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Astrophysics Part 3 2012
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  • 1. ASTROPHYSICS E3Stellar Types & Stellar Distances
  • 2. Types of Stars Red Giants Very large, cool stars with a reddish appearance. All main sequence stars evolve into a red giant. In red giants there are nuclear reactions involving the fusion of helium into heavier elements.
  • 3. Red Giants The fuel is expended much faster than in stars like our sun. Within a red giant is a core still increasing in temperature. When the temperature rises to 100 million degrees Kelvin helium fusion takes place.
  • 4. Red Giants There are now two layers of energy production;  the hydrogen burning shell,  the helium-burning core. This process eventually yields a carbon and oxygen core,that may eventually produce an iron core,in the most massive stars.
  • 5. Red Giants The fusion process stops with iron;  Iron represents the most stable form, in which protons and neutrons can exist. Once the Iron core is formed energy production comes to an end. The pressure forcing the star to expand no longer is present, gravity takes over.
  • 6. Red Giants Within seconds, iron core collapses with such a force,not even the space within the orbital structure of the atom is preserved. The layers within the iron core fall into the centre,at different rates,an outward shock wave is produced.
  • 7. Red Giants This shock wave is capable of driving off most of the mass of the star. For a star of size 10 solar masses;  85% of the mass is lost,  the star goes supernova.
  • 8. Types of Stars White dwarfs A red giant at the end stage of its evolution will throw off mass and leave behind a very small size (the size of the Earth), very dense star in which no nuclear reactions take place. It is very A comparison between the hot but its small size gives it a white dwarf IK Pegasi B very small luminosity. (center), its A-class companion IK Pegasi A (left) As white dwarfs have mass and the Sun (right). This comparable to the Suns and white dwarf has a surface temperature of 35,500 K. their volume is comparable to the Earths, they are very dense.
  • 9. Types of Stars Neutron stars A neutron star is formed from the collapsed remnant of a massive star (usually supergiant stars – very big red stars). Models predict that neutron stars consist mostly of neutrons, hence the name. Such stars are very hot. A neutron star is one of the few The first direct observation possible conclusions of stellar of a neutron star in visible evolution. light. The neutron star being RX J185635-3754.
  • 10. Types of Stars Supernovae A supernova is a stellar explosion that creates an extremely luminous object. The explosion expels much or all of a stars material at a velocity of up to a tenth the speed of light, driving a shock wave into the surrounding interstellar Crab Nebula medium. This shock wave sweeps up an expanding shell of gas and dust called a supernova remnant.
  • 11. Types of Stars Supernovae A supernova causes a burst of radiation that may briefly outshine its entire host galaxy before fading from view over several weeks or months. During this short interval, a supernova can radiate as much energy as the Supernova remnant N 63A lies within a clumpy region of gas Sun would emit over 10 and dust in the Large Magellanic billion years. Cloud. NASA image.
  • 12. Types of Stars Pulsars Pulsars are highly magnetized rotating neutron stars which emit a beam of detectable electromagnetic radiation in the form of radio waves. Periods of rotation vary from a few Schematic view of a pulsar. The milliseconds to seconds. sphere in the middle represents the neutron star, the curves indicate the magnetic field lines and the protruding cones represent the emission beams.
  • 13. Types of Stars Black Holes A black hole is a region of space in which the gravitational field is so powerful that nothing can escape after having fallen past the event horizon. The name comes from the fact that even electromagnetic radiation is unable to escape, rendering the interior invisible. However, black holes can be detected if they interact with matter outside the event horizon, for example by drawing in gas from an orbiting star. The gas spirals inward, heating up to very high temperatures and emitting large amounts of radiation in the process.
  • 14. Types of Stars Cepheid variables Cepheid variables are stars of variable luminosity. The luminosity increases sharply and falls of gently with a well-defined period. The period is related to the absolute luminosity of the star and so can be used to estimate the distance to the star. A Cepheid is usually a giant yellow star, pulsing regularly by expanding and contracting, resulting in a regular oscillation of its luminosity. The luminosity of Cepheid stars range from 103 to 104 times that of the Sun. Cepheid Variable Sim Real Data used by Astronomers
  • 15. Types of Stars Binary stars A binary star is a stellar system consisting of two stars orbiting around their centre of mass. For each star, the other is its companion star. A large percentage of stars are part of systems with at least two stars. Binary star systems are very important in Hubble image of astrophysics, because observing their the Sirius binary system, in which mutual orbits allows their mass to be Sirius B can be determined. The masses of many single clearly stars can then be determined by distinguished extrapolations made from the observation (lower left). of binaries.
