Modern AstronomyModern AstronomyStars & GalaxiesStars & Galaxies

Lecture 7Lecture 7The Milky Way Galaxy:The Milky Way Galaxy:Putting it all togetherPutting it all together

Geraint F. LewisGeraint F. LewisUniversity of Sydney 2007University of Sydney 2007

OutlineOutline

Galactic StructureGalactic Structure Where are the stars?Where are the stars? Rotation and spiral structureRotation and spiral structure Into the middleInto the middle Formation of the GalaxyFormation of the Galaxy Milky Way and friendsMilky Way and friends

The Discovery of the Milky WayThe Discovery of the Milky Way

Herschel’s UniverseHerschel’s Universe

Kapteyn’s Model (1922)Kapteyn’s Model (1922)

http:www-astronomy.mps.ohio-state.edu/~pogge/Ast162/Unit4

Shapley’s Globular ClusterShapley’s Globular Cluster

Views are obscured by dustViews are obscured by dust Discovering the true structure of the Discovering the true structure of the

galaxy took a lot of detective workgalaxy took a lot of detective work Eventually the mystery was solved when Eventually the mystery was solved when

Hubble discovered Hubble discovered Island UniversesIsland Universes

The Andromeda Galaxy, the Milky Way’s sister

Galactic MakeupGalactic Makeup

100-400 billion stars100-400 billion stars Disk, bulge & haloDisk, bulge & halo 25kpc (80,000 lyrs) across25kpc (80,000 lyrs) across 8kpc (30,000 lyrs) from the centre8kpc (30,000 lyrs) from the centre 300pc (1,000 lyrs) thick at the Sun300pc (1,000 lyrs) thick at the Sun 5kpc (16,000 lyrs) thick at the centre5kpc (16,000 lyrs) thick at the centre

Monty PythonMonty Python

Our galaxy itself contains 100 billion stars.Our galaxy itself contains 100 billion stars.

It’s a 100 thousand light years side to side.It’s a 100 thousand light years side to side.

It bulges in the middle, 16 thousand light years thick,It bulges in the middle, 16 thousand light years thick,

But out by us, it’s just 3000 light years wide.But out by us, it’s just 3000 light years wide.

We’re 30,000 light years from galactic central point.We’re 30,000 light years from galactic central point.

We go ‘round every 200 million years, We go ‘round every 200 million years,

And our galaxy is only one of millions of billionsAnd our galaxy is only one of millions of billions

In this amazing and expanding UniverseIn this amazing and expanding Universe

From the Meaning of LifeFrom the Meaning of Life

In context…In context…

Imagine each star in the Milky Way is Imagine each star in the Milky Way is the size of peathe size of pea

The 200 billion stars in the Galaxy would The 200 billion stars in the Galaxy would be a cube of peas 34m asidebe a cube of peas 34m aside

Scaling to the Milky Way, the typical Scaling to the Milky Way, the typical distance between peas is 220kmdistance between peas is 220km

The Milky Way would stretch between The Milky Way would stretch between the Earth and the Moonthe Earth and the Moon

The Galactic Disk: StarsThe Galactic Disk: Stars

82% of stars82% of stars Star on circular orbitsStar on circular orbits Many young stars (active star Many young stars (active star

formation)formation)

Metal rich stars (Pop I)Metal rich stars (Pop I) Pronounced spiral armsPronounced spiral arms Thin & thick disksThin & thick disks

The Galactic Disk: Gas & DustThe Galactic Disk: Gas & Dust

• ISM is ~10% mass of stars

• 1-2% of the ISM is dust

• Dust is visible in the infrared

• Dust made in evolved stars

• Made in supernovae

How do we see hydrogen?

The Galactic Disk: HydrogenThe Galactic Disk: Hydrogen

• Atoms start in a low energy state, with proton and electron unaligned.

•When atoms bump, the atoms can become excited, with the proton and electron aligned.

•The electron can return to the low energy state by emitting radio waves at 21cm.

•If we had radio eyes sensitive to this wavelength, the disk of the galaxy would glow brightly.

