What is Light years: Definition and 53 Discussions

The light-year is a unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometres (9.46×1012 km) or 5.88 trillion miles (5.88×1012 mi). As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in vacuum in one Julian year (365.25 days). Because it includes the word "year", the term light-year is sometimes misinterpreted as a unit of time.The light-year is most often used when expressing distances to stars and other distances on a galactic scale, especially in non-specialist contexts and popular science publications. The unit most commonly used in professional astronomy is the parsec (symbol: pc, about 3.26 light-years) which derives from astrometry: it is the distance at which one astronomical unit subtends an angle of one second of arc.

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  1. D

    I Question about Special Relativity: Mary from planet B flys at 0.8c to Tom on planet A...

    Quite simple question. There are two planets A and B, they are 4 light years apart measured by tom on planet A. Say both A and B remain stationary. Mary flys 0.8c to tom on planet A. When she passes planet B she should see Tom or planet A 2.4 light years away, am I correct? What about tom now...
  2. Cerenkov

    B The distance to the nearest identical Hubble volume in light years?

    Hello. I was wondering if I could please have a little help converting a double exponential that is expressed in metres into light years? My math simply isn't up to the task. https://en.wikipedia.org/wiki/Multiverse Level I: An extension of our universe A prediction of cosmic inflation...
  3. TaurusSteve

    Stargazing Convert Light Years to Miles: 5.879e+12 Explanation and Conversion Guide

    5.879e+12. So that is what comes up if you convert one light year to miles. My question is, what does the 'e' mean? I see it quite a bit.
  4. S

    B We see a quasar 13 billion light years away. What does this mean?

    Summary: If the furthest quasars we can see are let's say 13 billion light years away from us, then does this mean that the distance between us and that quasar was 13 billion light years at 13 billion years ago? To anyone educated in physics this might be a silly question but to me this is...
  5. D

    Worm Hole light years away and look at Earth through a giant telescope

    Summary: I'd like to know if anyone else thinks of these things <mentor moved to general discussion> What if one day we can "warp" x light years away from Earth, build an enormous telescope, and look back at the Earth x years ago with resolution that rivals the American flag on the moon? Do...
  6. H

    B What do they mean when they say something is so many light years away?

    Just as the title says, I am trying to figure out what they are actually telling us when they say something is so many light years away. If you were to search the internet "what is the most distant object ever observed" you will be told it is a galaxy 13.3 billion light years away. Do we...
  7. T

    I Einstein Proved Right 500M Light Years Away

    It had previously been verified within our own galaxy. http://uopnews.port.ac.uk/2018/06/22/einstein-proved-right-in-another-galaxy/
  8. MermaidWonders

    MHB Solving for the Speed to Reach a Star 240 Light Years Away

    How fast would you have to go to reach a star 240 light years away in an 85-year human lifetime? Here, I know that I'm supposed to find $v$, but I'm having a hard time setting up my equation(s) in order to reach the final answer. :(
  9. chloeid

    How do I work out units of velocity from light years?

    Homework Statement I'm currently working through the following question: "A spaceship travels from Earth to the vicinity of the star that is measured by astronomers on Earth to be six light-years away. The spaceship and its occupants have a total rest mass of 32 000 kg. Assume that the...
  10. Tareq Naushad

    When I lit a laser ray for 10 seconds from a distant planet

    I am on Alpha 4 light years away from earth. On 1st Jan, 2016 I lit a powerful laser light towards Earth for 10 seconds and then switched it off. My friend on Earth knows that he has to detect that light after 4 years on the night of 1st Jan, 2020 from a space station using a very powerful...
  11. Saerryan89

    Time Travel: How Far & Fast to Reach Alien Neighbors?

    I was discussing with a friend about speed of light and time traveling and I realized something bizarre. I could be wrong so please correct my mistakes. So let's assume we want to go to our alien neighbour whose planet is 100 light years away. When we travel there with near light-speed, we...
  12. BWV

    Distances in km/s vs mega parsecs or light years

    I know it has something to do with the rate of expansion, but why are large intergalactic distances measured in km/s rather than units of pure distance like mega parsecs? What is the conversion?
  13. G

    Age of the observable Universe?

    Forgive my ignorance?.. If we can see 13.8-ish billion light years away how can the universe be the same age? Matter cannot travel at the speed of light, so how are we as far away (in light years) as the universe is old?
  14. RandyD123

    Very Powerful Telescope to see 10 light years away at high resolution....

    Lets suppose I had a very powerful telescope and I could see 10 light years away. Would I see cars and people on that planet as they were 10 years ago? Would I actually see the people moving and the cars moving on the ground?
  15. J

    Measuring Light Years: How Far Is the Andromeda Galaxy?

