A star is a glowing ball of gas held together by its own gravity and powered by nuclear fusion at its center

Our star is a typical one. However, knowledge gathered from the Sun can be applied to other stars easily.

Key Properties:

• Radius: ~ 700 000 km

• Mass:  ~ 2 × 1030  kg

• Escape Speed: ~ 600 km/s

• Period: 25 solar days

• Axial Tilt: 7.25 degree

• Temperature: ~ 6000 K

Period: It is observed from the spots on the surface. However different periods are measured from different locations.

➤ This shows differential rotation like Jupiter and Saturn.

Temperature: Observing the solar spectrum and using radiation laws an effective surface temperature is calculated

➤ Blackbody @ 5800 K

Surface: It has a surface but not a solid one; it is a gas ball. Therefore, surface is taken as the layer where radiation escapes to space

➤ The layer is photosphere which is ~ 500 km thick

• Solar interiors cannot be observed directly.

• Therefore, observation of surface and radiation are combined with theoretical models of solar physics and created what is called Standard Solar Models.

• In 1960s, it is discovered that the Sun's surface vibrates :

  • How? Internal pressure waves rushing toward to surface reflect off the photosphere

  • So, the waves will create surface patterns

  • which will allow us to study inner parts: similar to seismology.

  • It is called helioseismology.

• The Sun is the only object continously observed.

So what happens to the energy?

Since we see the Sun therefore energy reaches to the surface. How?

➤ Convection

• Hot solar gas moves outward

• Cooler solar gas (above) sinks

➤ Convection cells

Therefore energy transported to upper layers by physical motion of solar gas.

There is a hierarchy in the convective zone

• Deepest                  ______ larger cells (~ 10 000 km in diameter)

• Shallow (1000 km)  ______ smaller cells (~ 1 000 km in diameter)

• Surface                  ______ granuls

• Photosphere           ______ convection stops (it is too thin)

Then, radiation takes over the transport.

In atoms or ions:

• we observe transitions

• they create spectral lines: emission or absorbtion 

In normal conditions:

• when radiation from hot background source

• faces with cool foreground gas

they will create absorbtion lines. This is modified for the Sun and we observe:

• bright background lines and dark absorbtion lines

• are formed in roughly the same locations

  • the photosphere (photon-sphere: layer of photons)

  • lower chromosphere (chromo-sphere: layer of colors)

Spectral analysis can tell us what elements are present. This spectrum has lines from 67 different elements. The most common ones:

Element __ % of Number __ % of Mass

• H       __ 91.2                __ 71.0

• He     __   8.7                __ 27.1

• O       __   0.0078          __  0.97

• C       __   0.043            __  0.40

• N       __   0.0088          __  0.096

• Below photosphere:

  • the gas is dense

  • interaction between photon, e– , ions are common

  • therefore, radiation cannot escape directly

• In the solar atmosphere

  • the escape will depend on the energy of photon

  • if the energy corresponds to an electron transtion  ➤ absorbtion

  • if not photon is free ➤ radiation

The lines we see on Earth tend to come from higher, cooler levels in the solar atmosphere.

• The dashed lines indicate schematically the levels in the atmosphere where photons corresponding to different parts of the absorption line originate.

• The inset shows a close-up tracing of two of the thousands of solar absorption lines: the "H" and "K" lines of calcium at about 395 nm.

So, except the core, the spectral lines might represent the entire Sun.

• Sunspot pairs are linked by magnetic field lines.

• The Sun’s magnetic field lines emerge from the surface through one member of a pair and reenter the Sun through the other member.

• The leading members of all sunspot pairs in the solar northern hemisphere have the same polarity (labeled N or S).

• If the magnetic field lines are directed into the Sun in one leading spot, they are inwardly directed in all other leading spots in that hemisphere.

• The same is true in the southern hemisphere, except that the polarities are always opposite those in the north.

• The entire magnetic field pattern reverses itself roughly every 22 years. This cycle is named as solar dynamo cycle.

• The darkest regions have temperatures of about 2 million K (figure b).

Sunspot Cycle (or Solar Cycle)

• Annual number of sunspots throughout the 20th century shows the (roughly) 11-year solar cycle.

• At the time of minimum solar activity, hardly any sunspots are seen.

• About 4 years later, at maximum solar activity, about 100 to 200 spots are observed per year.

• The most recent solar maximum occurred in 2001.

• Sunspots cluster at high latitudes when solar activity is at a minimum.

• They appear at lower and lower latitudes as the number of sunspots peaks. They are again prominent near the Sun’s equator as solar minimum is approached once more.

• The blue lines in the upper plot indicate how the “average” sunspot latitude varies over the course of the cycle.

• Nuclear fusion requires that like-charged nuclei get close enough to each other to fuse.

• This can happen only if the temperature is extremely high:

  • over 10 million K.

• The first step      :  1H + 1H → 2H + positron + neutrino

• The second step  :  2H + 1H → 3He + energy

• The third step     :  3He + 3He → 4He + 1H + 1H + energy 

So, as a summary   :  4(1H) → 4He + energy + 2 neutrinos 

• The helium stays in the core.

• The energy is in the form of gamma rays, which gradually lose their energy as they travel out from the core, emerging as visible light.

• The neutrinos escape without interacting.

Age of the Sun

• Energy created = E (mass of final particles) - E (mass of initial particles)

• For each interaction this turns out to be 4.3 × 10–12 J.

• 1 kg of H = 6.4 × 1014 J of energy produced.

• The Sun has enough hydrogen left to continue fusion for about another 5 billion years.