The Sun: Our Star The Sun is an ordinary star and shines the same way other stars do.
Announcements q Homework # 4 is due today!
q Units 49 and 51 Assigned Reading
Today s Goals q Today we start section 3 of our Course: STARS. q We begin by investigating the closest star to us: the Sun: q The Sun q The Sun provides virtually all of the energy necessary for life on Earth: food, weather, etc. q Our main experience with the Sun is through the heat and light it provides (`sunshine ), and visually, as a yellow/whit-ish sphere in the sky
The Sun s ID v Radius: 700,000 km (109 times the Earth) v Mass: 2 x 10 30 kg (330,000 times the Earth) v Luminosity: 4 10 26 W v Average density: 1.4 kg/l (a little over water); however: v Central density: 160 kg/l (20 times the density of steel) v Photospheric density: ~1/3000 that of air (and about 1/2,400,000 that of water) v Surface Temperature: 5,800 K v Central Temperature: 15,000,000 K
The Sun is hotter towards the center: the limb is slightly darker
The Solar Constant n The S.C tells us the amount of energy the Sun produces: It is 1360 Joules per second per square meter or about the luminosity (=ENERGY PER SECOND) of 60 W bulb every square millimeter. n n n n A stable change of ~1% in the S.C. would change Earth s temperature by 1-2 degrees Random variation ~0.1% over days or weeks Long-term, cyclical variation 0.018% per year (period longer than the 22 years of the sunspots) Small random fluctuations do not affect Earth s climate
What is the Sun? n The Sun is a star, a fairly common and typical star n A star is a ball of gas held in hydrostatic equilibrium against its own self-gravity by the thermal pressure of the hot gas, cooling from center to the surface n This energy comes from nuclear fusion reactions at the center of the Sun (we will expand on this in the next lecture)
What is the Sun made of It is made of hot gas; there is no solid material in the sun! The gas is mostly hydrogen (71%) and helium (~27%), plus traces of heavier elements (2%) Why does the Sun have such a sharp edge? Because it is made of dense gas; it takes 100,000 years for a photon to travel from the Sun s center to the photosphere (and only 8.3 minutes to come to us!). Hot+dense gas = plasma
What is that Yellow-ish Sphere The bright part normally seen is called the photosphere, which is about 500 km deep. It is an almost perfect black body with a surface temperature of 5800 K.
Structure of the Sun
The Surface and Atmosphere of the Sun Although the sun appears to have sharp edge, its surface (Photosphere) actually has a complicated structure. Its atmosphere is complex, too. Name Temperature Spectrum " Photosphere: 5,800 K, continuum + abs. Chromosphere: 4,500-500,000 K, emiss.+abs. " Corona: 1-2 10 6 K, continuum+emiss.+abs. " Chromosphere and Corona are the `Atmosphere
Each square millimeter of the Photosphere emits the same Energy as a ~60 Watt light bulb The Sun surface (photosphere) would cool off very quickly if Energy were not produced continuously from below Corona (1-2,000,000 K) spicules (4500-500,000 K) (5,800 K) The light we see originates from the thin photosphere
Spectrum of the Photosphere n The photosphere: the region where the black body radiation at 5,800 Kthat we see is made n Thin gas: about 3,000 times thinner than air (you need to travel about 7,000 km into the Sun to find a density similar to that of air) n Its upper layer produces the absorption spectrum (Fraunohfer spectrum)
Fraunhofer Spectrum
The Photosphere is NOT Perfect: Granulation of the Photosphere Each Granule is about the size of Texas and lasts for only 5-10 minutes before fading away! It is due to the convection zone
Granules: convective cells n The granules are just densely packed convective cells n Convective cells are very similar to thunderstorms
Sunspots Sunspots, about 1000-1500 K cooler than the rest of the Sun's photosphere, appear as dark spots. Size range ~500-5,000 km Sunspots come and go with time. Big sunspots can live for several weeks. They are `magnetic hurricanes, regions with magnetic fields ~100 times stronger than the rest of the Sun.
