A105 Stars and Galaxies

This week’s units: 74, 75, 76, 78, 79 News Quiz Today Galaxies homework due Thursday Projects due Nov. 30

Today’s APOD

Announcements…• Kirkwood Obs. open Weds night

6:30-8:30 PM• Rooftop Session on Thurs, Nov.

16 @ 8 PM• Leonid Meteor Shower, Friday

AM or PM

Main Types of Galaxies

Spirals

Ellipticals

Irregulars

Hubble Deep Field

• Our deepest images of the universe show a great variety of galaxies, many of them billions of light-years away

Irregular Galaxies

Hubble Ultra Deep Field

Spiral Galaxy

Elliptical GalaxyElliptical Galaxy

Spiral Galaxy

disk bulge

halo

Spheroidal Component: bulge & halo, old stars,few gas clouds

Disk Component: stars of all ages, many gas clouds

Disk Component:stars of all ages,many gas clouds

Spheroidal Component:bulge & halo, old stars,few gas clouds

Blue-white color Blue-white color indicates indicates ongoing star ongoing star formationformation

Red-yellow Red-yellow color indicates color indicates older star older star populationpopulation

Properties of Elliptical Galaxies

• Round or elliptical in shape• Contain no visible gas or dust• No young stars or star-forming

regions• The largest galaxies are

ellipticals (and also some of the smallest)

Elliptical Galaxy: All spheroidal component, virtually no disk component

Red-yellow color indicates older star population

Properties of Irregular Galaxies

• Chaotic mix of stars, gas and dust

• No bulge or spiral arms• May contain star forming

regions• Usually small galaxies

Blue-white color indicates ongoing star formation

Classifying Galaxies

12 3

14

A. EllipticalB. SpiralC. Irregular

Classifying Galaxies

24

22

28

30

A. EllipticalB. SpiralC. Irregular

Thought Question

Why does ongoing star formation lead to a blue-white appearance?

A. There aren’t any red or yellow starsB. Short-lived blue stars outshine othersC. Gas in the disk scatters blue light

Famous Galaxies!

• The Local Group – about 3 dozen galaxies– Milky Way– Large and Small

Magellanic Clouds– Andromeda– M33– Dwarfs

About four times smaller than our Milky Way Galaxy

Near M31 more than twice the angular size of the

full moon visible with a good pair of binoculars

Messier 33

The Whirlpool

• Messier 51

• 23 million light years away

Messier 87

• Giant elliptical galaxy at the center of the Virgo Cluster of galaxies

• ~60 million light years away

• note – globular clusters– jet– other galaxies

Messier 82• About 12 Mly• very disturbed galaxy

The Mice

~ 300 Mly

The Puzzle of “Spiral Nebulae”

• Before Hubble, some scientists argued that “spiral nebulae” were entire galaxies like our Milky Way, while others maintained they were smaller collections of stars within the Milky Way

• The debate remained unsettled until someone finally measured their distances

How did Hubble prove that galaxies lie far beyond the Milky Way?

Standard Candles! Pulsating Stars

Hubble settled the debate by measuring the distance to the Andromeda Galaxy using Cepheid variables as standard candles

Cepheid Variable Stars

Because the period of a Cepheid variable star tells us its luminosity, we can use these stars as standard candles

Period = 3 days

Cepheid variable stars with longer periods have greater luminosities

The Nearest StarsDetermine distances of stars out to a few hundred light-years using parallax

Distances

Distances of GalaxiesGalaxies are too far away for parallax

techniqueUse “standard candles” (INVERSE

SQUARE LAW)Cepheid variablesupergiant starsplanetary nebulaesupernovae

Image “graininess” – The smoother the distribution of stars in a galaxy the farther away it is

Steps to the Distance Scale

How do astronomers measure distances to objects?

Brightness alone does not provide enough information to measure distance

Start with nearby objects, move to greater distances

Identifyingthe MainSequence

Apparent brightness of star cluster’s main sequence tells us its distance

Knowing a star cluster’s distance, we can determine the luminosity of each type of star within it, including the distances to Cepheids

Establishingthe brightnessof stars

Other Distance Methods

What clues give you information about distance?

The Distance ScaleCombination of methods allows

us to measure distances to nearby galaxies, and then to further and further distant galaxies.

Andromeda – about 2,500,000 LY distant

Virgo Cluster – about 50,000,000

LY distant

Most distant galaxies –

By measuring distances to galaxies, Hubble found that redshift and distance are related in a special way

“Redshift” of Galaxies

Remember the Doppler Shift?

The spectral lines of galaxies are redshifted, i.e. galaxies are moving away from us.

Plot the velocity of recession against the distance to the galaxy: the farther away a galaxy is, the faster it recedes from us!

Hubble Law

Distance - Velocity Relation

0

200

400

600

800

1000

0 10000 20000 30000 40000 50000 60000 70000

Velocity (km/sec)

Dis

tanc

e (M

pc)

Hubble’s Law

• The correlation discovered by Hubble was reported in 1929 as the formula:

Velocity of Recession = Ho x Distance

• Ho is now know as the Hubble constant, and is measured in kilometers per second per megaparsec

o

Hubble’s Law …

o

• The Hubble Space Telescope was launched to the distance-redshift relation

• Why is Hubble’s law so important?

Hubble’s constant tells us age of universe because it relates velocities and distances of all galaxies

Age = ___________ ~ 1 / H0

Distance

Velocity

Read Units 74, 75, 76, 78, 79

Galaxies homework due Thurs