Transcript The Milky Way Galaxy

The Milky Way Galaxy
The infinitude of creation is great enough
to make a world, or a Milky Way of worlds,
look in comparison with it what a flower or
an insect does in comparison with the
Earth.
Immanuel Kant (1724 – 1804)
German philosopher
WHAT DO YOU THINK?
1.
How big is the Milky Way Galaxy?
2.
Where is our solar system located?
3.
Is there really a HUGE black hole at the center
(and why weren’t we sucked into it in 2012 ??)
Our View of the Milky Way…
Our view

Clearly a “disk” shaped, with us inside

Optical view blocked by dust & cold gas

Infer we probably look like other galaxies
Spiral?
 Elliptical?
 Neither?

M51 (enhanced)
A spiral galaxy “edge on”
Some other spirals…
Our view

Infer we probably look like other galaxies
=> Milky Way is a Spiral Galaxy!
 We
see new, young stars, gas, dust
|
 We see the “pancake” shape across our sky
indicating a disk.
 We
can use other wavelengths to confirm structure
of spiral arms.
Drawings – not photos – of our galaxy!
Milky Way
Structure
•Disk
•Bulge
•Halo
Regions of the Milky Way Galaxy
diameter of disk = 100,000 l.y. (30,000 pc)
radius of disk = 50,000 l.y. (15,000 pc)
thickness of disk = 1,000 l.y. (300 pc)
number of stars = ~200 Billion
Sun is in disk,
28,000 l.y. out
from center
Mapping the Galaxy

Using Optical light
Cepheid variables in “globular clusters”
 Distribution around galactic center defines
location of our solar system


Using Radio Waves
Cool Hydrogen Gas emits 21-cm light
 Map out spiral arms

Mapping the Galaxy

Using IR light


Warm clouds of gas/dust locates star
formation sites in spiral arms
Using X-rays
Highest energy emissions from center of
galaxy
 Supermassive Black Hole

Views of the Milky Way
Mapping the Galaxy- Optical

Using Optical light
Cepheid variables in “globular clusters”
 Distribution around galactic center defines
location of our solar system

Cepheid Variable Stars in Globular Clusters

Henrietta Leavitt, along with Harlow
Shapely, used variable stars to determine
the direction & distance to the center
Globular Clusters



Not centered
around Sun
Centered
about 30,000
light years
away
Towards
Sagittarius
Mapping the Galaxy
Using Radio Waves
Cool Hydrogen Gas emits 21-cm light
 Map out spiral arms

Structure of Milky Way Galaxy

Disk
younger generation of stars
 contains gas and dust
 location of open clusters


Bulge


mixture of both young & old stars
Halo
older generation of stars
 contains no gas or dust
 location of globular clusters

The Star–Gas–Star Cycle
Stellar Orbits in the Galaxy
Stars in the disk all orbit the
Galactic center:
•
•
•
in the same direction
in the same plane (like planets
orbit our sun)
they “bobble” up and down


due to gravitational pull from
the disk
this gives the disk its thickness
Stellar Orbits in the Galaxy
Stars in the bulge and halo all
orbit the Galactic center:
•
•
•
in different directions
at various angles to the disk
they have higher velocities

not slowed by disk as they
plunge through it
Spiral Arms
M 51

Galactic disk does not
appear solid
•
Spiral arms are not fixed
strings of stars which revolve
like the fins of a fan.
Spiral Arms

Caused by compression waves
which move around the disk.
• Increase density of matter at
“crests”
•

M 51
density waves revolve at
different speed than individual
stars orbit Galactic center
Note how the spiral arms appear
bluer compared to the bulge or the
gaps between the arms.
Does the Milky Way have a Bar?
Mapping the Galaxy

Using IR light

Warm clouds of gas/dust locates star
formation sites in spiral arms
Where the new stars are….
Where the new stars are….
Views of the Center!
Infrared
Visual
The Center in Radio & X-Rays
Although dark in visual light,
there are bright radio, IR, and
X-ray sources at the center of
the Galaxy, known as Sgr A*.
Radio Image of Center of Milky Way
“SNR” = Supernova Remnant
Sgr A = brightest radio source in Sagittarius
11 years of
observation
in IR
6 stars
orbiting
unseen
central
mass ~ 4
Million x
Mass of Sun
Mapping the Center - IR
Mapping the Center in IR

Use IR telescopes to measure orbits of fastmoving stars near the Galactic center.



