ASTR 1514 1st Edition Lecture 35The Expansion of SpaceChapter 14.5Cosmology- The study of the Universe as a whole- Understanding the structure of the universeDistance Ladder- Concept that lies together different methods to measure distance 1.) Radar Ranging- Smaller distances (within the solar system) can be measured using radar- Radar ranging – aim a pulse of light at a planetary body and measure how long for thereflection to return, since light always travels at the same velocity, the distance can bedetermined. “Standard Candle” - Object that always has the same luminosity - So if luminosity is known and the brightness is measured then the distance can be determined. 2.) Spectroscopic Parallax- A star on the main sequence has a unique temperature and luminosity - Temperature can be measured from the spectrum - If brightness and temperature can be measured then the distance can be determined. Cepheid Variable Stars- Stars that vary in size and brightness during a certain part of their evolution, when they move through the instability strip.- Brightness will vary periodically - The variability is useful because a relationship between the period of the variability and the luminosity. - So if you can measure the period, and measure the brightness, then you can determinethe distance. - Largest distances measured using Cepheid variable stars is 29 million parsecs = 94 million light years. Type 1a Supernova- The explosion of a white dwarf star that has become larger than 1.4 solar masses- All White Dwarfs explode when they reach the same mass, so the luminosity produced is the same. - Type 1 a supernovae are a standard candle. - These are extremely luminous so they can be seen from large distancesThe Properties of the Universe: Hubble Law- Before modern cosmology people believed that the universe was 1. Static (unchanging in time) 2. Infinite and 3. Eternal (no beginning or end). Obler’s Paradox- If the universe is static, infinite, and eternal, and uniformly filled with stars, the sky should be as bright as a star all the time- The sky is not bright all of the time, this means that the universe is not static infinite or eternal. The Universe is Expanding- Therefore it is not static- We measure speeds with Doppler shifts: all galaxies except the nearest have a redshift: observed wavelength is greater than the rest wavelength. - Redshift spectral lines = movement away - We define a number “z” for the redshift- The larger the z is, the faster the galaxy appears to be moving away. Hubble’s Law- We observe that the velocity at which a galaxy is moving away from us is proportional to the distance to that galaxy - The slope line is called the Hubble constant, it is measured to be 72 km/s/Mpc. - So two galaxies 1 Mpc apart will be moving apart at 72 km/s. - The observation that nearby galaxies move away slowly, and distant galaxies move away faster is most easily explained if the Universe is expanding uniformly. - All galaxies are NOT moving away from us, so we are NOT the center of the universe.Look-back Time- The speed of light is constant- We get all our information about distant galaxies from light- When we look at a distant galaxy, we are not looking at it as it is now, but as it was when the light left that galaxy and started traveling toward us. - So distant galaxies are also younger. - Distant galaxies have a large ‘look-back time’ - Galaxies will be farther apart in the future- Galaxies were closer together in the past- Hubble Time: When separation between galaxies was