AAS #4: Supernova expands as we watch!

By Phil Plait | January 6, 2009 10:32 am

Astronomers using the Chandra X-Ray Observatory have just released an extraordinarily cool animation. Composed of images taken between 2000 and 2007, it shows the supernova remnant Cas A literally expanding as you watch it!

See the little white dot in the center? That used to be a star, a big one. About 330 years ago*, that star blew up in a titanic supernova explosion. Several octillion tons of material screamed outward from the star at a good fraction of the speed of light, leaving behind an ultradense neutron star, the remains of the star’s core. So, that dot in the center? It’s an object a dozen kilometers across that has about the same mass as our entire Sun. It has a density of about 100 million tons per cubic centimeter. That’s roughly the mass of every single car in the United States, crushed into a cube the size of, oh, say, a mini marshmallow.

Yeah.

The outer layers of the explodey star are what you’re seeing here in this animation. Rushing outward at thousands of kilometers per second, they are heated by the explosion itself, by the decay of radioactive materials forged in the blast, and by magnetic processes when the spinning neutron star interacts with the gas. All of these together heat the gas to millions of degrees, and it glows in X-rays. The Chandra X-Ray Observatory can see these high-energy photons, and so astronomers use it to observe Cas A (so called because it is in the constellation of Cassiopaeia, and is also a radio source, which were traditionally named after letters in the alphabet).

In fact, Chandra observes this supernova remnant quite often, so the astronomers took the images made over the past 8 years and created this animation. It shows the expansion of the debris as the gas moves outward from the point in space where the original explosion happened. It’s like watching a nuclear blast in super duper slomo.

Still images showing the expansion. Credit: NASA/CXC/SAO/D.Patnaude et al.

But don’t be fooled by the apparent peacefulness and calm you see here. Supernovae explosions are among the most violent events in the Universe. If one were to happen within 20 light years of the Earth, it could cause a mass extinction through the destruction of our ozone layer. Happily, no star capable of blowing up is that close. Cas A is about 10,000 light years away, so we’re in no danger from it either.

The flip side of that is that the iron in our blood, the calcium in our bones, and even the gold in our jewelry is created in supernovae blasts. The incredible heat and pressure of the explosion forces the gas to undergo nuclear fusion, alchemically creating the heavy elements life is based on. A supernova could wipe out life on a nearby planet, but we literally owe our existence to such supernovae. I could go into some kind of "Lion King" Circle of Life speech, but I think you get my point.

An interesting tidbit: it’s been thought for years that high-energy particles that slam into our atmosphere all the time, called cosmic rays, were accelerated to their high speeds in supernovae remnants like Cas A. Interestingly, this animation indicates the gas is expanding more slowly than it should given how much energy is released in the explosion, which in turn means that lost energy is going somewhere. Astronomers think it’s going into the production of those cosmic rays, which eat up about 35% of the explosion energy.

Animations like this are more than just cool to look at (though they are very cool to look at). They can reveal a huge amount of information about the explosions as well. One of my favorite things about astronomy — and I have a lot of favorite things — is that we can create gorgeous images and movies, and they are a wonder to see… but they also teach us stuff. And not only that, the stuff we learn is fundamental to our very existence: information on how we got to be here, how the elements in our bodies were made, and even how the Universe itself changes over time.

How amazing is that?


* Whenever I mention distances and time, people get confused. Casa A is 10,000 or so light years away, so don’t I mean 10,330 years ago? No, I don’t. This is terribly confusing, and someday I’ll write up a total explanation, but because of relativity, Einstein, and the speed of light, you can think of time flowing at the same speed as light. Literally, as far as we are concerned, that star really did blow up 330 years ago, not 10,330.

CATEGORIZED UNDER: Astronomy, Cool stuff

Comments (63)

  1. MarkH

    “…that star really did blow up 330 years ago, not 10,330.”

    No, it really didn’t, and there’s no confusion. The light from the nova didn’t get here instantly from 10,000 light years away; it took a long time. Phil, I get what you’re saying, and completely disagree with it. The star *is* that far away, and the light *did* take that long to get here. There’s no advantage in saying it didn’t.

  2. Andrew

    Devil’s advocate: what advantage is there in saying that it did?

  3. MarkH: That is incorrect. I suggest you read up about simultaneity and relativity. It doesn’t matter how long it took to get here; the speed of light is the upper speed limit of the Universe, and so not only can we not know what is happened “right now” at Cas A, relativistically speaking the question itself is meaningless. They way we see it through our telescopes is the way it is “right now”.

