THE SOUTHWORTH PLANETARIUM 70 Falmouth Street Portland, Maine 04103 (207) 780-4249 usm.maine.edu/planet 43.6667° N 70.2667° W Founded January 1970 2022-2023: XII Sunrise: 6:19 a.m. Sunset: 6:53 p.m. Civil twilight ends: 7:22 p.m. Sun's host constellation: Leo the Lion Moon phase: Waning gibbous (81% illuminated) Moonrise: 9:02 p.m. Moonset: 12:08 p.m. (9/15/2022) Julian date: 2459837.16 "I find that a duck's opinion of me is very much influenced over whether or not I have bread." -Mitch Hedberg THE DAILY ASTRONOMER Wednesday, September 14, 2022 About Those Galactic Distances... For the first time this school year, we return to the woefully neglected Pandora's Jar,* the vessel in which we store subscribers' questions: questions that we strive to answer in a timely fashion. And often fail to do so. :-( The first query of the 2022-23 school year was written by subscriber Theo K, who, after having seen and been amazed by the Webb Space Telescope images, wondered how astronomers were able to determine the distances to other galaxies. Brilliant question. The distance to even the closest major galaxy, Andromeda, equals 2.2 million light years. Many galaxies are located billions of light years from us. How on Earth -and, yes, we phrased it that way on purpose- can we humans possibly measure such unfathomably vast distances? To explain the method, we turn our attention to the "Cosmic Distance Ladder." *Cepheid review* First, we need to review the Cepheid variable method that enables astronomers to determine the distances to the closest galaxies. Cepheids are variable stars whose variability is directly related to luminosity. The longer the variability period, the more luminous the star. By observing the amount of time separating successive maxima (times when the brightness is at maximum), an astronomer can measure the Cepheid's true brightness . Through use of the distance modulus equation, one can determine the Cepheid variable star's distance if its intrinsic brightness (absolute magnitude M) and apparent brightness (apparent magnitude m) are both known. [image: download.png] The distance modulus formula: m = apparent magnitude, M = absolute magnitude; d = distance (in parsecs). A Cepheid variable's period is directly related to its absolute magnitude. By comparing the true to apparent brightness, the star's distance is known. By extension, the distance to the star's host galaxy will also be determined. The problem with the Cepheid variable method is distance limitation. The method is valid out to 20 million light years. While a considerable distance, the cosmos is immensely larger. In order to measure the distances of more remote galaxies, astronomers generally rely on a technique related to the Universal expansion. In 1929, Edwin Hubble (1889-1953) announced his discovery that the Universe is expanding in all directions. This expansion is a consequence of the Big Bang, the primordial event that created the cosmos. Ever since its inception around 13.8 billion years ago, the cosmos has been growing steadily larger. Consequently, the galaxies within it appear to move away from each other unless they happen to be so close that gravitational attraction overwhelms the expansion.** Hubble devised a law relating a galaxy's recession speed and distance: * v = H x d* where v = a galaxy's velocity; H = Hubble constant; d = distance in megaparsecs. (One megaparsec equals 3.26 million light years.) The Hubble Constant is measured in units of kilometers per second per Megaparsec. One of the most recent Hubble constant estimates, published by a research team led by a University of Oregon astronomer, cites a value of 71.5 for the Hubble Constant. Any galaxy moving at 71.5 kilometers per second away*** from the Milky Way would therefore be located 1 Megaparsec away from the Milky Way. A galaxy receding from the Milky Way at 143 kilometers per second would be 2 Megaparsecs away from us and so forth. The *Hubble Law *can tell us a galaxy's distance "simply" by measuring the galaxy's speed. How can astronomers measure this speed? By measuring the galaxy's "light shift." A galaxy consists of innumerable stars, all of which produce light. If the galaxy is moving away from us, that light will become elongated as a consequence of the recession. [image: 500px-Redshift_blueshift.png] At the top of this simplified diagram a star moves away from the observer. The star's emitted light is "stretched out" as a result of this motion. The elongation of light increases its wavelength, which decreases its frequency. The light is shifted toward the red end of the spectrum, as red light has a lower frequency than blue light, At the diagram's bottom, the star moves toward the observer and its light is compressed. The wavelength decreases and its frequency increases, shifting the light toward the "blue end" of the spectrum. [image: main-qimg-1882da28ef34be15ceca6073a1cdd664.webp] In reality, the emitted light is separated into its component colors Within the spectrum one finds a series of absorption lines that occupy specific "rest" wavelengths. The lines within a spectrum of a receding galaxy shift toward the red end of the spectrum. The greater the shift, the faster the recession velocity and the more distant the galaxy. Application of the Hubble Law enables astronomers to measure the distances to remote galaxies throughout the Virgo Super Cluster and throughout the Universe. *Now, wait a second! Didn't Pandora possess a box, not a jar? Well, in Hesiod's "Works and Days," he mentions that Pandora opened a jar, which in Greek is the word "pithos." Desiderius Erasmus Roterodamus (1466-1536), was believed to have changed the word to "pxyis," meaning "box" in this 1508 publication Adagia. Consequently, the container encapsulating all the ills of humankind became known as a box, not a jar. **The Andromeda and Milky Way Galaxies are currently moving toward each other and will collide within 4 - 6 billion years. ***One notable consequence of this revised value is that the Universe's age would be lowered to approximately 12.8 billion years To subscribe or unsubscribe from the Daily Astronomer: https://lists.maine.edu/cgi-bin/wa?SUBED1=DAILY-ASTRONOMER&A= <https://lists.maine.edu/cgi-bin/wa?SUBED1=DAILY-ASTRONOMER&A=1>