Download Goal: To understand the structure and makeup of our own Milky Way

Goal: To understand the
structure and makeup of our own
Milky Way Galaxy
Objectives:
1) Viewing our galaxy in the optical
2) Viewing our galaxy at other wavelengths
(to understand what can they tell us that
the optical cannot)
3) To learn about Formation of our galaxy
4) To understand Structure of our galaxy
5) To learn about Movements in our galaxy
In the optical
• Why does it look like this?
In the optical
• We are looking through our galaxy – much
like looking through a fog or through a
forest.
• Much like a forest, nearby brush masks
our view of the surrounding forest also
(thus the stars all over).
• We live in a
barred spiral
galaxy.
• We live
halfway out.
• The bright
clumps are
star forming
regions –
notice how
they lie on
the spiral
arms.
From far away
But how do we know if we can’t see
through the dust?
• Dust is a big problem when observing in
the optical.
• http://www.astro.livjm.ac.uk/courses/one/TEXTBO/INTE
RS02.HTM
• So, how do you get around
the dust?
Infrared and Radio!
• Infrared and Radio work great because radio waves are
bigger than the size of the dust.
• The dust can’t absorb the radio (would be similar to an
ant trying to catch a basketball).
Still can’t see everything
• What are the arcs above and below?
How do we see all our galaxy and map it?
• Use radio!
• Some wavelengths of radio give us specific molecules
(emission) – below is Hydrogen
Carbon Monoxide
But we still don’t get the full picture.
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How far away is everything?
We want to know things in 3D not 2D!
How do we do that?
To help lets consider this.
Imagine a star system with no planets.
An alien species colonizes this star system by building
orbiting homes.
The homes each have an optical light to light up their
own home.
However, the walls of their home are transparent to
optical light.
Their homes also emit infrared light, but the infrared light
gets absorbed by all the other houses.
How does one alien – without leaving his home map the
system of homes?
Map infrared and optical!
• Well, the alien can figure out where the homes are (in
the sky) – sort of.
• The alien will see a large bar of optical light which
represents the plane in the system all the homes orbit in.
• There will be some points above and below this bar
because of the nearest houses.
• The infrared map will have thick dark areas where there
are homes absorbing light – and we won’t see very far
there.
• However, the alien still can’t map them.
• Could we use a trick? Do the homes move with respect
to the alien’s home?
Orbits!
• The homes are in orbit around the star.
• So, the homes will move with respect to the alien’s
home.
• At any given position in the sky the radial
(outward/inward) and tangential (sideways, or motion in
the sky) motions of the other homes will depend on their
distance from the star!
• Can you detect the radial and/or tangential motions
somehow?
• A) no for both
• B) no radial and yes tangential
• C) yes radial and no tangential
• D) yes for both
Movement
• For the tangential direction – in a star system if
you could see individual objects you MIGHT be
able to watch them move.
• This is how we find asteroids and Trans
Neptunian Objects (TNOs) such as Pluto.
• However, if you have soooo many homes they
all merge together, then you won’t.
• Also, if the size of the system is so big that the
motions are too small to detect then you won’t
be able to see the tangential movement either.
Radial velocity
• However we can observe radial velocity!
• How? By using the Doppler effect!
• When an object moves towards us, the
wavelengths of light it emits (or sound on earth)
decrease (because the object is closer to us
when the wave finishes than when it starts – so
the shrink in the wave is the distance the object
travels in the time it takes to make the wave).
• When it moves away from us, the wavelength
increases.
• The fraction of the increase/decrease of the
wavelength just depends on the velocity of the
object!
Radio and Doppler shift
• In the radio, you are looking at specific wavelengths.
• Hydrogen for example has a very strong line at 21 cm.
• So, if you look near 21 cm you can get a spectrum from
say 20 cm to 22 cm.
• If you get a peak at 21.1 cm then you know the
Hydrogen you are looking at is moving away from us
(away because the wavelength is increased) at 1400
km/s.
• By using this we can map our the homes, and our
galaxy.
• Everything in our galaxy orbits around the center of our
galaxy – so we have one really big system.
Mapping the galaxy
• So, in each direction we look for the
brightness at each wavelength near
specific bands.
• You compare that to where you expect
each part to be in orbits.
• This gives you a map.
Other wavelengths useful
Formation of our galaxy
• Formations of spiral galaxies are
very much like the formation of
an individual star.
• You start with really huge area of
gas with some spin.
• It collapses to a plane (except
the center which have orbits in
the 3rd dimension so are move
oval).
