Stars

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This is an introduction to stars, including the basics of observing and classifying stars as well as their evolution and life cycle. This is a modification of a presentation I found online.

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Stars

  1. 1. Stars Anna Quider Institute of AstronomyUniversity of Cambridge 13 July 2009
  2. 2. Overview• Introduction to stars – What they are – What we can measure• The Hertzsprung-Russell Diagram• Star life cycles – Evolution of stars across the HR diagram
  3. 3. It contains about 1011 stars, plusgas and dust between the stars(the intersellar medium)
  4. 4. The Sun and all the stars we can see atnight are part of the Milky Way galaxy It contains about 1011 stars, plus gas and dust between the stars (the intersellar medium)
  5. 5. The Sun and all the stars we can see atnight are part of the Milky Way galaxyThe Galaxy is basically disk shapedwith a spheroidal bulge at the centre It contains about 1011 stars, plus gas and dust between the stars (the intersellar medium)
  6. 6. You are here!
  7. 7. A very ordinary star!
  8. 8. What is a Star?• In lay terms, a star is a big ball of burning gas• More technically, a star is a body which satisfies two conditions:• It is bound by self-gravity – Spherical due to the symmetric nature of gravity• It radiates energy from an internal source – Nuclear energy released by fusion reactions in its interior
  9. 9. What is a Star?• In lay terms, a star is a big ball of burning gas• More technically, a star is a body which satisfies two conditions:• It is bound by self-gravity – Spherical due to the symmetric nature of gravity• It radiates energy from an internal source – Nuclear energy released by fusion reactions in its interior
  10. 10. Violation of this condition What is a Star? leads to the death of the star in that it will explode violently, scattering its• In lay terms, a star is a big ball of burning far and constituent matter gas wide• More technically, a star is a body which satisfies two conditions:• It is bound by self-gravity – Spherical due to the symmetric nature of gravity• It radiates energy from an internal source – Nuclear energy released by fusion reactions in its interior
  11. 11. What is a Star?• In lay terms, a star is a big ball of burning gas• More technically, a star is a body which satisfies two conditions:• It is bound by self-gravity – Spherical due to the symmetric nature of gravity• It radiates energy from an internal source – Nuclear energy released by fusion reactions in its interior
  12. 12. What is a Star?• In lay terms, a star is a big ball of burning gas• More technically, a star is a body which satisfies two conditions:• It is bound by self-gravity – Spherical due to the symmetric nature of gravity• It radiates energy from an internal source – Nuclear energy released by fusion reactions in its interior Violation of this condition (i.e. if the fuel source runs out) also leads to the death of the star in that it will simply fade away
  13. 13. !""#$%&"%()*+,-./%)01%2.-31$14 !""#$%&"%()*+,-./%)01%2.-31$1 Observing Stars""#$%&"%()*+,-./%)01%2.-31$14 ! 567/1$%%%%%%%%%%%%%%%%%%%% (819)7 !"#$% " &((%)*+#+(*+,+(-$.*+#((+#*+/)% • " The information which we can gather 0%*+1-)*23%4+,+"#/5+67%)*4+#((+#*+/)% from an individual star is quite restricted 567/1$%%%%%%%%%%%%%%%%%%%% (819)7 " 84%+,-(*%34+*+-4(#*%+1#3*-)2(#3+9#:%(%/$*.+3#/$%4 " ;%*#-(%<+(=+#*+4*32)*23%+,+#/+67%)*+#/<+%/:-3/"%/* Image Spectrum
  14. 14. Observing Stars• ‘Apparent Brightness’ - amount of radiation falling per unit time per unit area (of detector)• This is the ‘radiation flux’, F. This is not an intrinsic property of the star since it depends on its distance from us
  15. 15. Stellar Luminosity• We measure the star’s flux, but the intrinsic property is the Luminosity• This is the amount of power radiated per unit time• Related to the measurable flux by: Inverse Square Law
  16. 16. Inverse Square LawApplies to radiation, gravitational and electric fields
  17. 17. Think of a star as a torch... 1. The apparent brightness of the torch changes with distance from your eyes2. Torches have different intrinsic strengths
  18. 18. Measuring Distances: Parallax• Observe ‘nearby’ star at the extremes of the earth’s orbit• Measure the difference in its apparent position relative to ‘distant’ background stars• Use trigonometry to deduce the distance of the nearby star
  19. 19. Using the Inverse Square Law• Measure distance to star (using parallax), measure flux  luminosity – Star’s power output in Watts• Find other similar stars, assume luminosities are the same  distances – Use particular types of star as ‘standard candles’ for determining distances to e.g. stellar clusters
  20. 20. 01%2.-31$14 Stellar Emission • Stars are emitting light. We can study this (819)7 light by putting it through a spectrograph (prism) and splitting the light into its component wavelengths.
