Download The formation of the galaxy is believed to be similar

The formation of the
galaxy is believed to
be similar to the
formation of the
solar system.
During the time that
stars were being
formed, our galaxy
didn’t have a disk.
All that existed was
an irregularly
shaped galaxy.
All the gas & dust
collapsed into a
disk.
Current models show that the abundance of
heavy elements should be greater than what it
is actually observed.
This may be because low heavy metal abundant
gas & dust from the halo is diluting the disk.
Astronomers are still unsure about the Milky
Way’s origins…
Measurement of the position and motion of gas
clouds shows that the Milky Way has a spiral
form:
The spiral arms cannot rotate at the same speed
as the galaxy; they would ―curl up‖.
The spiral arms cannot rotate at the same speed
as the galaxy; they would ―curl up‖.
Rather, they appear to be density waves, with
stars moving in and out of them.
This is similar to a water wave, the spirals ―seen‖
are patterns from by these density waves.
They are not large masses of matter moving.
Dust lanes indicate
high density gas (star
forming regions).
Young O & B class
stars & open clusters
are found just past
these lanes.
Further ahead we find
older K & M class stars
& globular clusters.
In essence, the density
waves are thought to
drive star formation.
Or perhaps it’s the other way around…
The star formation may contribute to
propagation of the arms (driving the density
waves).
The origin of the spiral arms is not yet
understood.
Much nonplussary…
A few theories…
 Gravitational effects from neighboring
galaxies.
 Instabilities of the gas near the bulge.
 Asymmetrical bar within the bulge
(causing an off-axis rotation).
 Possible merger with a dwarf galaxy.
The position of the Sun in the Milky Way Galaxy
is best described as
a) in the disk, slightly more than halfway out
from the center.
b) very close to the center.
c) in an open cluster in the disk.
d) in a globular cluster in the halo.
The position of the Sun in the Milky Way Galaxy
is best described as
a) in the disk, slightly more than halfway out
from the center.
b) very close to the center.
c) in an open cluster in the disk.
d) in a globular cluster in the halo.
The thickness of our galaxy's disk is determined
by the
a) circular speed of stars around the galaxy.
b) random motion of stars in the plane of the
disk.
c) random motion of stars perpendicular to the
disk.
d) the amount of matter in nucleus of the
galaxy.
The thickness of our galaxy's disk is determined
by the
a) circular speed of stars around the galaxy.
b) random motion of stars in the plane of the
disk.
c) random motion of stars perpendicular to the
disk.
d) the amount of matter in nucleus of the
galaxy.
When we observe stars near the center of the
Milky Way Galaxy, we detect light that was
emitted from those stars about
a) 200 million years ago.
b) 25,000 years ago.
c) 8 years ago.
d) 8 minutes ago.
e) when the galaxy was formed.
When we observe stars near the center of the
Milky Way Galaxy, we detect light that was
emitted from those stars about
a) 200 million years ago.
b) 25,000 years ago.
c) 8 years ago.
d) 8 minutes ago.
e) when the galaxy was formed.
The mass of our galaxy is best found by
a) counting the number of stars in the sky.
b) counting the star clusters in the sky.
c) counting the hot, massive main sequence
stars.
d) radio measurements of the amount of
interstellar hydrogen.
e) measuring the rotation of the Galaxy.
The mass of our galaxy is best found by
a) counting the number of stars in the sky.
b) counting the star clusters in the sky.
c) counting the hot, massive main sequence
stars.
d) radio measurements of the amount of
interstellar hydrogen.
e) measuring the rotation of the Galaxy.
You could best map out the overall spiral
structure of our Galaxy by finding
a) young open clusters and hydrogen gas.
b) evolved stars like planetary nebulae, RR Lyra
stars.
c) smooth, round globular clusters.
d) high velocity stars.
You could best map out the overall spiral
structure of our Galaxy by finding
a) young open clusters and hydrogen gas.
b) evolved stars like planetary nebulae, RR Lyra
stars.
c) smooth, round globular clusters.
d) high velocity stars.
Choose the best evidence that the disk of the
Milky Way Galaxy does NOT rotate like a solid
wheel.
a) Disk stars have Doppler shifts.
b) The brightest disk stars form spiral arm
shapes.
c) Disk stars rotate twice as quickly that are
twice as far from the Galactic center.
d) The rotation of disk stars around the Sun
decreases with distance according to Kepler's
laws.
Choose the best evidence that the disk of the
Milky Way Galaxy does NOT rotate like a solid
wheel.
a) Disk stars have Doppler shifts.
b) The brightest disk stars form spiral arm
shapes.
c) Disk stars rotate twice as quickly that are
twice as far from the Galactic center.
d) The rotation of disk stars around the Sun
decreases with distance according to Kepler's
laws.
Compared to stars like the Sun in the disk of the
Milky Way, stars that populate the extended
halo of the galaxy were born
a) earlier, so have had time to accumulate more
heavy elements.
b) later, so have used up their heavy elements.
c) earlier, from more nearly primordial
material, so have fewer heavy elements.
d) later, so have accumulated more heavy
elements from previous generations of stars.
