PHY 113 Lecture Notes - Lecture 16: Photosphere, Dimensionless Physical Constant, Stephen Hawking

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9 Jun 2018
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Chapter 16 The Sun
16.1 Physical Properties of the Sun
Rotation: Differential; period about a month Surface temperature: 5800 K
Apparent surface of Sun is photosphere
This is a filtered image of the Sun showing sunspots, the sharp edge of the Sun due to the thin
photosphere, and the corona
Interior structure of the Sun:
Outer layers are not to scale
The core is where nuclear fusion takes place
In astronomy, we differentiate
an object gives off,
and how much we measure
on Earth (per unit area).
Luminosity, L: total energy Flux, F: what we measure
Surface area of sphere:
Luminositytotal energy radiated by the Sunis the solar constant times know the distance
between the Earth and the Sun, R=1 AU.
Wthe equivalent of 10 billion 1-megaton nuclear bombs per second.
16.2 The Solar Interior
Mathematical models, consistent with observation and physical principles, provide
information about the Suns interior
In equilibrium, inward gravitational force must be balanced by outward pressure
Doppler shifts of solar spectral lines indicate a complex pattern of gas moving up and down
near surface
Solar density and temperature, according to the standard solar model
Hottest and densest near the center of the Sun.
Energy transport:
The radiation zone is relatively transparent; the cooler convection zone is opaque
Invisible top layer of the convection zone is granulated, with areas of upwelling material
surrounded by areas of sinking material
Discovery 16-1:
SOHO: Eavesdropping on the Sun
SOHO: SOlar and Heliospheric Observatory
Orbits at Earths L1 point, outside the magnetosphere
Multiple instruments measure magnetic field, corona, vibrations, and ultraviolet emissions
16.3 The Sun’s Atmosphere
Spectral analysis can tell us what elements are present, but only in the chromosphere and
photosphere of the Sun. This spectrum has lines from 67 different elements.
Spectral lines are formed when light is absorbed before escaping from the Sun; this happens
when its energy is close to an atomic transition, so it is absorbed.
Difficult to see directly: photosphere is too bright, unless Moon covers photosphere and not
chromosphere during eclipse.
Solar corona can be seen during eclipse if both photosphere and chromosphere are blocked
Corona is much hotter than layers below it must have a heat source, probably
electromagnetic interactions
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