The discovery that stars are clumped into galaxies represents the first advance in efforts to describe the actual shape of the universe and the distribution of stars in it.
Thomas Wright’s theory of galaxies was the first astronomical work to place our sun not in the center of the universe, but in a tightly packed cluster of stars that Wright called a galaxy. This discovery led science a giant step forward in its efforts to understand the vast universe of which our sun and Earth represent only tiny and very ordinary specks. Twenty-five years later, Herschel conducted careful observational studies that proved Wright was right.
For thousands of years scientist believed that the universe consisted of a vast spherical shell of stars, with Earth at its center. Nothing existed in the immense void between Earth and that shell of stars except the few planets and the sun.
By the mid-1600s, most scientists acknowledged that the sun, not the earth, sat at the center of the spherical universe. Some prominent scientists (Christian Huygens, for example) believed that stars were really holes in the black sphere of space where light from a luminous region of perpetual day beyond shined through.
Two men’s discoveries combined to establish the existence of dense clusters of stars called galaxies. Born in 1711, Englishman Thomas Wright taught mathematics and navigation but was a passionate amateur astronomer. As had many astronomers before him, Wright observed that the stars were not evenly spread across the sky. A seeming cloud of faint stars was densely packed along the band called the Milky Way.
This bothered Wright. He believed that God had created a universe of perfect order. That should mean that stars were neatly and evenly, perfectly, spaced across the heavens. Wright could not accept that the heavens were not perfect and so began to play with schemes for the placement of stars to make them really be uniform in their placement even though they appeared not to be.
Wright considered that the stars might be spread along the surfaces of a field of giant bubbles. If we were packed along one of those rings of stars, looking along the ring would cause us to see more stars than if we looked straight out from it. He then considered the rings of Saturn and proposed that the stars might be packed into wide rings or a thin disk. If we were in that disc, it would account for the uneven distribution of stars we saw, even if the stars were really evenly spaced across that disk.
In 1750 Wright published a book, An Original Theory on New Hypothesis of the Universe, in which he proposed this theory. He was the first to use the word galaxy to describe a giant cluster of stars. Five years later, famed astronomer and mathematician Immanuel Kant proposed a similar arrangement of the stars into a giant disk-shaped cluster.
English astronomer William Hershel (born in 1738) read with interest Wright’s theory. In 1785 Herschel decided to use statistical methods to count the stars. He surely couldn’t count them all. So he randomly picked 683 small regions of the sky and set about counting the stars in each region using a 48-inch telescope, considered a giant scope at the time. Herschel quickly realized that the number of stars per unit area of sky rose steadily as he approached the Milky Way and spiked in regions in the Milky Way.
The number of stars per unit area of sky reached a minimum in directions at right angles to the Milky Way.
This made Herschel think of Wright’s and Kant’s theories. Hershel concluded that his counting results could only be explained if most of the stars were compacted into a lens-shaped mass and that the sun was buried in this lens. Herschel was the first to add statistical measurement to Wright’s discovery of the existence and shape of galaxies.
The central galaxy of the Abell 2029 galaxy cluster, 1,070 million light years distant in Virgo, has a diameter of 5,600,000 light years, 80 times the diameter of our own Milky Way galaxy.