Sky & Telescope - USA (2019-11)

pMOTTLED STAR This surface map of the star Zeta Andromedae

reveals a giant spot at the star’s north pole (top) and multiple spots at

lower latitudes. Over time, the polar spot persisted, while the others

proved transient. Below: Comparison of various sources’ angular sizes

and instrument resolutions.

skyandtelescope.com • NOVEMBER 2019 25

0.01 mas 0.1 mas 1 mas 0.01 ̋ 0.1 ̋ 1 ̋ 10 ̋ 1 ́ 10 ́ 1°

Sgr A*

shadow Zeta Andromedae Betelgeuse

CHARA VLTI

(at 130-m

separation)

GTC VLT singletelescope

HST

4-inch

telescope

Naked

eye

Ceres

Angular size of object

Instrument

resolution

Jupiter at

opposition

Crab

Nebula

Full

Moon

Temperature (K)

270° 90°

3600 3800 4000 4200 4400 4600

0°

and for astronomers it’s a window into what makes the star

tick. Churning bubbles of plasma provide clues about how

energy percolates into space. Dark spots trace out magnetic

activity generated by swirling electrical currents far below.

But stars keep their surfaces well hidden from us. Look at

a star through a telescope, and you see just a brighter point

of light. A telescope’s ability to distinguish features on some

remote object is set by the diameter of its main lens or mirror

(as well as the wavelengths of light it collects). The wider the

telescope, the more detail it reveals.

The trouble with stars is that they are too far away to be

resolved as anything but pinpricks. Seeing details is akin to

discerning a mattress on the surface of the Moon. The larg-

est optical telescope on Earth, the Gran Telescopio Canarias

SU

RF

AC

E^ M

AP

:^ R

AC

HA

EL^

RO

ET

TE

NB

AC

HE

R;^

SIZ

E^ A

ND

RE

SO

LU

TIO

N^ C

OM

PA

RIS

ON

:^ G

RE

GG

DI

ND

ER

MA

N^ /

S&

T

mas = milliarcsecond