Last week was the 101st anniversary of the Great Debate. In terms of scale, it truly was the great debate of recent history — it was about the size of the universe! It was also about the nature of so called “spiral nebulae” and whether our Milky Way Galaxy was the only galaxy or one of many, referred to as “island universes.”
In honor of this special anniversary, Unistellar Citizen Astronomers are invited to participate in this week’s quest to observe deep-sky objects that were key parts of the Great Debate!
Quick facts about the Great Debate:
- Also known as the Shapley–Curtis Debate: it was between astronomers Harlow Shapley and Heber Curtis
- Held at the Smithsonian Museum of Natural History on April 26, 1920
- Harlow Shapley contended that our Milky Way Galaxy was actually the entire universe. He believed that “spiral nebulae” were clouds of gas, part of the Milky Way.
- Heber Curtis argued that the spiral nebulae were in fact “island universes” or distinct, distant galaxies.
- Today, we know that Curtis was correct about the main topic of the debate, though both astronomers gave correct and incorrect assertions.
eVscope image captured by Unistellar Citizen Astronomer Gerd Waloszek, from Germany
Pinwheel Galaxy
In 1916, Shapley’s colleague and astronomer Adriaan van Maanen claimed that he observed the Pinwheel Galaxy (M101) rotate around its center like a wheel. Shapley used van Maanen’s observation as part of his argument that spiral nebulae are part of our galaxy. If M101 was a separate galaxy that rotated on a timescale of years, it would have to rotate faster than the speed of light! It was later proven that van Maanen’s observation was incorrect. You can’t see M101 rotate on the timescale of a human lifespan — it takes hundreds of millions of years for a galaxy to rotate!
Today, we know that M101 is a distinct galaxy, home to about 1 trillion stars!
Quick Facts and Observing Tips:
- Distance from Earth: 21 million light years
- Constellation: Ursa Major
- Search for “M101” or “Pinwheel Galaxy” in the Explore tab of the Unistellar app.
- Depending on the sky quality at your location, you may want to leave the Enhanced Vision mode on for at least about 10 minutes
- Recommended Bortle Class is 5 or lower.
eVscope image mosaic of the Andromeda Galaxy and its galactic companions captured by Unistellar Citizen Astronomer Romain Girard, from Switzerland
Andromeda Galaxy
During the debate, the Andromeda Galaxy (M31) was referred to as the “Nebula of Andromeda.” Curtis argued that M31 was an island universe, mentioning its dark lanes which resembled the dust clouds in our own galaxy. In 1925, astronomer Edwin Hubble settled the debate when he measured the distance to M31, which was conclusive evidence that it was not within our own galaxy, but was a separate galaxy.
M31 is the closest major galaxy to us and is also a barred spiral galaxy like our Milky Way. The bar structure in the center of these galaxies is found in about half of all the spiral galaxies in our universe!
Quick Facts and Observing Tips:
- Distance from Earth: 2.5 million light years
- Constellation: Andromeda
- Search for “M31” or “Andromeda Galaxy” in the Explore tab of the Unistellar app.
- This will be a challenging object to observe since it will be visible early in the morning
- Depending on the sky quality at your location, you may want to leave the Enhanced Vision mode on for at least 5 minutes
- Recommended Bortle Class is 6 or lower.
eVscope image captured by Unistellar Citizen Astronomers Nicole Ruel and Jacques Bérard, from Québec
Bode’s Galaxy
Bode’s Galaxy (M81) was originally known as “Bode’s Nebula” since it was discovered in 1774, well over a century before the Great Debate. During the debate, Shapley also used van Maanen’s observation of M81’s rotation as part of his argument for spiral nebulae being part of our own galaxy.
We now know that M81 is a spiral galaxy about half the size of our Milky Way and contains over 250 billion stars!
Quick Facts and Observing Tips:
- Distance from Earth: 12 million light years
- Constellation: Ursa Major
- Search for “M81” or “Bode’s Galaxy” in the Explore tab of the Unistellar app.
- Depending on the sky quality at your location, you may want to leave the Enhanced Vision mode on for at least about 5 minutes
- Recommended Bortle Class is 6 or lower.
eVscope image captured by Unistellar Citizen Astronomer David Hein, from Australia
Tarantula Nebula
The Magellanic Clouds were mentioned in the debate and were thought to be our closest galactic neighbors until 1994. The Small Magellanic Cloud (SMC) and the Large Magellanic Cloud (LMC) are irregular dwarf galaxies that orbit our Milky Way.
The Tarantula Nebula is located in the LMC and is a giant stellar nursery. If the Tarantula Nebula was located at the same distance away from Earth as the Orion Nebula, it would cast visible shadows on Earth!
Quick Facts and Observing Tips:
- Distance from Earth: 160 thousand light years
- Constellation: Dorado
- The Tarantula Nebula is currently visible in the Southern Hemisphere.
- Search for “Tarantula Nebula” or “NGC 2070” in the Explore tab of the Unistellar app.
- Depending on the sky quality at your location, you may want to leave the Enhanced Vision mode on for at least a minute
- Recommended Bortle Class is 6 or lower.
eVscope image captured by Unistellar Citizen Astronomer 後藤干城 Tateki Goto, from Osaka, Japan
Hercules Globular Cluster
During the debate, Shapley used his knowledge of globular clusters to argue that our Sun was not in the galactic center based on their distribution and distance to us. Shapley specifically mentioned the Hercules Globular Cluster (M13) many times, including its distance to us (which was determined to be 36,000 light years at that point) as well as the nature of its stars.
M13 is a densely packed group of several hundred thousand stars that all formed together at the same time. The stars within M13 are so densely packed together that sometimes they collide and form new stars, called blue stragglers!
Quick Facts and Observing Tips:
- Distance from Earth: 25,000 light years
- Constellation: Hercules
- Search for “M13” or “Hercules Globular Cluster” in the Explore tab of the Unistellar app.
- Depending on the sky quality at your location, you may want to leave the Enhanced Vision mode on for at least a few minutes.
- Recommended Bortle Class is 7 or lower.
We encourage you to share your observations and join the conversation through our Facebook, Instagram and Twitter pages using the hashtag #UnistellarChallenge!
If you’d like to send us your observations by email, send them to community@unistellaroptics.com.
Clear skies! 🔭
