Galaxies

Galileo’s 8X magnification telescope revealed that the white light of the Milky Way is the combined light of many stars. Scientists in the 18^th detected faint elliptical nebulae, and Immanuel Kant combined this information along with the line shape of the Milky Way (Figure 1‑19) to reason that galaxies have a disk shape and we are within in one of those disks, the Milky Way. Scientists in the 19^th and 20^th century identified the arms of the Milky Way and the structure of galaxies in the Universe.

In the 18^th century, Charles Messier (1730-1817) was searching for comets, which Europeans viewed as signs from God. Nebulae distracted Messier, so he made a catalog of their locations in order to avoid mistaking them for comets. As it turned out, his catalog of 110 nebulae (blurry objects) turned out to be more important than any comet. His catalogue became the famous Messier list (Figure 1‑21). Many of the objects in the Messier list are nearby galaxies and globular clusters (clusters of stars in our galaxy). For example, Messier object M31 is the Andromeda Galaxy, which is in the third column and third row (Figure 1‑21), and globular cluster M5 (Messier 5) has hundreds of thousands of stars. The letter M stands for Messier, and 5 refers to the fifth object in his list. Other objects are stellar nurseries where clusters of stars are forming (Eagle Nebula M16) and dying stars (M57). With his small 4-inch telescope with his eye, Messier only dimly saw the objects in Figure 1‑21 .

Increasing the length of telescopes increases magnification, and increasing the diameter of mirrors increases resolution. In the 19^th century, the Herschels (brother-sister) and the Rosses (husband-wife) built the largest telescopes. William Herschel was a musician who became interested in astronomy, and he spent 16 hours per day polishing mirrors and making telescopes. Caroline, his younger sister, fed him so that William did not need to let go of the polishing stone (Figure 1‑23). Although Caroline did not directly polish mirrors, she was heavily involved in their scientific work.

The Herschel’s largest telescope (Figure 1‑24) was 40 ft long but never worked. Their next largest telescope was 20 ft long and was used to map the heavens. After William passed away, Caroline continued their work by cataloging the nebulae that they observed, which formed the basis of the New General Catalogue (NGC) of 6,200 nebulae.

One of the their most significant discoveries was the planet Uranus. They also mapped the Milky Way by setting their telescope at 683 positions and counting the number of stars at each position. Their observations revealed that many more stars were along the line of the Milky Way than in directions perpendicular to the Milky Way. This observation provided the first empirical evidence that the Milky Way was in the shape of a disk. The Herschels assumed that all the stars (or other visible objects) had the same intrinsic brightness, which was incorrect, and led to an error in their estimate of the size of the Milky Way. Herschel also thought that all the clusters and galaxies that he could see with his telescope were within the Milky Way, which was also incorrect.

Lord Rosse built a 72-inch diameter telescope in 1845, with which he was able to see individual stars in distant galaxies. The telescope was called the Leviathon, and it was even featured in one of Jules Verne's novels. The images from this telescope provided support for Kant’s island universe theory. Rosse's partner in astronomy was his wife, Mary Rosse. Mary had a darkroom in their castle, and she experimented with new photographic techniques. The human eye is a poor collector of the dim light of telescopes. The eye is primarily designed to collect bright light in the daytime; however, photographic plates could collect the dim light of stars for extended periods and form images of distant nebulae. Based on their observations, Lord Rosse drew an image of the Whirlpool Galaxy (Figure 1‑25).

In 1904, Kapteyn, a Dutch astronomer, observed that half of the stars were moving in one direction, and the other half of stars were moving in the other direction. This revealed that the Milky Way is indeed rotating, and he drew a whirlpool representation of the Milky Way that was like Rosse’s image in Figure 1‑25.

In 1912, Henrietta Leavitt (Figure 1‑26) discovered Cepheid variable stars, which pulsated at a rate that was related to their brightness. This enabled her to calculate the distance to the stars and thus the distances to the galaxies that contained them. This ability to determine the distances to galaxies was one of the most crucial factors in the development of the Big Bang model.