  4. PG

    As Phil said, this is really a conversation for another (LONG) post.

    He could have said, “The explosion entered our detection horizon 330 years ago,” or “We entered the explosions future light cone 330 years ago,” or “The explosion event entered our past light cone 330 years ago,” but if you define “now” as all events just entering our past light cone, Phil is right. If you define time the way Newton did (with “absolute time”, which we know was NOT correct), then the explosion happened 10,330 years ago. That’s the brief explanation, as I see it.

  5. AlexBenjM

    What I find amazing about the animation is that it reminds me the Universe is a dynamic evolving place. Staring at frozen snapshots of celestial phenomenon, it’s too easy to forget that and unconsciously think of Universe as a piece of frozen crystal.

    Would it be a new golden age of public appreciation of astronomy if we were able to launch devices into the outer-space designed to take continuous video recording of events in a wide range of wavelengths?

  6. The holidays make me expand, too.

  7. Bjoern

    I’ll side with MarkH here, and disagree with Phil. Yes, I know that the concept of simultaneity is essentially meaningless in relativity – but only if one considers different frames of reference! What is simultaneous in one frame of reference is not simultaneous in another (if the other is moving with respect to the first one). That’s indeed right.

    But if we consider only *one* reference frame and stay in that one (sensibly the one in which we ourselves are at rest; one could also sensibly choose the one which is at rest
    WRT the source of the CMBR, something like a “universal rest frame” – and yes, I know that that would not be an “absolute” frame of reference), there is no real problem – in that frame of reference, the SN did *really* happen 10330 years ago.

    Saying that the SN happened 330 years ago makes as much sense as saying that the CMBR we observe has just been emitted in the moment in which we observe it! In that case, Phil has no problem at all to say that it was emitted 13.7 billion years (+ about 390 000 years ;-) ) ago; so what’s the big difference to the SN we’re talking about here?

  8. IVAN3MAN

    I understand what you are saying, Phil, but expect the bloody young-Earth creationists to quote you out-of-context and use your words to give credence to their BS.

  9. A brief historical perspective…

    Q: When was Cassiopeia A discovered?
    A: More than 60 years ago, way back in 1941 the radio pioneer Grote Reber detected 3 strong radio sources falling roughly along the plane of the Milky Way in the constellations Sagittarius, Cygnus, and Cassiopeia. These later became designated Sagittarius A, Cygnus A, and Cassiopeia A because each is the strongest radio source in its respective constellation. I find it ironic that each of these three represents a completely different astrophysical object:

    1.) Sagittarius A is associated with the supermassive black hole at the center of the galaxy

    2.) Cygnus A is an extragalactic radio source associated with a distant radio galaxy (presumably the association with the plane of the Milky Way is entirely accidental)

    3.) And Cassiopeia A is the remnant of a supernova

    The interesting thing about Reber is that he was an amateur and completely self-funded. For more than a decade he was the only radio astronomer on the planet. I can’t think of another amateur scientist within the last century who contributed as much to science as Grote Reber did, and all without one dime of federal or state money.

  10. Ooops, forgot to include a good link if you want to read about Grote Reber:

    http://www.bigear.org/CSMO/HTML/CS13/cs13p14.htm

    A very cool guy.

  11. Nyx

    I have to ask for that long post, Phil. As an engineer, my education on relativity was in a single physics course broken into thirds. The professor was forced to cram Thermodynamics, Optics and Special Relativity down our throats in one semester, leaving me a little staggered. My understanding is that the faster you go, the more you effectively slow down time, therefore going the speed of light means that time is, for all intents and purposes, almost stopped? I really have a hard time wrapping my mind around these concepts.

    I look forward to a good post on relativity.

  12. Elmar_M

    Ok Phil, please explain because I am very, very interested in how it is possible that light from a source 10,000 lightyears away reaches us within 330 years. I could understand if it was a little less than 10.000 years (e.g. we moving towards the position the star was at when it exploded and the star moving away from us at the same pace,or something like that), but this difference seems very hard for me to understand.

  13. Levi

    Hey, what a coincidence! I just received my order for a book with a whole chapter on how explodey stars can bring about the end of the world.

  14. DaveS

    Fine, redefine the common meaning of “now”. Scientists redefine words all the time. But I thought astronomers referred to “looking back in time to close to the Big Bang” when they use new technology to look farther. If you define “now” to be whatever is hitting your eyeballs/CCDs/sensors, then you’re not really looking back in time, right? I mean, much farther away, the Big Bang is happening now, right, since that’s when the light’s hitting our eyes, under your definition of “now”?

