Nuclear fusion reaction– In essence, 4 hydrogen nuclei combine (fuse) to

form a helium nucleus, plus some byproducts (actually, a total of 6 nuclei are involved)

– Mass of products is less than the original mass– The missing mass is emitted in the form of energy,

according to Einstein’s famous formulas:

E = mc2

(the speed of light is very large, so there is a lot of energy in even a tiny mass)

Fusion is NOT fission!• In nuclear fission one splits a large nucleus

into pieces to gain energy

• Build up larger nuclei Fusion

• Decompose into smaller nuclei Fission

Harvesting Binding Energy

Most stable element in the universe

Small harvest by decay

Big harvest by fusion

The Standard Solar Model (SSM)

• Sun is a gas ball of hydrogen & helium• Density and temperature increase towards

center• Very hot & dense core produces all the

energy by hydrogen nuclear fusion• Energy is released in the form of EM

radiation and particles (neutrinos)• Energy transport well understood in physics

Standard Solar Model

Hydrostatic Equilibrium• Two forces compete: gravity (inward) and energy

pressure due to heat generated (outward)• Stars neither shrink nor expand, they are in

hydrostatic equilibrium, i.e. the forces are equally strong

Heat GravityGravity

More Mass means more Energy

• More mass means more gravitational pressure

• More pressure means higher density, temperature

• Higher density, temp. means faster reactions & more reactions per time

• This means more energy is produced

Does too much Energy lead to Explosion?

• No, there is regulative feedback:– More energy produced means more radiative

pressure– This means the stars gets bigger– This means density, temperature falls off– This means less reactions per time– This means less energy produced

How do we know what happens in the Sun?

• We can’t “look” into the Sun

• But: come up with theory that explains all the features of the Sun and predicts new things

• Do more experiments to test predictions

• This lends plausibility to theory

Details

• Radiation Zone and Convection Zone

• Chromosphere

• Photosphere

• Corona

• Sunspots

• Solar Cycle

• Flares & Prominences

Sunspots• Dark, cooler regions

of photosphere first observed by Galileo

• About the size of the Earth

• Usually occur in pairs• Frequency of

occurrence varies with time; maximum about every 11 years

• Associated with the Sun’s magnetic field

Sunspots and Magnetism

• Magnetic field lines are stretched by the Sun’s rotation

• Pairs may be caused by kinks in the magnetic field

The Solar Cycle

Understanding Stars

• “Understanding” in the scientific sense means coming up with a model that describes how they “work”:– Collecting data (Identify the stars) – Analyzing data (Classify the stars)– Building a theory (Explain the classes and their

differences)– Making predictions– Testing predictions by more observations

Identifying Stars - Star Names• Some have names that go back to ancient times

(e.g. Castor and Pollux, Greek mythology)• Some were named by Arab astronomers (e.g.

Aldebaran, Algol, etc.)• Since the 17th century we use a scheme that lists

stars by constellation– in order of their apparent brightness

– labeled alphabetically in Greek alphabet

– Alpha Centauri is the brightest star in constellation Centaurus

• Some dim stars have names according to their place in a catalogue (e.g. Ross 154)

Classification by Star Properties

• What properties can we measure?– distance– velocity– temperature– size– luminosity– chemical composition– mass

Distances to the Stars

• Parallax can be used out to about 100 light years

• The parsec:– Distance in parsecs = 1/parallax (in arc

seconds)– Thus a star with a measured parallax of

1” is 1 parsec away– 1 pc is about 3.3 light years

• The nearest star (Proxima Centauri) is about 1.3 pc or 4.3 lyr away– Solar system is less than 1/1000 lyr

Homework: Parallax

• Given p in arcseconds (”), use d=1/p to calculate the distance which will be in units “parsecs”

• By definition, d=1pc if p=1”, so convert d to A.U. by using trigonometry

• To calculate p for star with d given in lightyears, use d=1/p but convert ly to pc.

• Remember: 1 degree = 3600”• Note: p is half the angle the star

moves in half a year

Our Stellar Neighborhood

Scale Model

• If the Sun = a golf ball, then– Earth = a grain of sand

– The Earth orbits the Sun at a distance of one meter

– Proxima Centauri lies 270 kilometers (170 miles) away

– Barnard’s Star lies 370 kilometers (230 miles) away

– Less than 100 stars lie within 1000 kilometers (600 miles)

• The Universe is almost empty!

• Hipparcos satellite measured distances to nearly 1 million stars in the range of 330 ly

• almost all of the stars in our Galaxy are more distant

Reminder: Three Things Light Tells Us

• Temperature – from black body spectrum

• Chemical composition– from spectral lines

• Radial velocity– from Doppler shift

Luminosity and Brightness

• Luminosity L is the total power (energy per unit time) radiated by the star, actual brightness of star, cf. 100 W lightbulb

• Apparent brightness B is how bright it appears from Earth– Determined by the amount of

light per unit area reaching Earth– B L / d2

• Just by looking, we cannot tell if a star is close and dim or far away and bright

Brightness: simplified

• 100 W light bulb will look 9 times dimmer from 3m away than from 1m away.

• A 25W light bulb will look four times dimmer than a 100W light bulb if at the same distance!

• If they appear equally bright, we can conclude that the 100W lightbulb is twice as far away!

Same with stars…

• Sirius (white) will look 9 times dimmer from 3 lightyears away than from 1 lightyear away.

• Vega (also white) is as bright as Sirius, but appears to be 9 times dimmer.

• Vega must be three times farther away

• (Sirius 9 ly, Vega 27 ly)

Distance Determination Method

• Understand how bright an object is (L)is (L)• Observe how bright an object appears (B)appears (B)

• Calculate how far the object is away:

B L / d2

So

L/B d2 or d √L/B

Homework: Luminosity and Distance

• Distance and brightness can be used to find the luminosity:

L d2 B

• So luminosity and brightness can be used to find Distance of two stars 1 and 2:

d21 / d2

2 = L1 / L2 (since B1 = B2 )

i.e. d1 = (L1 / L2)1/2 d2