    I was reading up on the Andromeda Galaxy and it was saying that it was 2.2 to 3 million light years away from earth. So my question is how do we measure how far it is in light years?
  16. Constantine

    Flaw in Traveling to Destination Several Thousand Light Years Away

    Where has my thinking gone wrong in the following? A man begins traveling to a destination several thousand light years away, accelerating to a velocity of near-light speed. Time dilations and space contractions would result in the trip lasting only a few years from his perspective. From the...
  17. N

    Reading of planets billions of light years away

    We can see galaxies/planets light years away determine their composition such such as if they have water et cetera. My question is if light takes billions of years to reach us from there how is that we can determine such things? Is the speed at which a telescope can zoom faster than the speed at...
  18. N

    Light Years: Subjectivity and Distance Measurement

    Light years-I just don't understand how something as subjective as visibilty, is used as a major indicator of distance. Isnt that subjective. What if some species had a million times the eye power than us humans, wouldn't they view the distance of let's say stars very differently because it...
  19. Crazymechanic

    Exploring Light Years and the Concept of Time in Physics

    Well today I was reading much about the subject of light and photons.And we agree that photons are massless and taht is the reason they travel at c. But if something travels at c then it means it has no time as is said and if that thing traveling at c has no time then it means for a observer...
  20. T

    Why did we stop at 46.5 billion light years?

    It's been bugging me. We've looked 13.5 billion years into the past, supposedly only about 500 million years short of the big bang, at objects 46.5 billion light years away. What were the limiting factors? What stops us so close from the finish line from peeking into the very beginning? If it's...
  21. B

    Calculating Distances 1 Billion Light Years Away

    If something is supposedly 1 billion light years away from us, how are we able to calculate this..? Wouldn't we have to send a light out and/or x/-ray out to that point and have it bounce back, calculating the time it takes to bounce back to us..? If so, it will take 1 billion years to get there...
  22. H

    10 light years but destroyed 6000 years ago?

    https://lh5.ggpht.com/NsJ2tAGpQI0QP2_P31FfOXryx0O3em1mSGLvBjfZcdcGux20sksxwyPJNvLGlMSs7Aw6pmA=s167 The Pillars of Creation are 10 light years away from us, but are said to be already destroyed 6000 years ago. In the picture above, it say that we still have a millenneum to observe them. But...
  23. I

    Unit Conversion: Astronomical Units, Parsecs and Light Years

    Hi everyone, It's been years since I solved a physics problem for school. Now that I'm back at it I could use some help on this problem: Question "The astonomical unit (AU) is defined as the mean centre to centre distance from Earth to the Sun, namely 1.496 x 10^11 m. The parsec is...
  24. J

    Number of stars within 150 light years of us?

    I would like to know how many stars are within 150 light years of Earth. There must be a website somewhere that has the number, or perhaps a graph correlating distance with number of stars. I've done some searching, but you folks are much smarter than I am. Thanks
  25. T

    Converting light years to meters and miles

    Homework Statement A light-year is the distance that light travels in one year. Find this distance in both miles and meters. Homework Equations Speed of light (c) = 3.00x10^8 m/s The Attempt at a Solution 10^15 m/s __________ 3.00x10^8 m/s That is to find meters...I think I can only find...
  26. U

    Travelling to a Star 40 Light Years Away: Time and Distance Calculations

    A star is 40 light years from Earth. (a) How far would you measure this distance to be if you traveled it in a spaceship moving at 1.00X10^8 m/s (b) How long would the trip last (for you)? Can someone please help me out with this question and what equations I should use to solve this...
  27. S

    Measuring Distances Across Billions of Light Years

    I hear a lot about astronomers using type IA supernovae to accurately measure large scale distances over billions of light years to do things like measure the rate of expansion of the universe. I doubt that the calculation of this distance is simply based on the inverse square law. So my...
  28. R

    13.1 billion light years object observed.

    Last year Astronomers observed a self-destructing star 13.1 billion light years away. Happening only 630 million years after the big bang. While it is amazing that we are able to observe objects that far away, questions arose. Now for a moment, let's imagine our universe in it's 3 axis (y, x...
  29. D

    How can we see stars past 13.5 billion light years away?

    okay. i understand that stars can 46 million light years away despite the universe only being 13.5 billion years old. The universe is expanding. However what i dont' understand is how can we see a star that is say 20 billion lights years away if it takes 20 billion light years for the light to...
  30. E

    Likelihood of light being absorbed/re-emitted in 50 light years

    What's the likelihood of light striking some interstellar gas and being absorbed, then re-emitted, and the re-emitted light actually being the light we see rather than the original light from the source? Also, what about the likelihood of reemission after passing through the heliosphere...
  31. F

    Universe 93 billion light years?

