Our Sun – Physical Properties

Diameter: 1,400,000 km, 864,000 miles 4.5 light-seconds 1,300,000 Earths could fit inside! 109 Earths would fit across the diameter of the sun!!

Mass: 2 x 1030 kg or 330,000 times Earth’s mass!! Density: 1.41 g/cm3

What planet has this same composition?

Surface temp: 5800 K, 5500oC, 11,000oF Luminosity – total energy output at all wavelengths = 4 x 1026 watts/second (more than 6 moles of 100 watt light bulbs)

Trivia… 4.5 million metric tons of H are converted to He every second! Expected lifetime: 10 billion years! Distance from earth: 1 A.U. = 93,000,000 miles = 150,000,000 km = 8.33 light minutes!

Trivia 1 rotation takes 27.5 days at the equator, but 31 days at the poles! Now that’s differential rotation. How was this determined?

That’s right – sunspots!

The Sun’s Structure 3 Interior Layers 3 Atmosphere Layers The core produces the energy The radiative zone The convective zone 3 Atmosphere Layers Photosphere Chromosphere Corona

The Core 16,000,000 K (a star’s core must be at least 8,000,000 K to start fusing H to He. so hot that there are no real atoms, only a soup of protons, electrons, and some larger atomic nuclei (He and C). all radiation produced is gamma ()

Radiative Layer Made of H, He gases. Not hot enough for fusion to take place, but so hot that normally transparent gases have become opaque to light. Photons of light produced by core bounce from one atom to another in a “random walk”, like a gigantic pinball game.

Radiative Layer A given photon may take 100,000 years to reach the next layer. As photons travel, they slowly lose energy, shifting down towards the X-ray region of the spectrum. Temperature of this layer falls with increasing distance from core.

Convective Zone Made of H, He gases…still hot enough to be opaque to light. Currents of gas move vertically, like water boiling in a pan. Energy is transported by moving hot mass (convection), not by radiation. This layer is like earth’s mantle.

The tops of convection cells can be seen near the sunspots The tops of convection cells can be seen near the sunspots. They are called granules, or granularity.

The Photosphere The innermost of the sun’s atmosphere layers. H, He gas finally cools enough that it becomes transparent to light. Our sunlight originates from this layer. This is the surface that we see. Only 300 km thick.

Actual color of photo- sphere … is slightly greenish.

The Chromosphere 2nd atmosphere layer. Glows in red H- light (the red line from the level 3 level 2 electron transition in H atoms). Tends to filter out the slightly greenish color of the photosphere, so we see yellow light from sun. Several thousand kilometers thick.

The Corona Millions of kilometers thick, but extremely low density. Sun’s magnetic field agitates corona, raises temperature back up to about 2,000,000 K. We can only study corona during a total solar eclipse, or from space with specially designed telescopes.

Features on the Sun’s Surface All features are produced by sun’s wacky magnetic field. Prominences & flares. Sunspots Coronal Holes Coronal Mass Ejections

Differential Rotation If the sun were solid and magnetic field rotated in an orderly way, there would be no storms or surface features on the sun, but… …differential rotation winds up and tangles the sun’s magnetic field, resulting in surface storms. Process is not very well understood.

There’s still a lot we don’t know For example, why doesn’t the sun have activity all the time? After all, the magnetic field should be winding up and tangling constantly. Does the sun produce the same strength of magnetic field all the time? Is it structured differently at some times than at others?

Prominences & Flares When a loop of the sun’s magnetic field projects out from the surface, some of the hot gas from the photosphere may flow along the field lines in arcs or loops, called prominences.

A loop prominence – lets us visualize the magnetic field.

Flares Sometimes, the magnetic field lines disconnect from the sun. Hot gas trapped inside the new loop of magnetic field travels outward from the sun as a solar flare.

Sun spots Where the loops of magnetic field penetrate the sun’s surface, they tend to cool it. The result is a darker, cooler area…a sunspot. Sunspots occur in pairs of (+) and (-) polarity. Sunspots are still about 3500 K – hot enough to melt anything on the earth, but 2000 K cooler than the surrounding surface.

Umbra & Penumbra Just like the parts of the shadow of a solar eclipse, the darkest part of a sunspot is the umbra. The surrounding, slightly less dark area is the penumbra.

Sunspot Cycle The number of sunspots varies from year to year, along with the overall magnetic activity of the sun. We’re used to hearing of an 11 year cycle. That’s only for the overall number of sunspots.

Sunspot Cycle The real cycle is 22.2 – 22.4 years long, and includes 11 years of the magnetic field with (+) polarity, then another 11 years with (-) polarity. We also see sunspots migrate from high latitudes to nearer the equator as the cycle progresses.

Sometimes, the cycle quits! During the period 1645 to 1715, very few sunspots were observed. We call this 70 year period the Maunder Minimum. At the same time, European weather watchers recorded a mini ice age across Europe. Temperatures fell several degrees year round, and winter storms were worse than normal.

Coronal Holes Another surface feature is the coronal hole. These are actual holes or windows in the sun’s corona, where solar wind from the photosphere can easily blow through. When one of these points towards the earth, the velocity and density of the solar wind increases.

