Today: Ch. 19: Our Galaxy, the Milky Way
Transcript of Today: Ch. 19: Our Galaxy, the Milky Way
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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