The Ever Expanding Universe Part III: Beyond the Stars
With Bessel’s determination of the distance to a star in the 18th century, astronomers thought all the stars in the sky would soon be measured, but 100 years later only a handful of star distances were discovered. Most stars were too far away to be measured by parallax. In fact the average star in the night sky is 2000 light years away, far too vast a distance to measure even a fraction of a parallax angle! New techniques would have to be found for measuring distance throughout the Milky Way. In 1912 Henrietta Leavitt, working at the Harvard College Observatory discovered a type of star whose light output (luminosity) varied over a regular period that could determine its absolute magnitude. Recall that some stars brighten and dim over time for various reasons. Leavitt’s star, discovered in the constellation Cephus, brightens and dims as gasses expand and contract in the star. Once astronomers know a stars absolute magnitude they can determine its distance because luminosity is directly related to the square of the distance to a star. There are about 700 Cepheid variable type stars in the Milky Way galaxy, the North Star Polaris being the most famous. Cepheidsbecame crucial in determining distance throughout the Milky Way. And Cepheids would have a starring role in making the first measurement to a Galaxy. Meanwhile, as astronomers built larger and larger telescopes throughout the 18th and 19th century, they began observing fuzzy objects that were neither stars nor planets. These “nebulae”, for nebulous, or mysterious, were catalogued by astronomers like William Herschel and Charles Messier who speculated on their origin and distance. Herschel built a massive reflector telescope that required assistants who would lower and raise it with pulleys and levers while Herschel stood on a ladder of scaffolding fighting the chilly night air to make his observations. Messier was looking for comets but will always be remembered for finding the most interesting deep sky targets for amateur astronomers worldwide. But what were these fuzzy objects and were they inside or beyond the Milky Way? Recall how human understanding of the size of the Universe was expanding since the time of the Greeks. And by the 19th century the Universe was a large as the farthest stars measured in our galaxy. But like people today that wonder what is beyond our Universe the question remained was there anything beyond the Milk Way?
In the 20th century Edwin Hubble would answer the “nebulae” question and expand the Universe to unimaginable dimensions. Edwin Hubble returned to the U.S. from college in England landingthedream of a lifetime job working at the largest telescope ever built. Starring through the eyepiece of the 100 inch reflector built atop Mt. Wilson in Los Angeles, Hubble began observing the mysterious “nebulae” in the constellation Andromeda. Working over many nights Hubble discovered a Cepheid in the nebulae and was able to determine its distance as millions of light years away. Hubble’s fuzzy object was the Andromeda galaxy millions of light years beyond the Milky Way! Hubble quickly began observing other galaxies throughout the Universe andby applying Einstein’s new equations about mass, light, and energy, Hubble determined that the further a galaxy is the faster it’s moving away from Earth. In fact the Universe of billions of galaxies is acceleratingoutward in all directions and extends an incredible 13 billion light years at least. Hubble’s discovery of the ever expanding physical Universe perfectly complemented the expansion of the human mind to understand the growing boundaries of our world whose dimensions were fought and won over thousands of yeas of scientific work!
Answer the following questions.
1. Describe a Cepheid variable type star. Why are they important to astronomers? Name one.
2. What did Herschel and Messier catalogue in the 18th and 19th century? Where does the word nebula come from? What was Messier looking for?
3. Where did Edwin Hubble work? What did he discover about the relationship between galaxies and distance? What is happening to the Universe today? How big is it today?