Hubble, Leavitt, and the Night the Universe Got Bigger
For 300 years after Galileo, most astronomers thought the Milky Way <em>was</em> the universe. What we now call other galaxies were labeled 'spiral nebulae' and assumed to be clouds within our own system. Two people, a decade apart, turned that assumption upside down.
Henrietta Swan Leavitt at Harvard
Henrietta Leavitt joined the Harvard College Observatory in 1893 as a member of the 'Harvard Computers' — a group of women hired to measure and catalog the enormous archive of photographic plates the observatory was accumulating. She was assigned to variable stars in the Magellanic Clouds, at the time thought to be nearby star-forming regions.
Because all the stars in a Magellanic Cloud are roughly the same distance from us, differences in their apparent brightness reflect real differences in their intrinsic brightness. In 1912, working with 1,777 variables in the Small Magellanic Cloud, Leavitt noticed a clean pattern in the subset of Cepheid variables: their period of pulsation was tightly correlated with their apparent (and therefore, at fixed distance, their absolute) brightness.
This period–luminosity relation was the crucial insight. If you could measure the pulsation period of any Cepheid anywhere, you knew its absolute brightness. Compare that to its apparent brightness and you had its distance. Cepheids were suddenly a distance measuring tape hundreds of times longer than parallax could reach.
The Great Debate, 1920
By 1920 the question of the 'spiral nebulae' had reached boiling point. Harlow Shapley argued they were within the Milky Way; Heber Curtis argued they were separate 'island universes' — full galaxies at enormous distances. The two debated publicly at the Smithsonian in April 1920. Neither side won — the observations to settle it did not yet exist.
Edwin Hubble at Mount Wilson, 1923–24
In the fall of 1923, Edwin Hubble was using the 100-inch Hooker telescope on Mount Wilson — then the largest in the world — to photograph the Andromeda 'nebula' (M31). On a plate taken October 4, he spotted a variable star in one of its spiral arms. Follow-up photographs let him measure its light curve.
It was a Cepheid. Applying Leavitt's period–luminosity relation, Hubble calculated the distance to Andromeda: over 900,000 light-years. (The modern figure is 2.5 million light-years — his estimate was off by a factor of 3 because the Cepheid calibration itself was still being refined, but the qualitative point was decisive.) The Milky Way was known to be under 100,000 light-years across. Andromeda was, by any reasonable measure, well outside.
In 1925 Hubble announced the result. The 'spiral nebulae' were other galaxies. The universe was suddenly at least a thousand times larger than anyone had officially conceded, and populated with objects like our own.
Then it got bigger
Hubble kept measuring. Over the next four years he collected distances to about two dozen galaxies (using Cepheids where he could, and other indicators where he could not), and combined them with redshift measurements made by Vesto Slipher at Lowell Observatory. He plotted distance against velocity in 1929 and got a nearly straight line: farther galaxies are receding faster.
| Quantity | Hubble, 1929 | Modern |
|---|---|---|
| Distance to M31 | ≈ 900,000 ly | 2,530,000 ly |
| Hubble constant H₀ | ≈ 500 km/s/Mpc | ≈ 70 km/s/Mpc |
| Age of universe (inferred) | ≈ 2 × 10⁹ yr | 13.8 × 10⁹ yr |
This linear relation — v = H₀d — is Hubble's law, and it is the observational foundation of the expanding-universe cosmology. The universe wasn't just bigger than had been thought; it was getting bigger, and the recession of distant galaxies was the ongoing evidence.
Frequently asked
- How does the period–luminosity relation actually work?
- Cepheid variables pulsate because a layer of doubly-ionized helium in the star opacifies and clears cyclically. The period of pulsation depends on the star's size, which in turn depends on its luminosity. The result is a clean logarithmic relationship: longer periods mean higher intrinsic brightness.
- Is the Hubble constant known now?
- Approximately — but not precisely. Different methods give ~67 km/s/Mpc (from the cosmic microwave background) and ~73 km/s/Mpc (from the local distance ladder). The 8% discrepancy is called the 'Hubble tension' and is one of the most active problems in modern cosmology.
- How many galaxies are there?
- The observable universe contains roughly 100 billion to 2 trillion galaxies, depending on how you count small ones. The Milky Way is one of them; our nearest big neighbor, Andromeda, is 2.5 million light-years away and on a collision course.