Lecture 18: The Galaxy, Our Home in Space

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Date: April 6, 1995
Reading Assignment: pp. 533-542

Description : understand the mass and core of our Galaxy

Objectives

be able to describe how Newton's laws are used to measure the mass of our Galaxy

be able to describe spiral density waves and how they might explain the observed spiral structure in our Galaxy

be able to describe why we need dark matter to explain the observed orbits in our Galaxy

be able to list and describe the possible types of dark matter that may be found in our Galaxy

be able to describe the center of our Galaxy and how it differs from our neighborhood

be able to describe what Cosmic Rays are and how they might be produced in our Galaxy

Lecture Outline

Slide # 1: The Galaxy- Our Home in Space (GRAPHICS) Slide # 2: Today's Lecture

overview of our Galaxy

shape, mass, size

spiral arms

red and blue stars

the mass of our Galaxy

dark matter

Slide # 3: The View from Earth (GRAPHICS)

the Milky Way is a faint band of stars that circle sky

Slide # 4: The Galaxy is a Disk? (GRAPHICS)

most stars are in a narrow band

most stars seem to be in a disk-shaped arrangement

Slide # 5: Hershel's Observations (GRAPHICS)

Hershel only saw about 5% of our Galaxy

this view was distorted by variable amounts of dust

Slide # 6: The Curtis-Shapley Debate (GRAPHICS)

what is the size of our galaxy?

what is the nature of spiral nebula?

Slide # 7: Variable Stars

eclipsing binary

white dwarf's in binary systems

accretion disks, nova, supernova

supernova

pulsars

pulsating variables

Slide # 8: Cepheid Variables

very high luminosity stars

found in globular clusters

by measuring the period, you can find the luminosity

from the luminosity and the brightness, you can calculate the distance

Slide # 9: Pulsating Variable Stars (GRAPHICS)

some stars pulse in size and temperature

SIZE CHANGE IS ONLY ABOUT 15% OF RADIUS

they are NOT in hydrostatic equilibrium

Slide # 10: Light Curves of Pulsating Variables (GRAPHICS)

Cepheid Variables

Slide # 11: Period-Luminosity Relationship (GRAPHICS)

the pulsation period of Cepheid variables is directly related to their average

Slide # 12: Distribution of Globular Clusters (GRAPHICS)

Shapley's result

Slide # 13: Hershel's Observations (GRAPHICS)

the real Galaxy

Slide # 14: Cepheids in M31 (GRAPHICS)

observations by Edwin Hubble

Slide # 15: The Current Picture

our Galaxy is 30 kpc in Diameter

it contains 100 billion stars

there are other Galaxies beyond our own

100 billion galaxies

the Universe is really big

Slide # 16: A New Debate

what is the nature of Gamma-Ray bursts

are they inside or outside our Galaxy?

A Debate on April 22 at the Natural Hisotry Museum

Lamb vs Paczynski

Slide # 17: Our Galaxy

disk

where all young blue stars are found

bulge

spherical region near Galaxy's center

nucleus

central region of the Galaxy

halo

region around the disk and bulge

Slide # 18: Our Galaxy (GRAPHICS)

side view

Slide # 19: Observing the Disk

the disk has lots of gas and dust

dust absorbs visible light

optical telescopes do not work well for disk observations

Slide # 20: Radio Telescopes

atomic hydrogen (HI) emits a 21 cm spectral line

a forbidden line

this line can be detected with radio telescopes

21cm = radio wavelength

radio telescopes can be used to map the distribution and velocity of the gas

velocity is determined using the Doppler shift

Slide # 21: Some Real Observations

VLA Pictures

and pictures from the VLA

Slide # 22: Spiral Arms

HI gas in our Galaxy is distributed in spiral arms

arms connect to the bulge

most young stars are also in these arms

regions of on-going star formation

Slide # 23: Spiral Arms (GRAPHICS)

top view (M51)

Slide # 24: Constellation Corner (GRAPHICS)

Constellation De Jour

Slide # 25: Bootes (GRAPHICS)

Fairfax 11pm - April 4 - NE - 4.0

Slide # 26: Bootes (GRAPHICS)

Fairfax 11pm - April 4 - NE - 4.0

Slide # 27: Sagittarius (GRAPHICS)

July 4 - 11pm - South - 4.0

Slide # 28: Where is the Center of Our Galaxy?

the center must be somewhere in the Milky way

which direction is it?

Slide # 29: Sagittarius (GRAPHICS)

July 4 - 11pm - South - 4.0

Slide # 30: Sagittarius (GRAPHICS)

July 4 - 11pm - South - 4.0

Slide # 31: A Few Words about Astrology

people born in December are classified as Sagittarians

the Sun spends most of its time in the Constellation of Ophiuchus and Scorpius

astrological signs have not changed with the precession of the equinoxes

Slide # 32: Red and Blue Stars

stars form from gas clouds

gas clouds are found in spiral arms

young stars must also be found in spiral arms

massive stars evolve and die very rapidly

so any massive main sequence stars will be found in the spiral arms

Slide # 33: The Distribution of Red Stars

older stars are red in color

many of these stars are found in the disk, but not concentrated in the spiral

red stars are also in the bulge and the halo

Slide # 34: Our Galaxy (GRAPHICS)

side view

Slide # 35: Disk and Bulge (GRAPHICS)

m104

Slide # 36: Orbits of Stars

stars in the disk of the Galaxy have circular orbits

stars in the bulge and halo have random orbits

Slide # 37: Disk Orbits (GRAPHICS)

stars and gas are rotating in the same direction

Slide # 38: Disk Orbits (GRAPHICS)

top view

Slide # 39: Halo and Bulge Orbits (GRAPHICS)

everything orbits around the nucleus

random orientations and shapes

Slide # 40: The Mass of the Galaxy (GRAPHICS)

we can use the circular orbits of stars to measure the mass of the Galaxy

Newton's law of gravity

Slide # 41: Observations

distance to star

position of star relative to Galactic nucleus

Doppler shift of star

Slide # 42: Observations (GRAPHICS)

geometry

Slide # 43: Results (GRAPHICS)

a rotation curve

Slide # 44: Results

the mass of the Galaxy is about 1011 times the mass of the Sun

there is much more mass that the stars, dust, and gas we observe

some mass is NOT detected in any electromagnetic wavelength

Slide # 45: Dark Matter

matter detected because of its gravity

has no electromagnetic emission that we have detected

over 90% of our Galaxy is composed of Dark matter

also called the "Missing Mass"

Slide # 46: What is Dark Matter made of ?

we don't know

it can't be anything too normal, or we would see it

we have some good candidates, but no single one explains all the observations

Slide # 47: Dark Matter Candidates

black holes

brown dwarfs (Machos)

neutrinos

subatomic particles (WIMPS)

Squarks

Higgs particles

Sneutrinos