Chapter 16

Charles HakesFort Lewis College 1



Chapter 16

Hubble’s Law


Outline

• Review• Hubble’s Law


Probable Job Opportunity

• The Academic Success Program regularly funds Astronomy Tutors/ Study Group leaders. If you might be interested in this for next semester, please let me know via email.


Possible explanations for Dark matter include everything except

A) WIMPs

B) Brown dwarfs

C) Black holes

D) Dark dust clouds

E) MACHOs


Possible explanations for Dark matter include everything except

A) WIMPs

B) Brown dwarfs

C) Black holes

D) Dark dust clouds

E) MACHOs


Count every “F” in the following text:

FINISHED FILES ARE THE RES

ULT OF YEARS OF SCIENTI

FIC STUDY COMBINED WITH

THE EXPERIENCE OF YEARS...A=2 B=3 C=4 D=5 E=6


Count every “F” in the following text:

FINISHED FILES ARE THE RES

ULT OF YEARS OF SCIENTI

FIC STUDY COMBINED WITH

THE EXPERIENCE OF YEARS...


Extending the Distance Scale

• Variable Stars• Tully-Fisher Relationship• Supernovae• Cosmological Redshift


Figure 14.7Variable Stars on Distance Ladder

• Greater distances can be determined than typically available through spectroscopic parallax, because these variables are so bright.


Figure 15.12Local Group


Tully-Fisher Relationship


Figure 15.9Galactic “Tuning Fork”

• Galaxies are classified according to their shape (Hubble classification)• Elliptical• Spiral• Irregular


Figure 15.10Galaxy Rotation

• Rotation rates can be determined using Doppler shift measurements• Blue shift indicates moving towards you• Red shift indicates moving away from you


Tully-Fisher Relationship

• Rotation speed can be used to determine a galaxy’s total mass.

• A close correlation between rotation speed and total luminosity has been observed.

• Comparing (true) luminosity to (observed) apparent brightness allows us to determine distance

• Distance scale can be extended to ~200 Mpc.


Figure 15.11Extragalactic Distance Ladder


Which of these does not exist?

A) a .06 solar mass brown dwarf

B) a 1.6 solar mass white dwarf

C) a six solar mass black hole

D) a million solar mass black hole

E) a 2.7 solar mass neutron star


Which of these does not exist?

A) a .06 solar mass brown dwarf

B) a 1.6 solar mass white dwarf

C) a six solar mass black hole

D) a million solar mass black hole

E) a 2.7 solar mass neutron star


Supernovae

• Type II Supernovae • Are a result of a very massive star’s core

collapse• Can vary in brightness, since the cores

can vary in size.• Therefore, they are not a good distance

indicator.


Supernovae

• Type I Supernovae • White dwarf, carbon detonation• Are a result of a white dwarf exceeding

its Chandrasekhar limit (1.4 Msolar).• They are all about the same size.• They are very good distance indicators

(Standard Candles).


Standard Candles

• Standard Candles are easily recognizable astronomical objects whose luminosities are confidently known.• Term usually only refers to very luminous objects

• Type I supernovae• Other objects might include

• Rotating spiral galaxies• Cepheid variables• Main sequence stars


Figure 15.11Extragalactic Distance Ladder


Chapter 16

Hubble’s Law


Thought Experiment• You observe (with a telescope) several cars driving

on US 160. They are all moving away from you. What pattern can you detect?Car distance speedCar 1 15 miles 5 mphCar 2 105 miles 35 mphCar 3 54 miles 18 mphCar 4 240 miles 80 mphCar 5 81 miles 27 mphCar 6 165 miles 55 mph


Cosmological Redshift


Figure 16.1Galaxy Spectra

• Early 20th Century astronomers observed that most galaxies were moving away from us.


Figure 16.2Hubble’s Law

• Hubble plotted the recession velocity against the distance of the galaxies, and found a direct relationship.


Hubble’s Law

recessional velocity = Ho x distance

• Ho is Hubble’s constant, the slope of the line on the previous plot

• Precise value is somewhere between 50-80 km/s/Mpc• Tully Fisher and Cepheid variable measurements

suggest higher values (70-80 km/s/Mpc)• Type I supernovae suggest lower values (50-65

km/s/Mpc)• Modern accepted value ~70 km/s/Mps


Hubble’s Law


• Exercise: if Ho = 50 km/s/Mpc, what is the recessional velocity of a galaxy that is 500 Mpc away?


