Last-modified: $Date: 1996/10/12 01:59:57 $
Version: $Revision: 2.0 $
URL: http://astrosun.tn.cornell.edu/students/lazio/sci.astro.html
Posting-frequency: semi-monthly (Wednesday)
Archive-name: astronomy/faq/part5

------------------------------

Subject: Introduction

 sci.astro is a newsgroup devoted to the discussion of the science of
astronomy.  As such its content ranges from the Earth to the farthest
reaches of the Universe.

 However, certain questions tend to appear fairly regularly.  This
document attempts to summarize answers to these questions.

 This document is posted on the first and third Wednesdays of each
month to the newsgroup sci.astro.  It is also available via anonymous
ftp in the directory <URL:ftp://seti.tn.cornell.edu/pub/lazio/> and it
is on the World Wide Web at
<URL:http://astrosun.tn.cornell.edu/students/lazio/sci.astro.html>.

Questions/comments/flames should be directed to the FAQ maintainer,
Joseph Lazio (lazio@spacenet.tn.cornell.edu).

------------------------------

Subject: Copyright

 This document, as a collection, is Copyright 1995,1996 by T. Joseph
W. Lazio (lazio@spacenet.tn.cornell.edu).  The individual articles are
copyright by the individual authors listed.  All rights are reserved.
Permission to use, copy and distribute this unmodified document by any
means and for any purpose EXCEPT PROFIT PURPOSES is hereby granted,
provided that both the above Copyright notice and this permission
notice appear in all copies of the FAQ itself.  Reproducing this FAQ
by any means, included, but not limited to, printing, copying existing
prints, publishing by electronic or other means, implies full
agreement to the above non-profit-use clause, unless upon prior
written permission of the authors.
 
 This FAQ is provided by the authors "as is," with all its faults.
Any express or implied warranties, including, but not limited to, any
implied warranties of merchantability, accuracy, or fitness for any
particular purpose, are disclaimed.  If you use the information in
this document, in any way, you do so at your own risk.

------------------------------

Subject: E.00 Sun and Planets

[Dates in brackets are last edit.]

    E.01 What is the "Solar Neutrino Problem?" [95-09-20]
    E.02 Could the Sun be part of a binary (multiple) star system? [95-08-27]
    E.03 When will the Sun die?  How? [95-08-23]
    E.04 Could the Sun explode? [95-07-07]
    E.05 How are solar system objects and features named? [95-11-29]
    E.06 Where can I find pictures and planetary data? (ref)
    E.07 Could Jupiter become a star? [95-07-07]
    E.08 Is Pluto a planet?  Is Ceres?  Is Titan? [95-08-18]
    E.09 Additional planets:
      09.1 What about a planet (Planet X) outside Pluto's orbit? [95-08-27]
      09.2 What about a planet inside Mercury's orbit? [95-08-27]
    E.10 Won't there be catastrophes when the planets align in the
         year 2000? [96-07-14]

------------------------------

Subject: E.01 What is the "Solar Neutrino Problem?"
Author: Bruce Scott TOK  <bds@ipp-garching.mpg.de>

A middle-aged main-sequence star like the Sun is in a slowly-evolving
equilibrium, in which pressure exerted by the hot gas balances the
self-gravity of the gas mass. Slow evolution results from the star
radiating energy away in the form of light, fusion reactions occurring
in the core heating the gas and replacing the energy lost by
radiation, and slow structural adjustment to compensate the changes in
entropy and composition.

We cannot directly observe the center, because the mean-free path of a
photon against absorption or scattering is very short, so short that the
radiation-diffusion time scale is of order 10 million years. But the main
proton-proton reaction (PP1) in the Sun involves emission of a neutrino:

        p + p --> D + positron + neutrino(0.26 MeV),

which is directly observable since the cross-section for interaction with
ordinary matter is so small (the 0.26 MeV is the average energy carried
away by the neutrino).  Essentially all the neutrinos make it to the
Earth. Of course, this property also makes it difficult to detect the
neutrinos. The first experiments by Davis and collaborators, involving
large tanks of chloride fluid placed underground, could only detect
higher-energy neutrinos from small side-chains in the solar fusion:

        PP2:    Be(7) + electron --> Li(7) + neutrino(0.80 MeV),
        PP3:    B(8) --> Be(8) + positron + neutrino(7.2 MeV).

