4 ms·

In addition to multiple sibling comments answering your actual question, I'd like to point out the following: > when we say that a star is N light years away a

by Beldin 3y ago

In addition to multiple sibling comments answering your actual question, I'd like to point out the following: > when we say that a star is N light years away and therefore it would take N years to reach at the speed of light, is that not true? That is not what "N light years away" means. It means that the light we're right now seeing from that star was emitted N years ago. But stars don't stay put. If you point a rocket at the star and fly N light years of distance, you won't end up inside the star -- it'll have moved on. This seems pedantic (and it is), but consider the expansion of the universe. You're seeing light from N years ago - when the universe was smaller. Suppose expansion is more or less uniform, i.e. something like "every 1000 years, each meter becomes 0.1% longer" (it isn't, but reasonable approximation). Then: the greater the original distance was, the faster the star is moving away due to expansion of the universe. After all, each meter in between expands x% per second, so more meters in between is greater increase in distance every second. If the distance is great enough, then each year, the universe expands that distance so much, it increases by more than 1 light year. That means that light from that star can nerve reach us again (unless the universe starts contracting). This phenomenon is called the Cosmological Horizon.

raattgift 3y ago
The millisecond pulsars in the international pulsar timing arrays https://ipta4gw.org/ https://ipta4gw.org/ are all within the Milky way. There is no metric expansion of space within the Milky way. The pulsars (and our solar system) are all on ~Keplerian orbits through the galaxy, and relative motion between us and them is small. > If the distance is great enough It's not, in this context. > Stars don't stay put Close enough, in this context. The uncertainties ("red noise") in PTA timing residuals from scattering in the interstellar medium and local properties (mostly internal structure) of the pulsars dwarf the uncertainties in relative motion. PTAs also care about the pulse timing not the carrier of the pulse. That is, the beam points at us with a predictable period and that period evolves (in parts per million) in the presence of nanohertz gravitational waves. The beam itself is broadband (radio to gamma rays) and any spectral line structure could be anything (most likely the interstellar medium). Detailed redshift studies look at the spread of spectral lines, particulary the Lyman and Balmer series, and pulsars are what you get when there's no hydrogen left to fuse. ("Finally, pulsars have broadband continuum spectra, so if there are gas clouds along our line of sight, pulsars can be used to probe the ISM via absorption by spectral lines of Hi or molecules. Such absorption spectra can be used to estimate pulsar distances" -- end of §6.2 of Condon & Ransom's Essential Radio Astronomy advanced undergraduate one-semester-course textbook (2016 ed.), web version at https://www.cv.nrao.edu/~sransom/web/Ch6.html https://www.cv.nrao.edu/~sransom/web/Ch6.html ) I have a related comment in this thread if you're interested.

by Beldin 3y ago

raattgift 3y ago

The millisecond pulsars in the international pulsar timing arrays https://ipta4gw.org/ https://ipta4gw.org/ are all within the Milky way. There is no metric expansion of space within the Milky way. The pulsars (and our solar system) are all on ~Keplerian orbits through the galaxy, and relative motion between us and them is small. > If the distance is great enough It's not, in this context. > Stars don't stay put Close enough, in this context. The uncertainties ("red noise") in PTA timing residuals from scattering in the interstellar medium and local properties (mostly internal structure) of the pulsars dwarf the uncertainties in relative motion. PTAs also care about the pulse timing not the carrier of the pulse. That is, the beam points at us with a predictable period and that period evolves (in parts per million) in the presence of nanohertz gravitational waves. The beam itself is broadband (radio to gamma rays) and any spectral line structure could be anything (most likely the interstellar medium). Detailed redshift studies look at the spread of spectral lines, particulary the Lyman and Balmer series, and pulsars are what you get when there's no hydrogen left to fuse. ("Finally, pulsars have broadband continuum spectra, so if there are gas clouds along our line of sight, pulsars can be used to probe the ISM via absorption by spectral lines of Hi or molecules. Such absorption spectra can be used to estimate pulsar distances" -- end of §6.2 of Condon & Ransom's Essential Radio Astronomy advanced undergraduate one-semester-course textbook (2016 ed.), web version at https://www.cv.nrao.edu/~sransom/web/Ch6.html https://www.cv.nrao.edu/~sransom/web/Ch6.html ) I have a related comment in this thread if you're interested.