Henrietta Swan Leavitt (July 4, 1868) was an American astronomer. After graduating from Radcliffe College, she joined the Harvard Observatory as a volunteer assistant and soon obtained a regular position as part of a team measuring the brightness of stars. Her excellent work led her to become the head of the photographic stellar photometry department at Harvard University. She devised new methods of analysis and was able to determine a basic sequence of standard brightness magnitudes, for a large sample of stars. Leavitt then expanded her work to selected areas of the sky and produced the North Polar Sequence, this became the Astrographic Map of the Sky. As part of this work Leavitt discovered 4 novas and 2,400 variable stars. These variable stars are also called pulsating starts or Cepheid variables because their brightness vary regularly from a few days to months. Leavitt discovered that the period of brightness fluctuation is determined by the pulsating star’s luminosity. Improvements in technology allowed for the further calibration of the curve for period and luminosity. Leavitt’s research produced very valuable results that have been used by the Hubble telescope to determine the distance of several Cepheid stars and importantly the distance from Earth to the galaxy Andromeda. Leavitt worked in the Harvard observatory until her death in 1921.

Ernst Mayr (July 5, 1904) was a world leading evolutionary biologist. His earlier work on speciation and founder populations was very instrumental in the development of the modern synthetic theory of evolution. This theory brings together the work of Charles Darwin and Gregor Mendel, including the mechanism of gene mutations, recombination, chromosome structure/function, and natural selection. His work on reproductive isolation led him to propose a definition of species that is currently used. He wrote many books; among them his seminal work on the modern theory of evolution is explained in his book, Systematics and the Origin of Species.

Nettie Maria Stevens (July 7, 1861) was an American biologist and a geneticist. She started her career as a school teacher but pursued her dream of doing biological research. She obtained her doctoral degree at Bryn Mawr College and stayed in that institution throughout her scientific career, starting as research fellow in biology, a reader in experimental biology, and finally as associate in experimental biology. Stevens first research project was studying the morphology of protozoans but soon become interested in regeneration and sex determination. This research led her into the area of cytology, the study of differentiation in embryos, and chromosomes. Stevens most significant work was her finding out that a particular combination of the chromosomes led to the determination of sex in an individual. She showed that male mealworms reproductive cells contain X and Y chromosomes while female reproductive cell only have X chromosomes. Importantly, Stevens work demonstrated a direct link between genes and inheritance of characteristics. At that time there was a debate over whether sex was determined by the environment or genes. Her work was not readily accepted; however, another male researcher produced the same results and with time she was proven correct.

Jocelyn Bell Burnell (July 15, 1943) was a British astrophysicist. She received a bachelor’s degree in physics and went to Cambridge University where she obtained a doctoral degree in radio Astronomy. She helped in the construction of a large radio telescope and while monitoring quasars, she discovered regularly and rapid radio pulses. Using more sensitive equipment the research group, including her adviser Antony Hewish, determined that the radio pulsars were coming from rapidly spinning neutron stars. For this work the Nobel Prize was awarded to her advisor and a collaborator, several scientists complained about leaving Bell Burnell out of the prize. After her discovery, she moved to other institutions ending up as a Dean of science at the University of Bath and a visiting professor at Oxford University. She was created commander of the Order of the British Empire, and a Dame. She became a member of the Royal Society, served as a president of the Royal Astronomical Society, and the Institute of physics.

Rosalyn Sussman (July 19, 1921) was an American Physicist that developed radioimmunoassay techniques for which she received the 1977 Nobel Prize for Physiology or Medicine. Radioimmunoassay is used to screen minute amounts of biological substances in body fluids. For example, RIA is used to detect the hepatitis viruses and other substances in blood banks; and to calibrate the effective amount of drugs and antibiotics. She lectured in physics and became the assistant chief nuclear physicist in the radioisotope service at the Veteran Administration Hospital.

Rosalind Elsie Franklin (July 25, 1920) was an English chemist and crystallographer. She studied physical chemistry at Cambridge University and after graduation she was offer a research position at this institution. However, this was interrupted by the Second World War. She served as a London air raider warden and at the British coal research association where she did research in the physical chemistry of coal and carbon for the war. After receiving a doctoral degree from Cambridge, she moved to Paris where she used X-ray diffraction to study the structural changes of graphite and its uses in coking. Back in England she joined the biophysics laboratory at Kings College in London where she produced her most influential work. Using her expertise in X-ray diffraction, she produced clear diffraction patters of DNA. Franklin’s results allowed her to find out the density of DNA, that DNA existed in a helical conformation, that the sugars were in the inside, and the phosphate backbone in the outside of the helix. This work was the basis on which the double strand structure of DNA was elucidated by Watson and Crick. She moved to the Crystallography Laboratory at Birkbreck College where she investigated the structure of the tobacco mosaic virus and its nucleic acid composition. She died before the Nobel Prize was awarded to Watson, Crick, and Wilkins (Franklin’s superior) for the elucidation of the structure of DNA.

Gregor Mendel (July 20, 1822) was a Czech botanist, Augustinian prelate and a geneticist who proposed the laws of heredity. Before Mendel performed his famous genetic experiments, he had previous scientific training in physics and mathematics. This helped him to design a series of very detailed and controlled experiments to investigate how traits or characteristics were inherited. Mendel is considered the father of genetics because his experiments revealed that inheritance of traits or characteristic are passed from one generation to the next as discrete units, later called genes. This falsify the widely believe idea that inheritance happened by the blending of the parents’ characteristics. Mendel found two Laws that control how simple traits are inherited; simple traits are those that depend on the expression of one gene only. Mendel’s work was the start of genetics; scientists following on Mendel’s footsteps extended his ideas to complex traits, those depending on the expression of two or more genes. Mendel’s results were not popular or well-known at the time, after many years his research was discovered and became popular mainly because science had advance and scientist could understand its significance. Mendel’s work was pioneering, visionary, and scientifically excellent.

Stephanie Louise Kwolek (July 31, 1923) was an American chemist whose main interest, research, and scientific contributions were in polymers. She received a chemistry degree from the Carnegie Institute of Technology and wanted to become a physician. Kwolek took a detour from her medicine plans to work as a laboratory chemist at the rayon department at the DuPont Company. At the time she joined this company, a new area of promising chemical research in synthetic fibers opened. She decided to stay with this company for a few years, but she ended up working at DuPont until her retirement. Kwolek’s most influential work was her investigation in aromatic polyamides. She found that these compounds form strong, stiff, and non-flammable fibers when coming out of solution. She determined the right solvents and polymerization conditions. This investigation led Kwolek to study other similar compounds, and discovered one that produced very stiff rods, with superior tensile strength. Commercially, these fibers are called Kevlar, and is used in high strength tie-cord, to reinforced boat hulls, and for light weight bullet-proof vests.