  21. 21. Stellar Emission• Stars emit their radiation thermally (rather than via atomic transitions)• In physics, a ‘black body’ is an object that absorbs all radiation that falls upon it, thus appearing black in colour• Practically, black bodies also radiate (in order to retain their thermal equilibrium)• Stars are approximately black body emitters
  22. 22. Blackbody Temperatures • This plot is the intensity of the radiation vs wavelength • The peak intensity shifts to longer wavelengths as temperature decreases • We can use this to derive stellar temperatures
  23. 23. Finding temperatures from real observations • We could use a spectrometer to measure a star’s spectrum • Flux vs wavelength • From the shape, we can determine its temperature • Which stars are the hottest here? Which are the coldest?
  24. 24. But...• It takes a lot of time to get a spectrum so is there a way to determine a star’s peak wavelength (and therefore it’s temperature) without taking a spectrum?
  25. 25. Filters and colours• Alternatively, we can compare the star’s flux in two different wavebands – range of wavelengths, eg• This can be done more easily than taking a spectrum of the star• The bands are defined by standard filters – U (ultra-violet): 300-400nm – B (blue): 400-500nm – V (visual): 500-600nm
  26. 26. Filters No sharp cut-off
  27. 27. The Sun in Different Filters
  28. 28. Colour Indices• Compare the ratio of the star’s flux in two filters, e.g. B and V, to find its ‘colour’• Blackbody peak shifts to shorter wavelength as temperature increases• See more flux in the B (short λ) filter relative to the V filter (longer λ) for a hot star• This means that we can deduce temperatures from these measurements – F(B)/F(V) large for hot stars – F(B)/F(V) small for cooler stars
  29. 29. Spectral Classification• The Harvard classification system was developed in the 1890s by Annie Jump Cannon• Still in use today• The classes are based on features in the stars’ spectra….• ….but actually it’s more useful to order the stars by their temperature or colour
  30. 30. Absorption Lines
  31. 31. Absorption Lines•The cooler outer regions of a star absorb photons fromthe hotter inner regions•Different elements absorb light at different frequencies•Atoms in different states absorb different frequencies
  32. 32. Titanium Oxide ‘Metals’Helium lines
  33. 33. Stellar Classes• The spectral classes, ordered according to temperature: – O: > 25,000K – B: 11,000 - 25000K – A: 7,500 - 11,000K  Sirius – F: 6,000 - 7,500 – G: 5,000 - 6,000K  The Sun! – K: 3,500 - 5,000K – M: < 3,500K  Betelguese
  34. 34. Very red (cool) Very blue (hot)
  35. 35. Luminosity vs Temperature• We have just seen how ‘colour’ (derived from flux) reflects temperature• There is also a correlation between luminosity (the intrinsic property) and temperature• If we plot the luminosities and temperatures of a large, representative sample of stars, we produce a ‘Hertzsprung-Russell’ diagram• Stars of the same type all lie in the same area of the HR diagram
  36. 36. 80% of stars lie across this diagonal. This is the ‘Main sequence’
  37. 37. 32
  38. 38. HR Diagram• About 80% of stars lie on a diagonal line across the plot – Main sequence – These are ‘dwarf’ stars• Giants lie above the main sequence – Sub-types populate separate areas• White dwarfs lie below the main sequence• This is the general case. Now let’s look as some specifics.
  39. 39. Open clusters • Found in disk of galaxy • E.g. the Pleiades • Contain 10 - 1000 stars • HR diagrams may contain less red giants • Predominantly young stars
  40. 40. Pleiades - HR diagram Few giants Predominantly main sequence stars
  41. 41. Globular Clusters • Found well away from galactic plane, in ‘halo’ of galaxy • E.g. M80 • Contain 105 - 106 stars • Blue end of main sequence not present • Many more red giants • Older stellar population
  42. 42. HR diagram for M80 Many giantsNo blue mainsequence
  43. 43. HR Diagram Summary• The HR diagram is a plot of luminosity vs temperature for a population of stars• Stars of different types lie in different places on the HR diagram• 80% of stars lie on the Main Sequence• HR diagrams will look different for different stellar populations• Stars ‘evolve’ and move around the HR diagram. To understand this we need to study the life cycle of a star.