Recently, however, the GALLEX experiment, using a gallium-solution detector
system, has observed the PP1 neutrinos to provide the first unambiguous
confirmation of proton-proton fusion in the Sun.

There is a "neutrino problem", however, and that is the fact that
every experiment has measured a shortfall of neutrinos. About one- to
two-thirds of the neutrinos expected are observed, depending on
experimental error. In the case of GALLEX, the data read 80 units
where 120 are expected, and the discrepancy is about two standard
deviations. To explain the shortfall, one of two things must be the
case: (1) either the temperature at the center is slightly less than
we think it is, or (2) something happens to the neutrinos during their
flight over the 150-million-km journey to Earth. A third possibility
is that the Sun undergoes relaxation oscillations in central
temperature on a time scale shorter than 10 Myr, but since no one has
a credible mechanism this alternative is not seriously entertained.

(1) The fusion reaction rate is a very strong function of the temperature,
because particles much faster than the thermal average account for most of
it. Reducing the temperature of the standard solar model by 6 per cent
would entirely explain GALLEX; indeed, Bahcall has recently published an
article arguing that there may be no solar neutrino problem at
all. However, the community of solar seismologists, who observe small
oscillations in spectral line strengths due to pressure waves traversing
through the Sun, argue that such a change is not permitted by their
results.

(2) A mechanism (called MSW, after its authors) has been proposed, by which
the neutrinos self-interact to periodically change flavor between electron,
muon, and tau neutrino types. Here, we would only expect to observe a
fraction of the total, since only electron neutrinos are detected in the
experiments. (The fraction is not exactly 1/3 due to the details of the
theory.) Efforts continue to verify this theory in the laboratory. The MSW
phenomenon, also called "neutrino oscillation", requires that the three 
neutrinos have finite and differing mass, which is also still unverified.

To use explanation (1) with the Sun in thermal equilibrium generally
requires stretching several independent observations to the limits of their
errors, and in particular the earlier chloride results must be explained
away as unreliable (there was significant scatter in the earliest ones,
casting doubt in some minds on the reliability of the others).  Further
data over longer times will yield better statistics so that we will better
know to what extent there is a problem. Explanation (2) depends of course
on a proposal whose veracity has not been determined. Until the MSW
phenomenon is observed or ruled out in the laboratory, the matter will
remain open. 

In summary, fusion reactions in the Sun can only be observed through their
neutrino emission. Fewer neutrinos are observed than expected, by two
standard deviations in the best result to date. This can be explained
either by a slightly cooler center than expected or by a particle-physics
mechanism by which neutrinos oscillate between flavors. The problem is not
as severe as the earliest experiments indicated, and further data with
better statistics are needed to settle the matter.

References:

[0] The main-sequence Sun: D. D. Clayton, Principles of Stellar Evolution
    and Nucleosynthesis, McGraw-Hill, 1968. Still the best text.
[0] Solar neutrino reviews: J. N. Bahcall and M. Pinsonneault, Reviews of
    Modern Physics, vol 64, p 885, 1992; S. Turck-Chieze and I. Lopes,
    Astrophysical Journal, vol 408, p 347, 1993. See also J. N. Bahcall,
    Neutrino Astrophysics (Cambridge, 1989).
[1] Experiments by R. Davis et al: See October 1990 Physics Today, p 17.
[2] The GALLEX team: two articles in Physics Letters B, vol 285, p 376
    and p 390. See August 1992 Physics Today, p 17. Note that 80 "units" 
    correspond to the production of 9 atoms of Ge(71) in a solution 
    containing 12 tons Ga(71), after three weeks of run time!
[3] Bahcall arguing for new physics: J. N. Bahcall and H. A. Bethe,
    Physical Review D, vol 47, p 1298, 1993; against new physics: J. N. 
    Bahcall et al, "Has a Standard Model Solution to the Solar Neutrino 
    Problem Been Found?", preprint IASSNS-94/13 received at the National
    Radio Astronomy Observatory, 1994.    
[4] The MSW mechanism, after Mikheyev, Smirnov, and Wolfenstein: See the
    second GALLEX paper.
[5] Solar seismology and standard solar models: J. Christensen-Dalsgaard 
    and W. Dappen, Astronomy and Astrophysics Reviews, vol 4, p 267, 1992;
    K. G. Librecht and M. F. Woodard, Science, vol 253, p 152, 1992. See
    also the second GALLEX paper. 

