On this night a century ago, the Universe got much bigger.
For millennia, humanity has wondered at the size of the Universe, and we have continually refined our estimates of distances to planets, stars, and other celestial bodies. Honestly, the Universe didn’t get bigger 100 years ago, but our understanding of its true scale grew significantly—and we got our first glimpse into the enormity of the cosmos.
On the night of October 5, 1923, the astronomer Edwin Hubble captured an image of what was then called the “Andromeda Nebula,” also known as “M31” by astronomy nerds who use the Messier catalog for referencing objects. This image led to a new estimate of the distance to M31, confirming that it lay outside our own Milky Way galaxy and at a vastly greater distance than previously imagined.
Actually, this is worth an historical aside… Back in the 18th century, Charles Messier catalogued numerous fuzzy objects (for lack of a better technical term) in the night sky. Motivated by a quest to find comets in our solar system, Messier decided to create a catalog of, well, non-comets. The aforementioned fuzzy blobs could easily be mistaken for comets in telescopes of the time, so Messier gathered a helpful catalog of 110 such objects worth ignoring in the pursuit of the fame and fortune of comet discovery.
But the nature of these blurry and diffuse astronomical targets remained elusive. Some appeared to be clouds of gas with stars embedded, which would likely make them part of our Milky Way Galaxy. Others, with more refined observations, seemed to be dense clusters of stars. But other so-called spiral nebula were something different altogether. At least, that’s what many astronomers speculated—they looked highly structured and appeared to be traveling away from us surprisingly fast—and the Andromeda Nebula was one of these spirals.
Indeed, some astronomers suspected that these “spiral nebulae” were in fact galaxies like our own—conglomerations of hundreds of billions of stars like the Milky Way in which we reside. But how could they prove that?
That night a century ago, Hubble was using the Hooker 100-inch telescope at the Mount Wilson Observatory in Southern California. He was making images on photographic plates—large pieces of glass coated with photographic emulsion, which could collect light gathered by the telescope’s 100-inch mirror for an extended period of time. Keeping the Hooker trained on M31 for 45 full minutes allowed Hubble to capture its faint light and to create this image.
Take a close look at this particular plate. It’s labeled with an identification number, “H335H,” which appears backward in the upper right corner of the image (because it’s written on the other side of the glass). The first “H” refers to the Hooker telescope, followed by the plate number and the terminal “H” for Hubble’s surname. You can see the date written near the middle of the plate, and you might also notice a few stars labeled “N.” Those are new stars, or “novae,” that Hubble picked out in the image.
But in the upper right, one “N” is crossed out, and “VAR!” written (with some excitement, one imagines) next to it. That’s Hubble realizing that, based on earlier images of M31, that point of light was actually a variable star—specifically, a Cepheid variable, which grows brighter and dimmer over a period of time that depends on its intrinsic brightness. Thus, if you measure the period of such a star (the time it takes to go from brightest to dimmest and back again), you know how much light it’s putting out—and you can calculate how far away it is!
The Cepheid in M31 is very very faint, and thus very very far away. This discovery quelled any doubts that M31 was distant—and huge! A collection of stars just like our Milky Way, a separate galaxy from our own. The Andromeda Galaxy. (A mere nebula no more!)
However iconic this image might be (at least for astronomers), it’s easy to mythologize an artifact such as this. Hubble acquired this photographic plate after many, many nights of observing at Mount Wilson; it’s special but not singular. He identified the star as a variable because he pored over image after image after image of M31. And he based his distance estimates on the painstaking work of Henrietta Swan Leavitt and other astronomers who established the period-luminosity relationship of Cepheids.
We can celebrate this milestone of a century ago as one step along a very long path… Which continues to this day.
You can learn more about Hubble’s “VAR! plate” on the Carengie Observatories website, but… Another 100th anniversary is coming up this month!
The initial prototype of the first planetarium star projector occurred in October 1923 in Jena, Germany. Planetariums have changed a lot over the intervening years, and we’ll be celebrating the history of our remarkable theaters for the next eighteen months or so—concluding a century after the public opening of the first planetarium at the Deutsches Museum on May 5, 1925. You can learn more on the centennial homepage.
One of the most powerful functions of modern, digital planetariums is providing an immersive portal to a three-dimensional model of the Universe, painstaking assembled over decades from observations such as Hubble’s—with more data being added all the time! Indeed, we’re ingesting data on a regular basis here at Morrison Planetarium, thanks in large part to our remarkable engineering team.
Our very own Dan Tell, Manager of Planetarium Engineering at the California Academy of Sciences, will be presenting a keynote address at a kickoff gala event for the planetarium centennial, which will take place in Jena on October 21, in just a few weeks. You can learn more about Dan on our staff biographies page and watch the live stream on the centennial homepage (Dan is scheduled to go on at 10:05 a.m. Pacific Time). Dan’s talk will focus on both the history and the future of planetariums, a great way to inaugurate the centennial celebrations.
We now know the distances to planets, nebulae, star clusters, and galaxies with astonishing precision. Stars in the night sky are astonishingly far away—hundreds of thousands of times farther away than objects in our solar system—and the Milky Way measures nearly 100,000 light years in diameter, when means that it takes light, traveling at 300,000 kilometers (186,000 miles) per second, nearly 100,000 years to travel from one side of the galaxy to the other.
And the Andromeda Galaxy? M31 lies over than two million light years away. While the farthest galaxies observed by Hubble and JWST are billions of light years distant. Hubble’s observations from a century ago provided an early indicator of how vast the Universe truly is.
Perhaps no experience can allow us to internalize these enormous scales, but planetariums give us one opportunity to wrap our heads around them. Every day at Morrison Planetarium, presenters take audiences on a guided Tour of the Universe (you can even watch the tour livestreamed, albeit less immersively, every Wednesday at 4:30 p.m. Pacific on the Morrison Planetarium Facebook page).
Modern planetariums and modern astronomy have both built on the experiences of the last century. If you have a chance to look up at a clear night sky this autumn, you might even be able to spot the Andromeda Galaxy if it’s dark enough—and you can ruminate on Hubble’s exciting find. But even if you can’t get outside to see the sky, you can visit your local planetarium to get a glimpse into the cosmos and to celebrate 100 years of bringing the sky down to Earth.
(Image courtesy of Carnegie Institution for Science.)