Wednesday March 17th 1999
Announcements:

Lecture notes:

Class 26

Review session for test
Test will focus on big issues not tricky.

Constellations        Stars
Pegasus
Ursa Major
Ursa Minor              Polaris
Bootes                    Arcturus
Orion                      Betelgeuse Rigel
Lyra                        Vega
Hercules
Cassiopeia
Cepheus
Andromeda
Cygnus                    Cyg X-1
Auriga
Canis Major             Sirius
Canis Minor             Procyon
Gemini
Perseus
Tarus

Have spent much of the lectures in this section talking about how people get the distances to far away objects - key to
understanding universe.

There is one key principle for many distance determinations that I have called the "2 out of 3 principle"
If you know 2 of the following 3 things you can get the other.
1 apparent brightness
2 luminosity - absolute brightness
3 distance.

Have already learned about parallax - uses geometry - only good for nearby stars. 

Spectroscopic parallax - uses spectral type to get luminosity HR diagram. Use 2/3 principle to get distance - have luminosity and apparent brightness - works for stars in our region of the disk.

Cepheid Variables and RR Lyrae stars - use pulsations to get luminosity - PL relation for Cepheids - all RR Lyrae have just
less than 100 L_(.)
Use 2/3 principle to get distance for Cepheids works throughout our Galaxy and nearby galaxies - with HST  Works out to Virgo cluster - about 18 Mpc.  Historically these were the key to telling us that there are
galaxies outside our own = Hubble.

RR Lyrae - shorter period - lower mass - less luminous
Cepheid - longer period - higher mass - more luminous

Cepheid variables - a type of pulsating high mass star.  1-60 day periods of pulsation.  Henrietta Leavitt discovered in 1908-1912 that:  Longer period Cepheid variables are more luminous than shorter period Cepheid variables - Period - Luminosity relation - can figure out Luminosity by measuring period.  Then have luminosity and apparent brightness so can get distance.  High mass - 100 to 10000+ L_(.) can see far away (throughout Galaxy)

RR Lyrae Stars - a type of pulsating lower mass star - .6-.9 M_(.)  .5 - 1 day periods of pulsation.  All have approximately the same  luminosity - a bit less than 100 L_(.).  Then have luminosity and apparent brightness and can get distance.
Quite common - while can't use too far away they are still very useful.

Standard Candles
 brightest globular clusters
brightest planetary nebulae
peak brightness of supernovae

Hubble's Law - measure redshift of optical emission lines
V=H₀D
V=Velocity in Km/s
D=distance in Mpc
H₀=70 (km/s)/Mpc
D=V/H₀

One thing that is very important is to have a feel for the sizes of things 1.3 pc to nearest star other than Sun
8.5 Kpc to center of our Galaxy
18 Mpc to Virgo Cluster
700 kpc to Andromeda Galaxy

If redshift is larger distance is larger.  If period of a Cepheid is larger luminosity is larger.

More dark matter in our galaxy than visible matter.

Most distant known quasar is farther than most distant known Cepheid variable

Parts of our Galaxy
Disk - radius of optically visible disk = 13kpc
    spiral structure - most of young stars are in the spiral arms
    stars in disk are "metal" rich - elements heavier than H He
enriched by previous supernovae.
takes 240 million years for Sun to orbit one time.

Nuclear bulge - 2kpc radius relatively little gas dust star formation most of stars are old cool low metal

Halo spherical cloud of thinly scattered stars and globular clusters only about 2% as many stars as disk
very little gas dust old cool low metal stars = Pop II

Center of our Galaxy - what do we think is there?  2.6 million solar mass black hole.  Why do we think this?  watch stars/gas
move and use Newton's law of gravity.  How do we see objects in the galactic center?  radio IR X-ray gamma ray.  Optical and UV are hopeless because too much obscuring dust.

Dark Matter in Milky Way
most of mass in dark matter
detect not at any wavelength.  use its gravity -how it pulls on things to infer it is there.  total mass of Galaxy about 1500
billion solar masses.

What is the dark matter?  don't know - not normal fusion powered
stars - "jupiters" less than .08 solar masses never started fusion - exotic subatomic particles.

We think other galaxies also have large amounts of dark matter - infer it is there by its gravity - how it affects galactic
rotation.

Types of galaxies
Spirals - like MW - have disk and spiral structure - also can have bars.
Ellipticals - round or elliptical in shape - little gas/dust/star formation - products of mergers
Irregulars - chaotic shape - no obvious bulge or spiral arms -
examples LMC SMC

Should be able to recognize pictures of basic galaxy types.
Spiral unbarred
Spiral strongly barred
elliptical
irregular

should also be able to recognize globular clusters and planetary nebula