THE SOUTHWORTH PLANETARIUM
70 Falmouth Street      Portland, Maine 04103
(207) 780-4249      usm.maine.edu/planet
43.6667° N    70.2667° W
Founded January 1970
2022-2023: XLV
Sunrise: 6:44 a.m.
Sunset: 4:10 p.m.
Civil twilight ends: 4:41 p.m.
Sun's host constellation: Libra the Scales
Moon phase: Waning crescent (2% illuminated)
Moonrise: 5:03 a.m.
Moonset: 3:19 p.m.
Julian date: 2459906.21
"To the mind that is still, the Universe surrenders."
-Lao Tzu

THE DAILY ASTRONOMER
Tuesday, November 22, 2022
A Few More Pandoras

Prior to the beginning of the DA holiday break, we wanted to try to lighten
the load of Pandora's Jar: the vessel into which we place pending astronomy
questions. They are inscribed on parchment, rolled into a cylinder, tied
with twine, and gently inserted into place like ancient scrolls in the
Apollo Library. And there they remain until we withdraw them and provide
answers. We have now answered a few more: the questions and responses are
below.

*How was the Sun’s corona first discovered?*

The Corona, the outermost region of the Sun’s atmosphere, is rendered
visible to observers during a total solar eclipse. This rarefied solar
layer can only be observed when the solar disc is entirely blocked. Even
though the Corona extends millions of miles into space and its estimated
temperature exceeds one million Kelvin, it is also quite diffuse: about 10
million times less dense than the material contained within the
*photosphere*, often mistakenly referred to as the Sun’s “surface.”

So, this corona has been seen throughout human history. However, the first
person who was said to have recognized this material as having been part of
the Sun rather than the moon was the Italian-French astronomer Giacomo F.
Maraldi (1665–1729).

Spanish astronomer Jose Joaquin de Ferrer y Cafranga (1763–1818), the man
who coined the term “Corona,” observed a total solar eclipse in 1806 and
likewise believed that this light was part of the Sun’s atmosphere and was
not associated with the moon. “Corona” is taken from the Latin word for
“Crown.” French astronomer Pierre Janssen (1824–1907) provided the first
observational evidence supporting the notion that the Corona was part of
the Sun by noting alterations in the coronal intensity in relation to
changes in the sunspot cycle.

Tangential note: It was by studying the corona that Sir Joseph Norman
Lockyer (1836–1920) discovered the second element, which had not yet been
observed on Earth. He named his element “helium,” after “Helios,” the Greek
god of the Sun.

The *solar corona* is only visible during a total solar eclipse. As seen in
the photograph above, the Corona appears as a diffuse “ring” of light
surrounding the eclipsed Sun. Image: Luc Viatour taken of the 11 August
1999 eclipse.




*Would it be possible to 'move' the Earth sometime in the future, while
keeping everything on Earth going as it is, before the Sun gets too big?*

Greetings!

First, I should mention that we would need to “move” Earth prior to the
stage at which the Sun expands to the red giant stage. This transformation
is due to occur in 5–6 billion years, when the Sun exhausts its core
hydrogen reserves. However, the Sun’s luminosity (energy output per second)
is slowly increasing as a consequence of the thermonuclear core reactions.
Astronomers estimate that the Sun’s luminosity increases by 6% every
billion years. Consequently, Earth will be rendered uninhabitable in
approximately 1.1 billion years.

In order to keep Earth habitable, we’d have to move to the Sun within 1.1
billion years and continuously shift its position away from the Sun because
the habitable zone, the region in which temperatures are conducive to
life’s survival, would also expand away from it. By the time the Sun
becomes a red giant, the habitable zone will have extended all the way out
to the orbits of Jupiter and Saturn. (Jupiter’s average heliocentric
distance is 483 million miles; Saturn’s mean distance is 914 million
miles.) See graphic below.

[image: EHZ.jpg]


The changing “Habitable Zone.” Earth is presently located within Earth’s
habitable zone, hence our continued existence on it. However, as the Sun’s
luminosity increases, this zone will expand away from the Sun. Earth will
be rendered uninhabitable in about 1.1 billion years. By the time the Sun
expands to the red giant stage, the habitable zone will be hundreds of
millions of miles farther away: in the region where Jupiter and Saturn
revolve around the Sun. Image: Astronomy Magazine

A few physicists have actually considered the possibility of shifting Earth
to protect it from the evolving Sun. One can well imagine that shifting
Earth safely constitutes one of the most challenging astro-engineering
problems. Perhaps the most feasible option discussed so far would be to
direct comets and asteroids with widths exceeding 100 kilometers around
Earth and then to either Jupiter or Saturn. The “pull” caused by these
repeated revolutions, though quite small, could cumulatively direct Earth
gradually but inexorably away from the Sun. Of course, this solution poses
many problems, namely, maintaining control of these bodies to ensure that
none of them eventually crashes onto Earth. Considering that the impact of
a 10-km wide asteroid ended the Cretaceous period, the consequences of a
100-km wide asteroid -taking into account the 1:10 ratio between the
impacting body’s diameter and that of the produced crater- could put an end
to Earth life itself.

