cosmic distance ladder is how we measure the distance to other galaxies.
cosmic distance ladder is how we measure the distance to other galaxies.
how do we measure the distance to other galaxies?
cosmic distance ladder
distance measurements in astronomy have historically been and continue to be confounded by considerable measurement uncertainty. in particular, while stellar parallax can be used to measure the distance to nearby stars, the observational limits imposed by the difficulty in measuring the minuscule parallaxes associated with objects beyond our galaxy meant that astronomers had to look for alternative ways to measure cosmic distances. to this end, a standard candle measurement for cepheid variables was discovered by henrietta swan leavitt in 1908 which would provide edwin hubble with the rung on the cosmic distance ladder he would need to determine the distance to spiral nebula. hubble used the 100-inch hooker telescope at mount wilson observatory to identify individual stars in those galaxies, and determine the distance to the galaxies by isolating individual cepheids. this firmly established the spiral nebula as being objects well outside the milky way galaxy. determining the distance to "island universes", as they were dubbed in the popular media, established the scale of the universe and settled the shapley-curtis debate once and for all.
observational cosmology
in the 1960s and 1970s, research to further understand the properties of seyfert galaxies was carried out. a few direct measurements of the actual sizes of seyfert nuclei were taken, and it was established that the emission lines in ngc 1068 were produced in a region over a thousand light years in diameter. controversy existed over whether seyfert redshifts were of cosmological origin. confirming estimates of the distance to seyfert galaxies and their age were limited since their nuclei vary in brightness over a time scale of a few years; therefore arguments involving distance to such galaxies and the constant speed of light cannot always be used to determine their age. in the same time period, research had been undertaken to survey, identify and catalogue galaxies, including seyferts. beginning in 1967, benjamin markarian published lists containing a few hundred galaxies distinguished by their very strong ultraviolet emission, with measurements on the position of some of them being improved in 1973 by other researchers. at the time, it was believed that 1% of spiral galaxies are seyferts. by 1977, it was found that very few seyfert galaxies are ellipticals, most of them being spiral or barred spiral galaxies. during the same time period, efforts have been made to gather spectrophotometric data for seyfert galaxies. it became obvious that not all spectra from seyfert galaxies look the same, so they have been subclassified according to the characteristics of their emission spectra. a simple division into types i and ii has been devised, with the classes depending on the relative width of their emission lines. it has been later noticed that some seyfert nuclei show intermediate properties, resulting in their being further subclassified into types 1.2, 1.5, 1.8 and 1.9 (see classification). early surveys for seyfert galaxies were biased in counting only the brightest representatives of this group. more recent surveys that count galaxies with low-luminosity and obscured seyfert nuclei suggest that the seyfert phenomenon is actually quite common, occurring in 16% ± 5% of galaxies; indeed, several dozen galaxies exhibiting the seyfert phenomenon exist in the close vicinity (≈27 mpc) of our own galaxy. seyfert galaxies form a substantial fraction of the galaxies appearing in the markarian catalog, a list of galaxies displaying an ultraviolet excess in their nuclei.
seyfert galaxy