Transcript Summary Of the Structure of the Milky Way

Summary Of the Structure of the
Milky Way
The following graphical data is meant to help
you understand WHY astronomers believe
they know the structure (dimensions, content,
etc…) of the Milky Way Galaxy.
This all sky map displays the locations of the most common stars (Main Sequence
K & M stars) down to 10’th magnitude. Notice that the distribution is fairly random
with equal number as stars in all directions. This suggests that the Sun is in the
center of this star distribution.
In fact, the Sun only appears to be in the center of the distribution because these
K & M stars are so faint that we cannot see them at great enough distances to
detect the larger structure of the galaxy.
The distances that appeared on the right are the distances to 10 randomly
chosen stars from this map. They illustrate the point that even though we
believe these stars to be most common, they are not good indicators of
galactic structure because they cannot be seen at sufficient distances to
resolve the structure of the galaxy.
Using these stars to map the galaxy would be similar to using the trees
nearest you to map a forest. You would only be able to see trees within a
few 10’s of meters before they began to obscure each other. So you could
only “see” that part of the forest near you, not the larger structure.
25 pc
143 pc
55 pc
4 pc
3 pc
25 pc
100 pc
18 pc
21 pc
19 pc
This all sky map displays the locations of some of the most luminous stars
(Spectral types O, B & A) down to 10’th magnitude. The distribution is NOT
random, but displays a faint, but noticeable, “wavy” pattern, similar in shape to the
ecliptic.
This “wavy” distribution suggests that these stars form a roughly planar structure
in space in which the Sun appears to be near the center.
The distances that appeared on the right are the distances to 10 randomly
chosen stars from this map. These stars are much more luminous stars but
still cannot be seen far away in the disk of the galaxy. However, they can be
seen at distances sufficient to reveal the disk-like nature of their distribution.
These stars are more useful in revealing the structure of the
galaxy, but are not sufficient to fully reveal the dimension s of our
galaxy. We must turn to even more luminous objects – star
clusters.
20 pc
53 pc
55 pc
41 pc
100 pc
63 pc
32 pc
167 pc
41 pc
44 pc
This all sky map displays the locations of over 600 open clusters in the Milky Way.
These clusters are more luminous than O or B stars since they contain many
hundreds of stars, some of which are O & B stars.
The “wavy” distribution is even more apparent in this map indicating the Sun is
part of a flat disk of stars.
The distances that appeared on the right are the distances to 10 randomly
chosen open clusters from this map. They can be seen to a much greater
depth into the disk of the Milky Way than single stars. Even so, there are
very few open clusters that are visible beyond 3,000 parsecs (about 10,000
light years). This limit is due to the obscuration of light by dust in the plane
of the Milky Way.
Close study of these open clusters can reveal some details of the galactic
structure. The next two slides illustrate this point.
2,600 pc
5,802 pc
480 pc
1,700 pc
800 pc
2,200 pc
3,000 pc
1,900 pc
2,100 pc
1,250 pc
Interstellar dust
causes extinction of
starlight by absorption
and scattering.
Interstellar dust grains
are microscopic
particles that
effectively scatter
short wavelength blue
light, but let the longer
wavelength red light
pass through. The
effect called
Interstellar
Reddening causes
stars behind dust to
appear dimmer and
redder.
Interstellar dust limits our ability to “see” within the disk of our
galaxy by the extinction of starlight over large distances. We can
only “see” at visible wavelengths to about 10,000 ly within the
Milky Way’s Disk
Open Cluster Distances
100%
Percent
This graph displays the percentage of 600
Open Clusters closer than the distance on the
abscissa (x-axis).
10%
You can see that 90% of this sample of Open
Clusters is within 3,300 parsecs or 10,000
light years. This limit is an artificial limit
caused by dust in mid-plane of the Milky Way.
1%
100
1,000
Distance, Parsecs
10,000
If you look carefully you can see that
the central point of this distribution is
slightly below the Sun. This is an
indication that the Sun is about 14
parsecs above the galactic mid-plane.
The study of these Open Clusters
50 how far the Sun is above the
reveals
mid-plane of the galaxy!. Now, how
cool is that!
