By Jean Balchin 04/07/2017

One day, when I was seven years old, I decided to camp out in the treehouse with my brother John.

It was a poorly planned venture from the start; we misjudged the cold, the ferocity of the mosquitoes and our own temperaments. John got spooked looking at the gnarled bark of the tree, reading faces in the knots of the wood. I fed him peanut butter sandwiches and told him to shut up. We fell silent when the clouds drifted away from the night sky and the stars began to shine like glowworms in the dark of a cave. I remember lying there, marvelling at the beauty of the Milky Way and pointing out the constellations to my brother.

Beatrice Tinsley

Our understanding of the evolution of stars, galaxies and the universe itself owes much to a British-born Kiwi astronomer and cosmologist called Beatrice Tinsley. Tinsley was born in the walled city of Chester, England, in 1941. She was the middle of three sisters, and grew up during the war time, when food and coal were rationed. After WW2, she emigrated with her family to Christchurch, New Zealand.

While studying in Christchurch, she married fellow physicist and university classmate, Brian Tinsley. Unfortunately, this prevented her from working at the University while he was employed there. After completing a master’s thesis in solid state physics, she and her family moved to Dallas, Texas, in 1963. However, she was restricted here too, by the sexism still rife within the academic community. In 1974, after years of desperately trying to balance home, family and career, she left her husband and two adopted children to take an assistant professorship at Yale University. Here she worked until her death from cancer in 1981.

Tinsley’s Thesis:

Though unfairly judged by many as just another Dallas housewife, Tinsley’s head was full of stars. Her brilliant academic record convinced the head of the astronomy department at the University of Texas to admit her. To this end, she commuted more than 200 miles from Dallas to Austin to work and study. Tinsley’s 1966 Ph.D thesis “Evolution of Galaxies and its Significance for Cosmology” argued that as internal changes owing to the evolution of stellar and non-stellar material occur in galaxies over long periods of time, determining distances based on the morphology of said galaxies is unreliable. Incidentally, Tinsley completed her thesis in about a third of the time it takes most people to do a Ph.D thesis. She was awarded marks of 99% and 100%; the first student in the department to achieve marks of over 80%.

A spiral Messier galaxy.

Prior understandings of galaxies

According to Edwin Hubble, galaxies “are enormous systems, and it is reasonable to sup­pose that their evolution is corre­spondingly slow.” Until Beatrice Tinsley came along, astronomers assumed that all galaxies — from the spiralling Milky Way, to the egg-shaped ESO 325-G004 — formed fairly quickly after the Big Bang and then changed little over time. At the time, astronomers could not see far back into the past; they were restrained by the limits of technology. Thus the distant galaxies they saw looked fairly similar to the galaxies right by us. Early cosmologists tracked galaxies in order to learn the universe’s fate. These galaxies functioned as markers to discern the rate at which the universe expanded. By comparing the speeds of galaxies in earlier epochs with those of the present day, they hoped to discern whether galaxies were slowing down at a sufficient rate to fall back in on themselves due to gravity, or whether they were flying outward at unrestrained speeds.

The belief that galaxies were uniform, “stedfast”, and drifting in “lone splendour”, to quote Keats, meant that the galaxy could be used as an cosmological yardstick. An astronomer estimated a galaxy’s distance by measuring its luminosity and angular width in the night sky. Reaching deeper into space, and thus viewing the universe as it was in the past, astronomers assumed that ever more distant galaxies would appear dimmer and smaller in a regular, systematic fashion.

Tinsley initially decided to take part in the enduring cosmological question of whether the universe was open or closed. However, upon examining many galaxies, with their different magnitudes and diameters, she began to question the nature of these changes. How were these galaxies changing over time? Tinsley decided to actually stimulate the evolution of a galaxy by setting up a numerical model, and tracking its changes in colour and brightness over “billions” of years. The stars within her hypothetical galaxy were born in numbers, radiated fiercely, and then expired silently within the pages of her thesis.

Long before the advent of our high-tech computers, which swiftly and easily run algorithms, Tinsley was calculating these gruelling numerical simulations. She constructed her model based on the best theoretical and observational evidence available at the time. Tinsley set up an initial population of stars and then decided their life span, and how soon new stars would be generated to take their place. At the time, it was uncertain whether a galaxy’s brightness depended more on the collective light emanating from its numerous long-lived, low-mass stars, or from the rarer, but brighter, short-lived, massive ones.

After completing this thesis, Tinsley continued to refine her models over the years. Each stimulation she created reinforced the notion that galaxies can undergo substantial evolution over time; far more than astronomers had previously thought. A bright, blue, young galaxy draws upon plentiful gas resources to create stars, and then gradually dims and reddens with age, as its stars mature and die over the millennia.

Observational findings

While some senior authorities initially disbelieved her conclusions, eventually Tinsley’s findings encouraged observers to start examining this predicted galactic evolution firsthand. Soon, astronomers had found direct confirmation of Tinsley’s theoretical findings. In 1977, Augustus Oemler Jr. and Harvey Butcher, balanced on Kitt Peak in Southern Arizona, used their 84-inch telescope to examine the light emanating from two galaxy clusters now known to be situated some 5 billion light-years away (hence 5 billion years back in time). Their findings matched Tinsley’s prediction perfectly. The galaxies in both clusters were radiating more blue light than the more reddish clusters near us today.

Beatrice Tinsley thus ushered in a new era in extragalactic research. Extraordinarily faraway galaxies that seemed mere smudges in Hubbles’ time can now be viewed today with thrilling clarity. We’re finding that over time, these galaxies can exhibit diverse and varied personalities. Some move silently and gently through the cosmos, evolving internally as Tinsley calculated. Other galaxies are menaces, colliding, sideswiping or merging with each other. Some even hungrily devour the odd hapless passerby. The universe truly is a magnificent place.

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