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ASTR 3130 (Majewski) Lecture Notes

OBSERVING THE SUN


REFERENCE: Birney's Chapter 15.

The only parts of the Sun observable are:

Interior: Stable, hydrostatic equilibrium (gravity = pressure). At each layer, constant temperature (T), pressure (P), density ().


From http://observe.arc.nasa.gov/nasa/exhibits/sun/sun_6.html

Convection better than radiation -- gases opaque, photons don't get far typically (say, few cm), absorbed, re-emitted in random directions; million year random walk from center to surface.

Measure of ability of gas to absorb radiation = opacity

Photosphere is top of convective zone -- origin of granulation we see on "surface" of Sun.

NOTE: conduction only important in degenerate stars -- WDs


Solar Atmosphere

Photosphere:

Limb darkening:


Other parts of photosphere - visible in white light picture:


Chromosphere:

Sits above photosphere:


Corona:


Techniques for Observing the Sun

Heliostat and Coelostat:

First problem we have is to track the Sun as Earth turns.

Second, we often find it useful to have the telescope fixed and simply reflect light into it -- allows giant telescopes to be made.

Note, it is not the sidereal rate, but the solar (human clock) rate that matters, so special telescopes tuned to this rotation speed are needed.

Also, should account for refraction of the atmosphere near sunset/sunrise.

Also, sun changes declination over the course of the year, so need to adjust on daily basis.

Most solar telescopes are built on the principle of the heliostat or the coelostat.

In a heliostat (see top figure below):

From Birney, Figs. 15.17 (heliostat, top) and 15.18 (coelostat, bottom).

In the coelostat (see bottom figure above):

McMath Solar Telescope

Image Projection:

Neutral Density Imaging

Spectroheliograph:

Sun through H-alpha filter:

Sun through Calcium filter:


From http://solar.physics.montana.edu/YPOP/Classroom/Lessons/Filters/Calcium.html

Solar Towers

Stellar Coronagraph:

As shown above, in a solar spectrograph, man-made disk blocks out photosphere to leave tenuous (but still bright in its own right) solar corona.

The concept of a coronagraph is not limited to use on the Sun:


Helioseismology:



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All material copyright © 2002 Steven R. Majewski. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 313 and Astonomy 3130 at the University of Virginia.