Download Chapter14- Our Galaxy - SFA Physics and Astronomy

Chapter 14
Our Galaxy
All sky view of the Milky Way
Questions

How many stars are in the Milky Way?
About 200 billion

How many galaxies are there?
About 40 billion (approximately)
Questions

How old is the Milky Way Galaxy?
It
is 15 billion years old and will remain
active for a another 10 billion years.
Recall:
The Sun is ~ 4.5 billion
years old.
Are all stars members of the
Milky Way?

No, most stars in the universe are in other
galaxies.

What is the structure of the Milky Way?

We know in general, that it has the shape of
a disk with spiral arms and a central bulge.
Where in the Milky Way is our
solar system located?

The solar system is located in a spiral arm
about 28,000 light years (8kpc) from the
center of the Milky Way.
Sketch of the Milky Way Galaxy
Artist’s conception of the Milky Way from its outskirts
Edge on schematic view of the Milky Way
Galaxy Formation
The Star-Gas-Star Cycle

Generations of stars continually recycle the same
galactic matter.

Supernova stir and heat the interstellar medium
and contribute new heavy elements.

Stars return mass to interstellar medium in two
ways
 Stellar winds (solar wind)
 Death events (planetary nebula and supernova)
The Star-Gas-Star Cycle
Evidence of Star-Gas-Star Cycle

A dying low mass star
returns gas to the
interstellar medium in
a planetary nebula.
(HST)
X-ray emission from a supernova
remnant Cassiopeia A. (Chandra Xray Observatory)
Atomic Hydrogen Gas

Most of the intersellar medium is
composed of atomic hydrogen gas (70%).

Atomic hydrogen emits a spectral line with
a wavelength of 21 cm – in the radio!

Using radio telescopes, Astronomers can
detect this 21 cm radiation coming form all
different directions.
Molecular Clouds

Cool clouds of molecular gas from out of hydrogen
and other elements.

Molecular clouds are formed from:
 H2 - Hydrogen gas
 CO – Carbon Monoxide
 H2O – Water
 NH3 – Ammonia

Gravitational forces in molecular clouds collect
molecules into dense cores, eventually becoming
protostars.
 Molecular
Cloud in the
constellation
Orion.
Completing the Cycle

After a few stars begin to form in a cluster,
their UV radiation begins to ionize the
surrounding gas of the cloud from which they
have formed.

The surrounding cloud material begins to
ionize and give off its own light.

We now have an ionization or emission nebula.
New stars forming
Molecular
clouds
The Eagle Nebula: a
complex of clouds where
new stars are forming
Distribution of gas in the Milky Way
The Milky Way in Motion
v
Rotation Curve for a merry go round
v = R
R
Rotation Curve for the
planets in our solar
system
The curve declines with
radius because the mass is
concentrated at the center
(the Sun)
The Milky Way’s Rotation Curve is flat,
indicating that the Milky Way’s mass is not
concentrated in the center bulge
This implies the existence of “Dark Matter”
Dark Matter/Missing Mass
Spiral Arms

Observational evidence suggests that they
result as a consequence of waves generated
by star formation.

Theoretical models indicate that
disturbances in the disk form spiral density
waves that are the source of the galaxy’s
spiral arms.
Galaxy M51 - A spiral galaxy with two prominent
spiral arms. (HST)
Red blobs are ionization
(emission) nebulae
Relationship of dust, gas, and new star clusters in a spiral arm
The Galactic
Center
In visible light, dusty
gas obscures the
galactic center.
Radio image of Sgr A* shows
bright filaments that trace out
magnetic field lines around a
central black hole
Radio image of galactic center shows
ionized gas swirling around Sgr A*
Star Velocities Near The Galactic Center
Galactic Center -Evidence of Black Hole (artist’s rendering)
How do we know that we are
located in the spiral arm?

In 1917 Harlow Shapley discovered that
the globular clusters form a huge spherical
system that is not centered on the Earth.
Finding Our Place
Early view
Sun at center
Globular Clusters
evenly distributed
Sun
Globular Clusters
unevenly distributed
Harlow
Shapley
The Great Debate - April 26, 1920
A galaxy is an
island universe!
And many nebulae
are galaxies.
Harlow Shapley
These are just nebulae
within the Milky Way!
Heber Curtis
RR Lyrae and Cepheid Variable
Stars

These are both pulsating variable stars.

Their pulsation periods are on the order of
a few days.

Using the period-luminosity relationship,
distances to other galaxies can be estimated
RR Lyrae &
Cepheids on H-R
diagram
Period-luminsoity relationship
Light curves for
RR-Lyrae and
Cepheid Variables
New Distance Ladder
Messier Catalogue
Many Messier objects are actually other galaxies
… and more galaxies still

In 1923, Edwin Hubble was examining photographic plates
of the Andromeda Nebula M31

Hubble located three novae, each marked with an "N.“ (he
was looking for Novae)

One of these novae, however, turned out to be a Cepheid
variable, a star that changes predictably in brightness

The "N" was crossed out and the star was relabeled
"VAR!“

This Cepheid, and others subsequently discovered in the
Andromeda Nebula, enabled Hubble to prove that the
Nebula was not a star cluster within our own Milky Way,
but a galaxy more than a million light years away.
Edwin
Hubble
Edwin
Hubble
http://www.seds.org
Right Ascension
Declination
Distance
Visual Brightness
Apparent Dimension
00 : 42.7 (h:m)
+41 : 16 (deg:m)
2900 (kly)
3.4 (mag)
178x63 (arc min)
Andromeda Nebula M3, is actually another spiral galaxy
End of Section