One star passed within 1 light-day of Sgr A*
using Kepler’s Laws, mass = 2.6 million M
What can be so small, yet be so massive?
Mapping the Galaxy in X-Rays
Using X-rays

Highest energy
emissions from
center of galaxy

Confirms
Supermassive
Black Hole
Chandra image of Sgr A*
Mapping the Galaxy in X-Rays
Chandra image of Sgr A*

Rapid flare rise/drop time (< 10 min)

Tiny emission region only 20 times the
size of event horizon of 2.6 million M
black hole.

Observations consistent w/
supermassive black hole at the center
of our Galaxy.

Energy from flare probably came from
a comet-sized lump of matter…torn
apart before falling beneath the event
horizon!
Missing Mass?

The edges of the galaxy orbit center “too
fast” to stay attached using gravity from
mass we “see”.

So there must be more mass we *don’t*
see pulling as well?

Dark Matter (Missing Mass)
Summary of Key Ideas
Discovering the Milky Way




A century ago, astronomers were divided on
whether all stars and nebulae are part of the
Milky Way Galaxy.
The Shapley–Curtis debate was the first major
public discussion between astronomers as to
whether the Milky Way contains all the stars in
the universe.
Cepheid variable stars are important in
determining the distance to other galaxies.
Edwin Hubble proved that there are other
galaxies far outside of the Milky Way.
The Structure of Our Galaxy


Our Galaxy has a disk about 100,000 light-years
diameter and about 2000 light-years thick, with a high
concentration of interstellar dust and gas. It contains
around 200 billion stars.
Interstellar dust obscures our view into the plane of the
galactic disk at visual wavelengths. However, hydrogen
clouds can be detected beyond this dust by the 21-cm
radio waves emitted by changes in the relative spins of
electrons and protons in the clouds.
The Structure of Our Galaxy



The center, or galactic nucleus, has been studied at
gamma-ray, X-ray, infrared, and radio wavelengths,
which pass readily through intervening interstellar dust
and H II regions that illuminate the spiral arms. These
observations have revealed the dynamic nature of the
galactic nucleus, but much about it remains unexplained.
A supermassive black hole of about 4 x 106 Msun exists in
the galactic nucleus.
The galactic nucleus of the Milky Way is surrounded by a
flattened sphere of stars, called nuclear bulge, through
which a bar of stars and gas extend. The entire Galaxy is
surrounded by a halo of matter that includes a spherical
distribution of globular clusters and field stars, as well as
large amounts of dark matter.
The Structure of Our Galaxy



A disk with at least four bright arms of stars, gas, and
dust spirals out from the ends of the bar in the galactic
nuclear bulge.
Young OB associations, H II regions, and molecular
clouds in the galactic disk outline huge spiral arms where
stars are forming.
The Sun is located about 26,000 light-years from the
galactic nucleus, between two major spiral arms. The
Sun moves in its orbit at a speed of about 828,000 km/h
and takes about 230 million years to complete one orbit
around the center of the Galaxy.
Mysteries at the Galactic Fringe
From studies of the rotation of the Galaxy,
astronomers estimate that its total mass is
about 1 x 1012 Msun. Much of this mass is
still undetectable.
Key Terms
dark matter (missing mass)
disk (of a galaxy)
distance modulus
galactic cannibalism
galactic nucleus
galaxy
halo (of a galaxy)
microlensing
Milky Way Galaxy
missing mass
nebula (plural nebulae)
nuclear bulge
rotation curve (of a galaxy)
Sagittarius A
Shapley–Curtis debate
spin (of an electron or proton)
spiral arm
synchrotron radiation
21-cm radio radiation
WHAT DID YOU THINK?
What is the shape of the Milky Way
Galaxy?
 The Milky Way is a barred spiral galaxy. A
bar of stars, gas, and dust runs through its
central region. It has at least four spiral
arms and is surrounded by a spherical
halo of stars and dark matter.

WHAT DID YOU THINK?
Where is our solar system located in the
Milky Way Galaxy?
 The solar system is between the
Sagittarius and Perseus spiral arms, about
26,000 light-years from the center of the
Galaxy.

WHAT DID YOU THINK?
Is the Sun moving through the Milky Way
Galaxy and, if so, how fast?
 Yes. The Sun orbits the center of the Milky
Way Galaxy at a speed of 828,000 km/h.
