Astronomy 1 – Fall 2014

Lecture 9, November 4, 2014

Reminder: You Are Invited!• Astro 1 Observing Session (Optional, Just for Fun)• WHEN: November 13th at 7-9pm

• Same time Nov. 20th if the 13th is cloudy

• WHERE: Broida rooftoop; take elevator to 6th floor. Turn right as you exit the elevator. Go up the stairs. A TA will great you.

• WHAT: 3 Celestron C8 Schmidt-Cassegrain telescopes and one C11.• Andromeda Galaxy, Uranus, Mars, Crab Nebula, Pleides, etc.• Help identifying the Celestial Equator, NCP (& Polaris), and the

Ecliptic• Constellations: Pegasus, Taurus, Summer Triangle, Orion, etc.

• Will you be able to see the moon?

Announcement

• Guest Lecturer this Thursday 11/06/14– Dr. Tucker Jones (SoCal Center for Galaxy

Evolution Studies Fellow)– Professor Martin will be observing with the Keck I

telescope on Mauna Kea. Wish her clear skies and a working laser guide star!

Previously on Astro-1

• What is the Moon made of?• How did the Moon form?• Tidal forces• What they are• Affect on Earch• Affect on Moon

Today on Astro-1• The Sun: From its inner depths to the Earth

– What holds the Sun up?• Internal structure

– What is the Sun made of?– Why does the Sun shine?

• How long will it shine?• What does it produce?• What is the solar neutrino problem?

– Does the Sun have a surface?• Sunspots and the sun cycle • The corona• The solar wind

The Sun is Heavy! Why Doesn’t It Collapse?

The Earth is a Solid Made of RockIt’s Size is Set by the Physical Size of Atoms.

The Sun is Made of Gas

… And the Earth’s Atmosphere Is Made of Gas

A Pressure Difference Holds Up the Atmosphere

What Do You Feel When You Dive Under Water?

Pressure Difference Balances GravityHow Many Forces Act on the Fish?

A Pressure Gradient Holds the Sun in Hydrostatic Equilibrium

Why is the Sun Hot?

The Sun is hot because …(iClicker Question)

A. It is made of fire.

B. Nuclear reactions make it hot.

C. The average density is low, so the average temperature must be high to produce enough pressure to hold up its weight.

D. It is hot due to the contraction of the pre-solar nebula.

E. Both C & D.

Model of the SunNote: We’ve said nothing about nuclear reactions!

Energy Conservation• If we push on a gas to compress it, it will heat

up because we’ve done work on it.• A gas must do work on its surroundings in

order to expand, so it will cool down.• You can demonstrate this with a bicycle pump.

– The tube heats up as you compress the air.– Press the valve on the inner tube with finger. As

the air expands, what do you feel?

Protostellar Contraction Generated Heat

• Why did the solar nebula collapse?– Right, gravity!– Gravity beat the pressure gradient. – The cloud collapsed.– Gravity did work on the gas, so it heated up as it

collapsed.– The pressure increased as the density and

temperature rose.– Contraction stopped when the hydrostatic

equilibrium was reached.

The Sun is hot because …

A. It is made of fire.

B. Nuclear reactions make it hot.

C. The average density is low, so the average temperature must be high to produce enough pressure to hold up its weight.

D. It is hot due to the contraction of the pre-solar nebula.

E. Both C & D.

How Long Will the Sun Shine?A Paradox.

• In the mid-1800s, Lord Kelvin and Hermann von Helmholtz calculated how long it would take the Sun to lose the heat generated from its contraction.

• The Sun loses its heat through radiation at its surface.– Time = (Work done by gravity) / (Solar Luminosity)– Time = 25 million years

• Why is this timescale a problem?

Can chemical reactions fuel the Sun?(iClicker Question)

Hint: Chemical reactions release about 10-19 J per atom. And, the luminosity of the Sun is 3.9 X 1026 Watts.

