Astronomy

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spacecraft show that the total
mass loss rate from Io is about
2,200 pounds (1,000 kilograms)
per second. At this rate, the
total mass lost by Io over the
lifetime of the solar system is
about 7x10^20 pounds (3x10^20 kg).
This is 0.33 percent of Io’s total
mass. So only a tiny percentage
of Io’s mass will disappear over
the lifetime of the solar system,
leaving the moon intact as the
Sun and the solar system age.
Julie Rathbun
Senior Scientist, Planetary Science
Institute, Claremont, California

Q: DOES (OR WILL) THE
PLANET VENUS GET HOTTER
AS YEARS AND CENTURIES
PASS, OR WILL IT STAY AT
THE SAME TEMPERATURE?
Robert Miniszewski
Buffalo, New York

A: The surface of Venus is hot
because its massive atmo-
sphere causes a huge green-
house effect. This effect could
date back billions of years to
early atmospheric formation,
or it could be related to
immense outgassing during a
geological cataclysm several
hundred million years ago.
The United States has not sent
a spacecraft to Venus since
1989, and we need new data to
determine this.
Either way, Venus is unlikely
to get hotter by itself. This is
because it is approximately in
radiation equilibrium; in real-
ity, heat out is a little bigger
than heat in, due to heat loss
from the interior. But all the
planets will heat up as the Sun’s
brightness increases over the
next several billion years as a
natural part of its evolution,
eventually scorching the sur-
faces of all the inner planets,
including Venus and Earth.
Robert E. Grimm
Director of Department of Space
Studies, Southwest Research Institute,
Boulder, Colorado

Q: WHICH SUPERNOVA DID

TYCHO OBSERVE IN 1572?

Gene Scovell

Los Osos, California

A: The supernova reported by

Danish astronomer Tycho

Brahe (and many others, inde-

pendently) occurred in the con-

stellation Cassiopeia. Tycho

noticed the “new” star on

November 11, 1572, after which

it brightened to about the mag-

nitude of Venus (–4) and was

visible during the day for about

two weeks. The supernova then

slowly faded until it was no

longer visible with the naked

eye in the night sky by March

1574. Keep in mind that the
      telescope was not invented until
      1608, so follow-up observations
      at the time were impossible.
      The supernova’s remnant
      was identified in the mid-20th
      century via telescope. Today,

astronomers refer to it by several

names, including B Cassiopeiae

and SN 1572, but it also carries

the informal name of Tycho’s

supernova. The remnant is

about 13,000 light-years away

within the Milky Way. It is

spherical and consists of an

expanding cloud of debris, led

by an outer shell of high-

energy electrons associated

with the initial shock wave

from the blast. A second shock

wave, called a rebound shock

wave, travels inward and has lit

up the debris.

Using the remnant’s spec-

trum and light ref lected off

nearby dust, astronomers have

classified Tycho’s supernova as

a type Ia event: the explosion of

a white dwarf that tips over its

allowed mass limit after accret-

ing matter from a nearby com-

panion. But recent work

published in September in

Nature Astronomy offers an

alternative scenario, based on

new observations of the rem-

nant and its environment: The

progenitor may have been the

merger of two white dwarf

stars, rather than a single white

dwarf stealing mass from a

stellar companion.

Alison Klesman

Associate Editor

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.

The Chandra X-ray Observatory has imaged Tycho’s supernova in detail. An outer expanding shell of high-energy

electrons associated with the initial shock wave is visible as blue. The debris left over from the explosion (red and

green) glows due to heating from a rebounding, inward-moving shock wave. NASA/CXC/SAO