Up until the invention of the telescope in 1608, humanity could only see bright objects in the sky, such as the Moon, planets, and stars. The telescope revealed many faint objects too dim to see with the naked eye. This included objects larger than stars or planets, which fell under the general category of nebulae.
Considerable disagreement existed in the early 20th century about the nature of nebulae. Certain astronomers theorized that these were star clusters within the Milky Way Galaxy. Others believed that some of these nebulae were actual ‘island universes,’ that is, massive collections of stars like the Milky Way galaxy. Hence, they must be far beyond the Milky Way. The barrier to resolution of this controversy was the inability to measure distance to these objects.
At this point, we must mention Henrietta Swan Leavitt, a photographic plate analyst at the Harvard Observatory. She studied variable stars in the Small Magellanic Cloud. Among the hundreds of variable stars she catalogued, she observed that some variable stars were brighter if they had a longer period. Since these stars, all in the Magellanic Cloud, were at similar distances, she noted that the luminosity (brightness) is related ONLY to period. Leavitt published papers in 1908 and 1912 confirming the relationship between luminosity and period of certain variable stars.
Research by others resulted in these types of variables being called Cepheid variables after Delta Cephei. The distance to Cepheids identified within the Milky Way was determined using the parallax method. Cepheids could now be used as standard candles to estimate distance. If you know the period, which can be observed, then you can calculate the brightness and, hence, the distance.
On the night of October 5, 1923, using the 100-inch Hooker telescope on Mt. Wilson, Edwin Hubble captured a photographic plate image of M31, the Andromeda “Nebula”. After comparing this plate to earlier images of M31, he realized that one of the stars was a Cepheid variable. He marked the plate directly with ‘VAR!” This plate still exists. See the link in the References of this script on KVNF.
Using Henrietta Leavitt’s relationship, Hubble calculated the distance to the Andromeda Nebula at 900,000 light years. While this is significantly less than the presently accepted value, it clearly established that the Andromeda Nebula was actually a galaxy far, far away. One hundred years ago next month, the Universe got bigger by orders of magnitude!
You have been listening to “Western Slope Skies”, produced by members of the Black Canyon Astronomical Society and KVNF Community radio. I’m Bryan Cashion.
REFERENCES
Carnegie Observatory image of Hubble’s Var! star in Andromeda galaxy
https://hubblesite.org/contents/news-releases/2011/news-2011-15.html
The Period-Luminosity Relationship: A Historical Review
