The Milky Way…The Milky Way…

Our GalaxyOur Galaxy

Felix Shih’s work at the Astronomy Club Felix Shih’s work at the Astronomy Club outing at Joshua Treeouting at Joshua Tree

Basic FactsBasic Facts

Greeks called it Gala (milk)Greeks called it Gala (milk) Until the early 20Until the early 20thth Century, the Century, the

Milky was thought to be the only Milky was thought to be the only galaxygalaxy

Only after better telescopes resolved Only after better telescopes resolved nebulae accurately were other nebulae accurately were other galaxies identified.galaxies identified.

The motivation for The motivation for The Great DebateThe Great Debate

Quite visibleQuite visible

A long exposure A long exposure picture though a picture though a telescope can get telescope can get you this, but the you this, but the Milky Way is Milky Way is obvious if the sky obvious if the sky is dark enough.is dark enough.

If you could see it from If you could see it from afar…afar…

Where the Sun is:Where the Sun is:

Factinos:Factinos:

A barred-spiral galaxyA barred-spiral galaxy 100-500 billion stars100-500 billion stars 100,000 LY across the disk100,000 LY across the disk

– If the Solar System was a CD, the Earth would If the Solar System was a CD, the Earth would be the Milky Waybe the Milky Way

– Pop I (young, hot) starsPop I (young, hot) stars Central bulge 15,000 LY thickCentral bulge 15,000 LY thick

– Pop II (old, red) starsPop II (old, red) stars Halo out to 75 kpc ~ 470,000 LY acrossHalo out to 75 kpc ~ 470,000 LY across

– Pop II starsPop II stars Sun located 28,000 LY from nucleusSun located 28,000 LY from nucleus 1 Rotation* takes ~ 240-270Myr1 Rotation* takes ~ 240-270Myr

– New data: seems to be turning 20% faster, New data: seems to be turning 20% faster, implying more mass than previously thought*implying more mass than previously thought*

*more later

Side ViewSide View

View from aboveView from above

Latest Spitzer Latest Spitzer pictures show only pictures show only two major arms, two major arms, the Scutum-the Scutum-Centaurus arm and Centaurus arm and the Perseus Armthe Perseus Arm– These end on the These end on the

central barcentral bar The other arms are The other arms are

minor, not ending minor, not ending on the central baron the central bar

EvolutionEvolution

The galaxy started out roughly sphericalThe galaxy started out roughly spherical Globular clusters, nuclear bulge formed Globular clusters, nuclear bulge formed

before the diskbefore the disk– Older starsOlder stars– Red-starred center, blue-starred diskRed-starred center, blue-starred disk

Disk formed later as infalling gas, Disk formed later as infalling gas, density waves formed younger starsdensity waves formed younger stars– Density waves carry the energy trigger to Density waves carry the energy trigger to

star formation (more in a moment)star formation (more in a moment)

DiagramDiagram

Sparke/Gallagher

Pop II stars here

Pop I stars here

Latest Model with Gas Latest Model with Gas StreamsStreams

Not seeing (V)Not seeing (V)

Seeing! (Spitzer IR)Seeing! (Spitzer IR)

Spiral Arms = star Spiral Arms = star formationformation

Density WavesDensity Waves

Like a traffic jam or people crowding Like a traffic jam or people crowding from a large parking lot into the smaller from a large parking lot into the smaller security area for a concert or gamesecurity area for a concert or game

Where the density increases the Where the density increases the probability for star formation is also probability for star formation is also increasedincreased

Hence, all else being equal, star Hence, all else being equal, star forming regions are bands twisted into forming regions are bands twisted into spirals by the rotation of the galaxyspirals by the rotation of the galaxy

Question.: Where are stars forming?Question.: Where are stars forming? Answer: Where you can see them!Answer: Where you can see them!

i.e. in the spiral armsi.e. in the spiral armse.g. where the e.g. where the density wavedensity wave is is

Just because you don’t see Just because you don’t see stars between the spiral arms stars between the spiral arms doesn’t mean that there’s doesn’t mean that there’s nothing therenothing there

Dustlanes

How Stars MoveHow Stars Move NotNot like planets in a solar system like planets in a solar system Vera Rubin discovered that, rather than Vera Rubin discovered that, rather than

falling off with distance, the star’s falling off with distance, the star’s rotation (or velocity) curverotation (or velocity) curve flattens out. flattens out.