------------------------------

Subject: E.02 Could the Sun be part of a binary (multiple) star system?
Author: Bill Owen <wmo@wansor.jpl.nasa.gov>, 
	Steve Willner <swillner@cfa.harvard.edu>

Very unlikely.  In the 1980's there was proposed a small companion, nicknamed
Nemesis, in a 26-million-year highly eccentric orbit, to explain apparent
periodicities in the fossil extinction record.  However, these periodicities
have turned out to be more imagined than real, so the driver for the existence
of Nemesis is gone.

Furthermore, such an object would be relatively close by, bright enough in the
infrared to have been detected easily by IRAS, and its high proper motion
should have been detected by astrometrists long ago.

One very slim possibility is that a very faint companion now located
near the aphelion of an eccentric orbit is not ruled out.  Such an
object would be hard to detect because its proper motion would be
small.  It's not clear, however, that an orbit consistent with the
lack of detection would be stable for the Sun's lifetime.

So the chances are that there exist no stellar companions to our Sun.

------------------------------

Subject: E.03 When will the Sun die?  How?
Author: Erik Max Francis <max@alcyone.darkside.com>

The Sun is a yellow, G2 V main sequence dwarf.  Yellow dwarfs live 
about 10 billion years (from zero-age main sequence to white dwarf 
formation), and our Sun is already about 5 billion years old.

Main sequence stars (like our Sun) are those that fuse hydrogen into
helium, though the exact reactions vary depending on the mass of the
star.  The main sequence phase is by far the most stable and
long-lived portion of a star's lifetime; the remainder of a star's
evolution is almost an afterthought, even though the results of that
evolution are what are most visible in the night sky.  As the Sun
ages, it will increase steadily in luminosity.  In approximately 5
billion years, when the hydrogen in the Sun's core is mostly
exhausted, the core will collapse---and, consequently, its temperature
will rise---until the Sun begins fusion helium into carbon.  Because
the helium fuel source will release more energy than hydrogen, the
Sun's outer layers will swell, as well as leaking away some of its
outer atmosphere to space.  When the conversion to the new fuel source
is complete, the Sun will be slightly decreased in mass, as well as
extending out to the current orbit of Earth or Mars (both of which
will then be somewhat further out due to the Sun's slightly decreased
mass).  Since the Sun's fuel source will not have increased in
proportion to its size, the blackbody power law indicates that the
surface of the Sun will be cooler than it is now, and will become a
cool, deep red.  The Sun will have become a red giant.

A few tens or hundreds of millions of years after the Sun enters its 
red giant phase (or "helium main sequence"; the traditional main 
sequence is occasionally referred to as the hydrogen main sequence to 
contrast the other main sequences that a massive star enters), the Sun 
will begin to exhaust its fuel supply of helium.  As before, when the 
Sun left the (hydrogen) main sequence, the core will contract, which 
will correspondingly lead to an increase in temperature in the core.

For very massive stars, this second core collapse would lead to a 
carbon main sequence, where carbon would fuse into even heavier 
elements, such as oxygen and nitrogen.  However, the Sun is not 
massive enough to support the fusion of carbon; instead of finding 
newer fuel sources, the Sun's core will collapse until degenerate 
electrons---electrons which are in such a compressed state that their 
freedom of movement is quantum mechanically restricted---smashed 
together in the incredible pressures of the gravitational collapse, 
will halt the core's collapse.  Due to the energy radiated away during 
the process process of the formation of this electron-degenerate core, 
the atmosphere of the Sun will be blown away into space, forming what 
astronomers call a planetary nebula (named such because it resembles a 
planetary disk in the telescope, not because it necessarily has 
anything to do with planets).  The resulting dense, degenerate core is 
called a white dwarf, with a mass of something like the Sun compressed 
into a volume about that of the Earth's.

