The Milky Way GalaxyThe Milky Way Galaxy

Structure & EvolutionStructure & Evolution

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Milky Way’s StructureMilky Way’s Structure

Nuclear Bulge in centerNuclear Bulge in center– 12,000 LY wide x 10,000 LY thick12,000 LY wide x 10,000 LY thick

– Made of old (mostly yellow & red) starsMade of old (mostly yellow & red) stars

– Contains much of the Contains much of the visiblevisible mass in the mass in the galaxy.galaxy.

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The Galactic CenterThe Galactic Center

The center of the galaxy is a very The center of the galaxy is a very busy place.busy place.

It’s obscured by dust and hard to see, It’s obscured by dust and hard to see, but we do know there’s a 4 million but we do know there’s a 4 million solar mass black hole inside!solar mass black hole inside!This intense X-ray and radio source is This intense X-ray and radio source is called called Sagittarius ASagittarius A..

Center of the galaxyCenter of the galaxy

There are so many old, red stars There are so many old, red stars there that “night” on a planet near the there that “night” on a planet near the galactic center would be twice as galactic center would be twice as bright as twilight is to us.bright as twilight is to us.

A fun place to live, except for all that A fun place to live, except for all that pesky deadly X-ray radiation.pesky deadly X-ray radiation.

Thin Disk surrounds nuclear bulgeThin Disk surrounds nuclear bulge– 120,000 LY wide, but <10,000 LY thick120,000 LY wide, but <10,000 LY thick

– Most new star formation goes on here, Most new star formation goes on here, so stars are much younger (Population I so stars are much younger (Population I type stars – all colors)type stars – all colors)

– Much free-floating gas & dust.Much free-floating gas & dust.

– BUT…contains < 50% of the BUT…contains < 50% of the visiblevisible mass of the galaxy.mass of the galaxy.

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A halo of stars and globular clusters A halo of stars and globular clusters surrounds the entire galaxy.surrounds the entire galaxy.– Contains almost exclusively Contains almost exclusively VERYVERY old old

stars (population II stars).stars (population II stars).– These stars are low in heavy elements, These stars are low in heavy elements,

meaning that they’re as old as the meaning that they’re as old as the universe (before supernovas formed universe (before supernovas formed heavier elements.)heavier elements.)

– Contains little Contains little visiblevisible mass, but about ½ mass, but about ½ totaltotal mass of galaxy. mass of galaxy.

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Total Mass of the galaxy is equivalent Total Mass of the galaxy is equivalent to 1 trillion stars, but the galaxy to 1 trillion stars, but the galaxy actually contains only about 200 actually contains only about 200 billion stars.billion stars.Loose gas & dust only account for Loose gas & dust only account for about another 200 billion stars’ worth.about another 200 billion stars’ worth.Where and what is the missing 600 Where and what is the missing 600 billion stars’ worth of mass?billion stars’ worth of mass?

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Where is our sun in the MW?Where is our sun in the MW?

We have a difficult problem in trying We have a difficult problem in trying to understand where we live in the to understand where we live in the galaxy.galaxy.

Imagine yourself being plopped down Imagine yourself being plopped down on a street corner in an unfamiliar on a street corner in an unfamiliar city, and being told to map the entire city, and being told to map the entire city.city.

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Here’s our galactic street corner.

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Where do we live in the galaxy?Where do we live in the galaxy?You’d only be able to clearly map the You’d only be able to clearly map the area right around you.area right around you.You might be able to guess at some You might be able to guess at some other streets if you could see light other streets if you could see light poles, trees, or rooftops.poles, trees, or rooftops.The other side of town would remain The other side of town would remain unknown.unknown.But if you could go 1000’ up in a But if you could go 1000’ up in a helicopter, it would be much easier.helicopter, it would be much easier.

From up above, we can see the layout.

It’s hard for us to know the layout of It’s hard for us to know the layout of our own galaxy, because we live in our own galaxy, because we live in the plane of the galaxy, not above it.the plane of the galaxy, not above it.