Open Clusters
Open Clusters
galaxy. There are many more Open
Clusters near the mid-plane of the
Galaxy than farther away from it. The
width of this distribution could be
interpreted to be about ±125 parsecs.
Many astronomers call this region the
“thin disk” or the “gas layer” within the
thicker disk of stars.
40
30
Number
Model
The Sun is near the mid-plane of the
This graph displays the distance of 600
Open Clusters above or below the
Sun.
20
10
0
-300
-200
-100
0
100
Height above/below the Galactic Mid-plane, Parsecs
200
300
Globular Clusters are very large
and ancient star clusters
These HR diagrams are for a star
cluster of progresively older ages.
This HR diagram are is typical for a
globular cluster. You can see tht it
most cllosely resembles that of a
very old star cluster.
•
•
•
A Cepheid is a star that pulsates between
a larger, brighter state and a smaller,
dimmer one. They are very luminous
variable stars, of a class that was especially
massive and hot, using up their fuel early,
leaving them in this pulsating condition.
The term cepheid originates from δ Cephei
in the constellation Cepheus, the first star
of this type identified, by John Goodricke in
1784.
. At the dimmest part of a Cepheid's cycle,
the ionized Helium gas in the outer layers
of the star is opaque, and so is heated by
the star's radiation, and due to the
increased temperature, begins to expand.
As it expands, it cools, and so becomes
less ionized and therefore more
transparent, allowing the radiation to
escape. Then the expansion stops, and
reverses due to the star's gravitational
attraction. The process then repeats.
• The luminosity of a Cepheid is found to correlate very closely to its period
of oscillation. The longer the oscillation period the greater the peak
luminosity of the star.
• Thus the luminosity of a Cepheid can be determined by measuring its
period of oscillation, making Cepheids good standard candles.
RR Lyrae variables are periodic variable stars, commonly found in globular clusters, and often
used as standard candles to measure galactic distances.
• This type of variable is named after the prototype, the variable star RR Lyrae in the constellation
Lyra.
• RR Lyraes are pulsating horizontal branch stars of spectral class A (and sometimes F), with a
mass of around half the Sun's.
• RR Lyrae stars pulse in a manner similar to Cepheid variables. They are much more common
than Cepheids, but also much less luminous. The average absolute magnitude of an RR Lyrae is
about 0.75, only 40 or 50 times brighter than our Sun. Their period is shorter, typically less than
one day, sometimes ranging down to seven hours.
Movie of RR Lyrae Variables
Please notice in the movie below that all of the variable stars (i.e. the
blinking stars) all have about the same maximum brightness. Since
these stars are all at the same distance (because they are part of the
globular cluster), they then must have the same luminosity. So these
RR Lyrae stars are good standard candles because they all have the
same luminosity.
http://apod.nasa.gov/apod/ap070415.html
This all sky map displays the locations of about 150 globular clusters in the Milky
Way. These clusters are more luminous than open clusters because they typically
contain 500,000 stars many of which are red giants. Also, they are not (generally)
buried in the dust of the Milky Way's disk, but are seen in the dust-free halo above
and below the disk.
The distribution of globular clusters is very different from the open clusters.
Globular clusters appear to be concentrated in one area of the sky towards the
constellations of Sagittarius, Scorpio and Ophiuchus.
The distances that appeared on the right are the distances to 10 randomly
chosen globular clusters near the center of their distribution on this map.
They are seen to be much farther away that the open clusters or the single
stars we examined earlier.
The average distance of just these ten randomly selected globular clusters
is 9,230 parsecs and is close to the accepted distance to the center of the
galaxy of between 8,000 and 10,000 parsecs.
A close examination of the distances and distribution of globular clusters
tells astronomers the location of the Sun and the true extent of the Milky
Way. The next two slides illustrate these points.
7,100 pc
24,500 pc
9,700 pc
6,700 pc
8,500 pc
8,200 pc
8,600 pc
10,500 pc
4,700 pc
3,800 pc
There are no Globular Clusters closer
than 3,000 parsecs (almost 10,000
light years) from the Sun. The number
of clusters begins to drop between
8,000 and 10,000 parsecs from the
Sun indicating that the center of this
distribution must be in that range.