A. Yes, the Sun has roughly 1057 H atoms to burn, so this will take a really long time.

B. Yes, the Sun is roughly 2% carbon, so it burns like coal for a very long time.

C. No, at a burning rate of 4 x 1045 atoms/s, all the Sun’s mass would burn in a fraction of a second.

D. No, at a burning rate of 4 x 1045 atoms/s, all the Sun’s mass would burn in just 10,000 years.

Can chemical reactions fuel the Sun?(iClicker Question)

Hint: Chemical reactions release about 10-19 J per atom. And, the luminosity of the Sun is 3.9 X 1026 Watts.

A. Yes, the Sun has roughly 1057 H atoms to burn, so this will take a really long time.

B. Yes, the Sun is roughly 2% carbon, so it burns like coal for a very long time.

C. No, at a burning rate of 4 x 1045 atoms/s, all the Sun’s mass would burn in a fraction of a second.

D. No, at a burning rate of 4 x 1045 atoms/s, all the Sun’s mass would burn in just 10,000 years.

What Keeps the Sun Hot Over Billions of Years?

• Great mystery of the 19th century.• Sun must be roughly 4.5 billion years old.

– Moon rocks from most heavily cratered regions are 4.5 billion years old.

– Most meteorites are 4.54 billion years old.– Oldest rocks on earth are also close to 4.5 Gyr.

• In the early 20th century, ideas from relativity and nuclear physics led to an understanding of why the Sun can shine so long.

The Sun’s energy is produced by hydrogen fusion:4 hydrogen nuclei 1 helium nucleus + energy

How do Protons Fuse?High Temperatures are Required to Overcome the Repulsion of Two Positively Charged Particles.

Pot

enti

al E

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gy

Separation ~ 10-15 m

The Proton-Proton Chain

How do the electron and the neutrino differ?

A. There is no difference between them.B. The neutrino has no charge, a much smaller mass

than the electron, and interacts weakly with matter. C. The neutrino has no charge, a mass the same as the

electron, and interacts weakly with matter. D. The neutrino has the same charge and mass as an

electron, and interacts weakly with matter. E. The neutrino has no charge, a much smaller mass

than the electron, and interacts strongly with matter.

Q16.4

How do the electron and the neutrino differ?

A. There is no difference between them.B. The neutrino has no charge, a much smaller mass

than the electron, and interacts weakly with matter. C. The neutrino has no charge, a mass the same as the

electron, and interacts weakly with matter. D. The neutrino has the same charge and mass as an

electron, and interacts weakly with matter. E. The neutrino has no charge, a much smaller mass

than the electron, and interacts strongly with matter.

A16.4

Prediction: Solar NeutrinosAbout 1014 neutrinos must pass through every square meter of Earth each secondDetect the flashes of light they make when they interact with a big tank of water deep underground.

Where Does the Sun’s Energy Come From?(iClicker Question)

A. Nuclear fusion in the center of the Sun creates new energy. Energy is not conserved.

B. Nuclear fusion converts mass into energy in the center of the Sun. A 4He nucleus is a bit less massive than four H nuclei.

C. Nuclear fusion converts energy into mass in the center of the Sun. A 4He nucleus is a bit more massive than four H nuclei.

D. Nuclear fission in the center of the Sun

E. Nuclear fission near the surface of the Sun.

Where Does the Sun’s Energy Come From?(iClicker Question)

A. Nuclear fusion in the center of the Sun creates new energy. Energy is not conserved.

B. Nuclear fusion converts mass into energy in the center of the Sun. A 4He nucleus is a bit less massive than four H nuclei.

C. Nuclear fusion converts energy into mass in the center of the Sun. A 4He nucleus is a bit more massive than four H nuclei.

D. Nuclear fission in the center of the Sun

E. Nuclear fission near the surface of the Sun.

Energy Production in the SunThermonuclear reactions can only occur in the Sun’s core — that’s the only place where pressures and temperatures are high enough

Failed Stars• A gas cloud less massive than the Solar Nebula

collapses until the density becomes so high that the particles are touching. Then the particles hold up the object in a manner similar to a solid, and no further collapse occurs.

