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> bcc: president @ sbau.org   
> Subject:    Questions for your "Astronomy FAQ" page.
> Date:    Sun, 11 Oct 2015 16:00:54 -0700
> From:    Mark Drysdale <drysdale.mark.r@gmail.com>

-------- Original Message --------
Subject: Re: Questions for your "Astronomy FAQ" page.
Date: 2015-10-11 23:00
From: outreach@sbau.org
To: Mark Drysdale <drysdale.mark.r@gmail.com>

Mark -

Greetings! Here's my shot at answering your questions.

"> 1. Supposedly in 4 billion years (which is 1 billion years before our sun dies, so maybe somebody will be around to worry about this) the Milky Way Galaxy collides with the Andromeda Galaxy and does a 'damped oscillation' dance before reaching some new galactic stasis. What is the probability (__%) that one or more of Andromeda's solar systems will 'slice through' our solar system? i.e., come close enough to do some serious wrenching of our planets from their normal orbits (if not outright planetary collisions)."

1. The exact time frame for the Andromeda/Milky Way collision is still not
settled, probably because of a lack of knowledge of the dynamics of all the
galaxies in the Local Group, but 4 billion years seems to be the consensus.

As for the Sun, if our models of stellar evolution are correct, in about
800 million years it'll have grown hot enough to boil off our oceans and
atmosphere. In roughly 5 billion years the concentration of hydrogen in the
core will have fallen to the point where it will enter the Red Giant phase,
and swell out to reach maybe Venus. Even though the surface will be cooler,
the proximity will result in a much hotter Earth.

In the galaxy collision, the probability that individual stars will have
close interactions is very small, but of course a bit larger toward the
cores. For the most part, it's like two diffuse shotgun blasts passing
through each other. The large clouds of gas and dust in spiral galaxies,
however, do have a much larger collisional cross-section, and the result
will be bursts of new star formation. The sky will look really cool.

> "2. They say that the universe started with a Big Bang. I've always assumed that this theory comes from (a) measuring the velocity* of a large number of Galaxies, and then (b) doing a linear** regression of their flight paths, and discovering that they all intersected at the ~same 3-dimensional point in space at the ~same time in the past. Is the "proof" of the Big Bang theory really that simple? Or are there more complex measurements/characteristics/parameters that perform more of the heavy lifting to support it?"
> * A vector = speed + XYZ direction wrt some arbitrary reference point
> ** Ok, maybe linear regression gets supplanted by polynomial regression under some circumstances (i.e., when the Galaxies -- or the precursors of Galaxies -- were close enough for interaction effects to be meaningful)>
2. There are multiple lines of evidence which converge onto our modern
model for the origin of the Universe in a Big Bang, approximately 13.8
billion years ago. The current best fit is called the Big Bang with initial
rapid inflation and collisionless dark matter.

In the early 1900's, Vesto Slipher and Hubble noticed a correlation between
the redshifts in the spectral lines of galaxies and their distance from us.
Galaxies twice as far away seemed to be receding twice as fast. This is a
characteristic of a uniformly expanding space, and the same relationship would
be observed from any other galaxy in space.

Humans are used to thinking in three dimensions, so this is a hard concept to
grasp, but there is no center to this expansion. The Big Bang created both space
and time, and is not expanding into anything. Since it happened about 13.8
billion years ago, your observable universe has a radius of about 13.8 billion
light years (in what's called lookback time, ignoring the expansion that's been
ongoing for those 13.8 billion years, making the actual bit of Universe occupied
by what you can see actually much larger). But I'm also the center of my own
observable Universe, and if I'm 100 miles "west" of you, I can see 100 miles
farther in the "west" direction than you can at any given instant, and you can
see 100 miles farther "east" than I can (directions are arbitrary).

Since the 1840s, we've known that the Universe is either finite in time, or
finite in extent (or both), because of something called Olber's Paradox.
Simply stated, if the Universe were infinite in space and time, and stars are
randomly distributed, and light does not get weaker over time, then in any
direction you looked, you would see the surface of a star, and the entire night
sky would be blindingly bright.

