Today Announcements: HW#8 due Friday 4/9 at 8:00 am. The size of the Universe (It s expanding!) The Big Bang Video on the Big Bang NOTE: I will take several questions on exam 3 and the final from the videos we watch. Sleep at your own risk. How do we measure properties of a star? How do we know what our sun (and other stars) are made of? How do we know the temperature? How do we know the size? From the spectrum of the EM radiation from the star ( Blackbody radiation ) ISP209s10 Lecture 20-1- ISP209s10 Lecture 20-2- Some Clicker Questions - #1 Some Clicker Questions - #1 What happens to a star if its surface temperature is increased and its size remains the same? A) It only gets brighter B) It only gets more red C) It gets brighter and more blue D) It only gets dimmer E) It gets dimmer and more red What happens to a star if its surface temperature is increased and its size remains the same? A) It only gets brighter B) It only gets more red C) It gets brighter and more blue D) It only gets dimmer E) It gets dimmer and more red Hint: recall how color correlates with temperature ISP209s10 Lecture 20-3- ISP209s10 Lecture 20-4-
A Stars Energy Source nuclear fusion The pp-chain in the Sun The sun generates its energy by a set of fusion reactions where 2 nuclei are stuck together and release energy. (the strong force in action) Conditions required for nuclear fusion (two things): High temperature: the central temperature of the sun is 15 million Kelvin. This is necessary to overcome the repulsion between the positively charged protons. High density: the probability of collisions must be high. Note: the Sun is balanced just right. It does not burn too fast or two slowly for us to have a potentially comfortable existence. ISP209s10 Lecture 20-5- time ISP209s10 Lecture 20-6- A star is born 1) Big clouds of gas (nebula) accumulate by chance thanks to gravity 2) Atoms heat up as more and more material falls towards the center under gravity as the gas cloud collapses to a smaller size 3) Eventually, the gas is hot enough that nuclear fusion reactions occur 4) Heat generated by fusion balances any further collapse (thermal pressure) Stars The mass of a star determines most properties of a star: lifetime, color, size, luminosity Massive stars are very bright and hot, but they don t last very long. Stars are a balance between gravity and pressure from the internal heat hydrostatic equilibrium Mass Lifetime By 0.3 M sun 1000 1.0 M sun 10 3.0 M sun 10 M sun 0.35 0.025 60 M sun 0.002 ISP209s10 Lecture 20-7- ISP209s10 Lecture 20-8-
Relative Sizes of Stars Hertzsprung-Russell Diagram:How a star evolves Blue hot Red - cooler ISP209s10 Lecture 20-9- ISP209s10 Lecture 20-10- Evolutionary Path of our Sun: HR Diagram Long parts: Hydrogen 10 By Evolutionary Path of our Sun: time line Helium Helium 1 By Hydrogen White Dwarf many BY Heavier stars have different possible endings: 1) M = 10-30 M sun => neutron stars (only few km across!) 2) M > 30 M sun => star collapses into a black hole ISP209s10 Lecture 20-11- ISP209s10 Lecture 20-12-
Some Clicker Questions - #2 Luminosity is relative to our Sun. The line in the HR diagram indicates the main sequence. What determines if a star lies on the main sequence? A) It is blue B) It is red C) How it produces energy D) Its temperature E) What the surface is made out of ISP209s10 Lecture 20-13- Some Clicker Questions - #2 Luminosity is relative to our Sun. The line in the HR diagram indicates the main sequence. What determines if a star lies on the main sequence? A) It is blue B) It is red C) How it produces energy D) Its temperature E) What the surface is made out of ISP209s10 Lecture 20-14- Some Clicker Questions - #3 Some Clicker Questions - #3 C A Luminosity is relative to our Sun. B D Where do we find White Dwarfs on the HR diagram? Hint 1: are white dwarves hotter or Colder than our Yellow sun? Hint 2: ask yourself Where is our sun Based on the luminosity ISP209s10 Lecture 20-15- C A Luminosity is relative to our Sun. B D Where do we find White Dwarfs on the HR diagram? Point A. (our sun is at point B) White => hotter than our sun Dwarf => smaller than our sun => Small luminosity ISP209s10 Lecture 20-16-
Some Clicker Questions - #4 Some Clicker Questions - #4 C A B D Where would we find our Sun on an HR diagram? Where would we find our Sun on an HR diagram? Point B. Luminosity is relative to our Sun. ISP209s10 Lecture 20-17- Luminosity is relative to our Sun. ISP209s10 Lecture 20-18- C A B D You could have either 1) guessed 2) Memorized the Temperature or 3) Made use that Luminosity is rel. To our sun. How do we measure big distances? Stellar Parallax Alpha Centauri is the next closest star to us. It is 4 light-years away from us. How do we measure such huge distances? Radar nearby things like the Sun Parallax up to several hundred light-years Spectroscopic parallax even bigger distances ISP209s10 Lecture 20-19-!! 1 arcsec = 1/60 degree From the angle!, we can figure out the distance from earth.! of 1 arcsec corresponds to a distance of 1 parsec (pc) = 3.24 ly ISP209s10 Lecture 20-20-
