Astronomy

Vega

Arcturus

Jupiter

Spica

Regulus Sun

MarsVenus

Capella

Procyon

Betelgeuse

Sirius

Rigel

W

N

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WWW.ASTRONOMY.COM 45

overcome rigid body forces and
become round is about 620
miles (1,000km). The main belt
asteroid Vesta is 326 miles
(525km) in diameter. In its
early history, Vesta’s interior
was at least partially molten
and may at one time have been
in hydrostatic equilibrium;
however, after cooling, Vesta
was battered out of round by
large impacts.
Farther from the Sun, ice
condensed along with silicate
grains to form planetesimals
consisting of a fragile mixture
of ice and rock. The minimum
diameter to achieve hydrostatic
equilibrium for an icy body is
comparatively small, about 310
miles (500km).
Due to rotational forces, icy
Ceres (620 miles [1,000km] in
diameter) is an oblate (f lat-
tened) spheroid. Some smaller
bodies in the outer solar system,
such as the trans-Neptunian
object Huya (310 miles [500km]
in diameter), appear to be
round and may also be in
hydrostatic equilibrium.
Thomas Prettyman
Senior Scientist, Planetary Science
Institute, Albuquerque, New Mexico

Q: DURING THE AUGUST
2017 SOLAR ECLIPSE,
I ASSUME I WILL SEE STARS
DURING THE DAYTIME.
WILL I SEE NEW STARS
AND CONSTELLATIONS?
David Pippin
Dearborn, Missouri

A: During totality, and even
for a short while before and
after, you should be able to see
the brightest stars and planets
while the Sun is covered by the
Moon. At the time of the
August eclipse, the stars you
may be able to see without
optical aid include Sirius,
Arcturus, Capella, and Rigel.
Visible planets include Venus
and Jupiter; although Mercury
and Mars will be in the sky,

they’ll be too faint for the

naked eye. I also stress “may”

because your location in the

United States will affect which

objects are visible above your

horizon. From your location in

Missouri, roughly 25 miles

(4 0k m) f rom where Astronomy

Senior Editor Michael E.

Bakich will be watching the

eclipse, Jupiter, Venus, Sirius,

and Arcturus should be visible.

While the sky is dark

enough to show some constella-

tions during an eclipse, it won’t

be as dark as a moonless night.

Additionally, because totality

will last less than three min-

utes, your eyes won’t have time

to adjust to the change in light

levels necessary to make out

more than the brightest objects.

It’s also important to note

that none of these stars or con-

stellations are necessarily new.

When a constellation is not

visible in the night sky, it’s

because those stars rise and set

while the Sun is up, which

depends on Earth’s position in

its orbit. This is why the con-

stellations we see change with

the seasons. You will get to see

stars and planets that are up

during the daytime while the

Sun is completely covered, but

they’ll be the same stars and

planets you could see at night

during other times of the year.

Alison Klesman

Associate Editor

Q: WHAT HAPPENS

TO STARS ONCE THEY

DIE AND COOL DOWN

COMPLETELY? IS THE END

RESULT DIFFERENT FOR

NEUTRON STARS, PULSARS,

AND WHITE DWARFS?

Rich Livitski

Seal Beach, California

A: The discovery of nuclear

fusion processes last century

was the seed for a detailed pic-

ture of the evolution of a star

from a protostellar gas cloud

through extinction as a white

dwarf or death in a supernova.

A star’s mass largely deter-

mines its fate; chemical com-

position plays a smaller role.

Stars with mass similar to

the Sun will end up as white

dwarfs — cores of carbon and

oxygen with hydrogen- or

helium-dominated atmo-

spheres — after their outer

layers of gas are lost as stellar

superwinds. Ultraviolet radia-

tion from the white dwarf ion-

izes the ejected gas, forming a

planetary nebula. The white

dwarf core can burn no addi-

tional fuel, and it gradually

cools until it no longer emits

heat or electromagnetic radia-

tion in the visible spectrum.

This stellar remnant is called a

black dwarf. No black dwarf

has been detected yet, as the

cooling time that a white dwarf

needs to reach this state is lon-

ger than the age of the uni-

verse. Stars of a few solar

masses also end their life cycles

as white dwarfs; however, these

objects have a mixture of car-

bon, oxygen, neon, and magne-

sium in their cores.

In the final moments of

stars more than eight times our

Sun’s mass, the outer layers fall

in at a tenth of the speed of

light, bounce off the rigid core,

and are ejected in an intense

supernova explosion. The core

of the star, which contains

traces of heavier elements such

as iron, is either left as a neu-

tron star or implodes as a black

hole. A white dwarf in a binary

system can also explode as a

supernova, leaving no remnant

and expelling iron and other

heavier elements generated

during the explosion into inter-

stellar space.

Borja Anguiano

Research Associate,

Department of Astronomy,

University of Virginia, Charlottesville

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.

During totality, some stars and planets may become visible in the

darkened sky, including Sirius, Arcturus, Capella, Jupiter, and Venus.

ASTRONOMY: RICHARD TALCOT T AND ROEN KELLY