Astro 1050 Mon., Nov. 4, 2002

Today: Chapter 12, The Milky Way

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)

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

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?

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

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

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

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

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.

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 Þ M then M-m Þ d

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

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

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

The Andromeda Galaxy

M51: The Whirlpool Galaxy

Sensing Hydrogen Gas

Radio emission at 21 cm wavelength

Penetrates gas and dust

Requires little energy to excite

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

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

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, M_effective grows with distance, so velocity does not drop in same way

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”

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

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

M51: The Whirlpool Galaxy

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

Degree of Organization
of the Spiral Arms

Different degrees of organization

Grand Design
Spirals: M51

Flocculent (“wooly”) Spirals

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 Þ500 pc

M51: The Whirlpool Galaxy

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.

M51: The Whirlpool Galaxy

Self Sustaining Star Formation

Cloud collapse Þ New stars

New stars Þ Supernova after few million years

Supernova Þ Shock Waves

Shock Waves Þ Nearby clouds collapse

Differential Rotation twists pattern into spiral

Two limiting cases of spirals

Grand Design: Density Wave

Flocculent: Self Sust. Star Form. + Diff. Rot.

In most Galaxies you have some combination of the two

The Nucleus of the Galaxy

Likely Black hole

High velocities

Large energy generation

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

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

A movie of stars at the core

www.mpe.mpg.de/www_ir/GC

Very cool, brand new, and worth a look!

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


	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)


	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


	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?


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


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


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


	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


	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.


	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 Þ M then M-m Þ d


	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


	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


	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


	Radio emission at 21 cm wavelength
	Penetrates gas and dust
	Requires little energy to excite


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


	Stars far from the center take longer to orbit galaxy

	If all mass is at the center get Keplerian Rotation:


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


	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”


“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


	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


	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



	Different degrees of organization
		Grand Design Spirals: M51

		Flocculent (“wooly”) Spirals


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 Þ500 pc


		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.


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

	Differential Rotation twists pattern into spiral


	Grand Design: Density Wave

	Flocculent: Self Sust. Star Form. + Diff. Rot.

	In most Galaxies you have some combination of the two


	Likely Black hole
		High velocities
		Large energy generation

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

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


	www.mpe.mpg.de/www_ir/GC

	Very cool, brand new, and worth a look!

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