Seeing The Universe

1. Seeing The Universe

2. Hans Lipperhey Dutch spectacle maker Born 1570 ? - Died 1616 On October 2nd 1608 applied for a patent on his new device for “looking into the distance” Patent refused since other opticians claimed that they had also constructed such devices Spectacles invented in the 14th century Benjamin Martin - 1760

3. Galileo Galilei (1564 – 1642) The Starry Message Published March 1610 Cor… what a big face First systematic study of the heavens with a telescope

4. Reflecting telescope: Invented by Newton light ‘captured’ by a curved mirror Radio telescope ‘light’ captured by curved metal ‘mirror’ Refracting telescope: light ‘captured’ by a curved glass lens. Design by Kepler

5. Telescopes: our ‘eyes’ on the Universe Definition a telescope is a device for collecting and bringing to a detector (eye/camera/..) electromagnetic (EM) radiation – e.g., light EM source Telescope Detector Flux data on EM source (STAR) Astronomer

6. Tripping the light fantastic Astronomers learn about the Universe by “reading” the many “stories” embedded in starlight To understand the message, however, we must first understand the language of the messenger Don’t do it Gort… Klaatu barada nikto

7. James Clerk Maxwell (1831 - 1879) First mathematics paper read before the Royal Society of Edinburgh when he was 16 years old Proved rings of Saturn had to be made of small particles Developed electromagnetic theory of light (1865)

8. An electromagnetic wave is a disturbance propagated as a variation in the local electric and magnetic fields at the speed of light No need to draw this diagram

9. Definition: A wave is a traveling disturbance Any form of change or disturbance that propagates (travels) from one region to another can be thought of as a wave Examples Sound waves Ocean waves Earthquakes Traffic lights The lecturer - a MWF wave

10. Frequency (f) = # wave crests passing per second Defining a wave Wavelength (l) Speed Wavelength (l) speed = wavelength (l) x frequency (f)

11. For all EM waves: speed = speed of light = c = 3 x 108 m/s c = l f (units: wavelength in meters, frequency in Hertz)  OOTETK There is no practical use for these radio waves Heinrich Hertz: discovered radio waves in 1888

12. Chapter and Verse What we have so far: Starlight  How stars tell us about themselves The messenger = electromagnetic (EM) radiation The means of collecting the message = telescope What we want to do now is learn how to ‘read’ the content of the message in detail To begin with let us see how starlight can tell us about the speeds of stars and the motion of planets

13. The Doppler Effect Christian Doppler (1803 - 1853) (1842) noted an apparent change in the observed wavelength of a signal (sound wave) as a result of motion either towards or away from an observer

14. The Doppler Effect Applies to all wave-like phenomena e.g., EM waves, and sound waves Everyday example =Sound of siren from emergency response vehicle • Pitch increases as vehicle approaches and decreases as it moves away – the change is an apparent change - not a change in the siren’s actual tone See 21st Century pages 134 - 135

15. No relative motion case 1 Wavelength 2 3 Both observers record same wavelength Wave “peaks” move outward at speed c

16. Motion towards left 1 2 3 1 3 2 Records longer wavelength Records shorter wavelength

17. So, the key idea for astronomy is source (star, galaxy,…) emits EM radiation at some fixed, but known wavelength l observer measures, however, a wavelength lobs Doppler effect “says” lobs < l if motion of source is towards observer lobs > l if motion of source is away from observer Our next step is to link measurements to numbers and to introduce Doppler’s formula

18. Doppler’s formula: light version If an object emits EM radiation at wavelength l, then the apparent change in wavelength (lobs - l), where lobs is the observed wavelength is, (lobs - l) / l = V/c where V is the relative velocity directly in the line of sight (the radial velocity), c = speed of light.

