The Sun, a Star

Astronomy 360 Notes (Physics 360/Geol 360) Dr.

Swez

Note: Slide which are important and should be reviewed are marked as

What Is the Sun?

A big, hot ball of gas

The Sun and planets are shown to the same scale. The small terrestrial planets and tiny Pluto are in the box---the Earth is the blue dot near the center of the box.

1. From Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for the updated and corrected version. Created by Nick Strobel using NASA images

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Physical Data on the Sun Mass 2x1030 kg

Watch planet orbits

Radius 110 REarth Use distance and trig

Luminosity 4x1026 W Use distance and value at Earth Also Stephan-Boltzmann Law

Surface Temp 5800K

Wien’s Law

Eq Rotation 25 days

Polar Rotation 33 days

Watch Sunspots

Composition: 71% H, 27% He, 2% Other

Examine spectrum

From: Explorations by Thomas Arny, page 325

The Mass Luminosity Law for stars

Where is the sun on this graph?

Remember the Stephan-Boltzmann Law?

Look ahead to p 363-365 text

A body of temperature T radiates an amount of energy each second equal to T4 per square meter. If we know the energy per square meter we can find the temperature.

= 5.67 x 10-8 watts m-2 K-4

From Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for the updated and corrected version.

The Photosphere (surface of the sun we see) is about 500 kilometers thick.

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Remember Wien’s Law (page 103 text)

Measure the body’s brightness at different wavelengths to find at which particular wavelength it is brightest.

Use Wien’s law to calculate the body’s temperature.

T = (3 x 106 / m)

Where Does the Energy Come From? Coal - lasts a few thousand years Gravothermal Energy

Gravity compresses Sun, which heats it

Could last 20 million years Nuclear Fusion

Energy is converted to mass (E=mc2) Could last 10 billion years

Mass-Energy Conversion Energy = mass x (speed of light)2

(1kg) x (3x108m/s)2 = 1017 joules watt = 1 joule/second

Annual US Energy Consumption ~1020 J The Sun fuses hydrogen into helium

The mass input is greater than the mass output

Some mass is converted to energy

Proton-Proton Chain Hydrogen fuses into helium Summary: 6 H -> 2H + 1 He + +

3x10-7 J Requires very hot temperatures and

high pressures (above 1 million K) The solar core will burn for a total of 10

billion years (How long do we have left?)

Know the full chain!

Neutrinos Neutrinos are small, neutral particles

They easily pass through matter Several light-years of lead can block them

Therefore they are very hard to detect Neutrinos can cause nuclear change in

atomic nuclei Underground detectors see a few each

day

Solar Neutrinos Solar neutrinos are the only direct probe of the

Solar core Detectors reported ~1/3 of the expected

neutrinos New results -> new physics Apparently, neutrinos oscillate between states So, the Sun produces the correct number of

neutrinos for current models There is no longer a ‘Solar Neutrino problem’

What if Hydrostatic Equilibrium Failed? What if gravity exceeded gas

pressure? The Sun would start to collapse The core temperature and pressure would

increase The nuclear fusion rate would increase Hydrostatic equilibrium would be restored!

What if gas pressure exceeded gravity?

Solar Atmosphere Photosphere

surface we see 5800 K contains sunspots

and granulation Chromosphere

cooler outer layer 4500 K contains spicules

Corona very thin outer

layer few million K visible during

eclipse contains flares and

prominances

Why are sunspots dark?

From The WEBSITE: http://www.kis.uni-freiburg.de/~pnb/granmovtext1.html

Granulation in the photosphere of our SUN. A time lapse movie.

A 27 x 27 Mm2 FieldCondensed into 35 minutes

“The series was observed with a fast frame selection system on June 5, 1993, at the SVST (La Palma) in cooperation with G. Scharmer (Stockholm) and G. W. Simon (Sunspot); N. Hoekzema (Utrecht), W. Mühlmann (Graz), and R. Shine (Palo Alto) were involved in the data analysis. Technical data: wavelength 468 ± 5 nm; exposure time 0.014 s; rms contrast (uncorrected) between 7 and 10.6 %. The images were registered, destretched, corrected for the telescope's point spread function, and subsonically filtered after interpolation to equal time steps. “

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Time lapse movies of solar granulation evolution.

The movies show time lapse series of the evolution of the solar granulation, which represents the top of the Sun's convection zone. At the centers of granules hot solar gas rises and radiates its heat rapidly into space; the gas then is diverted horizontally, and sinks back into the Sun in the darker intergranular lanes. The sizes of the granules range from approx. 250 km (the limit set by the telescope and the Earth's atmosphere) to more than 2000 km, with an average diameter of 1300 km. Lifetimes of granules typically range from 8 to 15 minutes. Horizontal and vertical velocities of the gas motion are 1 to 2 km/s.

Courtesy of: http://www.kis.uni-freiburg.de/~pnb/granmovtext1.html

H Alpha Sunspot Images taken at Holloman Air Force Base over a 10 day period.

From: http://www.sunspotcycle.com/Sunspots are cooler dimmer regions with strong magnetic fields.

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Can last months

Erupts over days

Complete Sunspot Data from http://www.sunspotcycle.com/

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How can astronomers detect magnetic fields in astronomical bodies

One way would be to place a magnetometer on the sun. (Impossible!).

A better way is to use the Zeeman effect; a physical process by which the magnetic field splits some of the spectral lines of a gas into more than one component. The magnetic field alters the atom’s electron orbits which in turn alter the emitted wavelength. (see page 345 text)

Sun Spot Data for a Period of about 10 Years

Solar Cycle #23 is in progress. The next sunspot maximum is predicted to occur in the year 2000.

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