Also, if the Universe started out infinitely small, then all parts of it would
have been in causal connection, and it would be in thermal equilibrium, with none
of the lumpiness we see in the form of stars and galaxies. Something called rapid
inflation, where space itself expanded faster than the speed of light, happened in
the first 10**-36 seconds after the Big Bang to let this happen. This doesn't
contradict Relativity, because no physical object was traveling through space faster
than the speed of light.

We see the results of this initial rapid expansion today in the minute anisotropies
in the Cosmic Background Radiation, which is essentially the surface of the fireball
of the Big Bang now redshifted by the expansion of the Universe into microwave
frequencies. Look 13.8 billion light years in any direction, and that's all you'll see.

The existence of the CBR (or CMB, for Cosmic Microwave Background), combined with
running the observed expansion backwards (accounting for the 1/r**2 of gravity, so
nonlinear) were two of the real clinchers for the reality of the Big Bang. Another
is that, in combination with the Standard Model of physics, it explains the abundances
we see for the various elements.

For a while, when things were closer together in a smaller Universe, gravity opposed
the expansion of space, but now the gravitational force has been overcome by the
inflationary properties of empty space, and the rate of expansion is accelerating.
This is the recently-discovered thing called Dark Energy. We don't know a lot about
its exact properties yet, but it seems to be that empty space has a pressure associated
with each bit of volume, causing it to expand. Perhaps this is related to the quantum
foam of virtual particles constantly popping in and out of existence throughout space.
This pressure causes that volume of space to expand, but after the expansion, the
pressure in each bit of volume remains the same, rather than decreasing as we're used
to seeing with gas pressure, for example.

We think that the Universe beyond what we can see is essentially infinite, because of
that initial rapid inflation, but our observations are limited by the speed of light.

Cosmology is pretty tough stuff to grasp at first. Here's a great website to peruse:
http://map.gsfc.nasa.gov/universe/

">> Like you, I reside in Southern California and sometimes look up (UNAIDED) into the night sky . . . .>
3a. . . . . Given that they say that our solar system is on the rim of our Milky Way Galaxy, during which months of the year am I looking into the core (the fat, meaty part in the center) of our Milky Way Galaxy, and during which months am I looking at the remainder of the 'rim' (what's left in between us and the closest*** entry into inter-galactic space)?"
> *** In the X-Y plane, not the Z dimension
>
3a. We're not quite on the rim of our galaxy, but definitely in suburbia. We think the
Milky Way is about 100,000 light years across, and we're about 30,000 light years out
from the center. The center of the galaxy is near the tip of the arrow in Sagittarius,
so in summertime in the northern hemisphere, it's to our south, most prominently seen
at reasonable hours of the night during August. The edge is to the northeast, toward
Cassiopeia. So, our wintertime Milky Way is looking toward the edge, and our summertime
Milky Way is looking more toward the center. In the southern hemisphere, the center of
the galaxy goes overhead, so they have spectacular winter (for them) skies.

"> 3b. . . . . Given the light pollution that nighttime Southern California enjoys, is it possible that any of the points of light that we might see are another Galaxy? Or are we always looking (again: unaided; no binoculars or telescopes) at local objects (stars within our own Milky Way Galaxy or planets within our own solar system)?"
>
3b. Essentially every star you see in the nighttime sky is part of our galaxy. There
can be supernovae in nearby galaxies (like SN1987a in the Large Magellanic Cloud) that
may temporarily reach eyeball visibility, but that's a transient thing. On a dark,
clear night away from light pollution, you can see the Andromeda Galaxy as a fuzzy patch,
making it the farthest thing you can generally see with the unaided eye, 2.5 million
light years away. From the southern hemisphere, you can see the Large and Small
Magellanic Clouds, which are small companion galaxies orbiting the Milky Way, but only on
the order of 180,000 light years away.


Hasta nebula - Chuck
--
Chuck McPartlin
www.sbau.org
--