19. Notes: by sign convention -V means motion is towards observer, and +V if motion is away For EM radiation (e.g., light rays) Motion towards  shorter wavelength observed (blue shifted) Motionaway  longer wavelength observed (red shifted)

20. With respect to light waves, the Doppler effect provides a ‘cosmic speedometer’ Applications include: Planetary rotation Stellar motion Binary stars Galactic rotation Motion of galaxies Expansion of the universe – Hubble’s Law Next topic Lab More in Astronomy 202

21. The Doppler Effect (lobs - l) / l = V/c observed actual Relative velocity Apparent change in wavelength due to the relative motion of the source Planetary rotation Velocity towards: lobs – l < 0 (blueshift) Velocity away : lobs – l > 0 (redshift)

22. Planetary rotation If a planet has distinctive markings - time transits. Works OK for Mars, Jupiter and Saturn Repeat time for distinctive feature tobe in same position = spin period of planet

23. Problem planets: Mercury: small, close to Sun, no distinctive features Venus: Dense atmosphere  no surface visible (in optical) • Solution for Mercury and Venus: Use radar and exploit the Doppler effect Radar image of Venus  UV image of Venus showing clouds in upper atmosphere. Rotation rate of clouds is about 4 days - but is this the rotation rate of the planet?

24. Planetary rotation by radar - view from above Limb moving away from observer VRot Reflected signal is red shifted lobs R Radar l lobs rotation Reflected signal is blue shifted VRot Limb moving towards observer

25. Procedure: we know l (part of radar design) and c measure red shift (or blue shift): (lobs- l) use Doppler equation to find VRot VRot = c (lobs - l) / l with VRot and a measure of the planets radius R, we can find the rotation period P: P = 2p R / VRot  Velocity = Distance / Time formula Circumference of planet

26. Rotation of Mercury Radar of wavelength l = 0.5 m (lobs - l) measured to be 5.0 x 10-9 m Doppler equation gives V(Mercury) = c (lobs - l) / l V(Mercury) = 3 m/s Period of rotation = 2 p (2440 x 1000) / 3 = 5.11 x 106 seconds = 58.7 days Radius of mercury in km Km  meters

27. Radar observations reveal Pspin(Mercury) = 58.7 days = 2/3 Porbit(Mercury) Conclusion: Mercury spins three times on its axis for every two orbits about the Sun The radar results were a complete surprise- astronomers expected to see synchronous rotation for Mercury: Pspin = Porbit [as in the case of our Moon] 87.97 days

28. Venus - unveiled (by radar) Radar obs. reveal: Pspin(Venus) = 243 days Big surprise (again): spin motion is retrograde i.e., spins east to west (see ASTRO, p. 244) Conclusion: Venus is a very slow rotator, and it spins in the opposite sense to its orbital motion Direction of orbital motion Venus Atmosphere rotates 60 times faster than planet (ESA Venus Express image shows a double polar vortex) Direction of spin

29. Fundamental Sun Result The Sun’s disk shows a Doppler variation - This indicates that it must be rotating At the Sun’s limb: (l-lobs) / l = 6.656 x 10-6 = V/c which indicates a rotation velocity of 2 km/s Red shift Motion away Blue shift Motion towards No DS on center line – since velocity is at right angles to LOS Also see Sun’s rotation through sunspot motion. Period of rotation varies from equator (25 days) to poles (35 days)

30. Stellar spectra Stars radiate most of their EM radiation at UV, visual and IR wavelengths They also show distinctive absorption lines due to specific atoms and in some cases molecules in their outer, cooler layers – the absorption lines fall at very specific wavelengths Absorption lines can be used to study stellar motion by measuring the Doppler shift of absorption lines Later on we will set up a star classification scheme based upon spectral lines Absorption lines Visual wavelengths of light

31. 1 orbit Binary Stars Periodic (Doppler) shift of stellar absorption lines

32. The Hubble Flow Observations: Distances from size variations “Big Result” formula: Small angular size  far away Large angular size  close Velocities from Doppler shift Edwin Hubble and Milton Humason (1920s) studied distant galaxies Milton Humason (1891 – 1972)

33. Small angular size  must be far away Large angular size  must be close Humason and Hubble Looked at elliptical galaxies

34. Assume all elliptical galaxies are the same physical size and use angular size and the “Big Result” formula to get distance (Dgalaxy) Doppler shift of absorption line features in the light from the galaxy provides the line of sight velocity (Vgalaxy)

35. Galaxy data: V(km/s) vs D(Mpc) reveals an incredible result – a straight line graph Velocity (km/s) Slope = H = Hubble’s constant Distance (Mpc)