White dwarfs are initially extremely hot.  But since the white dwarf
is supported by degenerate electrons, and has no nuclear fuel to speak
of to create more heat, they have no alternative but to cool.  Once
the white dwarf has cooled sufficiently---a process which will take
many billions of years---it is called an exhausted white dwarf, or a
black dwarf.

------------------------------

Subject: E.04 Could the Sun explode?
Author: Erik Max Francis <max@alcyone.darkside.com>

The short answer is no; the detailed answer depends entirely on what is
meant by "explode."  The Sun doesn't have anything like enough mass to
form a Type 2 supernova (whose progenitors are supergiants), which
require more than about 8 solar masses; thus the Sun will not become a
supernova on its own.

"Novae" arise from an accumulation of gases on a collapsed object,
such as a white dwarf or a neutron star.  The gas comes from a nearby
companion (usually a distended giant).  Although nova explosions are
large by human standards, they are not nearly powerful enough to
destroy the star involved; indeed, most novae are thought to explode
repeatedly on time scales of years to millenia.  Since the Sun is not
a collapsed object, nor does it have a companion---let alone a
collapsed one---the Sun cannot go (or even be involved in) a nova.

Under conditions not well understood, the accumulation of gases on a
collapsed object may produce a Type 1 supernova instead of an ordinary
nova.  This is similar in principle to a nova explosion but much larger;
the star involved is thought to be completely destroyed.  The Sun will
not be involved in this type of explosion for the same reasons it will
not become a nova.

When the Sun evolves from a red giant to a white dwarf, it will shed its
atmosphere and form a planetary nebula; but this emission could not
really be considered an explosion.

------------------------------

Subject: E.05 How are solar system objects and features named?
Author: Bill Owen <wmo@wansor.jpl.nasa.gov>,
	 Gareth Williams <gwilliams@cfa.harvard.edu>

Comets are named for their discoverers, up to three names per comet.

Minor planets are named by the Small Bodies Names Committee of the
International Astronomical Union Commission 20.  Discoverers of minor
planets may propose names to the SBNC and minor planets have been
named to honor all sorts of famous (and some not so famous) people and
animals in all walks of life.

Planetary satellites are named by the Working Group for Planetary
System Nomenclature of the IAU, in consultation with the SBNC (mainly
to avoid conflicts of names), and they *usually* defer to the
discoverer's wishes.  Names of satellites are usually taken from Greek
mythology or classical literature.

Features on Solar System bodies are named by the same commission, generally
following a specific theme for each body.  For instance, most features on Venus
are named in honor of famous women, and volcanos on Io are named for gods and
goddesses of fire.

For additional discussion, see
<URL:http://seds.lpl.arizona.edu/billa/tnp/names.html>.

The IAU Planetary System Nomenclature Working Group's Web site,
<URL:http://wwwflag.wr.usgs.gov/nomen/nomen.html>, has an extensive
discussion, as well as lists of names.

------------------------------

Subject: E.06 Where can I find pictures and planetary data?

See Part A of this FAQ, and
<URL:http://seds.lpl.arizona.edu/billa/tnp/>,
<URL:ftp://phobos.sscl.uwo.ca/pub/Space>,
<URL:http://bang.lanl.gov/solarsys/>,
<URL:http://www-pdsimage.wr.usgs.gov/PDS/public/mapmaker/mapmkr.htm>,
and <URL:http://wwwflag.wr.usgs.gov/USGSFlag/Space/>.

------------------------------

Subject: E.07 Could Jupiter become a star?
Author: Erik Max Francis <max@alcyone.darkside.com>

A star is usually defined as a body whose core is hot enough and under
enough pressure to fuse light elements into heavier ones with a
significant release of energy.  The most basic (and easiest, in terms of
the temperatures and pressures required) type of fusion involve the
fusion of four hydrogen nuclei into one helium-4 nucleus, with a
corresponding release of energy (in the form of high-frequency photons).
This reaction powers the most stable and long-lived class of stars, the
main sequence stars (like our Sun and nearly all of the stars in the
Sun's immediate vicinity).

