Two Women’s Contributions to Measuring Distances

Great Attractor and Galaxy Supercluster Map

Today is an appropriate day to celebrate the significant discoveries of Henrietta Swan Leavitt and Sandra Faber in measuring distances in the Universe as Feb 11th is recognized by the United Nations as its International Day of Women and Girls in Science.

Many amateur astronomers know of the significance of Henrietta Swan Leavitt’s discovery of the period-luminosity relationship in Cepheid Variable stars; the longer the period of a Cepheid, the more luminous it is. Edwin Hubble used this relationship with his observations of Cepheid Variable stars in the Andromeda Galaxy (M31) to estimate that the Andromeda Galaxy lies at a distance of 1.5 million light-years; thus resolving the Great Debate with Harlow Shapley conceding that spiral “nebulae” (what we now call galaxies) are located outside our milky way.

Plot prepared by Leavitt in 1912. Period on the x-axis and brightness on the y- axis for 25 Cepheids in the Small Magellanic Cloud (SMC). The relation also holds for luminosity because all stars in the SMC are at about the same distance from Earth.

The luminosity of a star is the total amount of light it emits from its surface. On the other hand, how bright a star appears depends on its luminosity and its distance from the observer because the light spreads out over a greater surface area, in accordance with the inverse square law.

Leavitt’s work was critical because once the luminosity is known, the brightness  from Earth can be measured and the distance estimated from the inverse square law.

Image Credit: Australian Telescope National Facility – CSIRO

Later in the 1970s, Sandra Faber found another luminosity relationship. Faber was following up on some of her thesis work that looked at absorption lines — those of calcium, sodium, or magnesium in the visible portion of a galaxy’s spectrum — when she spotted some evidence this relationship.

“Well, as I was taking the data, I could not help but notice that the more luminous galaxies had broader lines.” 

Sandra Faber interview in 2019

Further data analysis revealed a pretty good empirical relation between the luminosity and the velocity dispersion of stars near the centre of elliptical galaxies. This relationship became known as the Faber-Jackson relation after the work was published in a paper authored by Faber and Robert Jackson, her research assistant at the time. For any visible elliptical galaxy, the velocity dispersion can be measured from the width of the absorption lines and its luminosity estimated using the Faber-Jackson relation. The distance to the galaxy can then be calculated using the inverse-square law applied to its luminosity and its observed brightness from Earth.

Velocity dispersion (y-axis) plotted against absolute magnitude (x-axis) for a sample of elliptical galaxies, with the Faber–Jackson relation shown in blue. Image Credit: Wikipedia

Faber refined these methods as the head of a group, known as the Seven Samurai, on a project to measure distances and velocities of a sample of elliptical galaxies. The project failed to validate its original hypotheses on more accurate versions of the Faber-Jackson relationship but it did establish distances to 400 galaxies. Using these distances, they made a map of ellipticals around us and noticed that the recessional speeds were not exactly as predicted from a simple uniform Hubble law (due to the expansion of the Universe). Faber recalled, David Burnstein, one of he seven samurai remarking

“Look at this. There’s a whole region in Centaurus, and it’s moving at 1000 kilometres a second”

Dave Burstein remark, recollection by Faber in 2019,

Instead, large patches of the Universe were moving away from us too quickly or too slowly! This work culminated at a small meeting of cosmological experts in Hawaii in January 1986. The samurai sliced up the standard theories of the time, by announcing that the universe was expanding lopsidedly. A vast region of space 500 million light-years in diameter, containing hundreds of thousands of galaxies, was being drawn toward a huge concentration of mass later dubbed the Great Attractor. One attendee at the meeting said “In 20 years of science reporting, I have never seen such pandemonium at a scientific meeting”.

Faber has done significant research in other areas and has been recognized with numerous awards. Earlier this year in Jan 2020, the Royal Astronomical Society awarded her its Gold Medal in Astronomy. The award recognizes Faber “for her outstanding research on the formation, structure and evolution of galaxies, and for her contributions to the optical design of the Keck Telescopes and other novel astronomical instruments.”