Topic E: Astrophysics

1. Topic E: Astrophysics The following notes were taken primarily from Physics for IB by Chris Hamper and Physics Course Companion by Tim Kirk

2. Are they real?

3. E.1.1 • Outline the general structure of the solar system. • Students should know that the planets orbit the Sun in ellipses and moons orbit planets. (Details of Kepler’s laws are not required.) Students should also know the names of the planets, their approximate comparative sizes and comparative distances from the Sun, the nature of comets, and the nature and position of the asteroid belt.

4. http://solarsystem.nasa.gov/planets • Tons of interesting stuff about our solar system

5. Aug. 2006 the International Astronomical Union declared the official definition of a planet: • A “planet” is a celestial body that: a) is in orbit around the Sun b) has sufficient mass for its self-gravity to overcome rigid body forces so that is assumes a hydrostatic equilibrium (nearly round) shape c) has cleared the neighborhood around its orbit.

6. Our Solar System • 8 Planets – name them…. • Elliptical orbits – to have a circular orbit an object must have a very specific velocity. Any variations create and elliptical or hyperbolic shape • Moons • Period - 27.3 days • Which planets have them????

9. Phases of the moon

12. Our Solar System • Asteroids • belt between Mars and Jupiter • size – dust to hundreds kilometeres. • Comets • Similar to asteroids but made up of loose particles of ice and rock. • Tail is blown off by solar winds and melted by radiation. • Some orbit, others only pass the sun once • Planetoids??? • Pluto….

13. Relative size video. • http://www.wimp.com/starsize/

14. E.1.3 • Define the light year.

15. Super easy • Light year (ly) – the distance that a beam of light will travel in one year. • How far is that? (3 x 108m/s = c) • Used to measure distances outside of our solar system Other important units. • Astronomical unit (AU) – the average distance between the Sun and Earth • 1AU = 1.5 x 1011m • Used to measure distances inside our solar system • Parsec (pc) – 1parsec = 3.26 ly • Defined by making a triangle between the Earth, the Sun and a distant object. If the angle at the distant object is 1 arcsec then it would be 1 parsec away. (more later)

16. E.1.2 • Distinguish between a stellar cluster and a constellation • E.1.4 • Compare the relative distances between stars within a galaxy and between galaxies, in terms of order of magnitude.

17. Distribution of stars • Stars are not evenly distributed. • Stellar cluster – small groups of stars that gravitationally interact with one another. • Physically close to each other • Closest star, besides the sun is ProximaCentauri- 4.25ly

18. Distribution of stars • Galaxy – a very large number of stars bound together by gravity • Trillions of stars • 103 – 105 light years across • Each star is approx. 1 ly apart • Andromeda is about 2.5x106ly away • Galaxy cluster – small group of galaxies that gravitationally interact with one another • There are about 20 other galaxies we are clustered with. • Supercluster – bigger than a cluster

19. Virgo Cluster

21. Andromeda

22. How did galaxies get that way? • The simplest explanation is that • if all the gas is made into stars before the gas has time to form a disk, then you get an elliptical galaxy. • if the gas has time to stabalize into a disk before it is all used up, then you get a spiral galaxy. • Or perhaps some of the elliptical galaxies are made from merging of other types of galaxies. • Observations of distant galaxies indicates that spiral galaxies were more common in the past than they are today. • So maybe yesterday's spirals are todays ellipticals. • This is an active research area. One problem is that if most of the mass in galaxies is unaccounted for, we have a hard time understanding the dynamics of galaxy formation.

23. Constellations – groups of stars that are “linked” visually • Ancient civilizations played “connect the dots” • Located in the same general direction from Earth • Not necessarily close to each other • 88 total • Different ones are visible at different times during the year.

25. E.1.5 • Describe the apparent motion of the stars/constellations over a period of a night and over a period of a year, and explain these observations in terms of the rotation and revolution of the Earth. • This is the basic background for stellar parallax. Other observations, for example, seasons and the motion of planets, are not expected

26. Why do the stars move through the night sky? • Because the rotation of the Earth • It also matters where you are located on Earth • Ex. North Pole • http://www.yorku.ca/ns1745b/figs-ch1.html • This rotation takes 23h and 56min every time. • The effect is that it seams that the stars position at 12:00 changes each night. • This means that the Earth rotates 360º in 23h and 56min. • Which means 4min it will rotate 1º. • Which means it only takes 360 DAYS for the constellations to make one compete rotation.

