Re: stellar orbits in galaxies

In article <y937imy586i.fsf@xxxxxxxxxxxxxxxxxxxx>,
Allan Adler <ara@xxxxxxxxxxxxxxxxxxxx> wrote:

pausch@xxxxxxx (Paul Schlyter) writes:

and why?

Gravity of course!! What did you think?

I guess I didn't ask that very well. Of course it orbits the center of
mass of the inner stars and does so because of gravity. The same could
be said about the Earth with respect to everything closer to the Sun than
the Earth. The most important such thing is the Sun itself and it stands
out as more of a reason that the inner planets. Our Milky Way has at least
one black hole at its galactic center, so its significance, e.g., might be
compared with that of the other inner stars.

I guess the Sun is on the outer part of the spiral, so most of the mass
of the Milky Way is closer to the Sun.

The center of the galaxy is gravitationally much less dominant than
the center (= the Sun) of our solar system. One consequence of that
is that while the linear orbital velocity in our solar system
decreases with distance from the Sun (being inversely proportional to
the square root of the distance to the Sun for circular orbits), the
linear orbital velocity in our galaxy is more or less constant for a
large part of the galaxy - it's a bit as if, in our solar system, e.g.
Jupiter, Saturn and Uranus all had approximately the same linear
velocity. You can read more about that in Mihalas.

What kinds of general statements can be made about the orbits of the other
stars in the Milky Way? For example, are there some basic classifications
of orbit types?

There are basically two kinds of orbits:

Thanks for this explanation.

If so, how typical is the Sun's orbit?

The Sun's orbit is very typical for population I stars.

The same questions apply as well to other galaxies.

Is it fair to compare the orbits of population II stars with orbits of
comets?

Or perhaps asteroids? Anyway, why would you want to make such a comparison?
Our solar system is not a "miniature galaxy", just like an atom with its
electrons isn't a "miniature solar system".

Other spiral galaxies - not elliptical galaxies....

Are orbits in elliptical galaxies more like the motion of molecules in
a gas?

Not quite ---- the mutual gravitation between the individual gas
molecules is quite insignificant. Which means that in a gas, the
molecules can be considered to travel in straight lines between
collisions, while in an elliptical galaxy the paths of the stars are
definitely curved due to their mutual gravity between close encounters
(which can be considered the equivalent of a collision between two
molecules in a gas - an actual collision between two stars will
probably shatter both stars). If the gas is very thin (as in e.g. the
Earth's atmosphere at ca 120 km altitude and above), the mean free
path of the gas molecules between collisions is so long that the
molecules can be noticeably infunenced by an external gravity field
(the Earth's gravity) between collisions.

What would be a good book or article to read on stellar orbits in galaxies?

"Galactic Astronomy" by Mihalas is a classic: http://tinyurl.com/34h5hw

Thanks. I have it and should have read it. I'll look at it when I've finished
typing this.

Do so .....

When the Sun moves in its orbit, it bobs up and down in the equatorial plane
of the Milky Way. Why does it do that?

Since our galaxy is a pretty thin disk, there will be a force trying
to bring back any star which gets out of the plane of the disk to the
plane of the disk. If the star's velocity perpendicular to the disk
isn't high, it will be "pulled back" to the disk by this force - or
rather through the disk and out on the other side, where the process
repeats but in the other direction. This causes the Sun, as well as
other population I stars, to bob "up" and "down" when they orbit the
galaxy's center.

So, as regards perpendicular motion, it's kind of like someone drilled a
hole through the Earth and dropped a ball into the hole. Is there any force
that tends to attenuate the bounces? If so, when might we expect the ball
to stop bouncing?

In the case of a ball falling through a hole drilled through the Earth:
the friction against the air would soon stop the ball bouncing - it wouldn't
even bounce once --- at least not very far. Even a human falling through air
in our atmosphere will soon reach an equilibrium speed where the air resistance
equals his weight - that equilibrium speed is approximately 200 km/h.

If, for comparison, we look at the orbit of the Earth in the solar
system, is there any comparable bobbing motion?

Not at all as much, for the simple reason that the Sun is a sphere,
not a thin disk. However, our Moon, which follows an orbit inclined
some 5 degrees to the ecliptic, causes the Earth to bob up and down
once every draconical month. As seen from the Sun, this "up-and-down"
motion will be of some +-0.5 arc seconds in amplitude. Perturbations
by the other planets (most notably Venus and Jupiter) will cause a
similar motion but of longer period and smaller amplitude.

That's very interesting. Thanks.
--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://stjarnhimlen.se/
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