Henrietta Leavitt, the Woman Who Discovered a Cosmic Ruler

Henrietta Leavitt, the woman who discovered a cosmic ruler

Domingos Soares

In 1925, the American astronomer Edwin Powell Hubble (1889-1953) pub- lished a scientiﬁc article, where he unequivocally demonstrated that the ir- regular nebula NGC 6822 was in fact a stellar system outside our Milky Way system. A new area of astronomy was inaugurated: extragalactic astronomy, and the human horizons were expanded in a spectacular way! Hubble achieved this feat in a very simple way: he measured the distance to “Barnard’s galaxy”, whose catalog name is NGC 6822, and is located in the Sagittarius constellation. The distance he obtained was more than twice as large as the dimensions, known at the time, of the Milky Way. Therefore, NGC 6822 should be outside our own galaxy. Then began our journey towards the great realm of the galaxies! He used a very special ruler to measure this distance , a “cosmic ruler”, ca- pable of measuring distances unimaginable until then. And this ruler, which greatly expanded our horizons, was discovered by a woman, the astronomer, also American, Henrietta Swan Leavitt (1868-1921).

1 Henrietta Leavitt, whose name is written in the history of modern science for discovering a powerful astronomical method of measuring distances: the method of the Cepheid variable stars.

Before the advent of electronic computers, mathematical calculations were performed by teams of professionals called “human computers” or simply “computers”. Such practice was very common in all areas of the exact sci- ences, and especially in astronomy. Computers in astronomy were usually women, less because they were more careful with the exhaustive calculations, but because of the lower cost involved. Men’s wages were — and in many cases, nowadays, are still — higher than the salaries paid to women. Many female computers have become prominent astronomers in the history of astronomy. Henrietta Leavitt is an illustrious representative of this case. After receiving a bachelor’s degree in 1892, Henrietta was hired by astronomer Edward Pickering (1846-1919) from Harvard Observatory in the United States. Her function was that of a calculator, and she should work on the photographic catalog of the observatory, measuring star brightnesses. She realized that there were thousands of variable stars in the Magellanic Clouds

2 images, which we today know are satellite galaxies of the Milky Way. There was a special type, amongst the variable stars, called “Cepheid variables”. The characteristics that define a Cepheid variable are its great brightness — they are supergiant stars — and its variability period — the time interval of a complete cycle of brightness variation — from approximately 1 to 100 days. The variation of the Cepheid brightness is also distinctive. It exhibits a rapid increase until the maximum and then a slow decline until the minimum brightness. The cause of the variability is the pulsation of the star, i.e., the variability of its size, and the consequences of this variability on other features of the star such as, for example, density and temperature. The star has a greater brightness after it passes through its minimum size or radius. As its radius increases its brightness decreases. The star pulsates and its brightness varies during the pulsation. The main cause of the brightness variation of the star, however, is not the simple variation of its size, but is directly related to the variation of its surface temperature. The details of the physical mechanism that generates the Cepheid variability were first proposed by British astrophysicist Arthur S. Eddington (1882-1944). The Cepheid variables have this name because the first of them was discovered in the Cepheus constellation, named after Cepheus, in Latin, a king in Greek mythology. The star is the fourth brightest of the constellation and is therefore called Delta Cephei (or Delta of Cepheus), according to the astronomy convention, where “delta” is the fourth letter of the Greek alphabet.

3 Graph of a “light curve” of a ﬁctitious Cepheid variable star, with a period of a little more than 3 days. The brightness of the star, represented by the points, periodically varies between a maximum and a minimum value. Note that the variation to the maximum is relatively faster than the decrease to the minimum brightness. This is one of the features of Cepheid variables.

Henrietta discovered an important fact about the Cepheid variables of the Magellanic Clouds: the variables of greater brightness had a long period and those of smaller brightness a short period. And the relation between brightness and period was very simple, the brightness was directly proportional to the period. Since the stars were at the same distance from Earth, because they were at the Magellanic Clouds, the relation between the apparent brightness, which she measured, and the period was, actually, a relation between intrinsic — or absolute — brightness and period. This relation is also called “period-luminosity” relation.

4 Graph made by Henrietta Leavitt in a 1912 article, in which she describes the dis- covery of the period-luminosity relation for Cepheid variables in the Small Magel- lanic Cloud. The vertical axis records the apparent brightness and the horizontal axis the period. The two straight lines correspond to the relation for the maximum and for the minimum apparent brightness. The straight line means that there is a “direct proportion” between period and brightness.

