Astronomy 100Tuesday, Thursday 2:30 - 3:45 pm

Tom Burbinetburbine@mtholyoke.edu

www.xanga.com/astronomy100

What we will do today

• Go over exam

• Schedule for rest of class

• The Sun

Test

• Average score was 27.8 out of 40

• That is a 69.5%

• Grades ranged from a 12 to a 40

Test• We will go over test

y = 0.5138x + 13.093

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Exam 1

Exa

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y = 1.0971x + 17.927

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Homework score

y = 0.8996x + 21.12

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PRS score

Homework Assignment(Due March 31)

• Make up a test question• Multiple Choice• A-E possible answers• 1 point for handing it in• 1 point for me using it on test• The question needs to be on material that will be

on the 3rd exam• 15 people got extra HW credit for me using their

question (or inspiring a question)

I have

• Posted everybody’s exam, homework, PRS scores

• If there is a problem, you need to tell me now

• Goal is to get 20 credits with the Homework score (10% of grade) and 20 credits with the PRS score (10% of grade)

• Then you get perfect score for those parts of your grade

Schedule for Exam 3

• Mar 22 Tue The Sun’s Energy Source

• Mar 24 Thu Stellar Properties • Mar 29 Tue Evolution of Stars • Mar 31 Thu Synthesis of the Elements • Apr 5 Tue Stellar Remnants • Apr 6 Wed Review• Apr 7 Thu Exam 3

Schedule for Exam 4• Apr 12 Tue The Milky Way

• Apr 14 Thu Galaxies

• Apr 19 Tue Life of a Galaxy • Apr 21 Thu No class• Apr 26 Tue Dark Matter • Apr 28 Thu Cosmology and the Big Bang

• May 3 Tue Is there life elsewhere?• May 5 Thu Is there life elsewhere? • May 9 Mon Review • May 10 Tue Exam 4

Final

• Is scheduled for

• Friday 5/20 8:00 AM

• Totman Gym or Trailers GYM

• I know that is last day of finals week

• So …

I will

• Give a Final on the last day of class (May 12)

• And also a different Final on May 20

• I will take the top 4 scores of the 6 exams

• The finals will contain material that people tended to get wrong on the earlier tests

Sun

• Sun is a star

What is the most abundant element in the Sun?

• A) Helium

• B) Hydrogen

• C) Iron

• D) Lithium

• E) Lead

What is the most abundant element in the Sun?

• A) Helium

• B) Hydrogen

• C) Iron

• D) Lithium

• E) Lead

Why does the Sun shine?

• Nuclear fusion – the process in which two or more smaller nuclei slam together and make a larger nucleus

Is the source of the Sun’s energy

Figure 15.5

Strong Force is the force that holds nuclei together

The nuclei are positively charged

Figure 15.6

Sun’s interior

• 15 million Kelvin

• This temperature is hot enough where nuclei can fuse together

• Strong Force occurs over short distances and allows protons in the nucleus to stay together

• Hydrogen fuses into helium

Figure 15.7

Why is energy released when 4 Hydrogen atoms make one Helium atom?

• A) The mass of 4 Hydrogen atoms is greater than the mass of one Helium atom

• B) The gravity of a Helium atom is greater than the gravity of 4 Hydrogen atoms

• C) The gravity of a Helium atom is less than the gravity of 4 Hydrogen atoms

• D) Electrons are excited during the collision

• E) Neutrons are excited during the collision

Why is energy released when 4 Hydrogen atoms make one Helium atom?

• A) The mass of 4 Hydrogen atoms is greater than the mass of one Helium atom

• B) The gravity of a Helium atom is greater than the gravity of 4 Hydrogen atoms

• C) The gravity of a Helium atom is less than the gravity of 4 Hydrogen atoms

• D) Electrons are excited during the collision

• E) Protons are excited during the collision

Calculation• 4 protons (Hydrogen atoms) has a mass of

6.690 x 10-27 kg• Helium nucleus has mass of 6.643 x 10-27 kg• This is 99.3% of the original mass• 0.7% of the mass is turned into energy• E = mc2

• The resulting energy is found as kinetic energy of the helium atom, released gamma rays, and released neutrinos

Figure 15.7

Definitions

• Positron – postitively charged electron (antimatter)

• Neutrino – almost massless particle

1st Homework question (March 24)(beginning of class)

• I want you to detail the hydrogen fusion reaction (Steps 1 through 3) with words (written by hand) and pictures

• What is the solar neutrino problem?

• How was the Homestake Gold Mine used to detect neutrinos?

2nd HW question (due March 24th)(beginning of class)

• How much longer will it take the Sun to use up all its “fuel”?

• When the Sun uses up its fuel it will start expanding, which will be bad for people living on Earth

• I want an answer in years

• Show your work

Things you need to know

• Energy source for sun is four hydrogen atoms combining to produce one helium atom

• about 0.7% of the original mass is turned into energy during this process

• 10% of the Sun’s mass is hot enough to undergo fusion

• Mass of the Sun = 2 x 1030 kg

And

• Total lifetime = (energy available)

(rate [energy/time] at which sun emits energy)

• rate [energy/time] at which the Sun emits energy is equal to 3.8 x 1026 Watts (Joules/second)

And

• Time left = Lifetime – current age

• Current age = ~5 billion years

The 2 homework assignments will be due at the beginning of class

• Or they won’t be accepted

• Since I want to go over them next class period

• If you won’t be on class on Thursday or will be late, email it to me

Sun is in gravitational equilibrium

• The weight of overlying material is balanced by the underlying pressure of hot gas

• This keeps the Sun’s diameter stable

Figure 15.2

How did the Sun form

• Began as a cloud of interstellar gas

• Began to collapse

• Raised the interior temperature higher and higher

What happened next

• The temperature and density of the center of the cloud increased until nuclear fusion could take place

• Energy generated from the interior then equaled the energy radiated away

Questions