  • 16. Binary starsThere are three types of binary stars Visual binaries – these appear as two separate stars when viewed through a telescope and consist of two stars orbiting about common centre. The common rotation period is given by the formula: 4 2d 3 T2 G( M 1 M 2 ) where d is the distance between the stars. Because the rotation period can be measured directly, the sum of the masses can be determined as well as the individual masses. This is useful as it allows us to determine the mass of singles stars just by knowing their luminosities.
  • 17. Binary stars Eclipsing binaries – some binaries are two far to be resolved visually as two separate stars (at big distances two stars may seem to be one). But if the plane of the orbit of the two stars is suitably oriented relative to that of the Earth, the light of one of the stars in the binary may be blocked by the other, resulting in an eclipse of the star, which may be total or partial Eclipsing Binary Simulation
  • 18. Binary stars Spectroscopic binaries – this system is detected by analysing the light from one or both of its members and observing that there is a periodic Doppler shifting of the lines in the spectrum.
  • 19. Binary starsA blue shift is expected as the starapproaches the Earth and a red shift as itmoves away from the Earth in its orbitaround its companion.If λ0 is the wavelength of a spectral lineand λ the wavelength received on earth,the shift, z, is defined as: 0 z 0 If the speed of the source is small compared with the speed of light, it can be shown that: v z The speed is proportional to the shift c
  • 20. TOK Stars are a long way away. How can we claim we know anything about them?
  • 21. H-R diagram  The stars are not randomly distributed on the diagram.  There are 3 features that emerge from the H-R diagram:  Most stars fall on a strip extending diagonally across the diagram from top left to bottom right. This is called the MAIN SEQUENCE.  Some large stars, reddish in colour occupy the top right – these are red giants (large, cool stars).  The bottom left is a region of small stars known as white dwarfs (small and hot)
  • 22. H-R diagram (by Richard Powell) 22 000 stars are plotted from the Hipparcos catalog and 1000 from the Gliese catalog of nearby stars. An examination of the diagram shows that stars tend to fall only into certain regions on the diagram. The most predominant is the diagonal, going from the upper-left (hot and bright) to the lower-right (cooler and less bright), called the main sequence. In the lower-left is where white dwarfs are found, and above the main sequence are the subgiants, giants and supergiants. The Sun is found on the main sequence at luminosity 1 and temperature 5780K (spectral type G2).
  • 23. Astronomical distancesThe SI unit for length, the metre, is a very small unitto measure astronomical distances. There units usuallyused is astronomy:The Astronomical Unit (AU) – this is the average distancebetween the Earth and the Sun. This unit is more used withinthe Solar System.1 AU = 150 000 000 km or 1 AU = 1.5x1011m
  • 24. Astronomical distancesThe light year (ly) – this is the distance travelled by thelight in one year. c = 3x108 m/s t = 1 year = 365.25 x 24 x 60 x 60= 3.16 x 107 s Speed =Distance / Time Distance = Speed x Time = 3x108 x 3.16 x 107 = 9.46 x 1015 m 1 ly = 9.46x1015 m
  • 25. Astronomical distancesThe parsec (pc) – this is thedistance at which 1 AU subtends anangle of 1 arcsecond. “Parsec” is short for parallax arcsecond 1 pc = 3.086x1016 m or 1 pc = 3.26 ly
  • 26. 1 parsec = 3.086 X 1016 metres  Nearest Star 1.3 pc (206 000 times further than the Earth is from the Sun)
  • 27. ParallaxBjork’s Eyes Where star/ball Space appears relative to background Angle star/ball appears to shift Distance to star/ball “Baseline”
  • 28. Parallax Parallax, more accuratelymotion parallax, is the change ofangular position of twoobservations of a single objectrelative to each other as seen byan observer, caused by themotion of the observer. Simply put, it is the apparentshift of an object against thebackground that is caused by achange in the observers position. Parallax Applet
  • 29. ParallaxWe know how big the Earth’s orbit is, we measure the shift(parallax), and then we get the distance… Parallax - p (Angle) Distance to Star - d Baseline – R (Earth’s orbit)
  • 30. Parallax R (Baseline) tan p (Parallax) d (Distance)For very small angles tan p ≈ p R p dIn conventional units it means that 1.5 x 1011 1 pc m 3.086x 1016 m 2 1 360 3600
  • 31. Parallax 1.5 x 10111 pc m 3.986x 1016 m 2 1 360 3600 R R p d d p 1 d (parsec) p ( arcsecond)
  • 32. Angular sizes 360 degrees (360o) in a circle 60 arcminutes (60’) in a degree 60 arcseconds (60”) in an arcminute
  • 33. Parallax has its limits The farther away an object gets, the smaller its shift.Eventually, the shiftis too small to see.
