Astronomy 113 Dr. Joseph E. Pesce, Ph.D. Distances & the...

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© Dr. Joseph E. Pesce, Ph.D.

Astronomy 113Dr. Joseph E. Pesce, Ph.D. Distances & the Milky Way


Historical Overview: the Curtis-Shapley Debate

³What is the size of our galaxy?³What is the nature of �spiral nebula�?

³Occurred in 1920 in DC³Did not resolve issues³Both partly right and wrong

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The Curtis View³Our galaxy is small (10,000pc)³Spiral nebula are not part of our galaxy

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The Shapley View³Our galaxy is large (100,000pc)³Spiral nebula are gas clouds within our galaxy

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Our Galaxy

³Measuring the shape of our galaxy is hard: we’re inside!

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You are here


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View From Earth

The �Milky Way� is a faint band of stars that circles the sky

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The Milky Way

³We’ve known the Milky Way is composed of stars since Galileo (400 years ago)

³What does this tell us about the shape of the galaxy?

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A Disk

³Most stars are in a narrow band on the sky³Most stars seem to be in a disk-shaped

arrangement

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You are here

A big heap of stars!


Early Observations

³William Herschel, in Britain in the 1780s

³Assumed stars were of uniform brightness and distributed evenly

³Didn’t know about interstellar extinction

³Counted stars in different directions to infer

shape

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Herschel’s Results

³We are at the center of the galaxy³Spiral nebula are probably other galaxies

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You are still here

A big heap of stars!


Interstellar Extinction³Interstellar gas and dust absorb, redden, and

polarize light that passes through it.³Didn’t know about extinction prior to about

1900³Affects all observations

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What Herschel Saw

³He was only observing a very small portion (5%) of the galaxy

³His view was distorted by interstellar extinction

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You are here

Limit of Herschel’s observations


Globular Clusters³Shapley noticed that globular clusters are

located in one area of the sky, near the center of the Milky Way

³Perhaps they are orbiting the galaxy³How do you measure distances to these

clusters?

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Cepheid Variable Stars³Very high luminosity stars (red giants)³Found in globular clusters (and elsewhere)³Are variable³Period-luminosity relationship³By measuring the period, get luminosity³By knowing luminosity, get distance(How?)

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Cepheid Period-Luminosity Relationship

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Distribution of Globular Clusters14-16

You are here


Dust?What about dust?

³Most interstellar medium is in disk of galaxy³Most globular clusters are outside of disk

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Herschel & Shapley’s Results Combined

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Nucleus

Bulge

Halo

Disk


What About Other Galaxies?³Cepheids can be observed out to about 200

million ly

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Standard Candles³Objects with known intrinsic luminosity³Luminosity is the same wherever object is³Luminosity is known fairly accurately³Comparing absolute and apparent magnitudes

gives distance³Types

²Cepheids²Supernovae (Type I)²Red Giants²Others

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The Current Picture³ Our galaxy is 50 kpc in diameter

²It contains 100s of billion stars³There are other galaxies beyond ours

²100s of billions ³The universe is really big!

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Our Galaxy

³ Disk

²Where all the blue stars and ISM are found

³Bulge

²Spherical region near galaxy’s center; old stars

³Nucleus

²The central region of the galaxy

³Halo ²Spherical region surrounding disk and bulge; old

stars

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Schematic Picture14-23

Nucleus

BulgeHalo

Disk

Halo


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The Disk³Disk has lots of gas and dust – the ISM³Optical telescopes aren’t very useful³Infrared and radio observations

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The Disk14-25


Radio Observations³Atomic hydrogen (HI) emits a 21cm spectral

line (in the radio part of the spectrum)³Most of ISM is hydrogen³Can thus map the distribution of hydrogen gas

in the disk (velocity comes from doppler shift)

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21cm Emission Line³�Spin flip� of electron orbiting proton:

