3. The Sun.pdf

Lesson 2 The Sun The Sun, Our Star ▪ The Sun is an average star. ▪ From the Sun, we base our understanding of all stars in the Universe. ▪ Like Jovian Planets it’s a giant ball of gas. ▪ No solid surface. ▪ The Sun's rotation axis is tilted by about 7.25 degrees from the axis of the Earth's orbit...

Lesson 2 The Sun The Sun, Our Star ▪ The Sun is an average star. ▪ From the Sun, we base our understanding of all stars in the Universe. ▪ Like Jovian Planets it’s a giant ball of gas. ▪ No solid surface. ▪ The Sun's rotation axis is tilted by about 7.25 degrees from the axis of the Earth's orbit so we see more of the Sun's north pole in September of each year and more of its south pole in March. Vital Statistics ▪ ▪ ▪ ▪ ▪ ▪ Radius = 100 x Earth (696,000 km) Mass = 300,000 x Earth (1.99 x 1030 kg) Surface temp = 5780 K Core temp = 15,000,000 K Luminosity = 4 x 1026 Watts Solar “Day” = • 24.9 Earth days (equator) • 38 Earth days (poles) The Sun versus the Planets ▪ The Sun contains most of the mass (99.85%) of our Solar System. ▪ The planets are much smaller. Figure 22.1b Internal Structure of the Sun ▪ Like the Earth, the interior of the Sun is differentiated, however in the Sun; composition does not really change, only temperature and pressure. ▪ The distance from the center to its visible surface is 696 000 km – 110 times that of the Earth ▪ Main layers are core, radiative zone and convective zone ▪ The core extends from the center to 160 000 km (23% of its radius), temperature reach 15 million oC and diminish to the 8 million oC at the top of the core ▪ Because of inward force of gravity, the pressure at the core is 10 000 times that of the earth ▪ The nuclear fusion reactions on the sun convert 600 million tons of hydrogen into helium every second. Internal Structure of the Sun Internal Structure of the Sun ▪ The radiative zone is the thick layer surrounding the core energy passes through this layer in the form of electromagnetic radiation, it extends from the core to 490 000 km(70% of its radius) and has a thickness of 330 000 km and accounts for 43% of the Sun’s radius and 48% of its mass. ▪ This region is transparent to light. ▪ Why? • At the temperatures near the core all atoms are ionized. • Electrons float freely from nuclei • If light wave hits atom, no electron to absorb it. ▪ So: Light and atoms don’t interact. ▪ Energy is passed from core, through this region, and towards surface by radiation Internal Structure of the Sun ▪ The convective zone surrounds the radiative zone and has a thickness of about 200 000 km, when energy reaches this zone it heats the base of the overlying plasma layer to 2 million oC, as the heat decreases as the density of the plasma increases and carries thermal energy with it. ▪ This region is totally opaque to light. ▪ Why? • Closer to surface, the temperature is cooler. • Atoms are no longer ionized. • Electrons around nuclei can absorb light from below. ▪ ▪ ▪ ▪ No light from core ever reaches the surface! But where does the energy in the light go? Energy instead makes it to the surface by convection. The convective columns range from 500 -2000 km across, the center rising plasma is hotter and brighter, than the sides, where cooler plasma sinks. The pattern and of light and dark gives the Sun’s surface a grainy appearance know as solar granulation Solar Granulation ▪ Solar granulation results at the surface where hot plasma is rising and sinking. ▪ Hot rising material appears light; cooler sinking material appears darker. Figure 23.5 Structure of The Sun ◆ Because the sun is made of gas, no sharp boundaries exist between its various layers. Keeping this in mind, we can divide the sun into four parts: the solar interior; the visible surface, or photosphere; and two atmospheric layers, the chromosphere and corona. Structure of the Sun Photosphere ▪ The photosphere is the visible surface of the Sun. ▪ The photosphere is the region of the sun that radiates energy to space, or the visible surface of the sun. ▪ It consists of a layer of incandescent gas. ▪ A 50-500 km thick layer between the top of the convective zone and the rest of the solar atmosphere, at the base, the plasma is too dense for light to escape producing the surface that we see, the temperature at the top is 4300 oC to the base 5700oC, electromagnetic radiation from here heads off into space reaching the Earth 8 minutes later ▪ It exhibits a grainy texture made up of many small, bright markings, called granules, produced by convection. ▪ It is also the layer at which we measure the characteristic temperature of the Sun as a star. ▪ Most of the elements found on Earth also occur on the sun. Figure 23.6a The Photosphere ▪ This is the origin of the 5700 oC blackbody radiation we see. ▪ Why? • The hot convection cell tops radiate energy as a function of their temperature (5800 K). o l = k/T = k/(5800 K) → l = 480 nm (visible light) (Wein’s Law) ▪ This is the color of the light we see. ▪ That’s why we see this as the surface. Chromosphere ▪ The chromosphere is the first layer of the solar atmosphere found directly above the photosphere. ▪ It is a relatively thin, hot layer of incandescent gases a few thousand kilometers thick ▪ Its top contains numerous spicules, which are narrow jets of rising material. ▪ It is red in appearance and