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Astro 1050 Fri. Apr. 14, 2017 Today: Ch. 19: Our Galaxy, the Milky

Way

Reading in Bennett: Ch 12 this week, Ch. 13 for next week

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Chapter 12 The Milky Way Galaxy

•  Band of light running around sky in a “great circle” •  Name from Greeks and Romans: Milky Circle, Milky Road •  Galileo saw it was made of thousands of faint stars •  Great Circle suggests a plane of material with us in plane (like ecliptic)

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Jan. 23, 2002 9 pm

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The Milky Way (during the Leonid Meteor Shower)

•  Milky Way made of many faint stars

•  Bands of dark dust visible too

•  Many types of objects (eg. O, B stars, Hydrogen clouds) concentrated along plane of Milky Way

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Where are we within the plane? •  Great Circle suggests a

plane of material with us in plane (like ecliptic)

•  Similar brightness in all directions in plane.

•  Does that really mean we are located in the center?

Side View

Top View

•  That is an illusion: We can only see a limited distance in the disk because of dust

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To sort out location will need: •  Bright objects visible at large distances •  Objects above or below the plane – so not as

obscured

•  Ways to measure distances to those objects •  Ways to see material other than stars

–  Gas, dust, ???, mass distribution •  Ways to map motion of objects in our Galaxy

•  Examples of other Galaxies –  The Shapley-Curtis debate:

•  Are spiral nebulae external galaxies or a type of object within our own galaxy

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Distances to the farther stars

•  Parallax only works for nearby stars •  Spectroscopic “Parallax” works somewhat further out

–  Measure spectra and get Spectral Type and Luminosity Class –  From those get Luminosity and then use m-M to find distance –  Limited because need relatively bright stars to get high resolution

spectra

•  Need another way to find M, then use m-M to get distances

•  Variable stars and the Period-Luminosity relationship: –  Some stars vary in intrinsic brightness with time –  The larger, more massive, and brighter stars vary more slowly

•  Like the relative pitch of a large vs. small organ pipe

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Example of a Variable

•  Cepheid Variables named after prototype Delta Cephei •  RR Lyrae Variables names after prototype RR Lyrae

–  Related to presence of partially ionized He at right level of star •  Partially ionized material can act as a local energy source or sink

From our text: Horizons, by Seeds

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The Instability Strip •  If T too low partially

ionized He too deep to cause instability

•  If T too high partially ionized He too high to cause instability

•  Larger stars oscillate with longer periods

From our text: Horizons, by Seeds

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The Period-Luminosity Relationship •  Relationship discovered by

Henrietta Leavitt in 1912 –  stars in Small Magellanic Cloud –  all at roughly same distance –  but didn’t have absolute M, just

apparent M –  need absolute M to get distances

•  Calibrated by Harlow Shapley –  If you can find distance (so M-m)

for just one nearby Cepheid, you can convert Leavitt’s “m” scale to the “M” you want.

From our text: Horizons, by Seeds

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Calibrating the Period-Luminosity Relationship using Proper Motion

•  Suppose all planes fly at 500 MPH = 733 ft/sec –  Observe the angle that a plane shifts in 1 second of time

–  A plane that moves 10 in 1 sec (900 in 90 sec) is at 42,000 ft –  A plane that moves 20 in 1 sec (900 in 45 sec) is at 21,000 ft

•  Works for stars too: closer stars have faster “proper motion” –  Can only get average distances using average proper motion, since any

given star might be moving faster or slower than average •  Harlow Shapley found 11 Cepheids with proper motion

–  Used average proper motion, and average distance, to find average (M-m)

–  Let him replace Leavitt’s relative “m” axis with absolute “M” –  Now period ⇒ M then M-m ⇒ d

θθθ degft 42,000 distanceor

distanceft 733

deg 3.57arcsec 206265===

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Globular vs. open clusters •  Open Clusters

–  Typically a few thousand stars –  Not gravitationally bound –  Often contain young stars –  Concentrated in plane of Milky Way –  Distributed “randomly” around the

circle of the Milky Way

•  Globular Clusters –  Typically >hundred thousand stars –  Only contain older stars –  Are gravitationally bound –  Not strongly concentrated in plane of

Milky Way –  Not randomly distributed around the

circle of the Milky Way – more towards Sagittarius

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The Distribution of Globular Clusters •  Assume Globular Clusters

orbit around center of galaxy –  Center of Globular Cluster

distribution is 8.5 kpc in direction of Sagittarius

–  We are about 2/3 of the way out to one side – so “diameter” is approx. 25 kpc or 75,000 ly.

–  Dust within the galactic plane fools us with respect to distribution of ordinary stars

From our text: Horizons, by Seeds

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The Shapley-Curtis Debate: 1920 •  Are spiral nebulae really other

galaxies, or just swirling clouds of gas and dust within our own galaxy?