Seeing hydrogenSeeing hydrogen

Hydrogen: Stellar raw materialHydrogen: Stellar raw material

Dominant gas in the GalaxyDominant gas in the Galaxy Neutral hydrogen clouds (HI)Neutral hydrogen clouds (HI)

• 0.1-1000M0.1-1000M¯̄ & 80K & 80K Molecular complexes (HMolecular complexes (H22))

• 500,000 M500,000 M¯̄ & 10K & 10K Hot medium (Ionized)Hot medium (Ionized)

• Diffuse (from supernovae) & 10Diffuse (from supernovae) & 1066 K K

Spiral arms in hydrogenSpiral arms in hydrogen

Stellar Life CycleStellar Life Cycle

Star Birth Star Death

The BulgeThe Bulge

18% of star18% of star Very little gas & dustVery little gas & dust Little star formationLittle star formation Metal poor stars (Pop II)Metal poor stars (Pop II) Something energetic in centreSomething energetic in centre Stars on random orbitsStars on random orbits

Bulge OrbitsBulge Orbits

Unlike the Earth orbiting the Sun, the stars in the Bulge are not orbiting a single, massive object.

It is the gravitational attraction of all the stars together that keeps the bulge as a single object.

In effect, they are orbiting nothing!

Out into the haloOut into the halo

2% of the star2% of the star Virtually no gas or dustVirtually no gas or dust No star formationNo star formation Larger, emptier version of the Larger, emptier version of the

bulge bulge ~200 globular clusters~200 globular clusters Very metal poor starsVery metal poor stars Appears to be a very boring Appears to be a very boring

place!place!

Metal poor starsMetal poor stars

Stars must have formed a long time ago out of almost pristine material. Stars must have small masses to have lived this long!

Velocity of the SunVelocity of the Sun

The Sun orbits the Galaxy at 220km/sThe Sun orbits the Galaxy at 220km/s At this velocity, it takes 240 million years to At this velocity, it takes 240 million years to

complete a single orbitcomplete a single orbit It also bobs up and downIt also bobs up and down When the Sun was last at this location, it was When the Sun was last at this location, it was

the Early Triassic, a world with the first the Early Triassic, a world with the first dinosaurs, but no birds, grass or flowering dinosaurs, but no birds, grass or flowering plants!plants!

What about other stars?What about other stars?

How does the Milky Way rotate?How does the Milky Way rotate?

Is like a wheel?Is like a wheel? Like the PlanetsLike the Planets

We can tell by looking We can tell by looking at the Rotation Curveat the Rotation Curve

Using the Doppler ShiftUsing the Doppler Shift As objects move towards or As objects move towards or

away from us, their spectrum away from us, their spectrum of light is shifted via the of light is shifted via the Doppler effectDoppler effect

Towards red for things Towards red for things moving awaymoving away

Towards blue for things Towards blue for things coming towardcoming toward

Using this we can map the Using this we can map the radial velocities of stars in the radial velocities of stars in the galaxy and work out how the galaxy and work out how the Milky Way is rotating.Milky Way is rotating.

Rotation curveRotation curve

http://www-astronomy.mps.ohio-state.edu/~pogge

Rotating hydrogenRotating hydrogen

A flat rotation curve?A flat rotation curve?

Given the distribution of stars and gas in Given the distribution of stars and gas in the Milky Way, we would not expect a the Milky Way, we would not expect a flat rotation curve.flat rotation curve.

Either:Either: We have problems with physicsWe have problems with physics More matter is present than seenMore matter is present than seen

A lot (10-100x) more than seen!!!

The dark matter haloThe dark matter halo

The identification of a massive dark matter halo The identification of a massive dark matter halo enveloping the Milky Way has some important enveloping the Milky Way has some important implicationsimplications

Stars, gas & dust are made from protons & neutrons Stars, gas & dust are made from protons & neutrons (baryons).(baryons).

Dark matter cannot be dark baryonic matterDark matter cannot be dark baryonic matter So you, me, all the stars, gas and dust, everything you So you, me, all the stars, gas and dust, everything you

have seen in a NASA press release, all of it, are a have seen in a NASA press release, all of it, are a minor (tiny) player in a Milky Way dominated by dark minor (tiny) player in a Milky Way dominated by dark matter!matter!

The dark matter haloThe dark matter halo

Spiral structure Spiral structure

Where does spiral structure come from?

We have seen the disk doesn’t rotate like a solid wheel and so it would appear the spirals cannot be “fixed”!

The spiral structure is seem in hot, young blue stars and gas, but not in old red stars.

