vefsharp.blogg.se

Hubble's velocity-distance relationship, published in 1929, suggests that once we look beyond the gravitational effects of close galaxies within the local group, galaxies are moving away from one another. Where H 0 is Hubble's constant, v is the recession velocity and d is the distance. This is shown in the diagram below that shows the images and spectra of some of the galaxies he observed.

Hubble found that that those with a smaller image in a photograph had higher redshifts. The majority of galaxies therefore appeared to be moving away from our own galaxy. A few nearby galaxies had velocities that meant they were moving towards our own Milky Way, that is their lines were blueshifted but most exhibited redshift and hence had recession velocities. By measuring the amount of shift of specific spectral lines relative to those produced by reference arc lamps in the spectrograph he was able to calculate values for the galaxy velocities. He took long exposures of the spectra of faint galaxies. Hubble, aided by Milton Humason extended the work of Slipher by using the larger Hooker telescope. We now call these galaxies instead of island universes.

Taken together the evidence eventually convinced astronomers that "spiral "nebulae" such as Andromeda were in fact separate "island universes" of billions of stars like our own Milky Way but more distant. These speeds exceeded that of any known star in the Milky Way. Whilst he calculated that the Andromeda spiral was moving towards us at 300 km.s -1 he soon found others moving away at 1,100 km.s -1. He interpreted this as meaning that they were in fact moving away relative to us so their spectrum was shifted to longer (ie redder) wavelengths).

By 1925 he had shown that most exhibited redshifts in their spectral lines. He had started measuring the Doppler shift of spectral lines from spiral nebulae in 1912. Supporting evidence came from the work of Vesto Slipher at the Lowell Observatory in Arizona. By 1924 Hubble had calculated that the distance to the Andromeda Nebula was 900,000 light years. He could then apply the period-luminosity relationship to calculate the distance to the stars and hence the distance to the Andromeda Nebula that they were in. This allowed Hubble to observe the Cepheids over time and measure their varying brightnesses to determine their periods. Earlier, in 1908 Henrietta Swan Leavitt, working at the Harvard College Observatory discovered that Cepheid variables in fact obey a period-luminosity relationship the longer the period of a Cepheid, the more intrinsically luminous it is. These are periodic variable stars, that is stars that vary their brightness in a regular way. In 1923 he identified some of the stars within it as Cepheid variables. Hubble used the 100 inch Hooker Telescope on Mount Wilson, then the largest telescope in the world, to study the Andromeda Nebula, M 31. The advent of spectroscopy and photography in that late nineteenth century when used on the latest generation of large reflecting telescopes in the early twentieth century provided astronomers with the tools to study these objects. One of the unsolved problems in early twentieth century astronomy was the question of what were nebulae? These gaseous, fuzzy clouds were thought by some astronomers to be embryonic solar systems forming while others thought that they were "island universe" like our own Milky Way galaxy. Using the largest telescope of his time, the 100 inch Hooker telescope on Mount Wilson in California, he helped resolve one of the great debates in early twentieth century astronomy. He went on to have a profound influence on our understanding of the Universe. Trained initially as a lawyer, Edwin Hubble moved to astronomy in 1914, working at the famous Yerkes Observatory near Chicago. Edwin Hubble at the camera on the Hooker 100 inch telescope on Mt Wilson.