  • 34. Questions The parallax angle for Barnards star from the Earth is 0.545 arc secs. What is its distances in ly, parsecs and AU The parallax angle for 61 Cygni star from the Earth is 0.333 arc secs. What is its distances in parsecs and AU
  • 35. Solutions Using d = 1/p to find parsec 1.83 pc, 376000AU, 5.96ly 3pc, 9.78ly
  • 36. Another thing we can figure out about stars is their colors… We’ve figured out brightness, but stars don’t put out an equal amount of all light… …some put out more blue light, while others put out more red light!
  • 37. Usually, what we know is howbright the star looks to us hereon Earth… We call this its Apparent Magnitude “What you see is what you get…”
  • 38. The Magnitude Scale  Magnitudes are a way of assigning a number to a star so we know how bright it is  Similar to how the Richter scale assigns a number to the strength of an earthquakeBetelgeuse and Rigel, stars in Orion withapparent magnitudes This is the “8.9” 0.3 and 0.9 earthquake off of Sumatra
  • 39. The historical magnitude scale… Greeks ordered the Magnitude Description stars in the sky 1st The 20 brightest from brightest to stars faintest… 2nd stars less bright than the 20 brightest 3rd and so on... …so brighter stars 4th getting dimmer have smaller each time magnitudes. 5th and more in each group, until 6th the dimmest stars (depending on your eyesight)
  • 40. Later, astronomers quantified this system. Because stars have such a wide range in brightness, magnitudes are on a “log scale” Every one magnitude corresponds to a factor of 2.5 change in brightness Every 5 magnitudes is a factor of 100 change in brightness (because (2.5)5 = 2.5 x 2.5 x 2.5 x 2.5 x 2.5 = 100)
  • 41. Brighter = Smaller magnitudesFainter = Bigger magnitudes  Magnitudes can even be negative for really bright stuff! Object Apparent Magnitude The Sun -26.8 Full Moon -12.6 Venus (at brightest) -4.4 Sirius (brightest star) -1.5 Faintest naked eye stars 6 to 7 Faintest star visible from ~25 Earth telescopes
  • 42. However: knowing how bright a star looks doesn’t really tell us anything about the star itself! We’d really like to know things that are intrinsic properties of the star such as:Luminosity (energy output)andTemperature
  • 43. In order to get from howbright something looks… to how much energy it’s putting out… …we need to know its distance!
  • 44. The whole point of knowing thedistance using the parallax method isto figure out luminosity… Once we have both brightness and distance, It is often helpful to put we can do that! luminosity on the magnitude scale… Absolute Magnitude: The magnitude an object would have if we put it 10 parsecs away from Earth
  • 45. Absolute Magnitude (M) removes the effect of distance and puts stars on a common scale  The Sun is -26.5 in apparent magnitude, but would be 4.4 if we moved it far away  Aldebaran is farther than 10pc, so it’s absolute magnitude is brighter than its apparent magnitudeRemember magnitude scale is “backwards”
  • 46. Absolute Magnitude (M)Knowing the apparent magnitude (m) and thedistance in pc (d) of a star its absolute magnitude (M)can be found using the following equation: m M 5 log d 5Example: Find the absolute magnitude of the Sun. The apparent magnitude is -26.7 The distance of the Sun from the Earth is 1 AU = 4.9x10-6 pc Therefore, M= -26.7 – 5log (4.9x10-6) + 5 = = +4.8
  • 47.  What is the absolute magnitude of Sirius which is 2.7 parsecs away and has an apparent magnitude of -1.46 m M 5 log d 5 M = -1.46 – 5 log2.7 +5 = 1.4
  • 48. So we have three ways oftalking about brightness: Apparent Magnitude - How bright a star looks from Earth Luminosity - How much energy a star puts out per second Absolute Magnitude - How bright a star would look if it was 10 parsecs away
  • 49. Spectroscopic parallax Spectroscopic parallax is an astronomical method for measuring the distances to stars. Despite its name, it does not rely on the apparent change in the position of the star. This technique can be applied to any main sequence star for which a spectrum can be recorded.
  • 50. Spectroscopic parallaxThe Luminosity of a star can be found using anabsorption spectrum.Using its spectrum a star can be placed in a spectralclass.Also the star’s surface temperature can determinedfrom its spectrum (Wien’s law)Using the H-R diagram and knowing bothtemperature and spectral class of the star, itsluminosity can be found.