COOL TOPICS IN MODERN ASTRONOMY

(that have something to do with various kinds of

collisions)

Class 15 : The Sun

Basic properties of the Sun. Structure of the Sun. Nuclear fusion that powers the Sun.

Collisions of atomic nuclei!

I : Basic properties of the Sun

Distance: 150 million km. Radius : 700,000 km. Mass : 2.01030 kg. Luminosity : 3.851026 Watts. Temperature : 5800 K.

How do we know these facts? [See discussion on board]

Finally, temperature… Look at color of “glow”. As temperature increases, go from…

Precise measurements of Sun’s color give temperature of 5800 K.

REDYELLOWWHITEBLUE

COOL HOT

II: Structure of the Sun

How is the Sun put together?

Let’s unpeel it!

Core (< 200,000 km) T = 15 million K. Place where energy is produced (fusion).

Radiation zone (200,000-500,000 km) T = 7 million K. Energy transported outwards by radiation.

Convection zone (500,000-696,000 km) T = 2 million K. Energy transported by convection. Like boiling water in a pot.

Photosphere (696,000 km) T=5800 K. Place where the Sun becomes transparent. This is what we see as the “surface”.

But there’s more… Chromosphere (thin layer just above

photosphere) Looks pink!

Corona (surrounds Sun, very large) T = 1 million K. Very hot gas – can see with X-ray telescopes! Can only see normally during eclipse.

Solar wind Tenuous wind being blown by processes in corona. Flows into outer space (causes aurorae on Earth).

III : The Sun’s energy source

Sun powered by nuclear fusion Fusion = “merging” of 2 or more atomic

nuclei to form a bigger nucleus. In Sun, hydrogen nuclei collide and merge

to form helium nuclei. Actual process in Sun is called the “proton-

proton chain” (p-p chain).

Often called “hydrogen burning”… not hydrogen burning as you know it!

The p-p chain:1. Proton (p) + proton (p) deuterium (D) +

neutrino () + positron (e+) [& e+ + e- 2]2. D + p 3He + 3. 3He + 3He 4He + 2p

Overall result:4p 4He + neutrinos + energy

Step 1 is the hardest… Like charges (protons in this case) repel, so

they have to smash together very fast!

Mass of 4 protons: 6.694310-27 kg Mass of Helium-4: 6.646610-27 kg

4.7710-29 kg has been lost… This is converted into energy according to Einstein’s

formula E=mc2.

Sun “processes” 600 million tons of hydrogen per second… 4 million tons per second is completely converted into energy (0.7%).

ALL DRIVEN BY SUB-ATOMIC COLLISIONS!

HOW DO WE KNOW THIS IS CORRECT?

Two main lines of evidence:1. Can produce these kinds of nuclear

reactions in laboratories (and in bombs)… know their properties well.

2. Can observe neutrinos from the Sun!

Sudbury Neutrino Observatory (SNO)