The Great Debate

The Great Debate in Astronomy was an argument in 1920 between Harlow Shapley and Heber Curtis over the size of the Milky Way, and in turn, the size of the Universe.


Harlow Shapley believed that all the visible universe was contained in the Milky Way which he thought was 120,000 parsecs in diameter, with the Sun 2/3 of the way from the center to the edge of the Milky Way. To him, all nebulae seen in the sky, were in the disk of the Milky Way and there was nothing beyond the Milky Way.


Heber Curtis (who was director at the Allegheny Observatory) thought that nebulae and stars could exist beyond the Milky Way and that the Universe encompassed more galaxies than just the Milky Way.


Edwin Hubble gave the proof of the size of the Universe being larger than the Milky Way when he found a Cepheid variable star in the Andromeda galaxy. By observing the star, and comparing its brightness to known Cepheids within the Milky Way, he was able to determine that the Andromeda galaxy was about 778,000 parsecs away, much larger a distance than even Shapley thought the Milky Way was.


We now know that the Milky Way is just one of billions of galaxies in the visible universe and is only about 35,000 parsecs in diameter.

The Challenger Disaster

In January of 1986, I was 11 years old and had dreams of becoming an astronaut. I loved Star Trek and Star Wars, but would have preferred to be on the Enterprise, rather than the Millennium Falcon. I was waiting for my chance to go into space to be able to see the beauty of the Universe without the filter of Earth's atmosphere.
On January 28, 1986, the Space Shuttle Challenger was ready to launch into orbit with the first civilian, Christa McAuliffe, on board. Her job was to be the first teacher in space and to teach children around the world while orbiting the Earth. Along with McAuliffe were the commander of the mission Dick Scobee, the pilot Michael Smith, Mission Specialists; Ronald McNair, Elison Onizuka, and Judith Resnick, and the other Payload Specialist Greg Jarvis.
It was unusually cold for Florida that day, with temperatures below freezing. The cold weather caused the O-rings in the solid rocket boosters to warp and fail upon throttling the shuttle up. 73 seconds after launch, the Challenger and her crew were lost.
I was in sixth grade and in the middle of my music class at St. Peter's Cathedral in Kansas City, KS. We were singing so loud that we missed the announcement on the PA system. The PE teacher had to let our class know what happened. I will never forget that day.
In the aftermath of the Challenger Disaster, NASA and the US government created the Rogers Commission (named after the chairman, William P. Rogers) to determine what happened. On the commission were former astronauts, Neil Armstrong and Sally Ride, and famous physicist, Richard Feynman. It was determine that engineers thought that the O-rings would fail based on the temperature at launch, but were pressured to let the launch continue. In a demonstration to the commission, Richard Feynman showed how the O-rings would fail if exposed to frigid temperature, and would lose their elasticity to complete the seal on the booster rockets that would prevent liquid oxygen and liquid hydrogen from escaping. The seals failed, and caused a massive explosion seen the world over.
29 years later, I still remember everything about that day. However, it never diminished my desire to be an astronaut and go into outer space.

Top Row: Onizuka, McAuliffe, Jarvis, Resnick

Bottom Row; Smith, Scobee, McNair

 

RIP Challenger Crew - You will always be among the stars.

The Size of the Milky Way

For centuries, everything we thought about the universe was contained in a tiny package that centered on the Earth, and then the Sun after the heliocentric model was accepted. We only knew about what we could see and assumed that since it was observable, it had to be relatively close. If only we knew.

But first, we should see how the evolution of determining the size of the Milky Way galaxy itself took place.

The first actual attempt to determine the size of the Milky Way galaxy was done by William Herschel. He did a simple thing: he just decided to count all the stars he could see, and assuming that the Sun was the center of the galaxy (galactocentricism - yes, there is a word for it), was able to predict, correctly, that our galaxy was a disk. Of course, he was wrong about where the Sun is located with respect to the disk of the Milky Way.

The shape of our Galaxy as deduced from star counts by William Herschel in 1785; the solar system was assumed near center. (NOTE: The image shown is flipped 180 degrees on the horizontal axis from the original, as first published in the Philosophical Transactions of the Royal Society in 1785; the bifurcated arms of the illustration should be on the left.)