A coronal hole – a window to the interior.

You can see the solar wind blowing thru several coronal holes.

One of the worst events When the sun is in its active phase, the magnetic field sometimes gets tangled up so tight, that the sun blows off a portion of its entire corona. This is a coronal mass ejection (CME). A CME can be very damaging to electrical systems on the earth.

CME in progress

Watch a CME in progress http://sohowww.nascom.nasa.gov/

Because CME’s are made up of charged particles, they have magnetic and electrical fields. Their fields cause electrical systems to build up abnormally high voltages. In the winter of 2000-01, a CME knocked out power to all of eastern Canada & the northeastern US for nearly a week!

Satellites are also damaged by CME’s, so we have to spend $ on special shielding. Communications, especially broadcast radio & TV, can be knocked out by CME’s for hours at a time.

Missions to the Sun SOHO Ulysses Genesis

SOHO (Solar and Heliospheric Observatory) – a joint venture between ESA & NASA. Looks continuously at the sun from a fixed spot in space. Observes flares, CME’s & comets falling into the sun!

Ulysses – designed to orbit over the sun’s poles & provide a perspective that we can’t get from earth. This is ESA’s logo.

Ulysses mission

Genesis Mission The Genesis mission was designed to orbit the sun and collect samples of the solar wind in special gels. Genesis orbited at a point called the L1 Lagrange point - a place in space where earth’s gravity exactly cancels the sun’s gravity.

Genesis Mission In September, 2004, it returned these particles to Earth for examination. Unfortunately, its parachute didn’t deploy after re-entry into earth’s atmosphere and it crashed, but some of the samples were still useable. The samples are still being analyzed.

Genesis Solar Wind Sampling Mission

How does the sun make its energy? We’re coming to the end! Fusion of H to He occurs in a process called the proton-proton (or p-p) chain! Yes, you do have to know how this works! Bigger, heavier stars Fuse He to heavier elements like C and O, in a process called the CNO cycle.

A review of the symbols a proton: 11H or p+ (a proton is the nucleus of a normal hydrogen atom.) neutron: no electron: e- positron: e+ (a positively charged electron, “antimatter”)

more particles…actors on the stage neutrino:  (a tiny, nearly massless particle with no charge that barely interacts with normal matter.) gamma ray:  (the highest energy form of light)

Isotopes / nuclear symbols What does 42He mean? The 2 means 2 protons. The 4 means a total of 4 (protons + neutrons). Subtract the lower number from the upper to find the number of neutrons. How many p+, n0 in 5626Fe? 26 protons, 30 neutrons. neutrons are the nuclear glue that hold a nucleus together.

Act 1 – The first collision The first step in the p-p chain is the collision of 2 protons. One proton immediately turns into a neutron. The new neutron gets rid of its (+) charge by giving off a positron (e+) and a neutrino (). The resulting p+no is a deuterium nucleus.

p+  e+ p+no p+ e-  Leapin’ lizards, Batman! Can’t that positron combine with an electron and blow up?

Act 2 – a 2nd collision Another high speed proton (p+) collides with the deuterium nucleus (p+no) and sticks. This collision gives off a gamma ray () The result is a 32He nucleus: (p2no)+2.

p+no  (p2no)+2 p+ Pow! Zing!

Collision #3 – really bad driving Two 32He nuclei (p2no)+2 collide head-on to form a normal helium nucleus, 42He. In the process, they give off 2 protons and another gamma ray.

(p2no)+2 p+ (p2no2)+2  (a He nucleus) (p2no)+2 The 2 protons start the chain over.

Here’s the overall p-p chain

Accounting 101 What goes in: 6 protons (H nuclei) What comes out: 1 He nucleus 2 protons 2 positrons 2 neutrinos 2 gamma rays (4 gamma rays if you count the annihilation of the positrons!) How very Star Trek!

The Neutrino Problem One problem with our theory is that we can only detect about 1/3 of the neutrinos we ought to observe from the p-p chain. For the past several years, solar scientists weren’t sure if their model was correct.

The Neutrino Problem Neutrinos are detected by the flashes of light (scintillations) they produce as they interact with Cl atoms in big tanks of dry-cleaning fluid deep underground. (Underground, so less interference from cosmic rays.)

A neutrino detector tank in an underground mine.

Neutrino Problem Solved ! (?) Recently, researchers have determined that some neutrinos change type (or “flavor”) while they’re on their way between sun & earth. Counting these other flavors of neutrinos gives a total that’s close to what’s expected. Looks like our model may be OK after all.

Why did we go through all this? Our sun is the primary model we have for other stars. Next, we’ll examine how different kinds of stars live their lives & what they become after they die.

Sun in X-rays!

Credits solar-heliospheric.engin.umich.edu/ solar.physics.montana.edu/ sohowww.nascom.nasa.gov/ ulysses.jpl.nasa.gov/ helio.estec.esa.nl/ulysses/ genesismission.jpl.nasa.gov/ genesis.lanl.gov/ www.bbso.njit.edu/

More Credits csep10.phys.utk.edu/astr162/lect/energy/ppchain.html