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electronproton

High energy(from collision)

electronproton

Lower energy(Spontaneous de-excitation)

Photon emitted


Spiral Arms³ISM in disk is distributed in spiral arms³Most of the young stars in the galaxy are in

the spiral arms³The arms are sites of on-going star formation

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Spiral Arms


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Spiral Arms


Young Blue Stars³Stars form in gas clouds³These clouds are found in the spiral arms³Therefore young stars are found in spiral arms³Massive, young stars are bright and die rapidly³These bright, blue stars (and associated star-

forming regions) highlight the arms and make them easier to see

³Most stars are low mass, long-lived stars between the arms

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Red Stars³Most stars in the galaxy are low mass, long-

lived stars (red stars)³They are distributed throughout the disk and

are between the arms too³Red stars are predominantly in the bulge and

halo too

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Population I and II Stars³Population I – Like the sun

²A younger generation of stars found in the disk/spiral arms

³Population II – Older stars²An older generation of stars, found in the halo and

bulge

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Orbits of Stars³Stars in the disk have circular orbits

³Stars in the bulge have random, radial orbits

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Disk Orbits

Stars and gas are rotating in the same direction

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Nucleus of galaxy

Orbits of stars and gas

Disk of galaxy

Side View


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Top View14-36

Nucleus of galaxy


Where do the Arms come from?

³Winding?

*** don’t see this, so arms aren’t from orbiting stars

What causes them?

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Density Waves

³Ripples – waves travel through galaxy’s dust/gas: Spiral Density Waves

²Like traffic jams

³Galaxy rotates causing spiral structure

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Density Waves

³ Waves travel slower than gas/stars gas piles up

³ Compression causes star formation³ See arms easier because lots of hot, O & B

stars² But most stars (faint M dwarfs) are between arms

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Spiral Density Waves14-40


Halo and Bulge Orbits

³Radial orbits through nucleus³At any given time, stars are in various

positions, spherically around nucleus

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Disk and Bulge14-42


Disk and Bulge14-43


Our Galaxy14-44

Bulge (old stars)

Disk (old stars throughout)

Young stars in spiral arms

Nucleus

Halo (old stars)

Side View

Globular Clusters (mostly old)


Weighing the Galaxy³Use orbits of stars to measure mass of Galaxy³(Use Newton’s laws of gravity)³Measure orbital velocity³Measure distance from nucleus

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Rotation Curves

³Plots of orbital velocity versus distance from nucleus

³Gives measure of mass WITHIN each orbit

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Results³The mass of the Galaxy is about 1012 times the

mass of the sun (1 trillion)³There are 100-400 billion stars

³There is much more mass than all the stars, gas, & dust, that we don’t observe directly²Some mass is NOT detected through

electromagnetic means

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Dark Matter³Matter detected through its gravity

²Has no detectable electromagnetic emission

³Over 90% of the mass of the Galaxy is composed of this dark matter

³Found everywhere we can measure it’s gravitational influence. Universe is full of it

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What is Dark Matter?³We don’t know³It can’t be anything too normal or we would

detect it³Good candidates, but no observations yet

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Dark Matter Candidates³Black holes? (Probably not)

²Primordial ones expected but they have evaporated³Brown dwarfs? (Probably not)³Neutrinos? (Lots and lots of them, but not

massive enough)³Subatomic particles

²WIMPs²Higgs particles²Etc

³Detection may come soon….

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Nucleus of the GalaxyWhat’s in the

center of the Galaxy?

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Nucleus of the GalaxySGr A* - The

Dynamical Center

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The Monster in the Middle³Only radio, IR, and X-ray observations possible

(due to large amounts of dust)³Stars very close to dynamical center (Sgr A*)³From long-term observations and doppler

shifts of these stars => a large central mass³4.5 x 106 solar masses within a space of 1AU

³A Black hole!

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Thank You!


Astronomy 113 Dr. Joseph E. Pesce, Ph.D. Distances & the...

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