Hubble’s Law


• Exercise: if Ho = 50 km/s/Mpc, what is the recessional velocity of a galaxy that is 500 Mpc away?

• How long ago was that galaxy at your location?


Hubble’s Law


• How long ago was that galaxy at your location?• time = distance / velocity


Hubble’s Law


• How long ago was that galaxy at your location?• time = distance / velocity• 1 Mpc = 3.09x1019 km


Hubble’s Law


• How long ago was that galaxy at your location?• time = distance / velocity• 1 Mpc = 3.09x1019 km• 1/Ho has the units of time!


Hubble’s Law


• How long ago was that galaxy at your location?• time = distance / velocity• 1 Mpc = 3.09x1019 km• 1/Ho has the units of time!• 1/Ho gives the age of the universe.

(approximately)


Hubble’s Law

• Distances can be determined simply by measuring the redshift.

• The most distant objects show redshifts greater than 1.

• Relativity must be used to determine velocities approaching c.

• This is the “top” of the distance ladder.


Figure 16.3Cosmic Distance Ladder


Which of the following is inferred by Hubble’s Law?

A) The greater the distance, the more luminous the galaxy

B) The more distant a galaxy, the more evolved its members are

C) The larger the redshift, the more distant the galaxy

D) The larger the gravity lens, the more massive the galaxy cluster.


Which of the following is inferred by Hubble’s Law?

A) The greater the distance, the more luminous the galaxy

B) The more distant a galaxy, the more evolved its members are

C) The larger the redshift, the more distant the galaxy

D) The larger the gravity lens, the more massive the galaxy cluster.


What method would be most appropriate to determine the distance to a nearby galaxy?

A) Spectroscopic parallax

B) Cepheid variables

C) Hubble’s law

D) Radar ranging


What method would be most appropriate to determine the distance to a nearby galaxy?

A) Spectroscopic parallax


C) Hubble’s law

D) Radar ranging


What method would not be appropriate to determine the distance to a nearby galaxy?

A) Tully-Fisher relationship


C) Hubble’s law

D) Type I Supernovae


What method would not be appropriate to determine the distance to a nearby galaxy?

A) Tully-Fisher relationship


C) Hubble’s law

D) Type I Supernovae


What does the Hubble constant measure?

A) The density of galaxies in the universe

B) The luminosity of distant galaxies

C) The rate of expansion of the universe

D) the speed of a galaxy of known redshift

E) the reddening of light by intergalactic dust clouds


What does the Hubble constant measure?

A) The density of galaxies in the universe

B) The luminosity of distant galaxies

C) The rate of expansion of the universe

D) the speed of a galaxy of known redshift

E) the reddening of light by intergalactic dust clouds


Large-Scale Structure



• Use the scale of 1m = 1 A.U.



• Use the scale of 1m = 1 A.U.• The Earth is 1 m from the Sun



• Use the scale of 1m = 1 A.U.• The Earth is 1 m from the Sun• The Nearest star is near Albuquerque



• Use the scale of 1m = 1 A.U.• The Earth is 1 m from the Sun• The Nearest star is near Albuquerque• The center of the Milky Way galaxy

would be 4 times as far as the moon.



• Use the scale of 1m = 1 A.U.• The Earth is 1 m from the Sun• The Nearest star is near Albuquerque• The center of the Milky Way galaxy

would be 4 times as far as the moon.• The Andromeda galaxy would be near

Mars



• Redshift surveys of galaxies are used to determine the large-scale structure of the universe.


Figure 16.9First Galaxy Survey from the mid-1980’s


Figure 16.10The Local Universe


Large-Scale Structure• Redshift surveys of galaxies are used to determine

the large-scale structure of the universe.• Observed structure includes:

• Strings• Filaments• Voids

• The most likely explanation is a slice through “Bubbles.”

• Only a few of these “slices” have been completed.


Figure 17.1Galaxy Survey


Galaxy Survey

• The universe is homogeneous - it looks the same everywhere

• The universe is isotropic - it looks the same in all directions

• Cosmological principle - the universe is isotropic and homogeneous.


Three Minute Paper

• Write 1-3 sentences.• What was the most important thing

you learned today?• What questions do you still have

about today’s topics?

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Chapter 16

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