delay lines whose
   lengths are continuously
   adjusted to compensate for
   the arrival delay.

Getting Personal with

the Stars

This ability is giving astronomers a

chance to see what’s in store for the

Sun when its hydrogen fuel runs out

nearly 7 billion years from now. They

can do this by fi nding dying stars with

comparable masses to the Sun. Zeta

Andromedae was one such candidate.

Another is a star designated Pi^1 Gruis,

about 530 light-years away in the con-

stellation Grus, the Crane. In 2017, researchers published this

red giant’s picture.

“I was disappointed that it was round,” says Claudia Pala-

dini (ESO), who led the project. Pi^1 Gruis is one step removed

from shedding its gas and metamorphizing into a lumines-

cent cloud known as a planetary nebula. But planetary nebu-

lae are not round: Their glowing tendrils often stretch out

more in one direction than another. And that transition is a

bit of a mystery. “We don’t know how to go from something

round to that,” says Paladini (S&T: Nov. 2014, p. 20).

While Pi^1 Gruis was mum about how planetary nebulae

Instead of snapping

photos of stars, an

interferometer records

the interference pattern

created by combining

the light received by

two or more telescopes.

But this process only

works if the light has

traveled the same

distance.

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HOW INTERFEROMETRY WORKS