  44. 44. HR Diagram SummaryIn practice we could: is a plot of luminosity vs • The HR diagram temperature for a population of stars★classify a star from its spectrum, thus estimating itstemperature different types lie in different places • Stars of on the HR diagram★use the HR diagram to find its luminosity• 80% of stars lie on the Main Sequence★compare its luminosity with its measured flux to• HR diagrams will look different for differentderive its distance from us stellar populationsOr, Stars ‘evolve’ and move around the HR• for a star cluster at known distance: diagram. To understand this we need to★Plot the luminosities and temperatures on an HRdiagram the life cycle of a star. study★Deduce the cluster type, i.e. open or globular
  45. 45. Evolution of Stars
  46. 46. Star Formation• In between the stars in a galaxy, there is a lot of gas which we call the interstellar medium (ISM)• The gas exists in clouds – Small clouds support themselves against gravity using their internal pressure – Large clouds (with masses greater than typical stellar masses) have gravity which exceeds the internal pressure, so are unstable and collapse• Clouds fragment, forming multiple stars and hence star clusters
  47. 47. Star Formation Regions Young starsIonised gas Rosette Nebula
  48. 48. Protostars• The initial ‘free-fall’ phase of collapse is dominated by gravity• Gas still cool, radiates in the infra-red• As collapse progresses, internal pressure builds up, process slows• Star starts to heat up, makes transition to ‘pre-main sequence’
  49. 49. Main Sequence: Processes• For stars with masses at least 0.08 Msun• Central temperature reaches 107K, stars start burning Hydrogen (fusion) in their cores• Net effect: four protons turn into Helium nucleus from p-p chain:• This releases significant amounts of energy• The energy is transported to the star’s surface by radiation (light) or convection
  50. 50. Main Sequence: Timescales• This process of turning Hydrogen into Helium is the energy source for main sequence stars• It takes around 1010 years for a star to deplete the Hydrogen in its core• The star then moves off the main sequence• Massive stars evolve off the main sequence more quickly
  51. 51. Layers in the Sun
  52. 52. The Proton-Proton Chain
  53. 53. The CNO Cycle• Main core reaction in stars greater than 1.5Msun
  54. 54. Aside: Smaller ‘Stars’• Stars with masses less than 0.08 Msun never become hot enough to burn hydrogen• Smaller stars continue contracting, forming ‘brown dwarfs’ which are essentially failed stars• Jupiter is about 80 times less massive than a typical brown dwarf
  55. 55. Post Main Sequence• Hydrogen burning ceases and the core contracts, thus heating the star again• Helium now fusing in the core. Outside the core, a Hydrogen-burning shell forms• Star is now larger and cooler, but more luminous than before - Red Giant• When the Helium runs out, core collapses again, Carbon burning starts• This continues for all elements up to Iron• Evolution on HR diagram depends on mass
  56. 56. Relative Sizes
  57. 57. Shells of Fusion
  58. 58. Shells of FusionNo elements heavier than Iron (Fe) can be created in this way
  59. 59. Star Death• Earlier, we defined stars as bodies which fulfill two criteria: – They are bound by self-gravity – They have an internal fuel source• Violation of either results in star death• The actual endpoint of a star is governed by its mass
  60. 60. Massive Stars• A massive star (10-60Msun) will complete all stages of fusion shown on the ‘shell’ diagram• The Iron core rapidly loses energy and contracts again, forming an extremely dense neutron star• This leaves the envelope (mainly Hydrogen and Helium) unsupported so it collapses• The rapid heating leads to a thermonuclear explosion - a Supernova• Supernovae produce the elements heavier than iron
  61. 61. Supernovae• Supernovae are extremely luminous, with fluxes similar to those of entire galaxies• Most are seen in external galaxies (e.g SN1987a in the Large Magellanic Cloud)• We expect 1 SN every 30 years in our galaxy, but most are obscured by interstellar dust• They leave behind a neutron star (which may be a pulsar), plus a remnant shell• These remnants may be observed for centuries afterwards
  62. 62. Supernovae
  63. 63. Neutron Stars and Pulsars• Neutron stars are tiny, but very dense – E.g. radius ~10km, mass 1.5Msun!• Hard to detect unless they are pulsars• Discovered in 1967 by Jocelyn Bell. Her PhD supervisor won the Nobel prize….• Pulsars radiate beams from their magnetic poles (radio and optical)• These may sweep across the direction to the Earth as the star rotates• Incredibly accurate ‘clocks’
  64. 64. Supernova RemnantPulsar! Crab nebula
  65. 65. Supernova Remnant Crab nebula in x-rays
  66. 66. Supernova Remnant Crab nebula
  67. 67. Lower Mass Stars• Lower mass stars, such as the Sun, only form elements up to Helium via fusion• They undergo periods of instability while they evolve as giants• Eventually, pulsations in the star blow off the surface layers, revealing the hotter interior• The material which is blown off forms a ‘Planetary Nebula’• The central star, made mostly of Carbon, cools and contracts to become a White Dwarf• They have high temperature but low luminosity
  68. 68. Planetary Nebula
  69. 69. More Planetary Nebulae
  70. 70. Summary of Star Lifecycles• The formation, evolution and death of stars is a cyclical process• Starts off with big cloud of gas• Cloud collapses under gravity until it becomes hot enough to burn and shine• When the fuel runs out the star dies• Massive stars end in supernova explosions which returns material to the interstellar medium• This is recycled into new stars!
  71. 71. THE ENDAny questions?

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