Fortunately, we have plenty of time to contemplate the matter. That last
statement presumes that our species will even persist for 1.1 billion years
-4,400 times longer than the present duration of homo sapiens. If humans
are still extant by this time, presumably they would have devised a
solution to either shift Earth away from the Sun or to actually move to
another star system altogether. While the latter option would be the
logistically easier of the two, who knows what technology will develop in
the intervening time. After all, humans are known for their problem-solving
and, admittedly, problem-creating, capacities.

 *Since the Milky Way is spinning, if we tried to cross it in a ship, would
we just end up back where we started? In other words, would the rotation of
the Galaxy cause us to run right back into earth along the way*

Intriguing question. The answer of where you would end up depends entirely
on the speed and direction of your ship. To maintain our sanity, we will
neglect the harrowing effects of venturing close to Sagittarius A*, the
supermassive black hole in the galactic nucleus. Astronavigation,
particularly on an interstellar scale, is quite tricky because all the
objects within the Milky Way are rapidly moving. Conversely, if, let’s say,
you decided to manufacture a boat out of leather so you could sail across
the North Atlantic from Ireland to Newfoundland. You could expect a chilly
odyssey, but at least Newfoundland would remain in place. (We’re neglecting
the geological shift because it is conveniently negligible.)

The galaxy is in constant motion. However, unlike a merry-go-round or a
vinyl record, the galaxy doesn’t move as a single rigid body. The stars are
all moving either independently or in clusters. Regard the following
graphic which shows the location and direction of star streams within our
region of the galaxy.
[image:
The-galactocentric-U-and-V-components-of-velocity-for-1I-the-LSR-and-the-five-largest
(1).png]


So, one would have to define the “same place.” In this instance, one could
refer to the local standard of rest: an “average” of all the motions of
stars within one’s vicinity. Of course, this local standard is difficult
enough to establish within one local region of the galaxy, let alone the
entire Milky Way. One could also refer to the “galactic plane,” which would
be the average of the Milky Way’s “thickness” and its diameter. Measuring
the Milky Way’s thickness, which has been estimated at 1000 - 3000 light
years, is particularly challenging because the boundaries are vague and
always changeable as the stars are all moving along undulating paths within
it, like horses on a Merry Go Round.

As the ship’s captain, you decided to traverse the Milky Way so as to reach
the other side. Well, if you define the other side as the star whose
galactic coordinates are 180 degrees from Earth’s, you’d experience
difficulties because that star would move well away from this position by
the time you reached it. Even if your craft could approach light speed,
this star would be out of place as your journey’s distance would equal
about 75,000 light years. (Remember, we’re not at the galaxy’s outer edge,
but are instead about 25,000 light years from the nucleus.)

Let’s say, however, that you can know precisely the point directly opposite
our solar system relative to the nucleus along the galactic plane. You
would not encounter Earth unless you happened to be moving so slowly that
the journey required about 112 million years. Were this to be true, you
MIGHT encounter our solar system, which requires about 225 million years to
complete one revolution around the galaxy. But that “might” in bold
100-point print because the chances are vanishingly low. Remember that
according to the most recent estimates, our galaxy contains about 400
billion stars.

Your aim would have to be superb to hit Earth. However, the galaxy, itself,
wouldn’t spin you back here. You’d have much more control over your
trajectory than that.


*How can something so big as the sun be held together by gravity?*
The same way that something as unfathomably large as a galaxy, galaxy
cluster or supercluster can be held together by gravity: the matter
contained within. Yes, with an 865,000 mile diameter, the Sun’s volume is
enormous. So, too, is its mass. The Sun contains about 99.86% of all the
material within the solar system. (It is 333,000 times more massive than
Earth.) We often say that the massive particles are attracted to one
another gravitationally. One could also describe the situation from an
Einstein perspective by stating that all this matter deforms its local
space-time geometry to create a “gravity well” that holds the Sun together.

If, for instance, one were to expand Earth to the size of the Sun while the
mass remained constant, the material would dissipate because the
gravitational bonds connecting all the constituent particles would become
too tenuous. Gravity is the weakest of the fundamental forces (gravity,
electromagnetism, and the weak and strong nuclear force.) If you doubt this
assertion, look at your refrigerator magnet. The electromagnetic field
generated by that tiny magnet is easily overwhelming the gravitational pull
of an entire planet. However, as comparatively weak as it is, gravity
becomes powerful once you concentrate enough material within a small enough
region.



To subscribe or unsubscribe from the "Daily Astronomer"
http://lists.maine.edu/cgi/wa?A0=DAILY-ASTRONOMER