27. Sun rise • The Sun doesn’t make the same path through sky every day. • For us, the summers are high in the sky, winters are low on the horizon. • This is because the axis of rotation for the Earth and the axis in which we orbit around the sun aren’t the same angle.

29. Precession • The Earth is not a perfect sphere. • This means that depending on it’s location in it’s orbit, it will feel more or less pull from the Sun • This pulls on the Earths axis of rotation and makes it wooble. • This is technically called presession. • This means that the “North Star” won’t always be the north star. • Period – 26,000 years

32. Other plant’s movement • The word planet comes from the Greek word for wanderer. • Planets will shift back and forth in the night sky relative to the constellation background. • Apparent east/west motion comes from the Earth’s orbit around the sun. • See Diagram on board • Apparent north/south motion come from the other planet’s orbital plane being at a different angle from ours.

33. E.2: Stellar Radiation and Stellar Types

34. E.2.1 • State that fusion is the main energy source of stars • Students should know that the basic process is one in which hydrogen is converted into helium. They do not need to know about the fusion of elements with higher proton numbers. E.2.2 • Explain that, in a stable star (for example, our Sun), there is an equilibrium between radiation pressure and gravitational pressure.

35. “H” is Fuel • How does our sun work? • Fusion of hydrogen into helium that provides the energy, for our sun • Happens on the inside of the sun (Yes, there are different layers) • Produces neutrinos that leave the sun and travel to Earth

36. The proton-proton chain • This is the same reaction discussed in Topic 7. Each complete chain reaction produces 26.7MeV.

37. Remember you need 4 H to end up with one He • See simplified equation:

38. Star Stability • Gravity pulls inward • So much the sun should collapse. • Nuclear explosions push outward • These two have to balance out to be at pressure equilibrium • Ex. Balloon. • Rubber is like gravity • Air is like the explosions • If the temp changes the inside pressure will change and won’t be stable

39. E.2.3 • Define the luminosity of a star. E.2.4 • Define apparent brightness and state how it is measured.

40. Luminosity • Light measurements give us information about the temperature, size and chemical composition of a star. • Luminosity(L) is the total amount of energy emitted by the star per second. • Unit is watt (same as power) • Depends on the temp. • Ex. Two stars have same temp, the bigger one will give out more energy • Sun’s luminosity of 3.839 x 1026W

41. Apparent brightness(b) • Some stars appear brighter than others. • Brightness depends on: • How much energy is radiated (luminosity) • How far away it is located • Apparent brightness is the amount of energy per second received per unit area. • Unit is W/m2 • b = (L) / 4πd2 • d is distant to the star

42. E.2.5 • Apply the Stefan–Boltzmann law to compare the luminosities of different stars. E.2.6 • State Wien’s (displacement) law and apply it to explain the connection between the color and temperature of stars.

43. Black Body Radiation • Black bodies absorbs all wavelengths of light and reflects none. It also is a perfect emitter of radiation. • If temp is increased the energy available is increased. • Means the electrons can gain more energy and move into higher energy levels • Means more photons released, and their average energy is greater. • E = hf, Higher energy means higher frequency/shorter wavelength

45. Stefan-Boltzmann • Each peek represents the intensity(apparent brightness) of radiation at different wavelengths. • Total intensity is the area under the curve. • Power per unit area = σ T4 • σ = 5.6 x 10-8 W/m2K4 (Stefan-Boltzmann constant)

46. Stefan-Boltzmann • If a star has a surface area A and temperature T then the total power emitted (luminosity), L is given by: • L = σAT4

47. Stefan-Boltzmann • At the temperature increases, the peak wavelength is shorter • Relationship between peak wavelength and temp is Wien displacement law: • λmax = (2.90 x 10-3km) / T

48. Example • The maximum in the black body spectrum of the light emitted from the sun is at 480 nm. Given that the Sun’s radius is 7.0 x 108m, calculate the temperature of the sun, the power emitted per square meter, and the luminosity. • Answers: 6000K, 7.3 x 107 W/m2, 4.5 x 1026W

49. E.2.7 • Explain how atomic spectra may be used to deduce chemical and physical data for stars. • Students must have a qualitative appreciation of the Doppler effect as applied to light, including the terms red-shift and blue-shift. E.2.8 • Describe the overall classification system of spectral classes. • Students need to refer only to the principal spectra classes (OBAFGKM).

50. Stellar Spectra Remember: • Electrons only exist in certain energy levels • When excited only produce specific wavelengths. (Emission Spectrum) • When white light passes through same gas these wavelengths are absorbed. (Absorption spectrum)