Part 4: The Stars In Part 3 we looked at the Solar System, that is, we looked at all the parts except for the central element: the sun. In Part 4 we are going to look at stars. Since the sun is the closest star, we will now look at the sun, and then we will look at the other stars.

The Sun This image was taken by SOHO's EIT (Extreme-Ultraviolet Imaging Telescope) and is courtesy of the EIT Consortium.

The Sun: basic facts The sun is 93 million miles (150 million km) away from the earth, and makes an angle of 0.5o at the earth. This means that the sun has a diameter of about 800,000 miles (1,300,000 million km) – which is about 1/100 of the earth-sun distance (AU), and is about 100 times bigger (in diameter) than the earth.

The Sun: its structure The sun does seem to have a definite edge, or surface. We call this “surface” the photosphere. During solar eclipses, we see that the sun does seem to have an “atmosphere” above the “surface” (photosphere) of the sun. Although this atmosphere is very dim relative to the surface of the sun, it is bright compared to the blackness of space. This visible atmosphere of the sun is called the corona. It can extend as far as another radius of the sun. For an image, see: http://sunearthday.nasa.gov/2006/images/gal_045.jpg Corona visible during a solar eclipse – Marshal Space Flight Center.

The Sun: its structure When we look more closely at the photosphere we see relatively small spikes on the photosphere that measure about 400 miles wide and 4,000 miles high (small compared to the diameter of the sun of 800,000 miles). This is called the chromosphere and is considered part of the sun’s atmosphere along with the corona. See the web site: http://www.nasa.gov/mission_pages/solar-b/solar_019.html

Surface Temperature of the Sun As we saw in Part 2, when we look at the visible spectra of the sun, we see that it’s intensity peaks at about 500 nm (green light). From the equation:  = b/T (where b = 2.9 x 10-3m*K) we get: T = b/ = (2.9 x 10-3m*K) / 500 x 10-9m  6,000 K  10,000 oF .

Power output of the sun From the relation: P = AT4 where  = 5.67 x 10-8 m2 *K4 and the size of the sun (radius = 700,000 km = 7 x 108 m) A = area = 4r2 = 6 x 1018m2, we get: P = (1) * (5.67 x 10-8 m2*K4) * (6x1018m2) * (6,000K)4  4 x 1026 Watts.

Intensity of sunlight at the earth’s orbit At the earth’s distance from the sun (93 million miles = 1.5 x 1011m), the intensity of sunlight we receive is about I = P/A = (4 x 1026Watts)/(4**[1.5x1011m]2) | 1,600 W/m2. However, due to reflection off the atmosphere and due to day/night and slanting angle of sunlight during the day, the average intensity striking the earth is only about 250 W/m2.

Sunspots Images from NASA Marshal Space Flight Center http://solarscience.msfc.nasa.gov/surface.shtml

Sunspots When Galileo viewed the sun with the aid of a telescope, he noticed there were spots on the sun’s surface. Like the corona, sunspots are dark when compared to the very bright normal surface of the sun (at 6,000 K or 10,000 oF), but actually they are bright compared to the dark of space – the “dark” areas are still at 4,500 K. For images, see: http://www.nso.edu/current_images.html https://sohowww.nascom.nasa.gov/sunspots

Sunspots – cont While the sunspots themselves are relatively cool, the areas around sunspots are very active with the result that when there are more sunspots there is generally a little more power being put out by the sun. Solar flares often are seen in these areas around sunspots. Solar flares last about 20 minutes on average, and extend far out into the corona. They emit significant amounts of energy in all the various types of light from radio to x-rays. See: http://solarscience.msfc.nasa.gov/flares.shtml Solar Flair Marshal Space Flight Center

The Sun’s Rotation Sunspots with their associated flares rotate with the sun’s photosphere. Unlike solid objects, the sun is a gaseous object, and the rotation of the sun is different at different places. The equator of the sun rotates (counterclockwise as viewed from above the North pole like almost all of the rest of the solar system) with a period of about 26 days at the equator and up to about 30 days at higher latitudes.