•  Many spiral galaxies had much larger radial velocities than other objects within our own galaxy

•  Answered by new observations: •  In 1923/4 Edwin Hubble photographed and identified Cepheids in the

Andromeda Galaxy: –  Distance to Cepheids clearly showed Andromeda was outside our own

galaxy

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The Andromeda Galaxy

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M51: The Whirlpool Galaxy

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Sensing Hydrogen Gas

•  Radio emission at 21 cm wavelength •  Penetrates gas and dust so we can map Milky Way •  Requires little energy to excite

From our text: Horizons, by Seeds

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The Structure of our Galaxy –  The Disk Component

•  Stars, gas, and dust –  The spiral arms

•  Size: –  Luminous Diameter ~ 25 kpc –  Thickness 300 pc – 1 kpc

O stars and dust 30 pc Sunlike stars greater

–  The Spherical Component •  Old Stars, but little gas or dust •  The Halo

–  Globular clusters –  Isolated old stars

»  red dwarfs, giants, white dwarfs

•  The Nuclear Bulge From our text: Horizons, by Seedsp

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Disk vs. Halo Orbits –  The Disk Component

•  Stars, gas, and dust –  The spiral arms

•  Size: –  Luminous Diameter ~ 25 kpc –  Thickness 300 pc – 1 kpc

O stars and dust smaller Sun-like stars greater

–  The Spherical Component •  Old Stars, but little gas or dust •  The Halo

–  Globular clusters –  Isolated old stars

»  red dwarfs, giants, white dwarfs

•  The Nuclear Bulge

From our text: Horizons, by Seeds

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Differential Galactic Rotation

•  Stars far from the center take longer to orbit galaxy

•  If all mass is at the center get Keplerian Rotation:

•  If M is distributed, Meffective grows with distance, so velocity does not drop in same way

RGMv

MaP == or

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From our text: Horizons, by Seeds

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The Galactic Rotation Curve

•  Keplerian fall-off near center indicates compact mass at center •  Flat curve throughout disk indicates much distributed mass •  Lack of fall-off beyond visible “edge” indicates “dark matter”

From our text: Horizons, by Seeds

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Stellar Population and Galaxy Evolution

•  “Metal” abundance during time –  “Metals” are elements heavier than He –  A given star’s atmospheric abundance is approx. fixed at birth –  Interstellar metal abundance grows with each new generation of stars

•  Red giants and supernova eject new heavy elements into interstellar gas •  Orbits during time

–  A given star’s orbit is approx. fixed at birth – just plows through gas –  Orbits of gas clouds evolve with time since they can collide –  Orbits get more circular and disk-like with time

From our text: Horizons, by Seeds

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Traditional Model of Galaxy Evolution

•  Stars are stuck with their original orbits –  They plow through gas like bullets

•  Orbits of gas can evolve –  Gas clouds collide and only average motion (rotation)

survives •  Metal abundance grows with time

•  System starts out with little organized motion,few metals –  Halo stars form at this time

•  It contracts, velocities average out leaving only rotation –  Gas collapses to form the disk –  Disk stars form after this has happened

•  Some problems with traditional model –  Globular clusters not all same age –  Gap in ages between halo and disk objects –  Presence of some metals in even in oldest stars

•  System may have formed by merger of smaller galaxies –  Galactic Cannibalism

From our text: Horizons, by Seeds

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M51: The Whirlpool Galaxy

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Possible Origin of Spiral Arms •  Differential rotation

smears features out into spiral patterns

•  But can’t be whole story:

•  Number of times Sun has orbited the galaxy: –  10 billion yr/200 million

yr = 50 times

–  Spiral arms would have been wound up very tightly

•  Something must continuously rebuild them

From our text: Horizons, by Seeds

From Realm of the Universe by Abell et al.

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Degree of Organization of the Spiral Arms

•  Different degrees of organization –  Grand Design

Spirals: Two massive arms

–  Flocculent (“wooly”)

Spirals: lots of short arms instead of two long arms

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Degree of Organization of the Spiral Arms

•  Different degrees of organization –  Grand Design

Spirals: M51

–  Flocculent (“wooly”) Spirals

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Tracing the Spiral Arms

•  Arms NOT obvious if you look at: –  Old objects like the sun

•  Arms ARE obvious if you look at: –  Maps of gas clouds

•  21 cm Hydrogen •  Radio maps of CO

–  Far infrared observations of dust –  Young stars

•  O, B stars •  “HII” ionized hydrogen regions surrounding O,B stars

–  Clouds somehow form in arms , then dissipate between them –  Short lived objects only get a short distance from their places of

birth •  O stars, Lifetime = few million years, at 250 km/s ⇒500 pc

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M51: The Whirlpool Galaxy

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Density Wave Theory –  SPIRAL WAVE rotates with galaxy, but

slower than individual stars •  Like moving traffic jam after an accident has

been cleared

–  Gas (and stars) catch up with wave, move through it, eventually reach front

•  Just like cars catching up with moving traffic jam, eventually get through it

–  Gas is more crowded in wave – clouds collapse to form new stars

•  More collisions in the traffic jam

–  There are slightly more old stars in the arm too, because they speed up slightly coming into it and slow down slightly moving out of it.

–  But the best tracers are the things that mark recent cloud collapses: O,B stars, etc.

From our text: Horizons, by Seeds

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Self Sustaining Star Formation

•  Cloud collapse ⇒ New stars •  New stars ⇒ Supernova after few million years •  Supernova ⇒ Shock Waves •  Shock Waves ⇒ Nearby clouds collapse

•  Differential Rotation twists pattern into spiral From Realm of the Universe by Abell et al.

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Two limiting cases of spirals

•  Grand Design: Density Wave

•  Flocculent: Self Sustaining. Star Form. + Diff. Rot.

•  In most Galaxies you have some combination of the two

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The Nucleus of the Galaxy

•  Likely Black hole –  High velocities –  Large energy generation

•  At a=275 AU P=2.8 yr ⇒ 2.7 million solar masses

•  Radio image of Sgr A about 3 pc across, with model of surrounding disk

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A movie of stars at the core

•  www.astro.ucla.edu/~ghezgroup/gc/pictures/orbitsMovie.shtml

•  Very cool, and worth a look!

•  This is the best evidence to date for a massive black hole at the Galactic core. Now essentially “proven.”

Max Planck Institute for Extraterrestrial Physics