Compared to stars like the Sun in the disk of the
Milky Way, stars that populate the extended
halo of the galaxy were born
a) earlier, so have had time to accumulate more
heavy elements.
b) later, so have used up their heavy elements.
c) earlier, from more nearly primordial
material, so have fewer heavy elements.
d) later, so have accumulated more heavy
elements from previous generations of stars.
You observe two stars at the same distance. One
is in the disk of the Milky Way, the other in a
direction perpendicularly out of the disk.
Chances are that the disk star will be
a) less luminous, have a smaller Doppler shift and be
reddened by dust.
b) more luminous, have a smaller Doppler shift and be
reddened by dust.
c) more luminous, have a larger Doppler shift and be
reddened by dust.
d) more luminous, have a smaller Doppler shift and be
less reddened by dust.
You observe two stars at the same distance. One
is in the disk of the Milky Way, the other in a
direction perpendicularly out of the disk.
Chances are that the disk star will be
a) less luminous, have a smaller Doppler shift and be
reddened by dust.
b) more luminous, have a smaller Doppler shift and be
reddened by dust.
c) more luminous, have a larger Doppler shift and be
reddened by dust.
d) more luminous, have a smaller Doppler shift and be
less reddened by dust.
A cold thin cloud of interstellar gas embedded in
a hotter, thinner surrounding medium would
yield 21-cm radio spectral emission lines of
hydrogen showing
a) a single broadened emission line.
b) broad emission with a narrow spike-like
absorption core.
c) broad absorption with a narrow spike-like
emission core.
d) a narrow spike-like emission core and low
broad shoulders of emission.
A cold thin cloud of interstellar gas embedded in
a hotter, thinner surrounding medium would
yield 21-cm radio spectral emission lines of
hydrogen showing
a) a single broadened emission line.
b) broad emission with a narrow spike-like
absorption core.
c) broad absorption with a narrow spike-like
emission core.
d) a narrow spike-like emission core and low
broad shoulders of emission.
The primary reason that massive O-type stars are
not found in the galactic halo is because they are
a) too massive to be kicked into the halo from
the disk.
b) so massive that they settle into the thinner
disk.
c) too short-lived to have persisted from halo
formation until today.
d) closer to us in the disk than in the extended
halo.
The primary reason that massive O-type stars are
not found in the galactic halo is because they are
a) too massive to be kicked into the halo from
the disk.
b) so massive that they settle into the thinner
disk.
c) too short-lived to have persisted from halo
formation until today.
d) closer to us in the disk than in the extended
halo.
One model for the formation of the Milky Way
can be divided into 2 phases: a spherical gas cloud (halo)
collapsed to form the stars in the Milky Way's
spheroid, then rapidly rotating gas collapsed into
a disk-shaped configuration of stars. Since disk
stars have higher metallicity, which is most
likely? Gas ejected from the
a) spheroid stars enriched the material now in the disk
stars.
b) spheroid stars decreased their metallicity.
c) spheroid decreased its angular momentum.
d) disk stars puffed out the spheroid stars into a
rounder shape.
One model for the formation of the Milky Way
can be divided into 2 phases: a spherical gas cloud (halo)
collapsed to form the stars in the Milky Way's
spheroid, then rapidly rotating gas collapsed into
a disk-shaped configuration of stars. Since disk
stars have higher metallicity, which is most
likely? Gas ejected from the
a) spheroid stars enriched the material now in the disk
stars.
b) spheroid stars decreased their metallicity.
c) spheroid decreased its angular momentum.
d) disk stars puffed out the spheroid stars into a
rounder shape.
The orbital speed of an object depends only
on the amount of mass between it and the
galactic center:
The velocity should diminish with distance.
It doesn’t…
More than twice the mass of the galaxy would
have to be outside the visible part to
reproduce the observed curve below – a dark
halo.
What could this ―dark matter‖ be? It is dark at all
wavelengths, not just the visible.
• Stellar-mass black holes?
Probably no way enough of them could have been
created
• MACHOs – Brown dwarfs, faint white dwarfs,
and red dwarfs?
Currently the best star-like option
• WIMPs – weird subatomic particles?
Could be, although no direct evidence so far
A Hubble search for red dwarfs turned up too
few to account for dark matter.
If enough existed, they should have been
detected.
The bending of
spacetime can
allow a large mass
to act as a
gravitational lens.
Observation of
such events
suggests that lowmass white dwarfs
could account for
as much as 20%
of the mass
needed.
The rest is still a
mystery.
This is a view toward the galactic center, in
visible light: the two arrows in the inset
indicate the location of the center; it is entirely
obscured by dust.
These images—in infrared, radio, and X-ray—offer a
different view of the galactic center:
The galactic center appears to have:
•A ring of molecular gas 400 pc across
• Strong magnetic fields
• A rotating ring or disk of matter a few parsecs
across
• A strong X-ray source at the center
Apparently, there is an enormous black hole at
the center of the galaxy, which is the source of
these phenomena.
An accretion disk surrounding the black hole
emits enormous amounts of radiation.
These objects are very close to the galactic center.
The orbit on the right is the best fit; it assumes a
central black hole of 3.7 million solar masses.