L[Watts] intensity = 2 4! d Spectroscopic Parallax If we know L the luminosity (HR diagram), and measure the intensity, we can determine d, the distance to the source ISP209s10 Lecture 20-21- The Pace of Science Astronomers have studied the sky for thousands of years, but 90% of all astronomical information has been obtained obtained since 1900. For example, in 1900 scientists believed that the universe: Was infinitely old Was infinitely large Contained only one galaxy (the Milky Way) Did not change with time Was uniform throughout (problem with Olber s paradox) All of these are false! ISP209s10 Lecture 20-22- Edwin Hubble In 1922 Edwin Hubble measured the brightness of variable stars in the Andromeda galaxy. The structure of our local set of galaxies You are here (1pc = 3.24 light-year) He discovered that the Andromeda galaxy was about 2.5 million light years away. He was the first person to demonstrate the finite size of the Universe and the the Milky Way is not the only galaxy. ISP209s10 Lecture 20-23- Close-up view Zoomed out view ISP209s10 Lecture 20-24-
Something Else!?! Hubble s Law The further away an object is, the faster it is receding. Speed = H 0 x d The Hubble Constant: H 0 = 77 km/s/mpc ISP209s10 Lecture 20-25- Because he was able to measure distance, Hubble observed that on average all galaxies seem to be moving away from us. The speed is related to distance. Galaxies farther away are moving faster. No matter where you are, everything is moving away! Hubble Expansion ISP209s10 Lecture 20-26- The Universe The Universe is not uniform. The 200 billion galaxies are clustered into large clumps with voids in between Why? Answer: It must have started that way and gravity is slowly pulling things together ISP209s10 Lecture 20-27- How Did the Universe Begin? It looks like the Universe started about 14 billion years ago and has been expanding (space stretching) ever since. The model of what happened is called the Big Bang. There is a lot we don t understand. What came before? What caused the big bang? Why is there more matter than anti-matter in the Universe? ISP209s10 Lecture 20-28-
Evidence for the Big Bang The expansion of the universe: All galaxies appear to be moving away from us. The abundances of the lightest elements produced in the Big Bang: the universe is mostly hydrogen and helium. The cosmic microwave background radiation: It looks like we are in the middle of a big oven with a temperature of ~3 Kelvin. ISP209s10 Lecture 20 It looks like we are in the middle of an oven The spectrum is identical to that of a blackbody at 3 Kelvin! -29- Picture of the Universe at 300,000 years old WMAP observatory The splotches confirm the Universe was non-uniform In the beginning ISP209s10 Lecture 20-30- The History of the Universe What we see as we look away from the Earth We are effectively looking back in time. ISP209s10 Lecture 20 The pattern indicates that the Universe is 13.7 billion years old. -31- ISP209s10 Lecture 20-32-
Big Bang Timeline (the early moments) Inflation of the Universe Electro First nuclei formed Atoms formed Meters The existence of an unknown scalar field caused the rapid inflation of the Universe Years since the Big Bang Space stretched by 10 50 times! ISP209s10 Lecture 20-33- ISP209s10 Lecture 20-34- What about the future? Until very recently (past 5 years) we thought it was possible that the Universe might end in a Big Crunch. This would be the case if the mass of the Universe were large enough to halt the expansion and bring everything back together. In this model the Universe could be a neverending cycle of Big Bangs and Big Crunches. The microwave background measured by WMAP points to an ever expanding Universe. ISP209s10 Lecture 20-35- Our Future (Details) 3E+9 years from now, our Galaxy will collide with the Andromeda Galaxy, either merging Into 1 Super-Galaxy or ripping both apart. In 5E+9 years our sun fries Earth when it turns into a Red Giant In 1E+12 years the stellar era ends (Stars run out of nuclear fuel). In 1E+100 years, only remnant of stars remaining are black holes Eventually, the black holes will evaporate due to quantum Effects. All that remains in the universe is electrons, neutrinos, Photons. ISP209s10 Lecture 20-36-
Why does time always move in one direction? Inflation during the Big Bang resulted in a universe that had a very low entropy, much too low for its size. It was like the Universe started with all heads. Hence, everything in the universe moves toward reaching the correct amount of entropy. Time has a direction because going back in time would imply the entropy could be decreased. That is very improbable. The universe tends toward increasing entropy. What is time? ISP209s10 Lecture 20-37-