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Friday, June 21, 2024
Dr. Brown: Henrietta Leavitt and the distance to stars

Dr. William Brown is a professor of neurology at McMaster University and co-founder of the Infohealth series held on the second Wednesday of each month at the Niagara-on-the-Lake Public Library.  

Dr. William Brown

Special to The Lake Report

When Prof. Gösta Mittage-Leffler of the Swedish Academy of Sciences wrote Henrietta Leavitt in 1925 to inform her that he wanted to nominate her for a Nobel Prize, he was surprised to learn that she had been dead for three years.

Not the first potential laureate to lose out to death. She died of cancer at 53, largely unknown, except for a few astronomers familiar with her work and a paper published in 1912, “Periods of 25 Variable Stars in the Small Magellanic Cloud.”

For that work, the only credit she received from her boss, Edwin Pickering, the director of the Harvard College Observatory, was a brief acknowledgment in the opening line of the paper – “The following statement regarding the periods of 25 variable stars in the Small Magellanic Cloud has been prepared by Miss Leavitt.” That was it for credit.

It was said that Pickering was brilliant, gracious, compassionate and encouraging, yet “chose his women to work – not think.’”That summarizes how women in general and professional women especially, were treated by many men then and for much of the rest of the 20th century.

Leavitt, who loved her job at the observatory, was a quiet, kind, deeply religious woman of Puritan heritage and daughter of a congregationalist minister. She also was deaf.

Like the other dozen computer women, Leavitt was tasked with the painstaking job of analyzing thousands of photographic negatives taken by Pickering as part of his mission to map the heavens. Her specific job was to map the locations, apparent luminosities and periodicities of a species of variable stars, called cepheids, in the northern sky.

Cepheids are large stars, much larger than our sun, which expand and contract according to how much ionized helium there is in the outer layer of the star. They are also very bright and hence easier to spot at long distances

Measuring distances beyond our solar system and a few nearby stars in the Milky Way was impossible before Leavitt’s time. Trigonometric measurements based on the angle subtended by a star at two different locations on the Earth’s surface measured at the same time, works for the sun and its planets.

And extending the base of the triangle by taking measurements of the angle subtended by a star from opposite points in the Earth’s orbit about the sun (say, June and December), extends measurable distances to stars farther out within the Milky Way.

But until recent times when, with the aid of space-based satellites and the ability to resolve tiny differences in the angles subtended by stars, astronomers were stuck. For without accurate measurements to far distant stars, it was impossible to determine the size of the Milky Way and beyond.

What Leavitt found was that the apparent luminosity of cepheids, as seen from Earth, varied with their cycle period: longer cycle periods were associated with larger, brighter, more luminous stars and the converse for stars with shorter periods.

That relationship was the key to determining distances to faraway stars. But one piece was missing – how to determine intrinsic luminosities when only relative luminosities were known? The answer came a year later in 1913 with evidence from a Danish astronomer, Ejnar Hertzsprung, who found cepheids close enough to use the moving cluster method to determine their distances from Earth.

Now intrinsic luminosities could be determined directly from the cycle periods of cepheid stars and with the relative luminosities at hand from direct observations of the stars, distances to far away cepheid stars could be calculated with what became known as Leavitt’s law. Measuring distances to the far reaches of the Milky Way and beyond was now possible.

In 1924, Edwin Hubble used variable stars and more luminous O and B supergiant stars in the Andromeda and other nebulae, to determine their distances from Earth. All were far beyond our Milky Way and must therefore be other galaxies. We weren’t the only galaxy. Recent estimates suggest there might be trillions of galaxies. You do the math – there must be many habitable planets and probably life.

If that wasn’t enough drama, in 1929, Hubble published additional studies, this time employing the doppler effect, whereby galaxies moving away are red-shifted and those moving toward us are blue-shifted, to analyze the movement of galaxies.

These studies revealed that most galaxies were moving away from the Milky Way and one another and the farther away they were, the faster they were moving. It wasn’t much of a step to suggest that the universe had been much smaller at one time, an idea, which led directly to the Big Bang hypothesis.

These days the Big Bang hypothesis is taken for granted because the interim steps have been clarified, including the progression from the relative simplicity of the earliest stars to more complex stars, which through repeated cycles of creative fusion created all of the naturally occurring elements in the periodic table.

Indeed, the whole concept of evolution from simple to complex, applied first by Darwin and Wallace to all of biology, applies as much to the life of stars.

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