A1 20 Milky Way

Discovering our Milky Way
LACC: §24.1, 3, 5

• Where are we in the Universe?
• What are “spiral nebulae”?
• What is our Milky Way galaxy like?

An attempt to answer the “big questions”: where
are we? how did we get here?

Thursday, May 6, 2010 1

The Discovery of the Milky Way
• 1755 Immanuel Kant speculates that there may exist "Island
Universes" like our Milky Way.

• 1785 William Herschel studies star counts along several hundred
lines of sight in the galaxy.

http://cass.ucsd.edu/physics/ph162/lect1.html


Harlow Shapley (1915)
and Globular Clusters)

Sketch based on Shapley's original data,
uncorrected for interstellar absorption. The Sun
is located at the center of the axes (looking
roughly side-on), and the center of the Milky
Way inferred by Shapley is marked by the red X.
http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit4/milkyway.html

“Spiral Nebulae”
• 1920 Shapley Curtis Debate
Harlow Shapley took the position that the universe consisted
only of our Galaxy, which was very large -- about 300,000
light-years in diameter. The spiral nebulae, while distant, were still
part of our galaxy
Heber Curtis argued for a smaller galaxy - about
30,000 light-years in diameter - that was one of a
vast number of similar systems. The spiral nebulae,
he said, were separate star systems similar
to our own galaxy, and at great distances "from
500,000 to 10,000,000 light-years away"
• 1923 Edwin Hubble discovers Cepheid
Variable stars in Messier 31 - the Great
Nebula in Andromeda, estimating its
distance as nearly 0.3Mpc (modern value is
about 0.7Mpc), well outside our Galaxy.
http://ottawa-rasc.ca/features/marchHubble/index.html

Milky Way - Spiral Arms
(via radio observations)

http://www.astro.wisc.edu/goat/article/7/the-milky-way-using-real-data

The Size of the Milky Way

http://zebu.uoregon.edu/~imamura/123/lecture-2/lecture-2.html

What is a parsec?
An astronomical unit of length, equal to the
distance at which the radius of the Earth's
orbit subtends an angle of one arcsecond.
The name is a contraction of "parallax-
second."

1 parsec = 3.259 light-years = 206,265 AU
= 30.83 trillion km = 19.16 trillion miles.

The parsec is generally used by
astronomers in preference to the light-
year. For larger distances, the
kiloparsec (kpc) = 1,000 pc or
megaparsec (Mpc) = 1,000,000 pc are used.

http://www.daviddarling.info/encyclopedia/P/parsec.html


The Size of the Milky Way
This page was copied from Nick Strobel's Astronomy Notes.
Go to his site at www.astronomynotes.com for the updated
and corrected version.


A recent survey of stars

Milky Way - Bulge:
conducted with the Spitzer Space
Telescope is convincing
astronomers that our Milky Way

A Barred Spiral
Galaxy is not just your ordinary
spiral galaxy anymore. Looking
out from within the Galaxy's disk,
the true structure of the Milky
Way is difﬁcult to discern.
However, the penetrating infrared
census of about 30 million stars
indicates that the Galaxy is
distinguished by a very large
central bar some 27,000 light-
years long. In fact, from a
vantage point that viewed our
galaxy face-on, astronomers in
distant galaxies would likely see a
striking barred spiral galaxy
suggested in this artist's
illustration. While previous
investigations have identiﬁed a
small central barred structure, the
new results indicate that the Milky
Way's large bar would make
about a 45 degree angle with a
line joining the Sun and the
Galaxy's center. DON'T PANIC ...
astronomers still place the Sun
beyond the central bar region,
about a third of the way in from
http://apod.nasa.gov/apod/ap050825.html the Milky Way's outer edge.

Milky Way: What Do We See?

Our sun orbits every
225–250 million years
https://sites.google.com/site/earthsplaceintheuniverse/

Here's the Universe within 5000 The stars on the plot are all
light years, our little arm of the thousands of times brighter
Milky Way galaxy, the Orion Arm. than the Sun. The brightest star
Virtually every star we can see here is Rho Cassiopeia (ρ Cas,)
with the naked eye from Earth is some 100,000 times brighter
within this distance. than the Sun. At 4000 light
years away it is barely visible
to the naked eye.