Summary of Photosphere n n n n It is the `surface of the Sun, where most of the light we observe is coming from It is an almost perfect blackbody with temperature 5,800 K The photosphere shows `granularity ; the granules are convective cells of cooling gas Sun spots are magnetic disturbances (`magnetic hurricanes ) on the surface of the Sun that can live up to several weeks; they are regions about 1,000-1,500 K cooler than the surrounding regions (hence they appear to be as dark spots).
The Atmosphere of the Sun Atmosphere of the Sun: Name Temperature Spectrum " Chromosphere: 4,500-500,000 K, emiss.+abs. " Corona: 1-2 10 6 K, continuum+emiss.+abs.
Chromosphere "Reddish in color, which is the origin of its name (chromos meaning ``color'') "2000-3000 km thick "Faint relative to the photosphere "From 4,500 up to 500,000K: hotter than the photosphere and much less dense. "Contains thin columns of gas called spicules
Solar Prominence Composed of hot gas trapped in magnetic fields extending from one sunspot to another. They jut from the lower chromosphere into the corona.
Prominence
UltraViolet (UV) image of the Sun. It shows that regions of the Photosphere corresponding to Sunspots are more energetic than elsewhere. It also shows magnetic links (prominences) between Sunspots extending into the Chromosphere and Corona
Solar Flare A solar flare is a violet outburst that lasts an hour or less. It radiates X-ray, ultraviolet, and visible radiation, plus streams of high-energy protons and electrons. Solar Flares originate in/near Sun spots and juts into the chromosphere. A large flare can be a billion times more energetic than a large hydrogen bomb. Solar flares originate in/near sunspots, and are violent, but brief (~1 hour), outbursts of energy.
Picture of a Solar Flare
Corona - the outermost layer - made up of very diffuse (tenuous) but extremely hot gas (T~1 million K) - coronal emission is dominated by X-rays - carries very little energy - `stirred by the Sun s magnetic field
The heating of the corona The heating of the corona has puzzled scientists for a long time. How can heat go from cooler regions to hotter ones? It doesn t! Radiant heat penetrates the corona nearly undisturbed Corona heated by something else: whipping by magnetic force lines The corona is heated by energy outflowing from the sun's interior not as heat but as magnetic energy. The gas is heated by the motion through it of long lines of magnetic force, which act like whips. The gas is being whipped.
The heating of the corona The corona is heated by energy from the sun's interior not as heat but as magnetic energy. Heated by the whipping motion of lines of magnetic force
Solar Activity
Solar Wind The corona s high temperature gives sufficient kinetic energy to the atoms that the gas `escapes, creating the solar wind. The Sun is loosing mass, at a rate of 1.5E6 tons/s, but is only a small fraction of Sun s mass: 10-14 M o /yr
Auroras: the Northern and Southern Lights
Auroras n Auroras seen as far south as New England n Caused by a gust of solar wind (coronal mass ejections) n These powerful gusts are guided by Earth s magnetic field and can excite gases in the upper atmosphere, causing the air there to glow
Activity on the Sun Two key points to remember: 1. nearly all solar weather is a result of changes in the magnetic fields on the Photosphere. 2. Magnetic Fields are the result of convection in the Sun
The Sun s Rotation and Magnetic Field: Impact on Activity The Sun is a `ball of gas that rotates on its axis; the equatorial regions rotate faster (25 days) than the north or south (30 days).
Babcock Model
Sunspots are created at the `kinks of the magnetic field. After the maximum of activity (flares, mass ejection, many sunspots), the field relaxes again, and inverts polarity.
The differential rotation causes magnetic cycles: appearance of sun spots, and their cycling through a maximum every ~ 11 years
The Solar Cycle Solar activity peaks roughly each 11 years.
Survey Question The bulk of the violent surface activity on the Sun is due to seasonal variations in the Sun s. 1) energy output 2) radius 3) electric fields 4) magnetic fields