The Distance Ladder
Learning Objectives! What is the distance ladder? How do we measure distances to objects in our Galaxy and to other galaxies?! How are the concepts of absolute magnitude and apparent magnitude key to measuring distances?! What are the main techniques to measure distances? To what approximate distance can each technique be used?! What are Hubble s Law and Constant? How were they discovered? Can we use them to measure distances?! Are any distant galaxies blueshifted? Does the expansion of the Universe mean that we are at the center of the Universe? Are people expanding (and why or why not)?! How is the Universe structured? Are galaxies isolated?
Measuring Distances! To study the structure of the Universe we need to know the distance of different objects! Basic idea (other than gold standard parallax):! Determine the intrinsic brightness of an object! Absolute magnitude! Compare to the observed brightness in our sky! Apparent magnitude! Use the difference between apparent and absolute magnitude to determine the distance! No method is good for all distances! Different techniques have been developed for different distances
The Distance Ladder! Methods of finding distances in the Universe build on other methods! We step farther into the Universe with each method Parallax (from Gaia) as far as 8 kpc distance
Spectroscopic Distances! For nearby stars, we find distances by parallax! For more distant stars if we can determine a star s type (i.e. O, B, A F, G, K, M), we can use the H-R diagram to find its absolute magnitude! This is easy with star clusters, since we can match to the Main Sequence! This technique is called spectroscopic distance
Cepheid Variable Stars! Pulsating supergiant stars! They re bright visible out to about 30 million parsecs (100 million light years!)! Pulsation period is directly related to intrinsic luminosity (i.e. to absolute magnitude) Measure pulsation period from light curve
White Dwarf Supernovae! Because white dwarf (Type Ia) supernovae happen at a similar mass (about the Chandrasekhar limit), the explosions are always similar in energy! Absolute brightnesses of these supernovae should be the same! Problem: We have to wait for them to happen. Fortunately, they re fairly common! This method currently gets us out in distance to over 3 billion parsecs (10 billion light years) White dwarf Supernova
Galaxy Spectra! Since the light from galaxies comes mostly from stars, galaxy spectra look like a lot of spectra of stars added together
Redshifted Galaxies! In 1928, Hubble observed that the spectral lines of essentially all galaxies are redshifted! They appear to be receding (moving away) from us!! Exceptions are the 1 or 2 very closest galaxies (e.g. the Andromeda galaxy is blueshifted)! Redshift (z) in terms of observed (λ obs ) and emitted (λ em ) wavelength (where λ em is the wavelength we d measure on Earth for that line): z = (λ obs -λ em )/λ em! At low redshifts, the resulting Doppler velocity is just a fraction of the speed of light (c), i.e. v = cz
Spectrum shifts redder and redder blueshift redshift
Most Important Astronomical Discovery of the 20th Century Plot from Edwin Hubble s original 1929 paper which first described the expansion of the Universe
! Hubble noticed a correlation between redshift and distance! The farther a galaxy is from us, the faster it moves away from us! This can be described by the equation v=h o d Hubble s Law! Where v is velocity in km/s, d is distance in millions of parsecs (Mpc), and H o is the Hubble constant! Current best estimate of H o is about 70 km/s/mpc
What Does This Mean?! More distant objects appear to move away from us faster! From any galaxy s point of view, every other galaxy appears to be moving away from it! THE UNIVERSE IS EXPANDING!!!
What Does This NOT Mean?! We are NOT in a special location (because from any galaxy s point of view, every other galaxy appears to move away)! Note that planets, stars individual galaxies, cats that love teddy bears are NOT being pulled apart. Over small distances, the expansion of the Universe does NOT overcome the forces that bind atoms
Redshift and Distance! The Hubble Law gives us a new way of finding distances! Remember,! v=cz! v=h o d! Therefore, d=cz/h o! We can now measure distances to extremely distant galaxies just by taking a spectrum and using it to measure a galaxy s redshift
The Local Group
The Virgo Cluster! Over 1,000 galaxies! 50 million light-years away from our Milky Way galaxy! About 13 million lightyears across! Dominated by three giant elliptical galaxies
Structure of the Universe! Clusters of galaxies are grouped together in superclusters! Superclusters occur in filaments and sheets! Large voids of nothing between them
Next Time The Dawn of Time