What is Interstellar: Definition and 133 Discussions

In astronomy, the interstellar medium (ISM) is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space. The energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar radiation field.
The interstellar medium is composed of multiple phases distinguished by whether matter is ionic, atomic, or molecular, and the temperature and density of the matter. The interstellar medium is composed, primarily, of hydrogen, followed by helium with trace amounts of carbon, oxygen, and nitrogen comparatively to hydrogen. The thermal pressures of these phases are in rough equilibrium with one another. Magnetic fields and turbulent motions also provide pressure in the ISM, and are typically more important, dynamically, than the thermal pressure is.
In all phases, the interstellar medium is extremely tenuous by terrestrial standards. In cool, dense regions of the ISM, matter is primarily in molecular form, and reaches number densities of 106 molecules per cm3 (1 million molecules per cm3). In hot, diffuse regions of the ISM, matter is primarily ionized, and the density may be as low as 10−4 ions per cm3. Compare this with a number density of roughly 1019 molecules per cm3 for air at sea level, and 1010 molecules per cm3 (10 billion molecules per cm3) for a laboratory high-vacuum chamber. By mass, 99% of the ISM is gas in any form, and 1% is dust. Of the gas in the ISM, by number 91% of atoms are hydrogen and 8.9% are helium, with 0.1% being atoms of elements heavier than hydrogen or helium, known as "metals" in astronomical parlance. By mass this amounts to 70% hydrogen, 28% helium, and 1.5% heavier elements. The hydrogen and helium are primarily a result of primordial nucleosynthesis, while the heavier elements in the ISM are mostly a result of enrichment in the process of stellar evolution.
The ISM plays a crucial role in astrophysics precisely because of its intermediate role between stellar and galactic scales. Stars form within the densest regions of the ISM, which ultimately contributes to molecular clouds and replenishes the ISM with matter and energy through planetary nebulae, stellar winds, and supernovae. This interplay between stars and the ISM helps determine the rate at which a galaxy depletes its gaseous content, and therefore its lifespan of active star formation.
Voyager 1 reached the ISM on August 25, 2012, making it the first artificial object from Earth to do so. Interstellar plasma and dust will be studied until the mission's end in 2025. Its twin Voyager 2 entered the ISM on November 5, 2018.

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  1. Strato Incendus

    What can realistically go wrong on an interstellar journey?

    Alright, since I’m still stuck on my sci-fi story because I can’t exactly outline the mid-point plot twist with a realistic catastrophe on board an interstellar spaceship, I thought I’d widen the scope a little — towards full-on open brainstorming: What, if anything, can realistically go wrong...
  2. T

    I Voyager Spacecraft beginning to Power Down

    An interesting article on some of the history of, and discoveries by, the 44+ year old Voyager spacecraft s. The bean counters originally planned a 4 year mission for Voyagers 1&2. However, the scientists and engineers involved saw a golden opportunity that wouldn't repeat for another 176...
  3. C

    B How good are our ears? (Detectable Interstellar signals)

    How good are our ears / what can SETI and the like detect ? If we are listening for a radio signal broadcast by some advanced civilisation in our galaxy how sensitive would our "ears" have to be to detect a signal ? Obviously part of that equation would be how strong and how close the...
  4. Jarvis323

    I First Interstellar Object to Hit Earth

    I was hoping to hear some discussion about this eventually. It is one of the biggest events in all of human history if it is true isn't it? https://www.cnn.com/2022/04/13/world/interstellar-meteor-discovery-scn/index.html
  5. D

    Exploring Electron Rockets for Interstellar Travel

    Hi all, Why not build an electron rocket? Why won't this idea work? Seems like someone could just hitch an electron gun onto a spaceship. Boom! Electron Rocket. I was thinking about ion thrusters. These use ions to achieve thrust. They are problematic for achieving a high delta V because...
  6. elcaro

    Some ideas on interstellar space travel

    Science fiction is of course full of all kind of futuristic ideas about interstellar space travel and ways of propulsion, some more physically plausible then others. But within the current realm of what is physical possible, what could interstellar space travel be like? First you need a source...
  7. alan123hk

    Interstellar Spacecraft Voyager 1

    The interstellar spacecraft Voyager 1 was launched in 1977 and left the solar system to reach the endless interstellar space in 2012. It is now 2021. Voyager 1 has been sailing in space for 44 years. Why does it still have energy to send electromagnetic signals back to the earth? What does...
  8. E

    I Ray tracing in the movie Interstellar

    https://arxiv.org/abs/1502.03808 They are using Boyer-Lindquist coordinates for the Kerr metric. As far as I understand they also introduced a camera-fixed basis defining spherical coordinates ##\theta_{cs}## and ##\phi_{cs}## to keep track of the directions of the light rays relative to the...
  9. Giovanni Cambria

    A Is this solution for interstellar travel viable?

