Astro 1050     Fri., Nov. 8, 2002

   Today: Articles

                End Ch. 12, The Milky Way

                Start Ch. 13, Galaxies

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.

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 Sun in the Milky Way

Milky Way Reconstruction

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.”

Review Chapter 12: Milky Way

The discovery of the Galaxy

Variable stars as distance indicators

Globular clusters

The size and overall structure of the Galaxy

21 cm Hydrogen emission

Motions in the galaxy

The Halo

The Disk population

Spiral Arms

The Nuclear Bulge

The Rotation curve and the Galaxy’s mass

The origin of the galaxy

The Galactic Center

Chapter 13: Galaxies

Family of Galaxies

Classification

Properties of Galaxies

Distance; The Hubble Law

Size and Luminosity

Mass (including Dark Matter)

Evolution of Galaxies

Clusters

Mergers

The Hubble Deep Field

Types of Galaxies (pg. 254-255)

Spirals

      Sa                   Sb              Sc
   (large nuclei      Þ small nuclei)
(little gas,dust    Þ lots of gas, dust)

     SBa                SBb            SBc    (as above, with BARS)

Ellipticals

E0 E1 E2 E3 E4 E5 E6 E7
(spherical)               (highly elliptical)

Irregulars

Spiral types

The nuclear bulge is population II (old objects)

So the Sa – Sc sequence is consistent with
little gas Þ more gas

Properties of Galaxies

Distance

Use Cepheid Variables for close objects

Other objects for which Absolute Magnitude is know:

Supernova

Planetary nebula in certain emission lines

Use “Hubble Law” for more distant objects

(Correlation of distance with radial velocity)

Diameter and Luminosity

Obtain from angular size and magnitude, combined with distance

Mass

Rotation curves

Velocity dispersion

90 to 99% of mass is
“dark matter”

The Hubble Law using galaxies with visible Cepheid variables.

Hubble Law Example
v_r = H₀ d
with H₀=0.5 (mile/hr)/mile

Hubble Law Example using relative v_r and relative d

The Hubble Law using secondary distance indicators
H_o = 72 ±8 km/s/Mpc

The Local Group of Galaxies

Galaxies live in clusters

Rich clusters: thousands of galaxies

Poor clusters: Few than a thousand

The Coma Cluster

Evolution of Galaxies

Galaxies live in clusters

Rich clusters: thousands of galaxies

Poor clusters: Few than a thousand

Fundamental difference between stars and galaxies:

Stars live isolated lives:

They are much smaller than distance between them

They virtually never collide

Galaxies are not isolated

They are only slightly smaller than the distances between them

The can (and do) collide, and interact with gas within clusters

Effects of Collisions

Stars pass “through” each other, but orbits around galaxy disrupted

Gas clouds collide

Gas stripped away from stars

Collisions cause bursts of star formation

Ellipticals may be those galaxies which have suffered collisions

Spirals may be those galaxies which have not suffered collisions


	Today: Articles
	End Ch. 12, The Milky Way
	Start Ch. 13, Galaxies


		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.”


The discovery of the Galaxy
	Variable stars as distance indicators
	Globular clusters
The size and overall structure of the Galaxy
21 cm Hydrogen emission
Motions in the galaxy
	The Halo
	The Disk population
		Spiral Arms
	The Nuclear Bulge
	The Rotation curve and the Galaxy’s mass
The origin of the galaxy
The Galactic Center


	Family of Galaxies
		Classification
	Properties of Galaxies
		Distance; The Hubble Law
		Size and Luminosity
		Mass (including Dark Matter)
	Evolution of Galaxies
		Clusters
		Mergers


	Spirals
		Sa Sb Sc (large nuclei Þ small nuclei) (little gas,dust Þ lots of gas, dust)

		SBa SBb SBc (as above, with BARS)

	Ellipticals
		E0 E1 E2 E3 E4 E5 E6 E7 (spherical) (highly elliptical)

	Irregulars


	The nuclear bulge is population II (old objects)
	So the Sa – Sc sequence is consistent with little gas Þ more gas


Distance
	Use Cepheid Variables for close objects
	Other objects for which Absolute Magnitude is know:
		Supernova
		Planetary nebula in certain emission lines
	Use “Hubble Law” for more distant objects
		(Correlation of distance with radial velocity)
Diameter and Luminosity
	Obtain from angular size and magnitude, combined with distance
Mass
	Rotation curves
	Velocity dispersion
	90 to 99% of mass is “dark matter”


	Galaxies live in clusters
		Rich clusters: thousands of galaxies
		Poor clusters: Few than a thousand


	Stars pass “through” each other, but orbits around galaxy disrupted

	Gas clouds collide
		Gas stripped away from stars
		Collisions cause bursts of star formation

	Ellipticals may be those galaxies which have suffered collisions
	Spirals may be those galaxies which have not suffered collisions