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Milky Way Scales LectureTutorial: Page 123

• Work with a partner or two• Read directions and answer all questions carefully.

Take time to understand it now!• Discuss each question and come to a consensus

answer you all agree on before moving on to the nextquestion.

• If you get stuck, ask another group for help.• If you get really stuck, raise your hand and I will

come around.

Questions:

• How big is the Milky Way?• Where are stars forming (or not forming)?• How much mass is in the Milky Way?• What’s going on at the center?

Milky Way: A Spiral Galaxy• Our galaxy seems to be rotating: it has spiral arms

• These are dense concentrations of stars and gas.

• Stars orbit the galacticcenter, pass through thespiral arms as they go.

• Stars slow down andpile up in the spiral arms,like cars in a traffic jam.

Star Formation in Spiral Arms• Spiral arms contain gas and dust.

• Stars form out of nebulae in the spiral arms

• The spiral arms are denser than the rest of thedisk

The Disk contains gas,so stars are still formingthere. (Population Istars)

The Halo has very littlegas, and no new starsare forming there.

The halo of the galaxyis populated by oldstars. (Population IIstars)

Star Formation in the Milky WayStellar Populations

• Pop. I: Newer, disk & spiral arm stars,with higher percentage heavy elements

• Pop. II: Older, bulge and halo stars, withlower percentage of heavy elements

• Heavy elements (metals): anything thatisn’t H, He, or Li

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Measuring Distances

•To map the Milky Way Galaxy, we need tomeasure distances to stars.

• Parallax only works for nearby stars (withinabout 1000 light years)

• For more distant stars, we use StandardCandles

Standard Candles

Car Headlights are standard candles: We use them to determine the car’s distance

Standard Candles

• We can easily measure how bright a star appears(apparent magnitude)

•If we knew how bright the star really was (its absolutemagnitude) then we could calculate its distance.

• We need a star whose absolute magnitude is alwaysthe same, wherever we observe it.

• Such a star is called a “standard candle”

Cepheid Variables

In 1908, astronomer Henrietta Leavitt discovered a newstandard candle using variable stars

These stars are called Cephieds

They are named for δ Cephei, thefirst example of such a star.

Henrietta Leavitt

Delta Cephei

Measuring Distances with CepheidsCepheid stars change in brightness.

They pulsate in a very regular way.

Large, bright Cepheids pulsateslowly, while small, dim Cepheidspulsate quickly.

If we observe the period ofpulsation, we can figure out theabsolute magnitude &luminosity.

If we compare this to the apparentmagnitude, we find the distance!

Period-Luminosity Relation

• The connection between a Cepheid’spulse period and its luminosity.

Pulse Period (in days)

This relation can beused to determine theabsolute magnitude ofa Cepheid

Then, compare to theapparent magnitudeand find the star’sdistance.

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The CepheidMethod

Allows us to measurethe distances to starclusters throughout

the Milky Way.

Other types of “standardcandle” stars have beenfound which use the samemethod.

The Structure of the Milky Way

By measuring the distances to various parts of theMilky Way Galaxy, we map out its structure

The Milky Way is aBarred Spiral Galaxy

It has a straight structureat the center called a Bar

Mapping the Milky Way

A modern map of the Milky Way (computer-generated diagram)

The Sun is abouthalfway out from thecenter

Measuring the Mass of the Milky WayWe use the Sun’s motionaround the center of the MilkyWay

The greater the mass insidethe orbit, the faster the Sunhas move around the center.

This way we can measure themass of the Milky Way.

Total mass: about 400 billion MSun

Rotation Curve Example:Merry-Go-Rounds

• Every part of the merry-go-round “orbits” the center in the same amountof time– Inner part moves slow– Outer part moves fast

Solid body rotation

Rotation Curve Example:Our Solar System

The period of each planet depends on its distance from the Sun– Kepler’s 3rd Law: P2 = a3

– Planets farther away from the sun go much slower

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What does observing the light fromstars in other galaxies tell us?

The galaxy looks bright atcenter, so most of the starsare there, so most of themass should be at thecenter. Distance from the Galactic Center

Ligh

t fro

m st

ars

Distance from the Galactic Center

Mas

s

However the flatrotation curve tellsa different story!

Because there is a flatrotation curve thereshould be an equal amountof mass distributedeverywhere throughout thegalaxy’s disk and halo.

Distance from the Galactic Center

M

ass

Mass curvedeterminedfrom Light

Mass curvedetermined fromthe rotationcurve

So which mass curve isright?

The mass curve determined from therotation curve is our current bestmodel for how the mass of thegalaxy is distributed. Mass is evenlydistributed everywhere in the diskand halo.

Distance from the Galactic Center

Am

ount

of

Mas

s

Mass curvedeterminedfrom Light

Mass curvedetermined fromthe rotationcurve

Measuring the Mass of the Milky WayWe use the Sun’s motionaround the center of the MilkyWay

The greater the mass insidethe orbit, the faster the Sunhas move around the center.

This way we can measure themass of the Milky Way.

Total mass: about 400 billion MSun

Mass of the Milky Way

• The mass of the Milky Way is between 100 billionand MSun and 600 billion MSun

• Stars & Gas we see in the Milky Way can onlyaccount for a fraction of the total mass.

-What is it?

- Why can’t we see it?

Dark Matter

The Center of the Milky Way

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The Center of Our Galaxy• The density of stars in the Galactic Center is

much greater than in the Sun’s neighborhood.• They appear to be orbiting a supermassive black

hole at the center of the galaxy

Its mass is over3 Million MSun !

Chapter 16:

Galaxies

The Discovery of Galaxies• Up to the 1920’s, the Milky Way Galaxy

was thought to be the entire universe.

• “Spiral Nebulae” were assumed to beinside our own Milky Way galaxy.– Planetary systems in formation?– Strangely shaped clouds?

• In 1920, Shapley & Curtis debated thenature of “spiral nebulae” and the size ofour Galaxy.

A “Spiral Nebula”The Whirlpool Galaxy

We now realize that our galaxy is onlyone of billions of galaxies we can see.

These galaxies come in three maintypes:

Spiral, Elliptical & Irregular

Spiral Galaxies

M 100 NGC 300

• Typically bright, blue in color• Look like pinwheels (sometimes with

bar)

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A Spiral Galaxy Seen Edge On

Elliptical Galaxies

… are round, notflat like spirals

They are typicallyred in color.

Less gas and dustthan spirals.

The Coma Galaxy Cluster contains Ellipticals and Spirals

Some galaxies have characteristics of both ellipticals and spirals…

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Irregular Galaxies •Lack any distinct shape•Are generally smaller than spirals and ellipticals

Hubble Tuning ForkSpiral Galaxies (S): Classified according to spiral arms (a,b,c)

and presence of a bar (“B”)

Elliptical Galaxies (E): Classified according to shape (E0-E9)

Irregular Galaxies(Irr): Basicallyanything funky-looking!

A Barred Spiral Galaxy with only 2 arms.

TheAndromedaGalaxy

Our“Sister”Galaxy

Classifying Galaxies LectureTutorial: Page 127

• Work with a partner or two• Read directions and answer all questions

carefully. Take time to understand it now!• Discuss each question and come to a

consensus answer you all agree on beforemoving on to the next question.

• If you get stuck, ask another group for help.• If you get really stuck, raise your hand and I

will come around.