• The resulting object is hot. Its surface radiates like a blackbody at 8000 to 1800 K.

• The central temperature can be as high as a million degrees. Hot, but not hot enough to burn H by the proton – proton chain.

• We call these objects Brown Dwarfs. They would have become stars if they’d been slightly more massive.

Does the Surface of the Sun Shine in Gamma Rays?What wavelengths does the photosphere emit?

The energy produced in the central core of the Sun is transported to the surface

A. by radiation in the layers just outside the central core and by convection in the outer layers.

B. by convection in the layers just outside the central core and by radiation in the outer layers.

C. by convection from just outside the central core all the way to the surface.

D. by radiation from just outside the central core all the way to the surface.

E. only by convection.

Q16.5

The energy produced in the central core of the Sun is transported to the surface

A. by radiation in the layers just outside the central core and by convection in the outer layers.

B. by convection in the layers just outside the central core and by radiation in the outer layers.

C. by convection from just outside the central core all the way to the surface.

D. by radiation from just outside the central core all the way to the surface.

E. only by convection.

A16.5

It takes light about 200,000 years to get from the core to the surface (then 8 minutes to get to us)!

Convection transports heat in the outer layers of the Sun.

Heat Transport in the Sun

Granules are convection cells about 1000 km (600 mi) wide in the Sun’s photosphere. Inset: Rising hot gas produces bright granules. Cooler gas sinks downward along the boundaries between granules; this gas glows less brightly, giving the boundaries their dark appearance. This convective motion transports heat from the Sun’s interior outward to the solar atmosphere.

Scale of granules

Supergranules display relatively little contrast between their center and edges, so they are hard to observe in ordinary images. But they can be seen in a false-color Doppler image like this one. Light from gas that is approaching us (that is, rising) is shifted toward shorter wavelengths, while light from receding gas (that is, descending) is shifted toward longer wavelengths

This series of photographs taken in 1999 shows the rotation of the Sun. By observing the same group of sunspots from one day to the next, Galileo found that the Sun rotates once in about four weeks. (The equatorial regions of the Sun actually rotate somewhat faster than the polarregions.) Notice how the sunspot group shown here changed its shape.

Umbra

Penumbra

The Sun’s 22-year cycle is NOT uniform

Number of sunspots versus year, 1610-present

Maunderminimum

The number of sunspots on the Sun varies with a period of about 11 years. The most recent sunspot maximum occurred in 2000, next one will be in 2013 (predicted).

The surface (photo sphere) of the Sun is about 5800K, but the corona above it is about a million degrees. How?

Variations in the Sun’s magnetic field drive the activity on and above the solar surface

Rearrangements of the magnetic field cause eruptions and flares

(an ultraviolet movie)

Summary

• The Sun’s energy is produced by hydrogen fusion (E=mc2), occuring at 107 K in the nucleus of the sun.

• The standard model of the Sun– hydrogen fusion within 0.25 solar radius.– a radiative zone extending to about 0.71 solar radius– opaque convective zone– Photosphere (5800 K blackbody)– Chromosphere (hotter)– Corona (very hot; powered by magnetic fields)

• Neutrino’s escape from the Sun’s core and reveal neutrino oscillations

Summary• The Sun’s cycle is 22 long: Its magnetic field increases,

decreases, and then increases again with the opposite polarity, creating a 11 year cycle in Sun spot activity

• The magnetic-dynamo model suggests that many features of the solar cycle are due to changes in the Sun’s magnetic field. These changes are caused by convection and the Sun’s differential rotation.

• A solar flare is a brief eruption of hot, ionized gases from a sunspot group. A coronal mass ejection is a much larger eruption that involves immense amounts of gas from the corona.

Homework – Due 11/10/14• On your own: answer all the review questions

in chapter 16• To TAs: answer questions16.31, 16.32, 16.40,

16.48