Bob Sacamano;3595033 said:
Yes, but seriously. How are we able to measure something that is trillion miles away?
First step, get an idea of the scale of the universe. This was first done by parallax, measuring the apparent image shift of an object from two separate places. The greater the separation of those two places and the closer the object, the greater the parallax. The best example of this is holding a finger a few inches from ones face and alternately closing one eye and then the other. The finger seems to shift back and forth. The problem with doing this on stars is that they are so far away that the apparent shift is minuscule even through a telescope. What is more, early astronomers assumed that the brightest stars were the closest. They had no way of knowing that stars come in different sizes.
Eventually, however, they figured that out through trial and error. By measuring close stars six months apart (a perspective change of 186 million miles) they figured out that even the closest stars were very far away indeed. Stars beyond the closest ones are too far away even for that huge baseline to notice any image shift.
Two additional discoveries gave further yardsticks. The first was the color of stars. They are not all the same and physics predicted that certain colors indicate certain temperatures. Since stellar temperature is caused by nuclear fusion and since a greater density is needed for greater heat AND since greater mass is need for that greater density; an idea of the sizes of stars could be ascertained from their color. While odd-balls do exist usually at the end of a star's life, typically they range from cool, tiny (for a star) red dwarfs, to somewhat larger yellow dwarfs like our sun, to bright white stars like Vega to monstrous and short lived blue giants. Since luminosity can be determined to some degree by color, distance can, therefore, be guessed at by measuring the star's apparent brightness from Earth. Dimmer usually = farther away. I say guessed at because there are two curve balls here. One is that some stars are oddballs and their colors do not reflect their luminosity. The other is that intervening matter can make a star look dimmer than its distance would indicate.
The second (which may have been the earlier discovery) is spectral shift. Stars have spectra just like any other light source. Those spectra contain black absorption lines that indicate the presence of certain elements. These lines exist at specific places in the spectrum. Like the Doppler-effect in sound, all stars show these absorption lines shifted from their normal position to either the blue end or the red end. A blue shift means that the object is approaching. Red is retreating. What is more, the degree of the shift indicates how fast it is moving toward us or away from us.
Among the stars, the the red and blue shifts were pretty evenly matched in number. What was odd, however, is that certain nebulae had dramatic spectral shifts. A few, like Messier 31, the great spiral Andromeda Nebula were blue shift. Most, however, were red shifted and to a much greater degree than any star. By the early 20th century, astronomers understood the vast distances among stars. It was believed that billions exists arranged more or less in a disk shape with bizarre clumps of stars called globular clusters hovering just outside. This spectral shift data was something new and unexpected.
Early 20th century astronomers like Hubble and Harlow Shapely studied these spectral shifted nebulae and found that they seemed to be universes in miniature with thick dusty regions and their own tiny globular clusters. Obviously this together with the spectral shift data meant that what had been considered to be the universe was but a single island in the cosmic ocean; that these nebulae where sovereign galaxies in their own right. Relatively close galaxies like Andromeda are rushing toward us at breakneck speed. This accounts for the great blue shift. The vast bulk of galaxies, however, are rushing away from us. What is more, the faster they are moving the further away they are.
Recently, astronomers have been using supernovae of a particular variety to measure distance. Supernovae are the astonishing brightening of a massive star during the final hours of its life. When in progress, they are the brightest objects in the universe. A certain variety of supernovae is created when gas from a neighboring star gets dumped into either a white dwarf or a neutron star, I forgot which, causing it to reach critical mass and explode. These supernovae are always the same brightness and, therefore, can be used to measure cosmic distances. It was this method that caused scientists to discover that the expansion of the universe is accelerating.
Advances in measurement and in the understanding of cosmic physics with relativity has lead to greater and greater accuracy in distance measurements. As unimaginably gigantic as our own galaxy is (a beam of light would need 100,000 years to cross it), the universe is even more mind-****ingly enormous. The farthest objects, protogalaxies, are so remote that light reaching us today has been traveling for 13 billion years.
So to recap, scientists know how far away stars are by parallax, spectral shift, apparent brightness including supernovae and comparison to previously acquired data