    I have in mind a way to enable FTL travel. Is this way viable? In the paper: "Weighing the vacuum with the Archimedes experiment" we can see the dependency of the gravitational repulsion exerted by Casimir Vacuum on the energy between the plates. The force goes as E / c^2. In the papers...
  10. V

    Orbital shutter as a beacon for interstellar communication

    Summary:: Hypotetical structure in the style of Dyson ring In the style of Dyson's proposal of an orbital superstructure, I wonder if in the specialized literature wasn't suggested that an evolved civilization could make a kind of orbital shutter, in order to delivery optical signals...
  11. C

    B The composition of interstellar dust

    I have seen that interstellar dust contains the detritus of supernova explosions. But whet else does it contain? How uniform is it? Does its compositions vary significantly from place to place? This question started with wondering whether one could back out the origins of the Earth from it's...
  12. S

    My arXiv preprint on a crewed interstellar spacecraft

    Hi everybody, I would like to share with you a crewed interstellar spacecraft which I have designed and called Solar One. It employs a combination of 3 propulsion methods: nuclear fusion, beam-powered propulsion , and photon propulsion. Basically, several compact fusion reactors power a...
  13. B

    What Is Logical To Trade Across Interstellar Distances?

    So in a setting with interstellar travel I was thinking about interstellar trade. I know the method of travel dicates what is traded, so I will elaborate below: Constant Acceleration: Refueling is virtually not a concern. Stargates: Ships can accelerate for months to reach just bellow...
  14. wolram

    B Exploring Interstellar Space: What Voyager 2 Has Taught Us

    With voyager 2 now going to interstella space what have we learned?
  15. TOMMiB

    The Pressure in Interstellar Space.

    n = 11000 because n = N/V and V = 1m^3. Using P = nRT/V I get that P = 7.32x10^6 Pa. considering that the gas is in a high vacuum, this value seems far too high to be true. Please help?
  16. Ophiolite

    B Do we have another interstellar visitor?

    In 2017 the intriguing Oumaouma object was determined to have reached us from interstellar space. Now another object, this time larger and likely a comet, has been identified, apparently from outwith the system. More here on the BBC site and The Minor Planet centre. Unlike its predecessor...
  17. Skyland

    Sub Brown Dwarf, rogue life

    Summary: Can an interstellar planet be hot enough to heat it's terrestrial moon without igniting fusion? I am a science fiction writer, however I am studying astrophysics to better captivate audiences with real and believable plots, The idea in question is an interstellar sub brown dwarf gas...
  18. Z

    B Interstellar Dust: What Blocks Light From Distant Galaxys?

    We know interstellar space and intergalatic space is not empty. Couldn't small dust particles block the light of distant galaxys?
  19. D

    B Interstellar Flight Path

    When an object is propelled from a solar system, does that object sometimes travel in a curve depending on the direction it travels away from the solar system, relative to the direction the solar system is traveling and to some fixed point in space. And if it enters a second solar system, that...
  20. K

    I Interstellar Travel vs Universe Expansion: Is Andromeda Reachable?

    I read in a so called sci-fi website from my country that as the universe is expanding we may not be able to get to certain galaxies in a possible interstellar travel. That information seems ok for me. But then there was a comment by a reader concluding from the website post that there is coming...
  21. A

    A What volume of interstellar space is needed to form a star?

    So, let me preface by saying I’m neither a scientist nor a mathematician, so am requesting some talented help here checking the accuracy of my source information and math. Regarding star formation, I got curious about how much volume of space in the interstellar medium is actually required to...
  22. RandyD123

    B What the movie Interstellar got right and wrong?