Milky Way: What Do We See?
https://sites.google.com/site/earthsplaceintheuniverse/

Discovering our Milky Way
LACC: §24.1, 3, 5
• Where are we in the Universe? We are about 2/3
away from the center of our galaxy’s core (Hershel
and Shapley). Our galaxy is nowhere special.
• What are “spiral nebulae”? They are other galaxies;
Curtis suspected this (Shapley didn’t), Hubble
proved it using a Cepheid variable
• What is our Milky Way galaxy like? It is a barred-
spiral galaxy about 30 kpc wide with a Bulge
(barred), Disk (spiral arms), Halo (globular clusters)
An attempt to answer the “big questions”: where are we?
how did we get here?

LACC HW: Franknoi, Morrison, and
Wolff, Voyages Through the Universe,
3rd ed.

• Ch 24: Tutorial Quizzes accessible from: http://
www.brookscole.com/cgi-brookscole/course_products_bc.pl?
ﬁd=M20b&product_isbn_issn=9780495017899&discipline_number=19

Due at the beginning of next week’s ﬁrst class
period (unless there is a test that week, in which
case it’s due the same period as the test).
Be working on your Distance Ladders.


Our Milky Way Galaxy
LACC: §24.1, 3, 5

• Formation and Evolution
• Composition
• Recent Discoveries

An attempt to answer the “big questions”: where are
we? how did we get here?


The Formation of the Milky Way
A cloud of hydrogen/helium gas
begins to form myriad stars. As this
continues, the cloud may contract
somewhat and the assemblage of
stars begin to rotate around a
common center.... With rotation, there
is a tendency for the cloud to assume
a more oblate ellipsoidal shape and
begin to spin. The spinning produces
strings of stars in at least several
distinct arms. When well developed,
the stars have organized into a spiral
galaxy.

http://rst.gsfc.nasa.gov/Sect20/A2.html

http://www.youtube.com/watch?
v=n0jRObc7_xo&feature=related


Spiral Arms are Density Waves

http://abyss.uoregon.edu/~js/ast123/lectures/lec10.html

Spiral Arms are Density Waves


Composition of the Milky Way
This page was copied from Nick Strobel's Astronomy Notes. Go to his site at
www.astronomynotes.com for the updated and corrected version.
The components merge smoothly into each other
with the stellar halo among the disk and the inner
part of dark matter halo and the dark matter halo
among the disk, stellar halo, and bulge, etc. The
bulge is the elliptical-shaped center part of the
Galaxy about 1000 to 2000 parsecs in radius. It
had lots of star formation early on, so now it is
made of tens of billions of old, metal-rich! stars.
The disk is the thin pancake-shaped part about
400 parsecs thick and 15 to 20 thousand parsecs
in radius with the Sun 8000 parsecs from the
center. The disk contains over 98% of the dust
and gas in the Galaxy and has a few hundred
billion stars. Some stars continue to form so the
disk has some young metal-rich stars. The gas
and dust are found in a layer that is thinner than
the star layer (the gas/dust layer is the thin dark
line at the midplane of the disk in the picture above
and the star layer is the thicker light band).

The stellar halo is a roughly spherical distribution of hundreds of millions of old, metal-poor stars that has
increasing concentration of stars toward the center of the galaxy. It is about 20 to 30 thousand parsecs in radius
and it may contain small amount of hot gas, but the disk contains the vast majority. Most of the globular
clusters are found in the halo and, like the halo stars, the number of them increases toward the galactic center.
If the solar system was at the center, you would see approximately the same number of globular clusters in any
direction you looked in the sky. Since the globulars are found bunched up in one part of the sky, i.e., they are
swarming around some other point in the Galaxy, and we are not at the center. The dark matter halo is denser
toward the center. It extends further out than the stellar halo.


Stellar Population:
Population I Stars
Population I includes the younger stars in the disk/plane of
the galaxy. Because these stars formed recently, they have
all be enriched in heavy elements produced in previous
generations of stars.

http://ircamera.as.arizona.edu/NatSci102/NatSci102/lectures/milkywayparts.htm


Stellar Population:
Population II Stars
Population II is the older stars
that tend to lie around the
center and in globular clusters,
and hence have orbits that take
them well out of the disk/plane.
Many of these stars were
among the ﬁrst to form, and
hence they tend to be almost
pure hydrogen and helium, not
enriched by previous
generations of stars because
there were no previous
generations. From Gene Smith, http://
casswww.ucsd.edu/public/tutorial/Galaxies.html

http://ircamera.as.arizona.edu/NatSci102/NatSci102/lectures/milkywayparts.htm

Milky Way Satellite Galaxies

Not shown here is the
Canis Major Dwarf
Galaxy--discovered in
2003, it is 42 000 ly from
our galactic center (and
about 25 000 ly from us).