    Wouldn’t it be more precise to say “330 years ago we could have seen Cas A explode, 10,000 light years away, and now, 330 years later, we see an expanding gas cloud”?

  15. Bjoern

    @PG:
    You don’t need “absolute time” in order to say that the SN explosion happened 10330 years ago. In *any* reference frame which is at rest WRT the Earth (or moving only at speeds very small compared to light speed; oh, and the gravitational potential also shouldn’t be vastly different), the SN explosion happened 10330 years ago.

    A bit more mathematical: let’s choose a frame of reference which is at rest WRT Earth, with
    its origin at the center of the Earth, with t = 0 defined as “now”, and with the x-axis pointing to Cas A. Then the event “SN exploded” has the coordinates x = 10 000 light years (give or take a few ;-) ), y = 0, z = 0 and t = -10330 years. The event “SN is observed on Earth” has the coordinates x = 0, y = 0, z = 0, t = -330 years.

    In other frames of reference, these coordinates will obviously have different values (even when “t = 0″ is will be different, depending on your location, in a frame of reference which moves WRT to the one we chose above) – but that’s entirely irrelevant for the sake of discussion here, I’d say.

  16. Bandsaw

    Question: What do the changing colors signify? Do they imply a change in the energy the shell is experiencing (I would assume a decrease), or are they just different because the images were each processed independently?

  17. MarkH

    Phil, all I can say that you know pretty much how far away the star is; and you know what the speed of light is. Given that, you know how long ago the nova happened, and it wasn’t 330 years ago.

  18. TropicalBound

    Forgive my complete lack of knowledge here. I’ve been reading and listening to lectures about Relativity for quite some time, but have no formal education on the subject. Citing the Twins Paradox, if my twin travels for x amount of time in a spacecraft at near light speed, when he returns, he will be much younger than I. If light takes 8 years to make it to Proximus Centauri and back, and my twin did it in less time, doesn’t that mean light travels faster for someone outside Earth’s time domain?

  19. @Bjoern “The event “SN is observed on Earth” has the coordinates x = 0, y = 0, z = 0, t = -330 years.”

    Yeah, only it wasn’t for some strange reason. If you take the expansion speed of the supernova remnant and run it back in time then the supernova explosion resulting in Cassiopeia A should have been visible sometime around the year 1670 CE or so. Europe was crawling with astronomers at the time and they should have reported it but they didn’t. The usual explanation is interstellar dust, but I have some suspicions about that one. If we take the parameters for Cassiopeia A we have:

    RA = 23h 23m 26s
    Dec = +58deg 48′
    Distance = ~11,000 light-years
    Peak apparent magnitude = N/A

    But if we consider Tycho Brahe’s supernova observed in 1572 it was also in the constellation of Cassiopeia:

    RA = 0h 25.3m
    Dec = +64deg 9′
    Distance = ~7,500 light-years
    Peak apparent magnitude = -4 (as bright as Venus)

    So the two supernovae were separated by 5.35 degrees in declination and 61.9 minutes in right ascension (~7.4 degrees) or ~9 degrees of total separation. So an extra 3,500 light-years (46 percent) in distance and an angular shift of only 9 degrees makes the difference between being as bright as Venus and not being visible at all? I’m a bit skeptical of that but I don’t have a better explanation. I wonder if they have actually found the dust field that is blocking our view of Cassiopeia A.

  20. Elmar_M

    Ah ok, so Phil means that what we are seeing now is the remains of an explosion that happened 330 years before (not ago). Or in other words: This expansion of remains took 330 years from the start of the explosion until now.
    Is it that what you mean Phil? Versus: This explosion happened 330 years ago period.
    It still happened 10.330 years ago, for the person floating in an imaginary spaceship at the location of the star at this very moment.
    Correct?

  21. AFakeGuy

    Everything we see is in the past. When I’m looking at someone across the room it still took a few nano seconds for the light to get to my eye and probably a few tenths of a second for my brain to process that information. We can’t not see the present. If the supernova is 10,000 light years away then it happened 10,000 years ago. Whether we’re aware of it happening yet is irrelevant. It’s very self centered to say that just because we haven’t detected something yet doesn’t mean it doesn’t exist or didn’t happen.

    I respectfully disagree with Phil’s assessment.

  22. Todd W.

    A couple others have already touched on this, but those who say that it did explode 10,330 years ago are wrong, because they do not take into account that the Earth and Cas A are moving. To figure out exactly when, in an absolute sense, the star went supernova, we’d need to know the locations and paths of both the Earth and Cas A, in relation to each other, for at least the past 10K years or so, factor in how fast they are moving, etc., etc. Casually thinking about it, my mind begins to hurt at the complexity involved.