We have to find out indirectly…We have to find out indirectly…– by looking at other similar galaxies…by looking at other similar galaxies…

– by looking at globular clusters…by looking at globular clusters…

– by mapping the positions of bright by mapping the positions of bright sources (nebulae, bright stars.)sources (nebulae, bright stars.)

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The Andromeda Galaxy is similar to ours.

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We believe that our solar system is We believe that our solar system is about 2/3 out from the center toward about 2/3 out from the center toward the edge of the galaxy. Here’s why:the edge of the galaxy. Here’s why:

Our galaxy is surrounded by a halo of Our galaxy is surrounded by a halo of evenly-distributed globular star evenly-distributed globular star clusters.clusters.

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A globular cluster from a nearbyplanet.

If we lived near the center of our galaxy,we’d see globular star clusters evenlydistributed everywhere in the sky…

If we live nearer the edge of the galaxy,globular star clusters won’t be evenlydistributed – this is what we actually see.

How fast are we moving?How fast are we moving?By comparing how our position changes By comparing how our position changes relative to the average speed of the relative to the average speed of the galaxy’s globular clusters, we know that galaxy’s globular clusters, we know that the sun moves at a speed of 220 the sun moves at a speed of 220 km/second around the galaxy’s center.km/second around the galaxy’s center.The sun takes 1000 years to move 1 light The sun takes 1000 years to move 1 light year, or it takes 280 million years to orbit year, or it takes 280 million years to orbit the nucleus of the galaxy once.the nucleus of the galaxy once.The galaxy has only made about 50 The galaxy has only made about 50 complete rotations since the universe complete rotations since the universe began!began!

Here’s a problem: by looking at the Here’s a problem: by looking at the visible mass in the galaxy, and the visible mass in the galaxy, and the way that the galaxy rotates, we can way that the galaxy rotates, we can predict how fast the sun predict how fast the sun shouldshould move move through space, revolving around the through space, revolving around the center of the galaxy.center of the galaxy.

It moves 220 km/sec, but it should It moves 220 km/sec, but it should only move at 160 km/sec.only move at 160 km/sec.

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So what does this mean?So what does this mean?

The galaxy rotates so fast, that it The galaxy rotates so fast, that it should fly apart. (There’s not enough should fly apart. (There’s not enough visible mass to hold it together.)visible mass to hold it together.)

The way that the galaxy rotates tells The way that the galaxy rotates tells us where the galaxy’s mass is us where the galaxy’s mass is located.located.

If it rotated like a it rotated like a rigid object, (a rigid object, (a spinning DVD), spinning DVD), where most of the where most of the mass is located in mass is located in the the outer half of outer half of the diskthe disk, then the , then the outer edges would outer edges would move faster than move faster than the center.the center.

If the galaxy waslike the solarsystem, wheremost of the massis in the center,the edges wouldrotate much slowerthan the center.

Eventually, the galaxy’s arms would wind up, like a watch spring twisted too tight.

However, thegalaxy rotates atjust about thesame speedeverywhere.This means thatthe mass is evenlydistributed insideand outside thesun’s orbit.

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Dark MatterDark MatterSince more than half of the visible Since more than half of the visible (bright) mass is inside the sun’s orbit, (bright) mass is inside the sun’s orbit, this means is that there is a vast this means is that there is a vast amount of amount of dark matterdark matter in our galaxy, in our galaxy, about 600 billion sun’s worth.about 600 billion sun’s worth.

We can’t see this dark matter. We We can’t see this dark matter. We don’t even know what it is, but most don’t even know what it is, but most of it is found in the galaxy’s halo.of it is found in the galaxy’s halo.

There are >30 candidates for There are >30 candidates for the identity of the dark matter.the identity of the dark matter.

black holesblack holes dustdust neutrinosneutrinostiny red dwarf starstiny red dwarf starstypes of elementary particles that we types of elementary particles that we don’t yet know anything aboutdon’t yet know anything aboutdark energy – a strange repulsive dark energy – a strange repulsive “anti-gravity”“anti-gravity”