Rotation: Doppler ShiftRotation: Doppler Shift

Faster Rotation than Faster Rotation than PredictedPredicted

First predicted by the First predicted by the irritable Fritz Zwicky 1933irritable Fritz Zwicky 1933

Vera Rubin circa 1970 finds Vera Rubin circa 1970 finds undue rotation in many undue rotation in many galaxies, not just the Milky galaxies, not just the Milky WayWay

James Peebles and Jeremiah James Peebles and Jeremiah Ostriker showed Ostriker showed mathematically that visible mathematically that visible matter was insufficient to matter was insufficient to keep galaxies together, keep galaxies together, early 1970searly 1970s– At most, 20% is visible matterAt most, 20% is visible matter– All else Dark MatterAll else Dark Matter

An Astounding An Astounding Implication!Implication!

The only way a velocity curve could have The only way a velocity curve could have this shape would be if there was a lot this shape would be if there was a lot more mass distributed throughout, above more mass distributed throughout, above and below the disk of the Milky Wayand below the disk of the Milky Way

We can’t see it, therefore it’s called We can’t see it, therefore it’s called Dark Dark MatterMatter

5 times the amount of matter we 5 times the amount of matter we cancan see see– Diffuse like a gas, not compact like dwarfsDiffuse like a gas, not compact like dwarfs– Cold (non-radiating, slow moving)Cold (non-radiating, slow moving)

Dark Matter Dark Matter CandidatesCandidates WIMPSWIMPS

– Weakly Interactive Massive ParticlesWeakly Interactive Massive Particles Standard Model (next slide)Standard Model (next slide)

– To be found in the lab (maybe)To be found in the lab (maybe) MACHOSMACHOS

– Massive Compact Halo ObjectsMassive Compact Halo Objects– (maybe) Observed via gravitational microlensing(maybe) Observed via gravitational microlensing– Insufficient IR seen from the effectInsufficient IR seen from the effect

NotNot– Black HolesBlack Holes– Brown Dwarfs and other “dark” stars too dim to seeBrown Dwarfs and other “dark” stars too dim to see

These are of ordinary matter and will interactThese are of ordinary matter and will interact

DON’T memorize this table!

SupersymmetrySupersymmetry

Necessary for String TheoryNecessary for String Theory Labels: “s-” and “-ino”Labels: “s-” and “-ino” Symmetry as with quarks and leptonsSymmetry as with quarks and leptons Supersymmetric w and z bosonsSupersymmetric w and z bosons Too massive to yet be discovered in Too massive to yet be discovered in

colliderscolliders Good candidates because they are Good candidates because they are

weakly interactive and slow weakly interactive and slow (1000M(1000Mprotonproton))

But even But even en masseen masse not not enough massenough mass There’s more to the story of Dark There’s more to the story of Dark

Matter beyond velocity curvesMatter beyond velocity curves The amount of matter/energy in The amount of matter/energy in

the Universe will determine the the Universe will determine the fate of the Universefate of the Universe

So what it is and how much of it So what it is and how much of it there is in the subject of intense there is in the subject of intense studystudy

More in another PPTMore in another PPT

Leaving the curves Leaving the curves aside…aside… The movement of a star around The movement of a star around

the galactic nucleus is a the galactic nucleus is a complicated processcomplicated process

It’s tugged gravitationally by It’s tugged gravitationally by various and varying gravitational various and varying gravitational forcesforces– From above and belowFrom above and below– From within and withoutFrom within and without

“…“…Now I am become Now I am become Death, the Destroyer of Death, the Destroyer of Worlds…”Worlds…” The Shiva The Shiva

Hypothesis:Hypothesis:– Periodically the Solar Periodically the Solar

System’s Oort comet System’s Oort comet cloud is perturbed by cloud is perturbed by proximity to nearby proximity to nearby starsstars

– New comets streak New comets streak into the inner system into the inner system and may impact and may impact Earth, causing mass Earth, causing mass extinctionsextinctions

It’s just a hypothesis, but…It’s just a hypothesis, but…

Oh crap!