• Somehow you also form the
globular clusters.
http://www.ldps.ws/Mirror/Universe/galaxy.html
Components - Bulge
• The central 13000 light years of our galaxy
contains the Bulge.
• The Bulge is a bar aimed 45 degrees away from
where we are in the galaxy.
• On the outside of the bulge is a ring (called the
“5-kpc ring”) and contains a large fraction of the
molecular Hydrogen in our galaxy.
• This is also the location of the greatest amount
of star formation in our galaxy and would be the
Milky Way’s brightest feature to anyone in any
other galaxy.
http://www.ldps.ws/Mirror/Universe/galaxy.html
Disk
• The disk is about 2000 light years in total
thickness.
• The disk contains most of the stars in our
galaxy.
• Our galaxy has somewhere between 200 400 billion stars!
• The diameter of the disk is 100,000 light
years.
Spiral Arms
• A common feature in discs of Spiral
Galaxies are Spiral Arms.
• Spiral Arms are density waves which pass
through the galaxy.
• They also rotate around the galaxy, but
only with a period of 160 million years or
so.
• So, the materials in the galaxy actually
plow into the Spiral arms.
Structure of a Spiral Arm
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(diagram on board)
It starts with dust running into the arm.
This is called a dust lane.
The dust gets compressed by a factor of 4
at the start of the wave.
• Just behind that you start star formation.
Structure of a Spiral Arm 2
• The massive stars die in a few million years.
• So, just behind the start of star formation you
have supernovae.
• Just behind that you have bubbles from where
all the supernovae have merged.
• After that you are left with normal stars and
normal space which slowly cool until they hit the
next spiral arm in a few hundred million years.
• With this process, the Milky Way produces about
7 new stars per year.
3rd component: Halo
• Surrounding our galaxy is our Halo.
• This is where the Globular Clusters all lie.
• There are also a couple of dwarf galaxies
that our galaxy is currently absorbing:
• Sagittarius Dwarf Galaxy
• Canis Major Dwarf Galaxy
• But there is more…
Rotation curve
• In our solar system, as you move further from the sun,
your orbital velocity decreases.
• V2orbital= G Msun / R
• In a galaxy though, as you go further out the amount of
mass on the inside of your orbit goes up.
• So, if you plotted orbital velocity with distance, what
would you expect it to look like?
• A) should fall, but not as fast as for the solar system
• B) should fall, but very slowly
• C) should stay constant
• D) should go up
Expected
• From the mass we can see, you expect a
gradual decrease in the orbital velocity.
However
• The velocities stay flat or even INCREASE!
http://spiff.rit.edu/classes/phys301/lectures/mw/mw.html
Dark matter problem
• It turns out that most of our galaxy is
DARK MATTER.
• If you look at stars and gas and dust the
mass of our galaxy is about 100 billion
solar masses.
• However, the gravitational mass (M = V2*
R / G) is 1 trillion solar masses!
• 80-90% of our galaxy is mass we cannot
see!
What is dark matter?
• We have no idea.
• The ideas are:
• MACHOs – large objects which are too
dim to see.
• WIMPs – large atomic particles which
would not emit light
• Maybe others? We just don’t know.
• And the dark matter for our galaxy seems
to go out to 300,000 light years.
Back to stuff we DO know!
• Stars!
• There are 2 populations of stars in our
galaxy.
• Guess what we call them (hint REALLY
lame astronomy name coming up)?
Star populations
• Population I stars – told you it was lame –
are stars on the disk of our galaxy.
• They are newer stars (newer than
Population II).
• They have higher metals than Population
II – and are usually metal rich.
• Our sun is a Population I star.
Thick disc stars
• There are some stars that are sort of half way
between population I and II. These are called
the Thick disc stars.
• These are stars who have orbits which take
them into and out of the plane of the galaxy and
often are very elliptical.
• In essence they fill a thicker disc that the disc
stars.
• These stars are usually very old and have low
amounts of metals.
• Arcturus is a thick disc star.
Population II stars
• Population II stars are all very old (12
billion years).
• They are all located in the halo of our
galaxy (and most are in Globular
Clusters).
• All have very low metallicities.
• All Globular Cluster stars are Population II
stars.
Conclusion
• To understand our galaxy you need to look
at a multitude of wavelengths.
• Radio is the best type of light to map our
galaxy.
• Our galaxy has 3 components: bulge,
disk, and halo.
• There are 2 populations of stars.
• However most of our galaxy is made of
dark matter.