Features:
Besides their beautiful and often fragmentary spiral shape, the
spiral arms
have intricate coloration
in true color:
a complex mixture of
blue
(due to hot young blue
OB stars),
pink
(due to H II regions
caused mainly by illumination by the
OB stars),
and dark,
brown
star forming regions
often orgainzed
in dust lanes lacing the
spiral arms.
The OB stars
and associated H II regions
are always close to and often surrounded by
star forming regions because
the OB stars CANNOT wander far from their
star forming region of origin
in their lifetimes of <∼ 30 Myr
(see Wikipedia: Stellar evolution for
representative lifetimes of stars).
Recall orbital periods
for stars around
galaxy
barycenters are of order 200 Myr.
Dust lanes often appear on the trailing edge
of spiral arms and
the OB stars
and associated H II regions
on the leading edge
(Sh-274;
FK-569).
Note spiral arms
are bright because of the
OB stars
and associated H II regions
are very luminous.
The density of stars is about the
same throughout galaxy disks???.
The galactic bulges
are typically the brightest part of
spiral galaxies
because of high concentration of
relatively old
yellow
stars
(i.e., old
Population I stars and
Population II stars).
It seems that the processes probably overlap,
and so there is probably NO
absolute separation between the two kinds of
spiral arms
(see Wikipedia: SSPSF model
(stochastic self-propagating star formation model)).
The probable overlap of processes also means that the
classification of
spiral galaxies
into
flocculent spiral galaxies
(∼ 30 % of galaxies),
grand design spiral galaxies
(∼ 10 % of galaxies),
and multi-arm spiral galaxies
(sometimes lumped with flocculent spiral galaxies)
(∼ 60 % of galaxies)
is probably just an approximation.
For the relative abundances of the
spiral galaxies
by spiral arm
classification, see
Wikipedia: Flocculent spiral galaxy.
Below we discuss how the
spiral arms
form in
flocculent spiral galaxies
and
grand design spiral galaxies
treating the formation processes as distinct as a simplification.
The word FLOCCULENT means woolly or fleecy.
Flocculent spiral galaxies
have poorly defined, fuzzy arms.
The figure below
(local link /
general link: galaxy_triangulum_noao.html)
shows an example of a
flocculent spiral galaxy.
In brief, this is the story of
flocculent spiral arms:
The grand design spiral galaxies have well defined arms.
The figure below
(local link /
general link: galaxy_whirlpool.html)
shows an example of a
grand design spiral galaxy.
The spiral density waves are somewhat like traffic jams on a
highway at the location of a moving work crew. The jam moves
forward slowly as cars jam up there, but eventually the cars
pass through and speed on ahead.
The spiral density waves are in fact regions of compression
of gas and the accompanying star formation regions.
But because such stars and regions live only of order 10 Myr
and the orbital periods in galaxies are typically of order
hundreds of megayears, the hot young, blue stars, and the H II regions
never get far from their formation region in the
arms before they become extinct. So they stay part of the arms
for all or most of their lives.
One might say the
spiral density waves are like waves
rippling around in a bucket.
But that analogy CANNOT be pressed too far since water waves
are formed by pressure force and downward gravity.
In spiral galaxies, the gravity
is strong between matter
components and NOT downward. Pressure forces play a role too.
So the actual spiral density wave mechanism is complex
(Sh-275ff) and NOT yet fully
worked out (FK-570).
But vaguely the waves are self-propagating gas compressions and
gravitational perturbations.
Because of the viscosity of the gas, spiral density waves
should lose energy (kinetic and gravitational potential
energy) to heat energy that eventually is probably mostly
radiated away as light energy.
So something must pump energy into the
spiral density waves
or else they would fade after maybe a gigayear or so
(CK-392).
There are two proposed mechanisms, both of which may operate:
Of course, the
energy for the waves must then come ultimately from
the rotational kinetic or gravitational potential
energy of the galaxy matter, and so some spiraling in
toward the center of some matter must happen.
Since barred spirals
are in fact only of order half
of all spirals
(CK-393;
FK-583: but the
two authors disagree somewhat), there must be a second mechanism.
It seems to be that
spiral arms
form from a varying combination
of spiral density waves
and the SSPSF model mechanism.
So spiral arm
is complex---but that's what makes
spiral arms so beautiful.
To last spiral arms
need
interstellar medium (ISM)
and, for grand design spiral arms,
excitation by galaxy interactions
(i.e., strong
gravitational perturbations
between 2
galaxies).?????
What if the
ISM
gets depleted somehow or
there are insufficient excitations?
The spiral arms
can disappear and a
spiral galaxy
turns into
a lenticular (SO or SB0) galaxy.
The time scale for disapperance is probably of order a gigayear.???
What causes the bars of
barred spirals?
The bars
are thought to be some sort of
spiral density waves themselves
which FORM and DECAY on the time scale of gigayears
(see
Wikipedia: Barred spiral galaxy: Bars).
But they are spiral density waves
NOT in
galactic disks, but
in galactic bulges.
The bars are probably
excited by galaxy interactions
(i.e., strong
gravitational perturbations
between 2
galaxies).?????
And that's enough explication of
bars for us.
The mechanism that creates
flocculent spiral arms
is believed to be
stochastic self-propagating star formation (SSPSF).
Recall, we have discussed SELF-PROPAGATING
star formation in
IAL 21:
Star Formation: The Evolution of Star Formation Regions.
For an simulation of
stochastic self-propagating star formation (SSPSF),
see the animation
in the figure below
(local link /
general link: stochastic_self_propagating_star_formation.html).
The stars and dust clouds in these
spiral arms
are orbiting more
rapidly than the arms which are NOT fixed structures, but
spiral density waves
that move more slowly than the stars
and clouds that pass through them.
Question: The strong star formation in the arms which
leads to hot young, blue stars (OB stars)
and H II regions is:
The hot young, blue stars and the H II regions run ahead of
the compression, because they are moving faster than the
compression wave.
Answer 3 is right.
In addition to the two proposed mechanisms, there is the fact that
spiral density waves
in many cases CANNOT be separated from mechanism of
SSPSF model (stochastic self-propagating star formation)
discussed above in feature
Flocculent Spiral Galaxies and Self-Propagating Star Formation.