Today we celebrate the birth of Edwin P. Hubble. Born on this day in 1889, he would have been 126 years of age today. He would become one of the 20th century’s greatest astronomers, one who played a crucial role in our understanding of the universe and our place in it. His discovery of the expanding universe would change and revolutionize long-held ideas that the universe is a static, predictable Clockwork Universe to one that is a limitless, expanding, ever-receding void of space and time. That it is expanding, with galaxies receding from each other, being carried along on the fabric of space and time towards an ever receding horizon, would shake our fundamental notions of the universe and our place in it to their core. Hubble would discover this and go on to help usher in the modern age of telescopic astronomy, setting in motion a century of discovery that continues to this day.
1908: Henrietta Swan Leavitt, after observing variations in the brightness of certain variable stars, known as Cepheid Variables, notes that the star’s intrinsic brightness is linked to that variation. The greater the variation, the more intrinsically luminous the star is. Working at Harvard-College Observatory, she makes extensive studies of Cepheid Variables in the Small Magellanic Cloud, a satellite galaxy of our Milky Way. She, thus, formally codifies the “Period-Luminosity” relation, a first key step in building the model that would become Hubble’s Expanding Universe.
1912: As the second key step in what would become Hubble’s Expanding universe, Vesto M. Slipher, conducts Doppler studies on what was referred to then as the “Spiral Nebulae”, as their true nature was not yet known. Slipher measures their radial velocities relative to us. A notable example of such a galaxy was the great Spiral Galaxy, M-31, in Andromeda.
1919: Edwin Hubble is appointed to the Mt. Wilson Observatory by Carnegie Astronomer, founder and visionary, George Ellery Hale. Using the 2.5 meter Mt. Wilson reflector, Hubble studies the “Spiral Nebulae”, observing Cepheid Variables within them, most notably in M-31, the great galaxy in Andromeda. Not realizing that each of these objects is a separate galaxy containing hundreds of billions of stars, that they are the largest, gravitationally bound objects in the universe, Hubble and his colleagues are astounded to learn their true nature, dimensions and distance.
1929: putting together the work of Leavitt and Slipher, along with Milton Humason, Hubble formally codifies and publishes his Distance/ Velocity Law. Known as the Hubble Constant, it relates distance with recessional velocity: the further away an object is, the faster it is receding. Not only is this observation consistent with a uniformly expanding space, it is consistent with Einstein’s General Theory of Relativity.
Over the decades since its publication, Hubble’s Distance Velocity Law, otherwise known as the Hubble Constant, has been refined from it’s original value of 300-500 km/sec/M-parsec to 68 km/sec/M-parsec (1 “Mega Parsec” = 1 million parsecs; a parsec is the distance an object has to be such that its parallax shift is equal to 1 arc-second or 1/3,600 degrees). That is to say, for every million parsecs of distance, the observer and the observed galaxy are receding from each other at 68 km/sec.
Launched in 1990 aboard the Space Shuttle Discovery, the Hubble Space Telescope, named in Hubble’s honor, celebrated 25 years on orbit, April 24th of this year. It has pushed back the frontiers of space and time, revealing things not heretofore seen or imagined. The Hubble Ultra-Deep Field has allowed astronomers to observe galaxies whose light left them when the universe was only 500,000,000 years old, less than 4% of its current age of 13.7 billion years. This image required 841 orbits of the telescope and reveals approximately 10,000 galaxies! It is the deepest, most complete, most spectrally comprehensive image ever taken of the sky. Pointed at a seemingly blank expanse 2.4 arc-minutes wide or approximately one tenth of the angular diameter of a full moon as viewed from the Earth, this tiny patch of sky is located in the southern-hemisphere constellation of Fornax. It is a composite of Ultraviolet, Visible and Near Infrared images, all obtained with the telescope between September 24, 2003, through to January 16, 2004. In August and September 2009 additional exposures were made to add near infrared images using the newly installed WFC3 (Wide Field Camera 3). The newly enhanced image has been aptly named the eXtreme Deep Field.
Imagination is more important than knowledge
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