Below certain threshold temperatures and pressures, the fusion reaction
is not self-sustaining and no longer provides a sufficient release of
energy to call said object a star.  Theoretical calculations indicate
(and direct observations corroborate) that this limit lies somewhere
around 0.08 solar masses; a near-star below this limit is called a brown
dwarf.

By contrast, Jupiter, the largest planet in our solar system, is only
0.001 masses solar.  This makes the smallest possible stars roughly 80
times more massive than Jupiter; that is, Jupiter would need something
like 80 times more mass to become even one of the smallest and feeblest
red dwarfs.  Since there is nothing approaching 79 Jupiter masses of
hydrogen floating around anywhere in the solar system where it could be
added to Jupiter, there is no feasible way that Jupiter could become a
star.

------------------------------

Subject: E.08 Is Pluto a planet?  Is Ceres?  Is Titan?
Author: Andy Rivkin <asrivkin@lowell.edu>

While on the face of it, this seems a reasonably easy question with a simple
answer, like the "When does the 21st Century begin?" question there is no
hard and fast rule, no committee of astronomers who decide these things.
The best rule of thumb is that if people think something's a planet, it is.
Common criteria include orbiting the Sun rather than another body (although
sticklers find this troublesome) and being "large".  Some have suggested
using "world" as a neutral term for an interesting solar system body.  The
word "planet" originally meant "wanderer", so using a strict definition,
everything in the solar system is a planet!

When Pluto was discovered in 1930, there was no question as to whether
it was a planet.  The predictions made at the time imagined it to be
at least the size of the Earth.  As better data became available with
the discovery of Pluto's moon Charon allowing the determination of a
mass for Pluto, and with Pluto and Charon eclipsing each other in the
late 1980's--early 1990's, it was found that Pluto is much smaller
than the Earth, with a diameter of roughly 2300 km (or about 
1400 mi.).  In the last several years, a number of small bodies at about
the same distance from the Sun as Pluto have been discovered,
prompting some to call Pluto the "King of the Kuiper Belt" (the Kuiper
Belt is a postulated population of comets beyond Neptune's orbit) and
rally for its demotion from bona-fide planet to overgrown comet.

Is Pluto a planet?  It depends on what one thinks is necessary to
bestow planetary status.  Pluto has an atmosphere and a satellite.  Of
course, Titan has a much larger atmosphere, and the tiny asteroid Ida
has a satellite.  Most astronomers would probably consider stripping
Pluto of its status akin to stripping [the U.S. states of] Connecticut or
Vermont of statehood because Texas and Alaska later joined.

Is Ceres a planet?  Like Pluto, when it was first discovered there was no
doubt that it was.  Within a few years, however, Pallas, Vesta and Juno were
discovered.  While Ceres is the largest asteroid, the second, third and fourth
largest asteroids are roughly half its size, compared to Pluto, which is about
ten times larger than the Kuiper Belt objects found so far.  Ceres is also 
not thought to have undergone large-scale geological processes such as 
vulcanism, although Vesta has.  The consensus is probably that neither Ceres
nor any other asteroid is a "planet", though they are interesting bodies in
their own right.

Is Titan a planet?  In the 1940's a methane atmosphere was discovered around
Titan, making it the only satellite with a substantial atmosphere.  This 
atmosphere has long prevented observations of the surface, frustrating the
attempts of Voyager 1 and 2 and leading theorists to suggest a Titan-wide
global ocean of carbon compounds.  Recent observations have been able to 
penetrate to the surface of Titan, showing tantalizing glimpses of what may be 
continents on the surface.  The atmosphere combined with Titan's large size 
have led some to consider Titan a "planet", but what about Ganymede, which is
larger, or Mercury which is smaller and has no atmosphere?  Again, the general
consensus is that satellites are not planets.

