Astrophysics Part 4 2012


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Astrophysics Part 4 2012

  1. 1. ASTROPHYSICS E4Stellar Radiation & Stellar Types
  2. 2. Types of Stars (review) 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.
  3. 3. Cepheid variables The relationship between a Cepheidvariables luminosity and variability period isquite precise, and has been used as astandard candle (astronomical object that hasa known luminosity) for almost a century. This connection wasdiscovered in 1912 byHenrietta Swan Leavitt.She measured thebrightness of hundredsof Cepheid variablesand discovered adistinct period-luminosity relationship.
  4. 4. Cepheid variablesA three-day period Cepheid has a luminosity of about 800times that of the Sun.A thirty-day period Cepheid is 10,000 times as bright as theSun.The scale has been calibrated using nearby Cepheid stars, forwhich the distance was already known.This high luminosity, and the precision with which theirdistance can be estimated, makes Cepheid stars the idealstandard candle to measure the distance of clusters andexternal galaxies.
  5. 5. Cepheid variables
  6. 6. Distance measured by parallax: Distancemeasurement apparent spectrum by parallax brightness Chemical composition Wien’s Law of corona Luminosity d=1/p (surface L = 4πd2 b temperature T) L = 4πR2 σT4 Stefan-Boltzmann Radius
  7. 7. Distance measured by spectroscopic parallax / Cepheid variables: Apparent Luminosity spectrum Chemical brightness class composition Spectral type Cepheid variable H-R Surface temperature (T) Period diagram Wien’s Law Luminosity (L) Stefan-Boltzmann b = L / 4πd2 L = 4πR2 σT4 Distance (d) Radius
  8. 8. Newton’s Model of the Universe The universe is  infinite in extent,  contains an infinite number of stars,  is static and  exists forever
  9. 9. Obler’s paradox Why is the night sky dark? or Why isnt the night sky as uniformly bright as the surface of the Sun?If the Universe has infinitely many stars, then it should be.
  10. 10. Obler’s paradox If the Universe is eternal andinfinite and if it has an infinitenumber of stars, then the night skyshould be bright. Very distant stars contribute withvery little light to an observer onEarth but there are many of them.So if there is an infinite number ofstars, each one emitting a certainamount of light, the total energyreceived must be infinite, makingthe night sky infinitely bright, whichis not.
  11. 11. Obler’s paradox If we consider the Universe finite and expanding, theradiation received will be small and finite mainly for 2 reasons: There is a finite number of stars and each has a finitelifetime (they don’t radiate forever) and Because of the finite age of the Universe, stars that are faraway have not yet had time for their light to reach us. Also, The Universe is expanding, so distant stars are red-shiftedinto obscurity (contain less energy).
  13. 13. Doppler effect In astronomy, the Doppler effect was originally studied in the visible part of the electromagnetic spectrum. Today, the Doppler shift, as it is also known, applies to electromagnetic waves in all portions of the spectrum. Also, because of the inverserelationship between frequencyand wavelength, we candescribe the Doppler shift interms of wavelength. Radiationis redshifted when itswavelength increases, and isblueshifted when its wavelengthdecreases.
  14. 14. Doppler effect Astronomersuse Doppler shiftsto calculateprecisely how faststars and otherastronomicalobjects movetoward or awayfrom Earth.
  15. 15. Doppler effect Why is Doppler effect so important? In 1920’s Edwin Hubble and Milton Humanson realisedthat the spectra of distant galaxies showed a redshift, whichmeans that they are moving away from Earth. So, if galaxiesare moving away from each other then it they may havebeen much closer together in the past Matter was concentrated in one point and some “explosion” may have thrown the matter apart.
  16. 16. Background radiation In 1960 two physicists, Dicke and Peebles, realising thatthere was more He than it could be produced by stars,proposed that in the beginning of the Universe it was at asufficiently high temperature to produce He by fusion. In this process a great amount of highly energetic radiationwas produced. However, as the Universe expanded andcooled, the energy of that radiation decreased as well(wavelength increased). It was predicted that the actualphotons would have an maximum λ corresponding to a blackbody spectrum of 3K. So, we would be looking for microwave radiation.
  17. 17. Background radiationShortly after thisprediction, Penzias andWilson were working witha microwave aerial andfound that no matter inwhat direction theypointed the aerial itpicked up a steady,continuous backgroundradiation.
  18. 18. Background radiation In every direction, there is a very low energy and veryuniform radiation that we see filling the Universe. This is calledthe 3 Degree Kelvin Background Radiation, or the CosmicBackground Radiation, or the Microwave Background. These names come aboutbecause this radiation isessentially a black body withtemperature slightly lessthan 3 degrees Kelvin(about 2.76 K), which peaksin the microwave portion ofthe spectrum.
  19. 19. Background radiation Why is the background radiation an evidence for the Big Bang? The cosmic background radiation (sometimes called theCBR), is the afterglow of the big bang, cooled to a faintwhisper in the microwave spectrum by the expansion of theUniverse for 15 billion years (which causes the radiationoriginally produced in the big bang to redshift to longerwavelengths).
  20. 20. Big Bang The Big Bang Model is a broadly accepted theory for theorigin and evolution of our universe. It postulates that 12 to 14 billion years ago, the portion ofthe universe we can see today was only a few millimetresacross. It has since expanded from this hot dense state into thevast and much cooler cosmos we currently inhabit. We can see remnants of this hot dense matter as the nowvery cold cosmic microwave background radiation which stillpervades the universe and is visible to microwave detectors asa uniform glow across the entire sky.
  21. 21. Big Bang The singular point at which space, time, matter and energy were created. The Universe has been expanding ever since.Main evidence: Expansion of the Universe – the Universe is expanding(redshift)  it was once smaller  it must have startedexpanding sometime  “explosion” Background radiation  evidence of an hot Universe thatcooled as it expanded He abundance  He produced by stars is little  there isno other explanation for the abundance of He in the Universethan the Big Bang model.
  22. 22. Fate of the Universe Universe Closed Open FlatNot enough matter  Enough matter  Critical density density is not enough to density is such that Universe will onlyallow an infinite gravity is too weak start to contractexpansion  gravity will to stop the after an infinitestop the Universe Universe amount of timeexpansion and cause it expanding foreverto contract (Big Crunch)
  23. 23. Critical densityThe density of the Universe that separates a universe thatwill expand forever (open universe) and one that will re-collapse (closed universe).A universe with a density equal to the critical density iscalled flat and it will expand forever at a slowing rate. So, how do we measure the density of the Universe?
  24. 24. Critical density• If we take in account all the matter (stars) that we can see then the total mass would not be enough to keep the galaxies orbiting about a cluster centre.• So, there must be some matter that can not be seen – dark matter. This dark matter cannot be seen because it is too cold to irradiate.• According to the present theories dark matter consists in MACHO’s and WIMPS
  25. 25. Massive compact halo objects – brown and MACHO’s black dwarfs or similar cold objects and even black holes. Non-barionic weakly interacting massive WIMP’s particles (neutrinos among other particles predicted by physics of elementary particles)It seems that there is also what is called “dark energy”…
  26. 26. TOK Scientists claim our knowledge of the universe is based upon 5% of what is in the universe. Can we claim to know anything about the universe? Are there other ways besides Science to explain the universe? What happens when these alternatives meet? Is one right and the other wrong?