The Galaxy, Our Home in Space

Outline of Lecture 18
The Galaxy, Our Home in Space

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

variable stars

pulsating variables

the Curtis-Shapley debate

the size our Galaxy

the nature of spiral nebula

overview of our Galaxy

shape, mass, size

spiral arms

red and blue stars

the mass of our Galaxy

dark matter

Slide # 3 : Variable Stars (GRAPHICS)

eclipsing binary

white dwarf's in binary systems

accretion disks, nova, supernova

supernova

pulsars

pulsating variables

Slide # 4 : Eclipsing Binary (GRAPHICS)

binary stars can eclipse each other

orbits aligned so one star passes in front of the other

the total amount of light from the stars varies because of the eclipses

Slide # 5 : Binary Star Types (GRAPHICS)

Eclipsing Binary Stars

Slide # 6 : Eclipsing Binary

Light Curve

Slide # 7 : Algol

binary star system

star 1 = 3.7 solar mass B8V (main sequence)

star 2 = 0.8 solar mass K8IV (subgiant)

eclipsing binary star system

short orbital period and very close orbits

Slide # 8 : White Dwarfs in Binary Systems (GRAPHICS)

accretion disks and cataclysmic variables

nova explosions

type I supernova

Slide # 9 : Accretion Disks (GRAPHICS)

material forms disk as it is transferred to a compact star

disk emits energy from collisions and compression

possibly more energy than the two stars

Slide # 10 : Accretion Disks (GRAPHICS)

mass transferred to a compact star

Slide # 11 : Cataclysmic Variables (GRAPHICS)

the accretion disk can become unstable

the accretion disk can suddenly heat up and become very bright

the total light of the system increases

Slide # 12 : Nova (GRAPHICS)

hydrogen rich material compresses and then ignites (nuclear fusion) on the surface of a white dwarf

Slide # 13 : Type I Supernova (GRAPHICS)

white dwarf mass exceeds Chandarsekhar limit

star collapses and carbon detonation occurs

MUST OCCUR IN A BINARY SYSTEM

does not produce a neutron star

Slide # 14 : Type II Supernova

core collapse of SINGLE MASSIVE STAR

core made of degenerate iron

mass of core exceeds Chandrasekhar mass

electrons absorbed into nuclei

no pressure from electrons, so core collapses

very luminous

Slide # 15 : Pulsars

the lighthouse effect

Slide # 16 : 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 # 17 : Pulsing Variable Stars

internal pressure changes from an instability caused by ionization

ionization changes the stars OPACITY

these stars are always POST MAIN SEQUENCE

stars change luminosity regularly

Slide # 18 : Light Curves of Pulsating Variables

Cepheid Variables

Slide # 19 : The HR Diagram

10 Billion Years

Slide # 20 : Cepheid Period-Luminosity Relationship

discovered by Henrietta Leavitt in 1908

period and average luminosity of Cepheids are related to each other

Slide # 21 : Period-Luminosity Relationship (GRAPHICS)

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

Slide # 22 : The Curtis-Shapley Debate (GRAPHICS)

what is the size of our galaxy?

what is the nature of spiral nebula?

Slide # 23 : Historical Notes

occurred in April 1920

did not resolve the issues

more data was needed

both scientists were partially correct

and partially wrong

Slide # 24 : The Curtis View (GRAPHICS)

spiral nebula are not a part of our galaxy

our galaxy is a small part of a huge universe

the galaxy is small

size about 10,000 parsecs

Slide # 25 : The Shapley View (GRAPHICS)

spiral nebula are part of the Galaxy

just a normal gas cloud with a strange shape

our galaxy was very large

100,000 pc in size

Slide # 26 : The View from Earth (GRAPHICS)

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

Slide # 27 : The Milky Way

we've known these are stars since Galileo

easy to resolve even with a small telescope

what does this observation tell us about the structure of our Galaxy?

Slide # 28 : The Galaxy is a Disk?

most stars are in a narrow band

most stars seem to be in a disk-shaped arrangement

Slide # 29 : Early Theories

Thomas Wright- mid-1700's

suggested first disk model

Immanuel Kant - late 1700's

suggested spiral nebula might be other galaxies

Slide # 30 : Early Observations - William Hershel (GRAPHICS)

built a 48 inch diameter telescope

assumed stars were uniform in brightness and had different distances

assumed no interstellar extinction

counted stars in different directions to determine the shape of our Galaxy

Slide # 31 : Hershel's Result (GRAPHICS)

we are in the center of the Galaxy

spiral nebula are probably other galaxies

Slide # 32 : The Kapteyn Universe

around 1900, data still indicated that we were at the center of the Galaxy

the nature of spiral nebula was still being debated

Slide # 33 : Interstellar Extinction (GRAPHICS)

interstellar dust absorbs, reddens, and polarizes light

we didn't know about interstellar dust before 1900

all observations were limited by the absorption of light

Slide # 34 : Hershel's Observations (GRAPHICS)

Hershel only saw about 5% of our Galaxy

this view was distorted by variable amounts of dust

Slide # 35 : Globular Clusters (GRAPHICS)

Shapley noticed most of the globular clusters are located in one part of the sky

perhaps globular clusters are orbiting around the center of our galaxy

how do you measure the distance to these clusters?

Slide # 36 : Cepheid Variables (GRAPHICS)

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 # 37 : Distribution of Globular Clusters

Shapley's result

Slide # 38 : What about Dust? (GRAPHICS)

most interstellar absorption is inside the disk of our Galaxy

most globular clusters are outside the Galaxy's disk!

Slide # 39 : Hershel's Observations (GRAPHICS)

the real Galaxy

Slide # 40 : Other Galaxies

variable stars were detected in the Spiral nebula

nova and supernova detected first

Cepheid variables were detected in M31, the Andromeda Galaxy

Slide # 41 : Cepheids in M31

observations by Edwin Hubble

Slide # 42 : 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 # 43 : 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 # 44 : Our Galaxy

side view

Slide # 45 : 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 # 46 : 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 # 47 : Some Real Observations

VLA Pictures

and pictures from the VLA

Slide # 48 : 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 # 49 : Spiral Arms

top view (M51)

Slide # 50 : Constellation Corner

Constellation De Jour

Slide # 51 : Sagittarius

July 4 - 11pm - South - 4.0

the center must be somewhere in the Milky way

which direction is it?

Slide # 52 : Sagittarius

July 4 - 11pm - South - 4.0

Slide # 53 : Sagittarius

July 4 - 11pm - South - 4.0

Slide # 54 : 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 during this time

astrological signs have not changed with the precession of the equinoxes

Slide # 55 : 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 # 56 : 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 arms

red stars are also in the bulge and the halo

Slide # 57 : Our Galaxy

side view

Slide # 58 : Disk and Bulge

m104

Slide # 59 : Orbits of Stars

stars in the disk of the Galaxy have circular orbits

stars in the bulge and halo have random orbits

Slide # 60 : Disk Orbits

stars and gas are rotating in the same direction

Slide # 61 : Disk Orbits

top view

Slide # 62 : Halo and Bulge Orbits

everything orbits around the nucleus

random orientations and shapes

Slide # 63 : The Mass of the Galaxy

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

Newton's law of gravity

Slide # 64 : Observations

distance to star

position of star relative to Galactic nucleus

Doppler shift of star

Slide # 65 : Observations

geometry

Slide # 66 : Results

a rotation curve

Slide # 67 : 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 # 68 : 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 # 69 : 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 # 70 : Dark Matter Candidates

black holes

brown dwarfs (Machos)

neutrinos

subatomic particles (WIMPS)

Squarks

Higgs particles

Sneutrinos