The Sun & The Solar System

1. The Sun & The Solar System Chapter 26

2. The Sun’s Size, Heat & Structuresec 1 • Compared to Earth, the sun is enormous • Diameter = ~1,400,000 km • More than 3x the distance from Earth to the moon • More than 1 million Earth’s would fit inside • However, the sun is only an average size star.

3. The Sun’s Energy • All stars get their energy from fusion. • The combining of the nuclei of lighter elements to form a heavier element. • A star is a place of intense heat & pressure. • Atoms are torn apart, separating into their component nuclei & electrons • So, elements such as hydrogen & helium exist as plasma • Plasma is a 4th state of matter consisting of charged particles • The nuclei or ions have a positive charge • The elections have a negative charge • Normally, nuclei repel each other b/c they have similar charge, but due to the heat & pressure, the nuclei can fuse ~Some of their mass is converted to energy

4. Which element is lighter? • How many H nuclei are needed to fuse into one atom of He? • How many protons are in a He nucleus? • How many neutrons are in a He nucleus? • Where does the energy that is released come from?

5. The Sun’s Layers • Scientists have developed models of the sun’s structure even though they have never actually observed the interior. • The core: • Consists mostly of hydrogen & helium ions in a plasma state • Is 100x more dense than water • Temperatures can reach ~15,600,000°C • The radiative zone: • Lies around the core • Is also in the plasma state • Is cooler than the core • Near the core the temp is ~8,000,000°C • Near the convection zone, the temp is ~2,000,000°C

6. The Sun’s Layers • In the convection zone: • Rising & falling currents of plasma carry energy to the sun’s surface where it is radiated out into space as sunlight • The photosphere: • Is the visible surface of the sun • Granules form • Which are the tops of the convection currents • May be 1,000 km wide • May last 20 minutes • Is much cooler than the convection zone • Temp = ~6,000°C

7. The Sun’s Layers • The sun’s atmosphere • Chromosphere • Inner layer • Extends 1000s of km above the photosphere • Temp = ~20,000°C • Causes hydrogen to emit light with a distinctive reddish color • Features include • Solar prominences • Dense clouds of material suspended above the sun’s surface by magnetic fields • Can erupt off the sun in a few minutes or hours, extending 1000s of km before falling back to the sun’s surface • Corona • Thin outer atmosphere • A million times less bright than the photosphere • Temp ranges from ~1,000,000°C to ~3,000,000°C

9. Features on the Sun • Sunspots • Dark spots on the photosphere • Some are barely visible • May last only a few hours • Some are 4x larger than Earth’s diameter • May last a few months • Are actually very hot & bright • They look dark in pics b/c the surrounding photosphere is so much hotter & brighter • The magnetic field is about 1000x greater than the magnetic field of the photosphere

10. Features on the Sun • Move from left to right across the sun’s surface when viewed from Earth • This was the first hint that the sun rotates on its axis • b/c the sun is not solid, the rate varies from place to place • Equator ~25 days • Poles ~34 days • Number visible changes from day to day • Peak activity  more than 100 can be counted • Usually about 11 years between periods of peak activity • Low activity  several days may pass when no sunspots are visible

11. Features on the Sun • The solar wind & magnetic storms • The corona gives off a stream of electrically charged particles called the solar wind. • These particles (mostly p+ & e-) fly into space at ~450 km/sec & reach Earth in a few days. • There, they are deflected by Earth’s magnetic field. • Can be caused by: • Coronal holes • Solar flares • Outbursts of light that suddenly rise from areas of sunspot activity ~can last minutes to hours & increase in # with an increase in the # of sunspots

12. Features on the Sun • Earth’s magnetic field shields the surface from solar winds, which could harm living things • As the solar wind blows past Earth, some particles interact w/ Earth’s magnetic field & upper atmosphere causing auroras • Displays of color & light appearing in the upper atmosphere • Also called the northern or southern lights • Are common near the poles • Magnetic storms occur on Earth when particles thrown out by coronal holes & solar flares are added to the constant solar wind • Then auroras are also visible in middle latitudes • Electrical surges following solar flares may disrupt cellular phone service & damage unprotected electrical appliances

14. Observing the Solar System sec 2 • The Movements of Planets & Stars • For 1000s of years, the predominant model of the universe was the geocentric (Earth-centered) model • Believed Earth stood still at the center at the center of the universe

15. As long as 6000 years ago, astronomers noticed that the stars appeared to move across the sky, but did not move in relation to one another. • To explain this, they envisioned stars as holes in a solid celestial sphere that surrounded the earth like a shell. • They imagined that a very bright light source outside of the sphere was shining through the holes. • They concluded that the stars moved around the stationary Earth as the celestial sphere rotated

16. Early astronomers also noticed that the same constellations (groups of stars) were visible at the same time every year. • Often used as the basis for calendars • Not all points of light in the sky are fixed in constellations. • A few points seem to “wander” or change position over days, weeks, or months • These are planets • Early astronomers inferred correctly that planets are closer to Earth than the stars are. • Most of the time planets moved eastward in front of the background constellations • Periodically, however, the planets stopped moving eastward & moved westward for a few weeks, then resumed their eastward paths. ~This is called retrograde motion

17. Ptolemy’s Geocentric Model • Ptolemy • Greek astronomer, lived in Egypt, 2nd Century • Puzzled by retrograde motion • Developed a system that allowed him to predict where planets would appear in at a given time • Needed to account for retrograde motion • Imagined planets on small circular orbits epicycles • The center of each epicycle moved around Earth on a larger circular orbit called a deferent • Retrograde motion occurred when planet moved along part of the epicycle that the observer on Earth could see • Didn’t work perfectly, but still used until the 16th Century

