Physics 101 - Astronomy - Spring 2019

Class notes for day 19, March 28, 2019

Ch. 15 is about the Sun, the star that is closest to us. These notes cover some of the points I made on the PowerPoint slides.
Many of the images I showed of the Sun are from the Solar and Heliospheric Observatory (SOHO) spacecraft, which is in orbit around the sun near the L1 point.
See the SOHO website, see: http://sohowww.nascom.nasa.gov/
More recent pictures of the Sun also come from two spacecraft, called the STEREO mission, which are looking at the Sun from different points in Earth's orbit, so that we now have continuous coverage of almost the entire surface of the Sun. See http://www.nasa.gov/mission_pages/stereo/main/index.html
Another active spacecraft is the Solar Dynamics Observatory, SDO; see https://sdo.gsfc.nasa.gov/
or https://www.nasa.gov/mission_pages/sdo/main/

I pointed out some images of the Sun at the Solar Data Analysis Center website: https://umbra.nascom.nasa.gov/index.html/ 
Although the website is intended more for professionals, it has recent (last hour) images of the Sun at https://umbra.nascom.nasa.gov/newsite/images.html taken with various types of sensors in infrared, visible, UV, and X-rays.

See my PowerPoint or the textbook for a figure showing the layered structure of the Sun. The outer layers are all parts of the Sun’s atmosphere:
Corona
Transition zone
Chromosphere
Photosphere

The Photosphere is the “surface” of the Sun; it emits the light that we see. But it is just gas like the rest of the Sun. You could not land on it. The gas in the photosphere is continually moving around in convection cells, called granules. Convection occurs because hot gas rises and cooler gas sinks. For a nice picture of the Sun with granulation and a prominence, see: http://apod.nasa.gov/apod/ap040330.html
This granulation shows that convection is occurring under the surface of the Sun. On average these granules are about the size of Texas (1000 miles across).

The absorption spectrum seen in the light from the Sun tells us what elements are in the Sun’s chromosphere and most likely in the rest of the Sun, except the core. Absorption of specific wavelengths (colors) of light occurs in the Solar Chromosphere. This region is usually only visible to Earthlings during a total solar eclipse. Solar spicules are relatively cool and hence dark features that appear above the chromosphere. These spicules are jets of gas that rise up above the chromosphere, cool off, and then fall back down.

The Solar Corona is most obvious during a total solar eclipse. The Corona is very high in the "atmosphere" of the Sun, and so much energy is flowing through this region and the density is so low that the temperature of these regions is very high. As a result, the corona is very visible using X-rays. However, it is not very bright in visible light because the gas is very low density. All of this energy causes gas to “boil” off into space, or causes gas to be “pushed” off the surface of the Sun. This flowing gas is called the Solar Wind.

Sunspots are cooler regions of the Sun’s surface. Sunspots are also regions of intense magnetic fields. Just like regular magnets, sunspots come in pairs: one is a “North pole” and one is a “South pole”. Sunspots behave somewhat like a magnet, causing a magnetic field above the photosphere. Above the sunspot, the magnetic field causes the hot gas of the corona to concentrate along the field lines, which is best seen using ultraviolet cameras. I showed several pictures of the loops of hot gas that tend to move along these field lines.

The Sun has "differential rotation" which means different parts of the Sun rotate at different speeds. The Sun’s equator rotates faster than at the poles. Solar rotation “drags” the magnetic field around. Then it pops out of the surface, forming loops with a pair of sunspots at the base of the loop. All of the sunspot pairs in the north hemisphere have the same polarity or orientation. All of the sunspot pairs in the southern hemisphere have the opposite polarity (compared to those in the north hemisphere). Both the number and the location of the sunspots change during the Sunspot Cycle (or Solar Cycle) which lasts 11 years. The orientation of the N and S poles of the sunspot pairs alternates from each 11-year cycle to the next, so there is a secondary 22 year cycle. 
During the times in the Sunspot cycle when there are lots of sunspots, the surface of the Sun is very active. We talk about an "Active Sun" when many prominences, flares, and coronal mass ejections can be seen. Also during this time the corona becomes larger and more irregularly shaped. In 2013 the sun was in an active period, close to the Solar Maximum, when many sunspots are seen. The website www.spaceweather.com has a picture of the Sun and information about sunspots and other activity occurring on the Sun.

Solar Prominences are huge outbursts of hot gases that blow out into space, then often the gas cools and falls back onto the photosphere. Solar Prominences are usually less violent loops of hot gas that rise from the surface of the Sun. They are shaped by the magnetic fields of the Sun.
Solar Flares are much more rapid than prominences.

Coronal Mass Ejections (CMEs) occur 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. Coronal Mass Ejections throw huge amounts of material into space, which can have effects on Earth if it hits us. This is the topic of "space weather".

Near the end of class I showed an on-line video from NASA which discusses the solar cycle and the current solar minimum.
See https://www.youtube.com/watch?v=kBKJkU06ICQ
A consequence of the solar minimum is increased radiation in the atmosphere of Earth, which might be of concern to people who fly in aircraft at high altitudes.
See https://spaceweatherarchive.com/2018/10/24/atmospheric-radiation-increasing-from-coast-to-coast-in-the-usa/

The  following material will be covered in more detail in class on Tuesday:

The Core is the source of the Sun's energy, from the fusion of hydrogen nuclei (protons) into helium nuclei (which have 2 protons and 2 neutrons) with the emission of a neutrino and lots of energy in the form of X-rays and gamma rays. The sun’s core has a density about 20 times the density of iron, even though it is at an extremely high temperature, 15 million Kelvin. The core of the sun is at about 15 million Kelvin due to nuclear fusion reactions; and at greater and greater distances from the core the temperature decreases; the surface is at about 5800 K. (Recall that the classroom is at about 300 K.) Surrounding the core are shells with different characteristics.

 In the Radiation Zone energy is transported by direct radiation of electromagnetic radiation, as visible light, ultraviolet light, and X-rays, depending on the temperature.

In the Convection Zone energy is transported by the movement of matter (hydrogen and helium gas). Hot gas rises and cooler gas sinks. Above this region is the photosphere, which is the part visible to us.

We will have an exam on Thursday, April 4, over Ch. 11 through 16.

You should review these sections of the OpenStax book: 11.1-3, 12.1-5, 13.1-4, 14.1-4, 15.1-4, and 16.1-3.