Younger Dryas Boundary hypothesisYounger Dryas Boundary hypothesis

““The Younger Dryas The Younger Dryas hypothesis contends that a hypothesis contends that a cosmic impact occurred cosmic impact occurred 12,900 years that caused 12,900 years that caused the extinction of North the extinction of North American megafauna, the American megafauna, the disappearance of a widely disappearance of a widely distributed Clovis culture, distributed Clovis culture, and resulted in the and resulted in the development of farming in development of farming in place of a hunter-gatherer place of a hunter-gatherer culture in humans.” culture in humans.” – ““New evidence supporting New evidence supporting

Younger Dryas Boundary Younger Dryas Boundary hypothesis exposed”; hypothesis exposed”; Paul Paul Hamaker, Hamaker, Examiner.comExaminer.com

Dr. Ted Bunch, NAU / NASA

At the Galactic Core!At the Galactic Core!

The galactic rotation curve is not only a The galactic rotation curve is not only a strange shape, it indicates a lot more strange shape, it indicates a lot more mass in the nucleus of the galaxy, where mass in the nucleus of the galaxy, where stars are light weeks, not light years stars are light weeks, not light years apartapart

The following picture shows stars that The following picture shows stars that are orbiting a massive yet unseen objectare orbiting a massive yet unseen object

The observed speeds indicate the mass The observed speeds indicate the mass of the object > 1 million Mof the object > 1 million Msunsun!!

See the change in See the change in position over only a few position over only a few years?years?

Supermassive Black Supermassive Black HoleHole

““SagA*” (SagA*” (Sagittarius A-starSagittarius A-star)) Formed Formed

contemporaneously with contemporaneously with the nuclear bulgethe nuclear bulge

Strong radio sourceStrong radio source Feeds off close-by stars Feeds off close-by stars

and gas first, then and gas first, then becomes less active as fuel becomes less active as fuel diminishesdiminishes– Active galaxiesActive galaxies

4X104X1066 Solar Masses Solar Masses Less that Earth’s RLess that Earth’s Rorbitorbit

Turns every 11 minutesTurns every 11 minutes

Still HungryStill Hungry

Consumed Consumed something ~ something ~ Mercury’s Mercury’s mass 60 years mass 60 years ago (local ago (local time)time)

Brightened Brightened 100,000X (in 100,000X (in the x-ray band) the x-ray band) from the mealfrom the meal

You can see You can see the 3-year the 3-year illumination illumination tracetrace

Chandra X-Ray Telescope picture

SMBH Eats a StarSMBH Eats a Star

So many stars…So many stars…

The Drake EquationThe Drake Equation– Only a guess of a guess of a guessOnly a guess of a guess of a guess

SETISETI

Is Life Common?Is Life Common?

Stanley Miller in Stanley Miller in 1953 made 1953 made precursor precursor molecules in a molecules in a simulated early simulated early Earth atmosphereEarth atmosphere

That’s what That’s what Astronomy 104 Astronomy 104 talks about talks about

But are they/we But are they/we broadcasting?broadcasting?

The problem is The problem is broadbroadcastingcasting Our signals would be only Our signals would be only

1/101/103030 of a watt 10 pc away of a watt 10 pc away– NarrowNarrowcasting would work bettercasting would work better

NASA has recently beamed NASA has recently beamed “Across the Universe” to “Across the Universe” to Polaris, 131 pc, to celebrate Polaris, 131 pc, to celebrate the anniversary of its Deep the anniversary of its Deep Space Network, with 400kW of Space Network, with 400kW of powerpower

That’s our galaxy in That’s our galaxy in general…general…

Let’s see how other galaxies Let’s see how other galaxies stack upstack up