Do the spirals just rotate with the stars?

OverwindingOverwinding

So spirals should rapidly become over wound!

Density wavesDensity waves

Density waves are enhancements in the Density waves are enhancements in the surface density of the galaxysurface density of the galaxy

Density waves rotate slowly as coherent Density waves rotate slowly as coherent structurestructure

Stars pass through density waves, but Stars pass through density waves, but slow down as they do so.slow down as they do so.

Density wavesDensity waves

Density wavesDensity waves

Unlike stars, clouds of gas do not pass Unlike stars, clouds of gas do not pass through density waves unscathedthrough density waves unscathed

Gas clouds are compressed as they slow Gas clouds are compressed as they slow downdown

Gas clouds rear-end one anotherGas clouds rear-end one anotherBoth cause cloud collapse & forming new Both cause cloud collapse & forming new

stars, especially hot, luminous (OB) stars, especially hot, luminous (OB) stars. These light up the spiral arms.stars. These light up the spiral arms.

Density wavesDensity waves

OB stars evolve quickly, reaching the end of OB stars evolve quickly, reaching the end of their lives in a few million years.their lives in a few million years.

These stars explode before they leave the These stars explode before they leave the spiral arms, promoting even more star spiral arms, promoting even more star formation.formation.

The cooler, longer lived stars leave the spiral The cooler, longer lived stars leave the spiral arms and mix with the galactic population.arms and mix with the galactic population.

This is why spiral arms are obvious in OB This is why spiral arms are obvious in OB stars, but not older stars.stars, but not older stars.

Seeding density wavesSeeding density waves

What causes density waves?What causes density waves? Smaller perturbations can be induced in Smaller perturbations can be induced in

galactic disks via interactions with other galactic disks via interactions with other systemssystems

These grow to become pronounced These grow to become pronounced waves.waves.

Seen in all kinds of disks, including Seen in all kinds of disks, including protostellar disks!protostellar disks!

Journey to the centreJourney to the centre

So, what is occurring in So, what is occurring in the Galactic centre?the Galactic centre?

The region is clearly The region is clearly very energetic!very energetic!

Journey to the centreJourney to the centre

http://www.astro.ucla.edu/~jlu/gc/journey/

Journey to the centreJourney to the centre

http://www.astro.ucla.edu/~jlu/gc/

Right in the middleRight in the middle

What is this nothing which the gas and stars are rapidly What is this nothing which the gas and stars are rapidly orbiting?orbiting?

Only real possibility is a Only real possibility is a Black HoleBlack Hole!!Must have a mass of 2.7Must have a mass of 2.7££101066MM¯̄..Not a black hole from a single stellar collapse, but must Not a black hole from a single stellar collapse, but must

be built up over time.be built up over time.

We shall meet these again when we come to look at We shall meet these again when we come to look at quasars, but we have to ask “Just how active was the quasars, but we have to ask “Just how active was the young Milky Way”?young Milky Way”?

The formation of the Milky WayThe formation of the Milky Way The first stars formed in The first stars formed in

clouds of mainly hydrogen clouds of mainly hydrogen and helium (metal poor)and helium (metal poor)

Merge into a rotating Merge into a rotating protogalaxy of dark matter, protogalaxy of dark matter, stars, gas & duststars, gas & dust

Gas cools and collapses into a Gas cools and collapses into a disk, taking the dust with itdisk, taking the dust with it

Stars in the halo & bulge just Stars in the halo & bulge just age, while in the disk they age, while in the disk they continually formingcontinually forming

Nice picture, but it does not explain everything!!!

The Milky Way & friendsThe Milky Way & friends

The Magellanic Clouds

The Local GroupThe Local Group

Sagittarius DwarfSagittarius Dwarf

Magellanic StreamMagellanic Stream

Canis Major & Monoceros StreamCanis Major & Monoceros Stream

ConclusionsConclusions

The Milky Way does not live alone!The Milky Way does not live alone! We live in the Local Group, dominated We live in the Local Group, dominated

by little Dwarf Galaxiesby little Dwarf Galaxies The closest ones are being disrupted, The closest ones are being disrupted,

but where is this material going?but where is this material going? An important link to galaxy formation An important link to galaxy formation

and evolution which we will look at in and evolution which we will look at in more detail next week!more detail next week!

See you next week!See you next week!