Edwin Hubble and the Discovery of the Galaxies

Edwin Hubble and the discovery of the galaxies

Domingos Soares

The Sun, the planets, the asteroids — which are nothing more than small and shapeless “planets” —, the wonderful comets, those are the main in- habitants of such an extremely rich, mysterious and appreciated “universe”: the solar system! It is there that our so ill-treated Earth, the planet, where we were born, is located. Our closest neighbor, the Moon, Earth’s natural satellite, periodically shows up in beautiful and diﬀerent shapes, punctually marking weeks and months, during our annual voyage around the star, that we know by the name of Sun. But, where are we? Where, in such a great universe, in this enormous all-that-exists it is located the Sun and its family? That is what we intend to answer from the next paragraph and onwards: we need to discover the galaxies!

1 Edwin Powell Hubble

When we go to the countryside, far away from the bright lights of the big cities, we are able to start the journey that will lead us, in the end, to the discovery of the galaxies! And to the great American astronomer Edwin Powell Hubble (1889-1953). There, then, in a clear night, without clouds, looking up at the sky we see innumerable bright spots, which are, in its majority, the stars. Some of theses bright spots are the other planets of the solar system. But they are few, one can count all of them with the ﬁngers of both hands. Most of those luminous points, ranging from 1000 to 1500, are the other stars, similar companions to our Sun. They show up as luminous points because they are much farther away from us than the Sun is. Ancient sages and scholars — especially the Greeks — dedicated themselves to a careful and quantitative study of these luminous points. Some of them strayed away, “wander”, that is, moved in a rapid fashion through the heav- ens, during the annual cycle of the Earth around the Sun. Such wandering luminous points were called planets, a word that derives from the Greek word

2 for “wanderer”, or, “moving in a rapid way”. The other luminous points, the stars, kept their positions fixed, one with respect to the other. Stars formed, therefore, well-defined figures over the dark background of the sky, which were called constellations. In 1929, the International Astronomical Union, the top world astronomical entity, adopted 88 official constellations. It is al- ways instructive to remember that constellations constitute conventionalized arrangements of stars, which are close to each other when seen projected onto the sky. Stars of a given constellation may be at enormous distances one from another, when examined in the depths of space. But, in a way or another, all of them belong to the same family. This family is our galaxy! The galaxy of the Milky Way. The Milky Way, the “lactic path”, is a luminous and whitish stripe that extends itself through the whole sky. The above-mentioned stars sit themselves around that stripe and over it as well. We notice also that the term “galaxy” comes from the Greek word for “milky”.

3 Photo of approximately one third of the Milky Way obtained by the photographer Akira Fujii, in Australia. The Southern Cross and Orion constellations are at the left and right ends of the picture, respectively. Try to ﬁnd them.

Until about the year of 1600 astronomers undertook their studies using an extraordinary optical “instrument”, but that revealed itself limited for the ambitions of knowledge: the human eye. The great Danish astronomer Tycho Brahe (1546-1601) was the last to make professional use of such a fab- ulous instrument. And to get the most out of this instrument he invented, planned and constructed very large mechanical instruments called quadrants, sextants, etc. They were ﬁxed or movable and had sizes of the order of 3 to 6 inches. With these instruments and the eye, Tycho Brahe and his assistants made measurements of the positions of stars and especially of planets, which were of major importance for the development of modern science. But in

4 1608 something extraordinary happened. It came into the public domain the invention of a Dutch scholar that dedicated himself to the science of optics. His name is Hans Lippershey (1570-1619) and he invented the spyglass, a small refracting telescope. It is refracting because it uses lenses, here, two, a concave and a convex one. The great Italian sage Galileu Galilei (1569-1642) became aware of this invention in 1609 and immediately built his own instrument. He pointed it to the sky. The astronomical spyglass was born! The first significant advancement for the improvement of the access of mankind to the wonders of the cosmos. Galileu made several and important discoveries with his astronomical telescope. To our story, the important discovery was what happened when he pointed the instrument to the Milky Way. He verified that it was not so milky, but in fact it was constituted by thousands and thousands of stars! They were so close together and were so numerous that to the naked eye they gave the milky aspect, which originate its name. Only the most bright stars could be seen individually with a naked eye. Galileu’s astronomical spyglass — more than 400 years ago — was the forerunner of the great telescopes of nowadays. The telescopes that professional astronomers use in their researches have, in their vast majority, mirrors instead of lenses, as the main parts of the optical systems used to collect the light from distant astronomical objects. They are called, in this case, reflecting telescopes. The telescope which is the main character in the discovery of the galaxies is one of those. It is the reflecting telescope, with a mirror of 100 inches in diameter, located at Mount Wilson, in the American state of California. Those who have visited amateur observatories housed in public planetariums, which usually have reflecting telescopes with apertures from 10 to 20 inches, and admired the size of the telescopes, may well imagine the “monster” that we are talking about. With this telescope, Edwin Powell Hubble made his great discovery!

5 Photo of the Andromeda galaxy obtained by the amateur astronomer Jason Ware, in Texas, United States (see more details at http://www.galaxyphoto.com).

The English astronomer Thomas Wright (1711-1786) suggested in the XVIII century that the luminous “patches”, or nebulae, seen in the sky, among Milky Way’s stars, could be systems similar to the system of the Milky Way, but that seemed small in the sky for being at enormous distances from us. The German philosopher Immanuel Kant (1724-1804) enthusiastically adopted the idea and in a certain way was the most responsible for its dis- semination among scholars. His presentation of the problem became known as the “hypothesis of the island universes”. The idea was good and seemed correct, but it was necessary that the hypothesis was scientiﬁcally proved in order to be accepted once and for all. That is, using the rigorous methods of astronomical observations, the empirical laws and the scientiﬁc theories at

6 disposal. This is what Edwin Hubble realized.

Albert Einstein (1879-1955), Edwin Hubble and Walter Sidney Adams (1876-1956) — from left to right —, in 1931, at the 100-inch (2.5 m) telescope of the Mount Wilson Observatory, located in southern California, at the San Gabriel Mountains. Hubble made here the majority of his great discoveries about the extragalactic universe (Archives of the California Institute of Technology).

In 1923, using the 100-inch Mount Wilson telescope, he identiﬁed individ- ual stars in one of Wright and Kant’s nebulae, one of the largest of them, the “Great Andromeda Nebula”. It has this name because is seen in the sky re- gion where it is located the constellation of Andromeda. Through a detailed investigation of the luminous properties of those stars, Hubble managed to measure the distance to them and, consequently, to the “Great Nebula”. The result was astonishing: the distance to the nebula was much larger than the

7 size of the Milky Way itself! The conclusion was inevitable. That luminous “speckle” in the sky — one amongst many — was in reality a stellar system as magniﬁcent as that where the Sun and our Earth were located. They have come to be called “galaxies”, by analogy with the denomination of our Milky Way. And thenceforth, other nebulae were studied by Hubble, and the result was repeatedly conﬁrmed. Galaxies have been discovered!

The reﬂecting Hooker telescope with 100 inches of aperture. Through it, astronomers had the ﬁrst views of the universe beyond the Milky Way.

Now, of course this story is much too succinct. There is, for example, a woman — an astronomer, in fact — very important in all of this. Her name is Henrietta Leavitt (1868-1921). And what she has uncovered was fundamental to the discovery of the galaxies. Indeed, she deserves a complete story. Wait then.