    We all know that getting too close to the gravity well of a massive object like a black hole causes time to move more slowly for you than it would for people on Earth. But is it possible for a planet to exist close enough to a black hole and have a person actually stand on that planet? And if...
  23. Y

    I Interstellar Travel Without Propulsion or Time Dilation

    I hope this is the right forum for this question. Imagine alien tech allows them to travel 1 million light years to Earth instantaneously. No thrust, vector, propulsion was involved. They didn't have to approach the speed of light, with its attendant increase in mass. Having arrived at earth, in...
  24. S

    How noticeable is an interstellar spaceship in cruise?

    From Pirx´s Tale: What actually was found inside the swarm, though: But the conclusion was: No evidence whatever, in short. The Earth of Pirx´ Tale could not spot a dead ship - an asteroid-like object - at a distance of 400 million km. It was only through chance presence of a spaceship...
  25. C

    B First Interstellar Asteroid Found

    The first asteroid ever seen from another solar system is whizzing through our own, and astronomers are racing to observe the visitor before it slips away. Links: Nature http://www.skyandtelescope.com/astronomy-news/astronomers-spot-first-known-interstellar-comet/
  26. G

    Interstellar Trade: Value, Resources & Unobtanium

    Lets suppose that in SF universe, due to hyperspace, travel between stars don't require planet killer energies. A mothership can transport kilotons of goods with energy of a few Tsar bombs, travel takes months. What could be valuable enough to transport between star systems? Are there solar...
  27. Noisy Rhysling

    Which interstellar "empire" do you like the most?

    Despite its flaws I find the empire in The Mote in God's Eye timeline to be very effective. Doesn't get too much in the way, but is ruthless about maintaining order. "Istvan Dies", for example, tells the story of a planet that is slagged to bedrock when it rebels and won't surrender...
  28. Ranvir

    B Why is interstellar travel so hopelessly difficult?

    So, I have been thinking about TRAPPIST-1 and how far away that system is from us. It is 40ly away from Earth, according to our frame of reference. This is often put in a way that makes one think that even at speeds close to that of light, it will take almost 40 years to get there. The muons in...
  29. Wrath_of_sloths

    A problem with interstellar travel

    Hi guys, I am writing an undergraduate screenplay about physicist. The physicist is working on a mission with the aim to fly a spaceship with human crew to an extremely distant planet with an environment suitable for humans. Its really not that important for the story, because the film is...
  30. Dr Wu

    Traveling through the Interstellar Medium

    I understand that the interstellar medium (ISM) consists mostly of neutral hydrogen gas, plus a smaller fraction of helium gas. Furthermore, I gather that between 0.1 and 1.0 atoms per cm3 correspond to the average density values pertaining in our "Local Bubble". The above, however, refers...
  31. Steven Hansel

    I Will interstellar travel become more possible in future

    Let's say we already colonized some of the planets and moons in our solar system. Mars,Moon, Europa and etc. Planets like Mars and other has less gravity than earth, so the resource needed for space travel in our solar system will reduce by a lot if we launch spaceships there. Say, that we...
  32. H

    B Time Dilation, Interstellar Travel, and "The Forever War"

    Dear community, I have no formal education in physics, but I think I have understood some of the basic concepts and ideas in SRT. I am currently trying to find a solution to a problem that came up during my reading of Joe Haldeman's "The Forever War", a Sci-Fi novel that deals with the impact of...
  33. Jigsaw674

    I Interstellar travel/centrifugal couching

    Given: original project Orion https://en.m.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) My idea is the addition of a centrifugal couch to regulate gravity to crew during the acceleration and deceleration phases of the trip. The Orion concept, for an interstellar ark concept, relies on...
  34. newjerseyrunner

    B Do we currently have interstellar com capabilities?

    In 1974 we sent a signal out towards a cluster of stars 25,000 light years away at a power of 1000kW. If we had been on the receiving end of that signal, would we have been able to detect it? Basically if you could have two identical Arecibo observatories, what's the maximum range in which...
  35. S

    Help Calculating a interstellar Speed requirements

    Hello everyone, what a fantastic community this is. I was hoping someone here could help me calculate the speed required to reach a certain distance in a certain amount of time. How fast will Object X be moving if it clears 28,163,529 kilometers in 0.0001 microseconds. We are looking for the...
  36. U

    A Calculating the ionization rate in the Interstellar medium?