36. The Universe is Expanding Hubble’s law (1929) There is a systematic increase in the velocity of recession of galaxies with increasing distance Vgalaxy = H x Dgalaxy Where H = Hubble’s constant = 72 km/s/Mpc Age of the Universe ~ 1 / H ~ 13 billion years  OOTETK

37. The Universe is Expanding Uniformly • Hubble’s law tells us that • Every region of space is expanding at the same rate – that is uniformly • Galaxies are “dragged along” with the expansion of space rather than expanding into space • The Universe must be of a finite age • The Universe had an origin – the Big Bang

38. The rest of the starlight story Starlight  How stars tell us about themselves The messenger = electromagnetic (EM) radiation The means of collecting the message = telescope What the telescope detects is the energy from the star and what is specifically measured is the energy per square meter per second getting to the telescope’s detector (eye / camera / …..)

39. The electromagnetic spectrum longest wavelength - radio waves shortest wavelength - gamma (g) rays and between these we have…… g-rays, X-rays, UV, visible, IR, microwaves, radio waves increasing wavelength

40. An important point about waves they transmit information in the form of energy Definition: Energy - a measure of the ability to do work - units are Joules (J) Energy has many forms: - energy of motion - chemical energy - heat energy - solar energy - nuclear energy Work…. Work harder James Prestcott Joule (1818 -1889)

41. Radiant Energy The amount of energy E carried by an electromagnetic wave is related to its frequency: E = h f where h = Planck’s constant = 6.626 x 10-34 JS  OOTETK So, the greater the frequency the greater the amount of energy carried by the wave Hence: g-rays carry more energy than radio waves

42. So, now that we’re energized In astronomical applications we are often interested in energy transfer For example: the temperature of a planet is determined by the amount of EM energy it receives from the Sun Hence: We would like to know exactly how much EM energy the Sun radiates into space

43. Recap: what we have so far Light = periodic variation (wave) in the electric and magnetic field that travels at c = 3x108 m/s EM waves characterized by l (and f), but l f = c for all EM waves Energy - a measure of the ability to do work - units are Joules (J) EM waves carry energy E = h f Telescopes = starlight energy (flux) detectors

44. Terminology: Luminosity (L) = total amount of EM energy radiated at all wavelengths into space per second: units = Joules / sec. = Watts James Watt (1736 - 1819) Flux (F) = energy received per square meter per second: units = Watts/m2

45. The problem We want to determine the total energy output (the luminosity L) of a star, but it can’t be measured directly Why? - two minutes - talk to your neighbours Energy radiated in all directions – would need to build a detector around the entire star – would also need a “universal detector” sensitive to all wavelengths of electromagnetic radiation The solution Measure the flux (the energy received at a detector per second per square meter) and find a relationship between flux and luminosity

46. Take a source of EM radiation e.g., a star electromagnetic energy is radiated into space equally in all directions at a distance d from the star the energy radiated per second at the star will be spread out equally over the entire surface of a sphere of radius d d Star of luminosity L Sphere of radius d L = radiative energy at source (luminosity) Energy passing through surface per m2 per sec = FLUX (and this is what we measure)

47. A really useful result (honest): Energy flux a distance d from EM source of luminosity L: where 4p d2 is the surface area of a sphere of radius d ( see formula handout sheet) Trust me – I’m a doctor  OOTETK Measured

48. A fundamental result Question: what is the Sun’s luminosity? Idea for answer: measure Sun’s energy flux at the Earth’s orbit Symbol means Sun related value Procedure: determine F = Sun’s energy flux at Earth’s orbit + we know the Earth is 1 AU from the Sun theory: F = L / 4p (1.495 x 1011)2

49. The solar constant The energy per square meter per second received from the Sun at the top of Earth’s atmosphere is: F = 1370 W/m2 = Solar constant Determined by looking at the Sun’s energy spectrum (diagram of energy flux versus wavelength) at the top of Earth’s atmosphere Solar constant = area under graph Flux wavelength UV Visible IR Radio

50. The picture: 1.495x1011 m Sphere of radius 1 AU about Sun Sun 1 AU • Observations: F = 1370 W/m2 • Hence fundamental result 1370 = L / 4p (1.495 x 1011)2 L= 3.85 x 1026 Watts Earth’s orbit (where we measure the flux)