Sunspot cycle The number and location of sunspots varies over time in more or less a periodic fashion. Sunspots start appearing around 30o latitude, reach a maximum in number appearing at lower latitude, then start decreasing in number as they start forming near the sun’s equator. The next cycle of sunspot activity starts before the old cycle is finished. This cycle lasts about 11 years on average, with the maximum number of sunspots being between 100 and 200 per year, and the minimum number being between 0 and 15 a year. See: http://solarscience.msfc.nasa.gov/SunspotCycle.shtml

Sunspot activity Sunspots seem to be associated with magnetic fields. Sunspots usually appear in pairs, with one spot associated with a North magnetic pole, and the other with a South magnetic pole. Actually the sun’s overall magnetic field flips each cycle, and so actually the full sunspot cycle is actually composed of two 11 year cycles (on average) for an overall period of about 22 years.

Sunspot Activity Because solar output and solar flares are associated with sunspot activity, the sunspot cycle is important. Solar activity and solar flares in particular affect power transmission and communications, and may even influence the earth’s weather. It appears that there were few if any sunspots for about a 70 year period from 1645 – 1715 (sunspots were first observed in 1610 by Galileo and others), and this corresponds to the “Little Ice Age” in Europe and to an extended drought in the southwestern US. There are other possibilities for the “Little Ice Age”, but this is one of them.

The Solar Wind The solar wind is a mixture of ions and electrons that are streaming out from the sun. This wind decreases in density as it moves away from the sun, but it extends at least all the way to Neptune (as measured by our satellites). See the web page at: http://solarscience.msfc.nasa.gov/

Solar Energy – what is the fuel? We’ve looked at the surface and the atmosphere, but what is the source of the sun’s energy (its fuel), and how does it “burn” the fuel?

Source of the sun’s energy: possibilities a) Gravity is an energy source. As things fall down, the gravitational energy can be converted to other forms like kinetic energy, heat, and light. We use gravitational energy when we build a dam and store water above the dam. We then convert the gravitational energy into electrical energy using turbines.

Source of the sun’s energy: possibilities a) Gravity – Using the sun’s gravity as a source and knowing the present energy output of the sun, we figure the sun could only last a few million years using gravity as its source.

Source of the sun’s energy: possibilities b) “burning” = chemical energy. We burn fossil fuels like coal and oil to generate heat that can be converted into other forms of energy like kinetic energy (of cars and trucks) and steam which can drive a turbine to generate electrical energy.

Source of the sun’s energy: possibilities b) Chemical energy – If all of the sun were carbon and oxygen, the sun would only last a few thousand years burning this fuel into carbon dioxide. Note: we do NOT see lots of carbon, oxygen, and carbon dioxide in the sun. It is mainly hydrogen with a little helium and very small amounts of other atoms.

Source of the sun’s energy: possibilities Nuclear energy – either fission or fusion. c-1) nuclear fission (atomic bombs and nuclear reactors use this) converts very heavy atoms like uranium and plutonium into mid-size atoms and releases a tremendous amount of energy.

Source of the sun’s energy: possibilities c-1) Nuclear fission – we do NOT see any significant amounts of uranium or plutonium in the sun and we do NOT see any significant amounts of the mid-size atoms that should be the “ashes”.

Source of the sun’s energy: possibilities c-2) Nuclear fusion. This energy comes from combining light elements into slightly heavier elements (like hydogen into helium, or helium into carbon). This is the source of the extra energy in a hydrogen bomb. We do NOT have any fusion reactors, but we are working on them. The main problem is that it takes a fairly high density of atoms raised up to about a million degrees to get this process going.

Source of the sun’s energy: possibilities c-2) Nuclear fusion – This energy source could power the sun for 100 billion years if all of the hydrogen were converted into helium. We DO see lots of hydrogen and some helium in the sun. The sun’s gravity would keep the density high, and the sun’s gravity would also provide the initial energy to ignite the process. We think this is the fuel that the sun is using right now.

The sun’s interior Our present view of the sun is that nuclear fusion (converting hydrogen into helium) is occurring at the sun’s core where it is extremely hot (on the order of 15 million Kelvin or 28 million oF) and the gases are very dense – due to gravity. The energy initially leaves the core in the form of radiation, so the layer that surrounds the core is called the radiative zone.

The sun’s interior As the radiation moves out from the core through the radiative zone, it essentially spreads out and is converted into heat – the kinetic energy of the atoms. The layer where this occurs, above the radiative zone, is called the convective zone. The heat is then “convected” onto the surface of the sun.

The sun’s interior At the surface, the photosphere, the heat then radiates the energy into space. Magnetic fields associated with the hot atoms (actually ions since many electrons have been stripped off due to the heat – this is called a plasma) then cause the surface of the sun to erupt in the flares we talked about earlier.