------------------------------

Subject: E.09 Additional planets:

------------------------------

Subject: E.09.1 What about a planet (Planet X) outside Pluto's orbit?
Author: Ron Baalke <baalke@kelvin.jpl.nasa.gov>, 
	 contributions by Bill Owen <wmo@wansor.jpl.nasa.gov>,
	 edited by Steve Willner <swillner@cfa.harvard.edu>

Pluto was discovered from discrepancies in the orbits of Uranus and
Neptune.  The search was for a large body to explain the
discrepancies, but Pluto was discovered instead (by accident, if you
will, though Clyde Tombaugh's search was systematic and thorough).
Pluto's mass is too small to cause the apparent discrepancies, so the
obvious hypothesis was that there is another planet waiting to be
discovered.

The orbit discrepancies go away when you use the extremely accurate
measurements of the masses of Uranus and Neptune made by Voyager 2
when it flew by those planets in 1986 and 1989.  Uranus is now known
to be 0.15% less massive and Neptune 0.51% less massive, than was
previously believed.

[N.B.  These numbers come from comparing the post-Voyager masses to those in
the 1976 IAU standard.]

When the new values for these masses is factored into the equations,
the outer planets are shown to be moving as expected, going all the
way back to the early 1800's.  Dr. Myles Standish from JPL did the
analysis on this and published his results in the May 1993 issue of
the Astronomical Journal.

The positional measurements do not bode too well for the existence of
Planet X.  They do not entirely rule out the existence of a Planet X,
but they do indicate that it will not be a large body.

There's also some interesting info at
<URL:http://seds.lpl.arizona.edu/billa/tnp/hypo.html>

------------------------------

Subject: E.09.2 What about a planet inside Mercury's orbit?
Author: Joseph Lazio <lazio@spacenet.tn.cornell.edu>

In the mid 19th century, some observers reported seeing a planet
inside Mercury's orbit.  The planet was named Vulcan.  However, later
attempts to find the planet failed.  It is now believed that there are
no planets interior to Mercury.

------------------------------

Subject: Won't there be catastrophes when the planets align in the
	year 2000?
Author: Laz Marhenke <laz@leland.Stanford.EDU>, 
	Chris Marriott <chris@chrism.demon.co.uk>

There weren't any in the year 1982.

For starters, the planets only "align" in a very rough fashion.  They
don't orbit the Sun in the same plane, so it's impossible to get very
many of the planets in a straight line.  Nevertheless, any time they
all get within about 90 degrees of each other, someone will claim
they're "aligned."  The last time this happened was 1982 when dire
predictions were heard about how the "Jupiter effect" would lead to
world-wide disaster.

Second, even if they *were* all aligned, the effect on the Earth would
be miniscule.  It's true that the other planets' gravity does affect
the orbit of the Earth, but the effect is small, and lining up all the
planets doesn't even come close to making it big enough for anyone to
notice.  The effect on the Earth is dominated by Jupiter and Venus
anyway (Jupiter because it's massive, Venus because it's occasionally
very close to us).  All the other planets put together only affect us
about 10% as much as those two, so the fact that they're all in the
same general direction as Jupiter and Venus doesn't make much
difference.

Third, even if all the planets could produce a strong gravitational
effect on the Earth (which they can't, unless they find a way to
increase their mass by a factor of 10--100), it wouldn't result in the
"crust spinning over the magma" or some other dire effect, since their
gravity would be pulling on every part of the Earth (almost) equally.

The "(almost)" is because the other planets do exert tidal forces on
the Earth, which means they pull on different parts of the Earth very
slightly differently.  However, tidal forces decrease *rapidly* with
distance (as the third power), so these forces are very small: The
tidal force from Venus at its closest approach to Earth is only
1/17,000th as large as the Moon's, and we seem to survive the Moon's
tides well enough twice a day.  If the Moon raises tides of 1 meter
(three feet) where you live, Venus at its closest will raise tides of
1/20th of a millimeter, or about the thickness of a hair.  The other
planets have even smaller tidal effects on the Earth than Venus does.

Finally, it's worth remembering that the Earth is about 4.5 billion
years old.  Whilst these "alignments" may be rare in terms of a human
lifetime (occurring once every few decades), they've occurred
countless times during the time that life has existed on this planet,
and many, many times in the comparatively brief time that humans have
been around.  The fact that we're still here to talk about it is proof
enough that nothing *too* terrible happens!