18. Retrograde Motion

20. Copernicus’s Heliocentric Model • Nicolaus Copernicus • Polish astronomer • Proposed the heliocentric (Sun-centered) model of the solar system • Suggested that Earth: • Was a planet • Rotated • Revolved around the sun (along w/ the other planets) • Basis for our modern understanding of the universe • Observe retrograde motion b/c each planet orbits the sun from a different distance & at a different speed • Ex. Earth moves faster than Mars. When Earth begins to overtake/pass Mars, Mars appears to move West (backward) among the stars. After Earth has fully passed Mars, eastward motion appears to resume. • Ex. cars passing each other on the road

22. Tycho, Kepler, & Planetary Motion • Tycho Brahe • 16th Century Danish nobleman & observational astronomer • Like Ptolemy & Copernicus did not have the aid of a telescope • Devised his own instruments to observe the moon, planets, & other celestial objects • Unlike others, studied the movement of the moon & planets throughout their orbits, not just at certain points • Led Tycho to identify a # of unexpected occurrences • Particularly in Mars’ orbit • Most precise records made before invention of the telescope • Died in 1601 before he could apply his data • His assistant, Johannes Kepler, built on Tycho’s work

23. Tycho, Kepler, & Planetary Motion • Johannes Kepler • Discovered the unexpected occurrences that could be explained if the planets’ orbits were elliptical, instead of round • First Law of Planetary Motion • States that the planets travel in elliptical orbits with the sun at one focus • Instead of having only a center like a circle, an ellipse has 2 foci on opposite sides of the center (separated by a distance “d”) • b/c the sun is at one focus of the ellipse, a planet’s distance from the sun changes throughout the orbit

24. Other Foci is in space Sun is one Foci Distance

25. Not very elliptical Very elliptical F1 F2 L= length of major axis d= the distance between the foci • Eccentricity (e) is the measure of how flattened an ellipse is. dL e =

26. Eccentricity will be between 0 and 1 • e = 0 for a circle because the distance = 0 • e = 1 for a line because d = L Foci are on top of each other L Foci are so far apart, they are on the line

27. Tycho, Kepler, & Planetary Motion • Second Law of Planetary Motion • Equal Area Law • States that each planet moves around the sun in such a way that an imaginary line joining the planet to the sun sweeps over equal areas of space in equal periods of time • B/c a planet’s orbit is an ellipse w/ the sun at one focus, the equal area law means that the speed at which a planet travels around the sun is not constant. ~Planets travel faster when they are closer to the sun. (Kepler couldn’t explain why.)

29. http://honolulu.hawaii.edu/distance/sci122/Programs/p11/ellipse.htmlhttp://honolulu.hawaii.edu/distance/sci122/Programs/p11/ellipse.html *Remember planets move faster when closer to the sun!

30. Tycho, Kepler, & Planetary Motion • Third Law of Planetary Motion • Harmonic Law • The time it takes a planet to travel one orbit around the sun is its period • States that the period (P) of a planet squared is equal to the cube of its mean distance (D) from the sun or P2 = D3 • The formula is used to find the mean distance between the sun & a planet if the period is known, or to find the period if the mean distance is known. • The farther a planet is from the sun, the longer its period of revolution • b/c its orbit is larger • b/c it moves slower than planets closer to the sun

31. Isaac Newton & the Law of Gravitation • Isaac Newton • English scientist & mathematician • Explained what kept the planets in motion • His first law states that an object will move in a straight line at the same speed forever unless some external force changes its direction or speed. • Said that the force of gravity is what keeps the planets orbiting the sun. • The law of gravitation states that every mass exerts a force of attraction on every other mass, & the strength of that force is proportional to the masses & inversely proportional to the distance between them.

32. F = G m1 m2 d2 Isaac Newton & the Law of Gravitation • Gravity is determined by the mass & distance between 2 objects. • The formula to calculate Gravity is: • Force increases as mass increases or distance decreases • Force decreases as mass decreases or distance increases

33. Isaac Newton & the Law of Gravitation • The sun is more massive than the planets, so its force affects the planets more than the planets’ gravitational forces affect the sun. • The sun’s gravitational pull deflects the planets from the straight-line paths that they would follow under Newton’s first law of motion & keeps them in elliptical orbits around the sun. • Kepler had realized in his second law of motion that the planets did not move with constant speeds. • With the law of gravitation, Newton was able to explain why planets do not move with constant speeds.

34. More KE b/c greater gravitational pull by the sun • Faster to avoid being “sucked in” • Perihelion (closest) • According to Newton’s law of gravity objects must travel faster (more KE) when closer to the sun to avoid being pulled in by the sun’s gravity • More PE b/c less gravitational pull • Slower • Aphelion (furthest) For Earth- perihelion = 147 million km (Jan. 3) & aphelion = 152 million km (July 4) Avg = 149.6 million km (or ~93 million miles) = 1 astronomical unit (AU)

35. Apparent Diameter- the distance from Earth to the sun, moon and planets is constantly changing… When they are closer they appear larger. NY’s Summer Sun appears smallest NY’s Winter Sun appears largest Why does the size of the sun appear to change? animation

36. Homework • Read pgs 572-576 & answer # 5 • Read pgs 577-580 & answer # 1, 3, 5