•Lecture 7

Our Star

What is a Star?

• Large, glowing ball of gas that generates heat and light through nuclear fusion in its core.

• Stars shine by Nuclear Fusion:– For the sun and most of the stars: powered by the

reaction:

• 4 Hydrogen nuclei (4 protons) 1 Helium nucleus (2 protons and 2 neutrons) + energy

• The same process in H bomb

• E=mc2 at work: the total mass of helium is only 99.3% of the mass of 4 protons put together; the rest, 0.7% of the rest mass is turned into energy the powers stars

Sun: Our Star

Distance: 1.49 x 108 km = 1 A.U.

Mass: 1.99 x 1030 kg

Radius: 6.96 x 105 km

Density: 1.41 g/cm3

Luminosity: 3.8 x 1026 watts

How to calculate Sun radius :

• The angle diameter of Sun = 32’ (arcsec). • The Earth-Sun distance = 1 A.U.= km81.49 10x

Radius: 6.96 x 105 km

How to calculate Sun mass :2

21 . . (1 . .)zz z s

centripetal gravitational

M v M MF G F

AU AU

8

11 3 1 2

30 /

1 . . 1.49 10

6.67 10

zv km s

AU x km

G x m kg s

Mass: 1.99 x 1030 kg

12-23IR absorptions of H2O & CO2UV absorbed by O2 & O3

AMO: available solar energy for satellite solar cells.AM 1.5: AMO reduced by gases in earth’s atmosphere.

• Solar constant is the intensity of radiation (the flux of energy riching ) at AMO ( air mass zero)

• Stefan – Boltzman law:

How to calculate Sun luminosity :

2 11370sS Jm s 2 264 (1 . .) 3.86 10s sL AU S x W

21m

How to calculate Sun temperature :

2 44s sL R T 5800T K

Sun density

Composition of the Sun Percent by Mass

• H 73%

• He 25%

• O 0.8%

• C 0.3%

• N 0.1%

Hydrostatic equilibrium

The Sun’s Internal Structure

Layers of the Sun

Core of the Sun

The core is in the center, where fusion occurs.

• Temperature: 14 million K

• Pressure: 1 billion atm

• Plasma

• Nuclear Fusion

Transition region

– Convection to Radiation– 300 km thick

• Convection zone is similar to the Earth’s mantle.

• It lies between the photosphere and the radiation zone.

How does the Light Comes Out?

Photons are created in the nuclear fusion cycle.They collide with other charged particles and change their direction (random walk).They also decrease their energy while walking.It takes ~10 million year to get outside.

The random bouncing occurs in the radiation zone (from the core to ~70% of the Sun’s radius).

At T<2 million K, the convection zone carries photons further towards the surface.

How does the Light Comes Out?

Solar Neutrino

Neutrino is a subatomic particle.It is a by-product of the solar proton-proton cycle.It barely interacts with anything.

Counts of neutrino coming from the Sun are crucial to test our knowledge about solar physics.

Neutrino observatories use huge amounts of different substances to detect nuclear reactions with neutrino.So far theory predicts more neutrino than is seen.

Photosphere

• 6000 K

• Sunspots

Photosphere is the visible surface of the Sun.It consists of gas and is far less dense than air over the Earth’s oceans.

Sunspots

• Dark areas on the Dark areas on the photospherephotosphere

• CoolerCooler –appear dark –appear dark

• 11.2 year cycle11.2 year cycle– Dynamic Magnetic fieldDynamic Magnetic field

SOHO/MDI NASA & ESASOHO/MDI NASA & ESA

SOHO

NASA ESA

Chromosphere

• HotterHotter

• Less denseLess dense

• FlaresFlares

• ProminencesProminences

SOHO/EIT NASA/ESA

Chromosphere is a zone below corona, where the Sun’s UV radiation is produced (T~10,000 K).

Chromosphere

SOHO/EIT NASA/ESA

Corona

• The uppermost layer of the atmosphere

• Very hot: • 0.5 - 2 Million K• very low density: • 1-2 million km thick

– Visible during eclipse

• Coronal mass ejections

YOHKOHThe solar windThe solar wind is a stream of photons, ions, and subatomic particles outward from the surface.

Dynamic Magnetic field

• Sunspots

• Flares

• Prominences

• Coronal mass

ejections

Effects on the Earth

• Magnetic storms

• Auroras

• Climate– 1645-1715 few sun spots– "Little ice age"

Basic Properties of a Star

• Mass: from 0.1 solar mass to > 100 solar mass• Size: from 0.1 solar radius to > 1000 solar radius• Luminosity: from 0.01 solar luminosity to a million solar

luminosity• Life time: from a few million years to longer than tens of

billion years – Sun: 10 billion years lifetime (~5 billion years old now)

• Surface Temperature: from 3000 degree above absolute zero (3000 K 4900 F) to 30,000 K (54000 F)– Sun: ~6000 K

• Color: from violet to red– Astronomers classify stars based on their colors, or spectral types:

from hot (violet) to cool (red) are: O, B, A, F, G, K, M

Oh Be A Fine (Girl/Gal/Girl) Kiss Me

The Hertzsprung-Russell Diagram

Temperature or color

Luminosity orThe energy output

Main Sequence Stars

• For more than 90% of a typical star’s lifetime, it is on the Main Sequence of H-R Diagram– With stable Hydrogen burning in the core

– The luminosity, temperature and mass of main sequence star follow simple relation:

• High mass stars:– Hotter, more luminous, bigger, and have shorter lifetime

• Low mass stars:– Cooler, less luminous, smaller, and have longer lifetime

– For main sequence stars, if we know the color of the star, then we know its MASS, temperature and lifetime as well.

Main Sequence Stars

The most important property:Mass of a star

• To the first order, the mass of a star determines all the other properties of a star:– in particular, determines the lifetime, evolution, and

fate of a star– Lifetime:

• 10 solar mass star: a few million years• Sun: 10 billion years

– Fate of a star:• Low mass (M < a few solar masses) white dwarf• High mass star (>5 – 10 solar masses) supernova

explosion and neutron star• Very high mass star (> 20 solar masses)

supernova explosion and Black Holes

Fate of a low-mass star (Sun)

• Main sequence red giant planetary nebula white dwarf; M(white dwarf) < 1.44 solar mass

Fate of a High-mass star

• Main sequence (short) supergiant supernova neutron star; M(neutron star) < 3 solar mass or black hole

Black Holes

• After a massive star supernova, if the core has a mass > 3 M, the force of gravity will be too strong for even neutron degeneracy to stop.

• The star will collapse into oblivion.– GRAVITY FINALLY WINS!!

• This is what we call a black hole.• The star becomes very small.

– it creates a “hole” in the Universe– That not even light could escape

• Since 3 M or more are compressed into an infinitely small space, the gravity of the star is HUGE!

• WARNING!!– Newton’s Law of Gravity is no longer valid !!– Have to use Einstein’s general relativity to calculate the properties of

black holes