can only be seen during an eclipse or through special filters ▪ It is 3000 – 5000 km thick and has very low density Figure 23.6b The Chromosphere ▪ Very hot ▪ Same as the gas tubes we saw in class and lab. ▪ Energy from below excites the atoms and produces emission from this layer. ▪ Predominant element – Hydrogen. ▪ Chromosphere = color Corona ▪ The corona is the outer layer of the solar atmosphere. ▪ The temperature rises to 1 million oC at a distance of 10 000 km above the photosphere ▪ It extends out several times the diameter of the Sun. ▪ Left: Inner part of the corona just above chromosphere ▪ Right: Outer part of the corona as viewed in ultraviolet light ▪ Source of the Solar Wind Figure 23.6c,d The Solar Wind ▪ Solar wind: high-speed stream of particles (mostly protons and electrons) escaping the Sun’s gravity ▪ Moving matter, not just energy; particles travel at 500 km/s ▪ The Earth’s magnetosphere deflects most of these electrically charged particles. ▪ Particles that stream toward the Earth’s poles produce aurorae. ▪ Like steam above our boiling pot of water, the gas ‘evaporates’. ▪ Solar Wind carries away a million tons of Sun’s mass each second! ▪ Only 0.1% of total Sun’s mass in last 4.6 billion years. The Sun’s Magnetic Field ▪ Produced by rapid circulation of the plasma within the convective zone because plasma is an electrical conductor, moving charged particles produce magnetic fields, plasma contains charged particles ▪ Magnetic field on the earth arc from pole to pole ▪ Complicated by the fact that solar rotation varies with latitude, the Sun is a fluid whose rotation rate varies with latitude, plasma at the equator rotates faster around the Sun’s axis (25 days) than does the plasma around the poles (38 days) ▪ This “warps” the magnetic field lines over time. ▪ The Earth magnetic field polarity reverses every thousand to millions of years, on the sun this happens every 11 years Earth’s Magnetic Field Figure 23.7a Sunspots ▪ Sunspots: patches on the Sun’s surface that have cooled as upwelling plasma has been inhibited by strong magnetic fields that arc out of the surface. Always occur in pairs where field exits then re-enters surface. ▪ These areas become 1300oC-2700oC cooler than brigher regions of the photosphere, the cooler temperature makes them appear darker than other regions ▪ The number of sunspots varies over time as a consequence of the reversals in the Sun’s Magnetic field Figures 23.7b, 23.8a Solar Cycle ▪ As field lines become more warped over time, the solar magnetic field eventually collapses and then reforms. The number of sunspots increase to a maximum of 50 to 120 per month, then when the polarity reverses, the number decreases to nearly zero ▪ Sunspot numbers increase as lines become more warped then drop off sharply. ▪ This occurs over an 11-year period—the sunspot cycle. Names have been assigned to time intervals with an anomalous number of sunspots, from 1650 to 1710 there were hardly any sunspots Figure 23.8c ▪ Can see that Sun doesn’t rotate as a solid body? ▪ Equator rotates faster. ▪ This differential rotation leads to complications in the Solar magnetic field. Solar Storms: Flares ▪ ▪ ▪ ▪ Solar Storms- disruption of the Sun’s magnetic field triggers the ejection of particularly large amounts of high energy particles into space, there are different type of solar storms (below) Solar Prominence- outburst of glowing gas and plasma emerge from one sunspot on the Sun’s surface and then follow magnetic field lines back to other sunspot of the pair, these are huge arcs of plasma, mostly within the chromosphere and can last for hours, days, weeks Flare: an even brighter eruption/explosion of plasma that shoots out of the Sun’s surface and releases a huge amount of energy as well as X-rays and UV rays and particles into space all in the matter of minutes, extremely hot reaching 200 000 oC- 1 500 000 oC Material ejected from prominences and flares contribute to solar wind. Figure 23.9b Space Weather ▪ Conditions on the Earth caused by ionized particles coming from the Sun (solar wind) ▪ Some effects are • • • • • Disrupts satellites Hobbles communications systems Damages electrical grids Disrupts cell phones Affects GPS navigation Aurora Borealis • The solar wind passes out through the Solar System. • Consists of electrons, protons and other charged particles stripped from the Sun’s surface. • Interaction with planetary magnetic fields gives rise to the aurora. Atmospheric Composition ▪ Probably same as interior. ▪ Same as seen on Jupiter. ▪ Same as the rest of the Universe. The Solar Atmosphere ▪ ▪ ▪ ▪ Above the photosphere, transparent to light. Unlike radiative zone, here atoms not totally ionized. Therefore, there are electrons in atoms able to absorb light. Absorption lines in solar spectrum are from these layers in the atmosphere. The X-Ray Sun ▪ Q: At 1,000,000 K where does a blackbody spectrum have its peak? ▪ A: X-rays ▪ Can monitor the Solar Coronasphere in the X-ray spectrum. ▪ Monitor Coronal Holes Helioseismology ▪ Continuous monitoring of Sun. • Ground based observatories • One spacecraft (SOHO- solar and heliospheric observatory) ▪ Surface of the Sun is ‘ringing’ ▪ Sound waves cross the solar interior and reflect off of the surface (photosphere). Homework ▪ Page 75 #1-2 ▪ Sunspot assignment