How to use the period-luminosity relation to measure distances? Well, it is known that the brightness of a source seems to be the smaller the greater its distance from the observer. There is a very simple law: the apparent brightness is inversely proportional to the square of the source distance. That is, if we know the brightness and distance of a luminous source, and we measure its brightness at an unknown distance, we can, with a simple mathematical operation, calculate the distance to the source! For example, if a source has a brightness 4 times smaller than its brightness at a known distance, then it means it is at a distance 2 times greater. It is therefore enough to know the distance to a Cepheid variable from our own galaxy, which can be achieved

5 with appropriate methods for smaller distances. The period-luminosity relation for the Magellanic Clouds can be “calibrated” and then become a relation between period and brightness at a known distance, or “absolute brightness”. When we observe a Cepheid variable of unknown distance, it is enough to measure its period, what is simple, and the period-luminosity relation, calibrated as described above, gives the star’s absolute brightness. One mea- sures the apparent brightness of the star and applies the law of the inverse of the square of the distance to obtain its true distance. This a very powerful method, because the Cepheid variable stars are supergiant stars, which have intrinsic luminosities thousands of times greater than Sun’s luminosity. And hence can be observed even when are located very far away. That is how Edwin Hubble discovered the distance to Barnard’s galaxy. He identified Cepheid variables in the nebula, measured their periods, and derived the absolute brightness therefrom. He then measured the apparent brightness of each variable on the photographs he obtained and, finally, derived the distances to the variables, and therefore to the galaxy. The result was astonishing: the nebula was located at a distance a little more than two times the size of our own Milky Way! It was a stellar system located outside our galaxy! It was another galaxy! It was one of the greatest discoveries of astronomy, and a new era in astronomical research was inaugurated, the era of “extragalactic astronomy”. Barnard’s nebula has this name because it was discovered in 1884 by the American astronomer Edward E. Barnard (1857-1923). It was later cataloged by the Danish astronomer John L.E. Dreyer (1852-1926) in his “The New General Catalog of Nebulae and Clusters of Stars” with the number 6822, hence its name NGC 6822. It was the first system, after the Magel- lanic Clouds, where Cepheid variables were identified. Hubble found in it 15 variable stars, 11 of which were Cepheids. The observations were made with a reflecting telescope, whose mirror is 2.5 meters in diameter, located on Mount Wilson, in the American state of California. Hubble calculated the distance to NGC 6822 of 700 thousand light-years, using the calibration of the period-luminosity relation known at the time. The size of the Milky Way had been determined by the American astronomer — Hubble’s contemporary and greatest scientific rival — Harlow Shapley (1885-1972) as 300 thousand light-years, using the same calibration. The conclusion was definitive: NGC 6822 was not confined to the boundaries of the Milky Way!

6 The modern calibration of the period-luminosity of the Cepheid variables implies a distance of 1.6 million light-years to NGC 6822. Soon after, Hubble repeated the same procedure for the Great Andromeda Nebula (whose catalog name is M31) and for the Triangulum Nebula (M33). The result was the same: they were independent systems, located at distances of more than ten times the size of our own galaxy! The method of Cepheid’s variables remains a modern tool for measuring cosmic distances. One of the research projects of the Hubble Space Telescope (http://hubblesite.org) is the identiﬁcation of Cepheid’s variables in distant galaxies, aiming the precise determination of their distances. The most distant galaxy in which Cepheid variable stars were observed is the spiral galaxy M100 (also cataloged as NGC 4321), pertaining to the Virgo cluster of galaxies. It is located in the Coma Berenices (Berenice’s Hair) constellation, which is close to the Virgo constellation. The Hubble Space Telescope observed this galaxy and the astronomers responsible for the project discovered 20 Cepheid variables. They investigated about 40,000 stars to reach this result. Obser- vations were made over 2 months, during which 12 images of the galaxy were taken, each with an exposure time of one hour. The interval of 2 months, or 60 days, is appropriate for the detection of Cepheid variables, because, as seen above, the periods of the variables range from 1 to 100 days. The image shown here refers to one of the M100 Cepheid variables, located on the periphery of the galaxy disk. It is remarkable to detect the variation of the star brightness, especially since it is such a remote galaxy. This shows in a very strong way the success of the Hubble Space Telescope. The result would be impossible to achieve with telescopes on the ground. The analysis of all the 20 Cepheids identiﬁed in the galaxy, with the help of the period- luminosity relation, resulted in a distance of 56 million light-years.

7 A Cepheid variable star, located on the periphery of the M100 galaxy disk, is identiﬁed by the small square on the right. Part of the galaxy nucleus shows up at the lower left corner. The three upper squares show the star at their centers, illustrating its variable brightness. As one can see, the brightness of the star increases starting from the left picture (Image: Wendy L. Freedman/Hubble Space Telescope).