"Herschel-Galaxy" by Caroline Herschel. Licensed under Public Domain via Wikimedia Commons

It wasn't until the 1900s that a more extensive survey of the Milky Way took place to determine the size of the Milky Way. Jacobus Kapteyn (cap-tine) deduced that the Milky Way was similar to Herschel's drawing in that the Milky Way is disk-like, heliocentric, and about 20,000 parsecs in diameter.

Kapteyn's Model of a Heliocentric galaxy

Harlow Shapley immediately published a rebuttal stating that the Sun not at the center, but about 2/3 of the way out into the disk and the Milky Way was about 120,000 parsecs in size. Shapley used the distribution of globular clusters and found them more concentrated around the center of the disk of the Milky Way, instead of near where the Sun is located.

Shapley's Model to Determine the Size of the Milky Way

Who was correct? Actually, both were correct, but also, both were wrong. Kapteyn was around the correct size, but Shapley had the Sun in the correct relative location. This actually led to a debate between Harlow Shapley and Heber Curtis, referred to by astronomers as the Great Debate (or the Shapley-Curtis debate).

History Of the Observation of Galaxies

Galaxies were really only discovered recently in the history of astronomy. Up until the 1500s, the universe was thought to only compose a small sphere centered on the Earth, with all the stars on a fixed background at the outer edge of the sphere. With the dawning of the age of telescopes, things began (forgive the pun) to be seen more clearly.


Staring with Galileo, when he turned his telescope to the fuzzy patch of light that crossed the entire night sky, he noticed that the patch actually composed thousands of stars. He was able to conclude correctly that the Milky Way was actually part a large band of stars. The significance of his observation was not really known at the time.


Immanuel Kant was a German philosopher who also studied astronomy. He deduced that the Solar System was formed by the collapse of a rotating cloud (nebula). He also correctly concluded, using the knowledge that the Milky Way was actually a band of stars, that the Milky Way was a disk of stars. He also theorized that there could be many of the disk of stars which he called island universes.


The name "galaxy" comes from a Greek word, galaxias, which means "milky one", which is derived from the name of our galaxy, the Milky Way. Ancient astronomers though that the band of the Milky Way looked like milk that had been spilt across the sky, and the Greeks believed that it came from the breast of a sleeping Hera as she was nursing Heracles. When she woke up to discovered Heracles there, she pushed the baby away and what we see is the breast milk as it was sprayed into the night sky.


The first true catalogue of galaxies was not developed until the 1700s when Charles Messier started cataloguing objects in the night sky that were not stars or planets as he was searching for comets. In his catalogue, he discovered open and globular clusters, nebulae, and galaxies that were visible from the northern hemisphere (Messier was a French astronomer), and named them according to their order of discovery by him. A famous example of a galaxy that Messier catalogued was M31, which is commonly known as the Andromeda galaxy.


William Herschel and his son John also catalogued nebulae, clusters, and galaxies in the General Catalog (renamed New General Catalogue after updated by John Louis Emil Dreyer). The Andromeda Galaxy is NGC 224, so some objects in both the Messier Catalogue and the NGC overlap. The NGC does contain many more southern hemisphere objects than the Messier Catalogue.

Edwin Hubble took things even further when he began observing galaxies at Mount Wilson. He was provided significant evidence that these fuzzy patches of light in the Messier Catalogue and NGC were actually galaxies outside the Milky Way. Vesto Slipher was actually the first astronomer to provide evidence for extragalactic galaxies.

Hubble was able to differentiate between elliptical, spiral, and irregular galaxies, and in fact, thought that there might be an evolutionary sequence between the three types. He proposed something call the tuning fork diagram which shows what he believed to show the sequence. However, we now know that the sequence is not evolutionary, but is a good image to see the differences in galactic types.

Another thing Hubble used galaxies for was to determine the age of the Universe. He noticed that galaxies farther from the Milky Way seemed to be moving faster (recessional velocity). He knew the distances based on a standard candle, in this case Cepheid variable stars, and by using the Doppler shift in the spectrum, was able to formulate a simple equation to determine the age of the Universe:

v=H*d

• v is the recessional velocity in km/s
• d is the distance in Mpc = megaparsecs (a million parsecs)
• H is the Hubble constant in (km/s)/Mpc which today is approximately 67.80

The inverse of H gives the age of the Universe and converting everything, the age of the universe is approximately 13.8 billion years (give or take a couple of billion). This is also called the Hubble Time.