Astronomy - September 2015

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WWW.ASTRONOMY.COM 35

to spiral inward toward the
moon (rings, by the way, are
not affected by this process).
Most planets, by contrast,
rotate quickly, so their slow-
orbiting moons gently and
safely spiral outward (the
exceptions, Mercury and
Venus, have no moons, likely
due to the same effect).
Matthew Tiscareno
Cornell University
Ithaca, New York

Q: SIRIUS, OUR BRIGHTEST
STAR, IS ORBITED BY A
WHITE DWARF — A DEAD
STAR’S CORE. WHERE IS THE
PLANETARY NEBULA THAT
WOULD HAVE FORMED
WHEN IT SHED ITS SKIN?
Mark Socha
Saginaw, Minnesota

A: The white dwarf Sirius B
probably would have had a plan-
etary nebula around it at one
point, long ago. The planetary
nebula phase of stellar evolution
is a short-lived phenomenon,
astronomically speaking, gener-
ally lasting only about 50,000
years. Sirius B is estimated to be
over 100 million years old, so
any planetary nebula it might
have had is long gone.
After a low-mass star (low-
er than 8 solar masses or so)
evolves past the hydrogen-
burning main sequence phase
of its lifetime, it goes through
several other phases, such as

the red giant phase and the

asymptotic giant branch phase.

Eventually, the outer layers are

shed in a slow wind, and the hot

core of the star is exposed. This

hot central core, in the process

of becoming a white dwarf, has

temperatures of around 100,000

kelvins. This is hot enough to

emit a lot of ultraviolet radia-

tion, which ionizes the blown-

off material, causing it to glow

and become a planetary nebula.

There are two factors that

cause this phase to be short-

lived. One is simply that the

material blown off the star

continues to drift away and

eventually dissipates into the

interstellar medium. Addition-

ally, as the central star cools

and becomes a white dwarf, its

luminosity drops to the point

where it would be inefficient in

ionizing the gas, even if it were

present.

Brian Williams

NASA Goddard Space Flight Center

Greenbelt, Maryland

Q: AT TENS, HUNDREDS, OR

THOUSANDS OF LIGHT-

YEARS FROM A STAR, HOW

DO YOU DISTINGUISH

BETWEEN EXOPLANETS,

SUNSPOTS, AND DWARF

BINARY COMPANIONS?

Bill Fox

Loganville, Georgia

A: While the items you list all

produce drops in starlight as

they cross their stellar host, an

exoplanet’s signature is often

well characterized.

Starspots can occupy the

same area as an exoplanet (or

larger or smaller) and dim star-

light as they rotate in and out

of view. However, a number of

things distinguish them from a

planet transit. Spots do not

have sharp edges or circular

shapes, but they do appear

with each stellar rotation and

have a period that can mimic a

planet. Spots, however, move in

both longitude and latitude as

the star rotates, causing their

period to change slightly after

a few rotations. Stars with

spots also tend to have larger

amplitude variations in their

light, which are usually greater

than a planet transit depth and

lead to a sine-like light curve.

Stellar companions differ

from exoplanet transits in a

number of ways as well. Planets

do not shine in the optical-

light wavelengths, so they cross

the star as black disks. Binary

companions are self-luminous

and cause not only a different

shape of eclipse (V compared

to a U shape) but also a color

change in the light. This hap-

pens because they block some

light and emit some as well,

changing the total light color

we see during their eclipse.

Exoplanet transits are usu-

ally U-shaped due to both their

blackness and the host stars’

limb darkening — where the

center of the star is brighter

than the edge. They also keep

the same period in most cases

(exceptions are exoplanets in

highly elliptical orbits). We

measure the planet’s size by the

depth of its transit, given that

the star’s radius is known.

Thus, while it is often the

case that astronomers can dis-

tinguish a planet transit from

starspots or eclipsing binary

companions, it is not perfect.

Smaller planets, more variable

stars, and less than optimal

light curve data can be confus-

ing and fool us at times.

Steve Howell

NASA Ames Research Center

Moffett Field, California

Send us your

questions

Send your astronomy

questions via email to

[email protected],

or write to Ask Astro,

P. O. Box 1612, Waukesha,

WI 53187. Be sure to tell us

your full name and where

you live. Unfortunately, we

cannot answer all questions

submitted.

Spurious spots

The light curve created by an exoplanet passing in front of its star (top) is

different from the one caused by a starspot. These subtle inequalities tell

astronomers which is which. ASTRONOMY: ROEN KELLY