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The Milky Way “Milky Way”: A band of ____ and a _______

Milky Way probably looks likeAndromeda.

The band of light we see isreally 100 billion stars

Milky Way Composite Photo

• ______ in the center• Dark strip in the middle, from _____

Milky Way

Before the 1920’s, astronomers used a “__________model” for the galaxy

Tried to estimate our location in the galaxy by countingstars in different __________

Because some stars are _______ by dust, the trueshape of this group of stars was unclear.

Finding the Center

• Harlow Shapely studied_______ ________.

• He theorized that they mustorbit the true ______ of thegalaxy

A Globular Cluster

Finding the CenterShapely plotted the ________ of the globular star clusters.

He found that they are are not centered on the Sun….

…but are centered on a point about _______ light years fromthe Solar System.

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The Milky WaySize: The Milky Way isroughly _______ lightyears across, and about_____ light years thick.

Stars: The Milky Way iscomprised of over __________ stars!

Almost everything visiblewith the naked eye isinside the Milky Way

Parts of Our Galaxy

Disk: The ____ Resides in theDisk

Nuclear Bulge: The dense_______ region

Halo: Spherical regionsurrounding the disk where the_______ ________ live.

Parts of OurGalaxy

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?

3

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

• These are dense concentrations of _____ and ____.

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

• Stars ____ _____ andpile up in the spiral arms,like cars in a traffic jam.

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

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

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

Star Formation in the Milky Way

Stellar Populations

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

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

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

Measuring Distances

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

• Parallax only works for nearby stars (withinabout ____ 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(________ magnitude)

•If we knew how bright the star really was (its ________magnitude) 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”

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Cepheid Variables

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

These stars are called _________

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 pulsate_____, while small, dim Cepheidspulsate _______.

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

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

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 a_______ ______ 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 about_________ out fromthe center

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

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

This way we can measure themass of the Milky Way.

Total mass: about ___ _______ MSun

Mass of the Milky Way

• The mass of the Milky Way is between ___ billionand MSun and ___ 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?

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The Center of the Milky Way The Center of Our Galaxy• The ______ of stars in the Galactic Center is

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

hole at the center of the galaxy

Its mass is over__ _____ MSun !

Chapter 13

Galaxies

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

These galaxies come in three maintypes:

Spiral, ________ & _______

Spiral Galaxies

M 100 NGC 300

• Typically very bright, _____ in color• Look like _________ (sometimes with

____)

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… are ____, notflat like spirals

They are typically___ in color.

Less gas and dustthan spirals.

EllipticalGalaxies

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

TheAndromedaGalaxy

Our“Sister”Galaxy

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

and presence of a bar (“B”)

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

_______ Galaxies(Irr): Basicallyanything funky-looking!

A Barred Spiral Galaxy with only 2 arms.

Measuring Distances toGalaxies

• Too far for ______!

• Standard Candles:– Cepheid Variables (for Local Group)– Type Ia ___________

• Redshifts

Type Ia Supernovae• These are another _______ _____ used to

measure distances to galaxies• Matter from large companion falls onto a white

dwarf, causing its mass to exceed ____ Msun• The resulting explosion is a Type Ia supernova.

Supernovae Types

Type Ia: Exploding White Dwarfin Binary

Type II: “ordinary” supernovaecaused by an explodingmassive stars

Supernovae are Good StandardCandles

• They are allthe same_________

• They can beseen at verylarge________– (1000x

farther thanCepheids)

Supernova in galaxy NGC4526 (HST Image)

Galaxies in Motion• Motion of galaxies is measured using the _______ effect.• Spectrum will be redshifted if it is moving _____,

blueshifted if it is moving _______ us.

Non-moving galaxy spectrum

Redshifted Spectrum

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Galaxy Redshifts

• When we look at the spectrum of most galaxies,they are __________.

• This means they are moving _____ from us.

• Only a few exceptions: ___________ is movingtowards the Milky Way (may collide with the MilkyWay… in 3 billion years)

Hubble’s Law• E. Hubble measured the ________ to some of these

galaxies (using Cepheid variable stars)• He discovered that the redshift is proportional to the

galaxy’s _________.

•The relationship iscalled Hubble’s Law

Hubble’s Original Data

Hubble’s Law and DistancesHubble’s Law can be used to determine the ______ togalaxies which are too far away for us to see Cepheid

Variables or Type Ia supernovae

vr = H0 x d

d = vr / H0

Hubble Law

d = vr / H0

• d = distance to galaxy (Mpc)• vr = radial velocity of galaxy (km/s)• H0 = Hubble constant (70 km/s/Mpc)

Hubble Law Example: Find the distanceto a galaxy that has a radial velocity of

35,000 km/s.

d = vr / H0

d =

d =

Techniques for Measuring DistancesReview

1. ________

• Measure angle, p. D = 1/p

2. _______ _______ (standard candle)

• Measure Period, get luminosity

3. Type Ia _________ (standard candle)

4. _______ _____

• Measure velocity Vr . Use: Vr = Ho x D

(nearbystars)

(nearestgalaxies)

(distantgalaxies)

(wholeuniverse!)

9

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.