http://8minutesold.com/?p=135

The Galactic Core

A panoramic X-ray view, covering a 900 by 400 light year swath, shows that the center
of the Galaxy is a teeming and tumultuous place. There are supernova remnants: SNR 0.9-0.1,
probably the X-ray Thread, and Sagittarius A East. There are many bright X-ray sources, which
astronomers believe are binary systems—or pairs of orbiting objects—that contain a black hole
or a neutron star (the 1E sources). There are hundreds of unnamed point-like sources that
scientists think are solo neutron stars or white dwarfs, which all light up the region. In addition,
the massive stars in the Arches and other star clusters (the DB sources) will soon explode to
produce more supernovas, neutron stars, and black holes.

Sgr A*, the supermassive black hole that marks the center of the Milky Way
Galaxy. Sgr A* contains about 3 million times the mass of the Sun, and is gaining weight daily as
it pulls in more material. http://xrtpub.harvard.edu/edu/gcenter/


The Galactic Core: A
Supermassive Black Hole
This page was copied from Nick Strobel's Astronomy High-resolution infrared measurements of
Notes. Go to his site at www.astronomynotes.com for the orbits of the stars at the center show
the updated and corrected version. that a very compact mass---a super-
massive black hole---with about 3.7 million
solar masses lies at the center. The picture
below (courtesy of Andrea Ghez and the
UCLA Galactic Center Group) shows the
orbits of the stars around the black hole
from the years 1995 to 2006. At a distance
of 8 kpc for the Sun, the 0.2 arc second
scale bar in the ﬁgure corresponds to about
0.025 light years or 1600 AU. The object is
too compact to be a dense cluster of
stars---the Chandra X-ray Observatory's
observations of X-ray bursts from the object
place an upper limit of the diameter of the
object of the size of the Earth's orbit. An
expanding ring is also seen about 9000 light
years from the center. Other galaxy cores
have supermassive compact objects (the
Andromeda Galaxy, M32, Sombrero Galaxy,
M87, and many others).


Rotation Curves


Rotation Curves of Milky Way
To determine the rotation curve of
the Galaxy, stars are not used
due to interstellar extinction.
Instead, 21-cm maps of neutral
hydrogen are used. When this is
done, one finds that the rotation
curve of the Galaxy stays flat out
to large distances, instead of
falling off as in the figure above.
This means that the mass of the
Galaxy increases with increasing
distance from the center.
The surprising thing is there is very little visible matter beyond the Sun's orbital
distance from the center of the Galaxy. So the rotation curve of the Galaxy indicates
a great deal of mass, but there is no light out there. We call this the dark matter
problem, and states that the halo of our Galaxy is filled with a mysterious dark
matter of unknown composition and type.



What is Dark Matter?
Basically, according to careful observations, we know that up to
90% of the matter in galaxies must be in the form of "dark
matter" to account for the dynamics we observe. On top of that,
the dark matter appears to be distributed in a spherical halo
around the Milky Way, while the luminous matter is located
largely in the ﬂat disk.
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970417c.html

http://relativity.livingreviews.org/open?pubNo=lrr-2002-4&page=node9.html

Our Milky Way Galaxy
LACC: §24.1, 3, 5
• Formation and Evolution: protogalactic cloud,
globular clusters, population I and II stars
• Composition: Bulge--population I, random orbits;
Disk--population I, “normal” orbits; Halo--
population II, random orbits
• Recent Discoveries: Canis Major Dwarf Galaxy, our
nearest neighbor; Sagitarius A*, a supermassive
black hole in the galactic center (ir and x-ray); dark
matter halo (rotation curve)
An attempt to answer the “big questions”: where are
we? how did we get here?

LACC HW: Franknoi, Morrison, and
Wolff, Voyages Through the Universe,
3rd ed.

• Ch. 24, pp. 554-555: 11 (a--3 choices, b--1 choice, c--pick 2,
d--pick 1, pick 2).

• Ch 25: Tutorial Quizzes accessible from: http://
www.brookscole.com/cgi-brookscole/course_products_bc.pl?
ﬁd=M20b&product_isbn_issn=9780495017899&discipline_number=19

Due at the beginning of next week’s ﬁrst class
period (unless there is a test that week, in which
case it’s due the same period as the test).
Be working on your Distance Ladders.


A1 20 Milky Way

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