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Galaxies and Cosmology

The Discovery of Galaxies •  Up to the 1920’s, astronomers were not

sure exactly how far away galaxies were, and thus didn’t know how big they are!

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

•  In 1920, Shapley & Curtis debated the nature of “spiral nebulae” and the size of our Galaxy. (The debate was eventually settled using standard candles)

A “Spiral Nebula”The Whirlpool Galaxy

We now realize that our galaxy is only one of billions of galaxies we can see. These galaxies come in three main types: Spiral, Elliptical & Irregular

Spiral Galaxies

M 100 NGC 300

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

bar)

A Barred Spiral Galaxy with only 2 arms.

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A Spiral Galaxy Seen Edge On

Elliptical Galaxies

Ellipticals are round, not flat like spirals, and come in a wide range of sizes

They are typically “red” in color and have less gas and dust than spirals.

The Coma Galaxy Cluster contains Ellipticals and Spirals Irregular Galaxies • Lack any distinct shape• Are generally smaller than spirals and ellipticals

Hubble Tuning Fork Spiral 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): Basically anything funky-looking!

Galaxy Classification Lecture Tutorial: Page 139-142

•  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 next question.

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

will come around.

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Questions:

•  How do we measure the distance to a galaxy?

•  How do we detect a galaxy’s motion? •  How do we measure the mass in a

galaxy?

Extragalactic Distances

Galaxies are typically millions or billions of light years from our galaxy.

Typical distance units:

Kly = 1,000 light years

Mly = 1,000,000 light years

White Dwarf Supernovae •  Need a standard candle other than Cepheid

variable stars: Supernovae! •  Matter from large companion falls onto a white

dwarf, causing its mass to exceed 1.4 Msun •  The resulting explosion is a Type Ia supernova.

Supernovae Types

Type Ia: Exploding White Dwarf in Binary

Type II: “ordinary” supernovae caused by an exploding massive stars

Supernovae are Good Standard Candles

•  They are all the same brightness

•  They can be seen at very large distances –  (1000x farther

than Cepheids)

Supernova in galaxy NGC4526 (HST Image)

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

blueshifted if it is moving toward us.

Non-moving galaxy spectrum

Redshifted Spectrum

Hubble Law and Distance

The fact that the universe is expanding can be used to determine the distances to galaxies which are far, far away.

vr = H0 x d

If we measure vr we can calculate the

distance.

d = vr / H0

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Hubble Law

d = vr / H0

•  d = distance to galaxy (Mly) •  vr = radial velocity of galaxy (km/s) •  H0 = Hubble constant (22 km/s/Mly)

Techniques for Measuring Distances Review

1.  Parallax

•  Measure angle, use d = 1/p

2.  Cepheid method (standard candle)

•  Measure Period, get luminosity

3.  Type Ia Supernovae (standard candle)

4.  Hubble’s Law

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

(nearbystars)

(nearest galaxies)

(distant galaxies)

(whole universe!)

Hubble Law & Expansion

•  Expansion of pool balls. The balls farthest from the center are moving the fastest. –  Velocity is proportional to distance.

Measuring the Mass of Galaxies We can use the Sun’s motion around the center of the Milky Way

The greater the mass inside the orbit, the faster the Sun has move around the center.

This way we can measure the mass of the Milky Way and other galaxies.

Rotation Curve Example: Merry-Go-Rounds

•  Every part of the merry-go-round “orbits” the center in the same amount of 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 from stars in other galaxies tell us?

The galaxy looks bright at center, so most of the stars are there, so most of the mass should be at the center.

Distance from the Galactic Center

Ligh

t fro

m st

ars

Distance from the Galactic Center

Mas

s

However the flat rotation curve tells a different story!

Because there is a flat rotation curve there should be an equal amount of mass distributed everywhere throughout the galaxy’s disk and halo.

Distance from the Galactic Center

M

ass

Mass curve determined from Light

Mass curve determined from the rotation curve

Mass of the Milky Way

•  The mass of the Milky Way is about 400 billion Msun

•  Stars & Gas we see in the Milky Way can only account for a fraction of the total mass. (~10%!)

- What is it?

- Why can’t we see it?

Dark Matter