    (http://en.wikipedia.org/wiki/Observable_universe) If the universe is 13.7 billion years old, how is the diameter 93 billion light years? Light travels 1 light year for every year. Then if two particles were expanding outwards to form the size of the diameter, it would be...
  32. S

    Supernova energy reached by detector disk 1560 light years away

    Homework Statement A super nova releases 1.9E45 J of energy over a 10 day period. It is 1560 ly from earth. A detector, facing the star, is a disk of radius 7cm. How much energy reaches the detector? (The detector is in orbit, so it can face the star for the entire 10-day period) Homework...
  33. E

    Calculating strength gravity over light years

    How do I calculate the strength of gravity between two objects say that 2 years ago were 2 light years apart, during which they became 0.5 light years closer, if during those two light years, the stars lost 50% mass due to fusion and light, based on their mass "now", or based on how much...
  34. G

    Exploring 50 Giga Light Years of Universe

    Ok, someone was kind enough to point out to me recently the obvious fact that the universe must be much larger than what we can see. Suddenly I feel like I'm literally trying to fit all of it into my head! I've always had this nagging feeling about inflation, but it's never come up to the...
  35. K

    How do I calculate light years to years?

    I have a question, sparked by http://http://www.cnn.com/2009/TECH/12/16/super.earth.discovery/index.html" article, so I figured this would be the best place to put this. That is said to be 40 light years away... So if we're seeing a planet 2.7 times the size of Earth with liquid water on it...
  36. X

    Cmb 13.7 billion light years away

    Cosmic Microwave Background radiation is coming to us from about 13.7 billion light years away... is that correct? The reason its coming from 13.7 billion ly away is because the universe is 13.7 billion years old... is that correct? so when the universe was only 12 billion years old the...
  37. S

    The universe 700 trillion light years away.

    Our universe 700 trillion light years away. Could our universe 700 trillion light years away be alien to the observable universe? Containing objects that don't exist in the observable universe for example?
  38. B

    Size of the Universe is 13.7 billion light years?

    Hi. I'm not a student of physics, so please forgive me for my ignorance. I only have a very simple question. If the observable universe is about 93 billion light years in diameter, how could the universe have reached that size in only 13.7 billion light years? Doesn't it stand to reason...
  39. P

    0.16c how fast would it take you to get 4 light years?

    I was wondering, if you where going at 0.16c how fast would it take you to get 4 light years?
  40. G

    Why would entangled particles light years away need to be able to cancel?

    Ok, here's my problem... In the EPR experiment it is described that if entangled particles are required to be able to cancel each others spin's then no mater how far apart these particles are, if one is measured you can instantly infer the state of the other particle. Why would the other...
  41. M

    Assume that on earth we see something 1 billion light years away

    This is probably an elementary exercise, but my head spins trying to think about it. Assume that on Earth we see something 1 billion light years away (i.e. light took 1 billion years to reach us). To simplify, assume no relative proper motion, although the distance will change due to universe...
  42. R

    Viewing size of Galaxies 7 Billion light years away?

    Would a galaxy 7 billion LY away appear to be twice the expected size (angular size) as a consequence of the expansion of the universe? (Assuming the universe is 14 Billion years old). My math says yes. But of course, I am just using trig and geometry. Just curious.
  43. J

    Speed of Light vs Light Years

    Hi, This is probably a beginners question, but ill ask it anyway as it has me confused.I have read quite a few posts from clever people here (so I assume its correct) that the closer to light speed you get, the slower time gets, and also distances get shorter. Effectively meaning that, say for...
  44. P

    The Speed of Light: If My Arms Were 10 Light Years Long

    If my arms were as long as 10 light years and I waved, wouldn't they be going faster than the speed of light?
  45. A

    Assume I travel near c from a planet 5 light years away

    to the earth. I will arrive at Earth a little over 5 years from the time I depart, correct? How many years will I have aged as a passenger on the spaceship and how long will someone on Earth have aged during my journey?
  46. S

    There could be Intelliengent life 26 light years away

    http://www.guardian.co.uk/science/story/0,,1713604,00.html Here's another article http://science.monstersandcritics.com/news/article_1131203.php/Stars_searched_for_extraterrestrials I'am not sure if there would be aliens there but it better then nothing.I wonder if seti has looked at...
  47. R

    Balck hole within 20 light years?

    Can you get a thing like that within the sol's closest space? I mean, if there was one somewhere between us and say alpha centauri or barnard's star, is it possible that you cannot feel it's gravity force?
  48. T

    Gravitational Force: Separated Bodies 5 Light Years Apart

    Two bodies are separated in space through 5 light years.Suppose if one body is destroyed by some explosion .How long will the gravitational force act on other body.Whether it will affect only after 5 years?
  49. A

    Parsecs, trigonometric parallax and light years

    Hey I'm really confused about these things above (well except for light years). How would you get a distance in parsecs and in light years if there was an annual parallax of say 0.2 arc seconds. I'm just revising for exams but am really confused now! Thanks.
  50. Link

    Prove that 1 parsec = 3.26 light years?

    How do you prove that 1 parsec = 3.26 light years? My physics bok have a really harsh explanation for this. :frown: