Astronomy notes for Phys/Geog 182 Week 5 – Earth, sky, Sun, and Moon Week 6 – Motion in the Solar System Week 6/7 – How stars (& the Sun) work Week 7 – The cosmos in the large We will use a combination of simulations and worksheets Topics align with teaching standards

The recommended textbook is “Astronomy”, by A. Fraknoi, D The recommended textbook is “Astronomy”, by A. Fraknoi, D. Morrison, and S.C. Wolff Available free online at openstax.org

Openstax Astronomy Week 6 readings: For Monday class: Read sections 3.1 – 3.4 pp. 69-88 For Wednesday: Read section 15.1 pp. 523-534 Study Ch. 17 pp. 591-612 For Friday: Read Ch. 18 (skim) pp. 621-646 Look at key terms and chapter summaries, but skip the calculations in the boxed sections.

The Sun The closest star

Vital Statistics of the Sun Diameter is about 109 Earth diameters. Mass is about 333,000 times Earth’s mass. Surface temperature is about 5800 K (or 5500oC). Rotation period is about 25 days at the equator and 35 days at the poles (differential rotation). The core rotates once each week (this is new information!). Average density is about 1.4 times that of water. All the Sun’s energy is produced by fusion in a dense core that is at about 15.5 million Kelvin (or Celsius). The energy travels outward through various zones.

Solar Prominence – photo by SOHO spacecraft from the Astronomy Picture of the Day site link

SOHO (before launch, at ESA facility) (without solar panels) Other spacecraft looking at the Sun: STEREO (2 of these) SDO and others

Solar and Heliospheric Observatory (SOHO) (link) is in orbit around the sun near the L1 point

The closest star - The Sun

The Sun is a layered structure

The atmosphere of the Sun The outer layers are all parts of the Sun’s atmosphere: Corona Transition zone Chromosphere The Photosphere is the “surface” of the Sun; it emits the light that we see. The Convection and Radiation Zones are named for how energy is transported in the interior of the Sun.

Solar Granulation in the photosphere can be seen in movies taken by the SOHO cameras This granulation shows that convection is occurring under the surface of the Sun. On average these granules are about the size of a large state like Texas (up to 1000 miles across).

Above the granular photosphere is the chromosphere.

Next are the transition zone and the Solar Corona

The Solar Corona is most obvious during a total solar eclipse.

Coronal Hole, seen in X-ray images by Yohkoh. (link)

Solar Atmospheric Temperature So much energy is flowing through this region and the density is so low that the temperature of these regions is very high. All of this energy causes gas to “boil” off into space, or causes gas to be “pushed” off the surface of the Sun. This gas is called the Solar Wind.

Solar Spectrum in the visible region

Solar Spectrum This absorption spectra tells us what elements are in the Sun’s chromosphere and most likely in the rest of the Sun, except in the core. Likewise, spectra of stars tells us about their contents.

The “active Sun” refers to times when there are lots of sunspots and the surface of the Sun is very active in other ways. Many Prominences, Flares, and Coronal Mass Ejections can be seen. Also during this time the corona becomes larger and more irregularly shaped

Solar Prominences are huge outbursts of hot gases that blow out into space, then often the gas cools and falls back into the Sun.

are loops of hot gas that rise from the surface of the sun. They are Solar Prominences Solar Prominences are loops of hot gas that rise from the surface of the sun. They are shaped by the magnetic fields of the Sun. SOHO website: http://sohowww.nascom.nasa.gov/ For Mar. 28, 2014, there is a movie of 8 CMEs in five days: http://sohowww.nascom.nasa.gov/pickoftheweek/

Solar Flares are much more rapid than prominences.

A Solar flare on Nov. 11, 2003. SOHO obtained numerous images of the active Sun in fall 2003.

Coronal Mass Ejection Coronal Mass Ejection events (CME) is when an eruption on the surface of the Sun ejects large amounts of gas into space. These events are larger than solar flares and are less frequent.

The thermonuclear core produces energy by nuclear reactions which are caused by the high temperature, hence “thermo-nuclear”. (more on this later) Much of the energy comes out of the core in the form of X-rays or gamma rays, which travel without being absorbed through the radiative zone, which is transparent to these X-rays or gamma rays (g rays). The X-rays are then absorbed in the convective zone, and this heats the plasma in that zone, which undergoes convection, a motion which is similar to convection in any hot fluid. The convective zone has very large convection cells, and then above that zone is the photosphere, which has smaller convection cells.

FIGURE 10-21 The Solar Model (a) Thermonuclear reactions occur in the Sun’s core, which extends to a distance of 0.25 solar radius from the center. In this model, energy from the core radiates outward to a distance of 0.8 solar radius. Convection is responsible for energy transport in the Sun’s outer layers. (b) The Sun’s internal structure is displayed here with graphs that show how the luminosity, mass, temperature, and density vary with the distance from the Sun’s center. A solar radius (the distance from the Sun’s center to the photosphere) equals 696,000 km. (c) Ten most common elements in the Sun, by the numbers of atoms of each and by the percentage of the Sun’s total mass they each comprise. (a: NASA)