    I hope putting this in the high energy section is the right section (if not, please let me know which would be more appropriate!) I felt this was appropriate since the work I am doing is high energy astrophysics. So I'm doing some research this summer, and my tasks were to take some data...
  37. A

    Using orbital spin to accelerate an interstellar launch

    Ok, so I know I'm missing something here. But, you know how pool balls that rotate opposite direction increase the speed of one of them through kinetic transfer? Why don't we build a launch platform based on this approach and use the orbital spin of the Sun, Jupiter, and Saturn (gaining speed...
  38. Prez Cannady

    I Detecting organic compounds in interstellar dust

    Two questions: 1. What tools and techniques are typically used to determine and plot spatial distributions of compounds--particularly organic ones--in interstellar dust? 2. Is it presently possible to determine distributions of compounds along arcs distant from the galactic center? If so, can...
  39. B

    Debunking Interstellar Travel: Separating Fact from Fiction

    Hi Folks, What is your opinion on this article? It suggest that interstellar travel is a fantasy. http://www.scientificamerican.com/article/interstellar-travel-as-delusional-fantasy-excerpt/# Yet, I read articles about institutions like NASA investing in various conceptual propulsion...
  40. T

    Project Orion again - why not interstellar probe?

    I was watching recently that PBS Space Video on youtube about space propulsion and it got me thinking about Project Orion again. They talk about sending humans to another star system, but would it not be spectacular enough to send probe? The probe could scan and photograph planets there and...
  41. T

    Interstellar travel and Special Relativity

    I always ask myself this question, and I think it is better to ask people that really know about this. Due to the impossibilities that Special Relativity poses, such as any object with mass not being able to travel at the speed of light. If we don't manage to at least scratch that speed, I...
  42. G

    FTL Communication in an Interstellar Civilization

    There are lots of sci-fi settings in which Mankind has become an interstellar civilization and lives on numerous planets throughout its galaxy. However, if the fastest we can communicate is at the speed of light, we will have to wait ages to receive a message between planets. Does this mean...
  43. Quasi Lunatik

    About the use of wormhole in the interstellar movie

    I am new to this forum and don't quite know if these types of questions are appreciated here or not but i have a question about the use of wormhole in the interstellar movie. We saw that they used the wormhole to travel to a distant place on the universe. When the main characters went to the...
  44. mileymo

    Relative Time in Space: What Speed for 1 Day Equals 1 Day on Earth?

    What speed does a ship traveling through deep space need to travel at for 1 day on the ship to equal 1 day on earth?
  45. mileymo

    Travel to Light Speed: Acceleration & Deceleration

    How long/what distance would it take a spaceship (with a hypothetical propellant-less engine) to accelerate to near light speed, and secondly, how low long/what distance would it take to decelerate back to zero again?
  46. mileymo

    Gravity, Time Dilation & Black Holes: Q&A

    I'm wondering if a ship orbited a black hole for a year, how large would the black hole be for it to equal 10,000 years on earth?
  47. T

    Question about interstellar travel

    My physics professor told us something about interstellar travel, which I have a question on. He told us that scientists figured out a way to send objects into space and frequently increase velocities. He said they do this by sending probes and aiming them at planets and large bodies in space...
  48. robbertypob

    Interstellar Movie - Conservation of Angular Momentum

    OK, so I'm right in the middle of watching Interstellar and I've just seen the part where they have docked with the 'mothership' and they fire up their engines to get it to rotate. This is so they can generate a centrifugal force to simulate gravity. My question is - first the ship was not...
  49. A

    Interstellar Exocomets Density & Impact Probability

    I'm trying to find reliable research on the density of interstellar exocomets per cubic lightyear beyond our solar system, particularly in the direction of the Hercules constellation if possible. From this, I want to extrapolate the statistical probability of one being captured by our solar...
  50. S

    Relativity Expert on Startalk Podcast: Interstellar & Near Light Travel

    I really hate, and also not fully understand it, when people talk about near light travel and they only mention about the stationary observers (people on earth). I forget her name but the relativity expert on the Startalk podcast about the movie interstellar, along with Neil, said that if you...