This graph displays the distance from
the Sun of 105 Globular Clusters
located in the constellations of
Sagittarius, Scorpio and Ophiuchus.
Milky Way Globular Clusters
25
20
Thus the Globular Clusters reveal the
location of the center of the galaxy
even though we cannot see it directly
because of dust in the galactic midplane.
Number
15
STOP
Galactic
Center
10
5
0
0
1
2
3
4
5
6
7
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9
Distance from Sun, kpc
10 11 12
13
About half of the Globular Clusters are
located within a sphere of 5,000 parsec
radius from the center. The number
drops rapidly at increasing distances.
90% of all Globular Clusters are within
25,000 parsecs from the galactic
center. 80
This graph displays the distance from
the galactic center of 145 Globular
Clusters.
Milky Way Globular Clusters
70
60
Thus the Globular Clusters define the
size of the galaxy by delimiting the
edge of the galactic halo.
Number
50
40
30
20
STOP
Outer
Boundary
of Galactic
Halo
10
0
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
Distance from Galactic Center, kpc
Where is M13: Open Clusters
On the web, search for and open the program called “Where is M13”.
Using the preset filters, explore the distribution of
1. Nearby Stars
2. Open Clusters
3. Globular Clusters
The following three images show parts of the galaxy with the Sun (the Orange
dot) and the distribution of the three sets of objects listed above.
What does each distribution tell us of our place in the galaxy?
The distribution of nearby stars suggests that we inhabit a small spherical distribution
of stars with the Sun at the center.
What does each distribution tell us of our place in the galaxy?
The distribution of open clusters suggests that the Sun inhabits a thin disk of stars with a
radius of a ten thousand light years and a thickness of a few thousand light years (like a
hockey puck) with the Sun very near the center of the distribution.
What does each distribution tell us of our place in the galaxy?
The distribution of globular clusters suggests that the Sun is far from the center of the
galaxy, about 28,000 light years (equivalently 9,000 pc).
Summary So far…
• The Distribution of stars can reveal part of the disk-like
nature of the Milky Way galaxy, but are not “deep”
enough probes to fully reveal the structure of the Milky
Way.
• Open clusters can define the thickness of the Milky
Way’s thin disk where star formation is active.
• Globular clusters allow astronomers to know the
direction to the center of our galaxy, in spite of the
obscuring dust that prevents us from seeing it directly,
and tells astronomers the distance of the Sun from the
galactic center and the largest dimension of the Milky
Way – the Halo.
• In addition, more complex analysis of globular clusters
has yielded the orbital speed of the Sun as it orbits the
galactic center and the age of the Milky Way.
• Finally, when knowledge of the orbital speed of Sun is
combined with its distance from the galactic center an
estimate of the mass of the galaxy can be obtained.
This all sky map displays the locations of very many distant galaxies
beyond the Milky Way.
The distances that appeared on the right are the distances to 10
randomly chosen galaxies on this map. As you can see, galaxies
are visible many millions of light years away except in the Zone
of Avoidance located near the galactic mid-plane.
This zone where almost no external galaxies can be seen is due
to the obscuring dust in the Milky Way. We believe there are
many galaxies in that zone but we cannot see them through the
dust of our own galaxy.
Further, the larger circular region of the zone near the center is a
silhouette of the bulge of our own galaxy.
33,000,000 pc
50,000,000 pc
53,000,000 pc
76,000,000 pc
62,000,000 pc
42,000,000 pc
75,000,000 pc
4,200,000 pc
16,400,000 pc
92,000,000 pc
These final two slides present
images of other galaxies thought
to be similar to the Milky Way in
Structure
You should be able to attach numbers or
dimensions to each of the labeled objects on
the following images that reflect the
dimensions of the Milky Way and the position
of the Sun
Halo
Boundary
Analogs to the Milky Way Galaxy
Spiral galaxy NGC 4565
Bulge
Gas Layer
Sun
Disk
Disk
Halo
Disk
Bulge
Bar
This is the
current model
of our Milky
Way Galaxy
with a central
bulge, a bar,
two dominant
spiral arms
and several
smaller spiral
arm
fragments.