  23. Phil Plait and MarkH are both right (and both wrong in suggesting the other is mistaken). It all depends on the coordinate system you use. MarkH uses regular Minkowski spacetime coordinates (the ones we know and love), while Phil uses lightcone coordinates. The whole discussion is the same as insisting that one should use polar instead of Cartesian coordinates, or even more trivial: whether we should use metres or feet.

  24. Bjoern

    @ToddW:
    I’d think that the usual speeds for stars aren’t so large, that the figure of 10,330 years would have to be changed much. A rough estimate would be that the speeds would be around 100 km/s (probably smaller), i. e. about 1/3000 of light speed. Hence the distance can change by only about 10,o00 light years/3,000, i. e. about 3 light years. And hence the time also is only incorrect on the order of a few years.

    @Pieter Kok:
    As I already pointed out, e. g. in the case of the CMBR, Phil has no problem with saying that this radiation was emitted 13.7 billion years ago (i. e. using Minkowski coordinates). So why should we handle this example differently (and here use lightcone coordinates)? And after all, Minkowski coordinates are the one “we know and love”, as you yourself say – so why use others, which make it more confusing?

  25. Another Eric

    All this talk of events happening then and now, I think it’s time to clear this issue up with a video:

    http://www.youtube.com/watch?v=zvd3kaupZ60

  26. @Bandsaw “Question: What do the changing colors signify? Do they imply a change in the energy the shell is experiencing (I would assume a decrease), or are they just different because the images were each processed independently?”

    From Phil’s link:

    “In these images, the lowest-energy X-rays Chandra detects are shown in red, intermediate energies in green, and the highest energies in blue.”

    It’s quite interesting that the outer part of the blast front appears to be entirely blue which means it’s the highest energy X-rays being emitted there. I suppose that the electrons in this region are accelerated to the highest speeds and produce the highest frequency synchrotron radiation.

    One other pet peeve of mine (don’t worry it’s not BA’s doing but rather the article) is when it says:

    “Scientists have used the movie to measure the expansion velocity of the leading edge of the explosion’s outer blast wave (shown in blue). The researchers find that the velocity is 11 million miles per hour”

    Miles per hour??? As DeForest Kelly might have said, “Damn it, Jim! You’re a scientist. Use the friggin’ metric system”. I had to pull out my trusty calculator to convert it to 4,900 kilometers per second or 1.6 percent the speed of light. Now I can understand how fast it is. :)

  27. Bjoern, coordinate changes are tyoically used to make a problem more tractable. I don’t know why one would use lightcone coordinates here. You have to ask Phil.

  28. …and a plane on a treadmill won’t take off. :D

  29. Law Mom

    So what would this have looked like 330 years ago to the casual observer?

  30. @Law Mom “So what would this have looked like 330 years ago to the casual observer?”

    Well, it would have looked like… uh… uh… nothing, or perhaps a 6th magnitude star depending upon who you ask.

    On the other hand, a very similar supernova went something like this. Sometime in early November 1572 a new star arose whose brightness rivaled Venus. It was visible to the naked eye well into 1574 or, in other words, for more than a year. That’s what Cas A should have looked like to the casual observer if it hadn’t been for all that pesky dust. :)

  31. Bjoern

    @TropicalBound:
    The details of the answer to your question depend on the frame of reference – but the general answer is: no, light does not travel faster for the twin.

    Measured by you on Earth, your twin did not make it in less time than the light – it took him a little bit more than 8 years, a bit more time than the light.

    Measured by your twin, he did make it in less time than 8 years – but you also have to take account “length contraction”: measured by him, the distance to Alpha Centauri wasn’t a bit more than 4 light years but only, say (depending on his speed) 1 light year. So he took only a bit over 2 years, measured by him. But measured by him, the light took *also* only about 2 years (a bit less time than he); the light was still faster than he, even measured by him.

  32. Tacoma

    Yeah this is one of the few comments sections on this site that I’ve read – and only because Phil included the weird 330-year thing. I was curious as to his explanation for claiming the event occurred 330 years ago and not 10,330 years ago.

    Imagine Phil and I are in a room with a pair of wristwatches. He sends a signal to me which takes 1 second to reach me across the room. We meet up at the center of the room later. He says he sent the signal at noon. I received the signal one second after noon. But we discuss it and we agree that the event of sending did not occur when I received the signal – the event of sending happened when Phil sent it!

    Which means the signal (the light from the supernova) began to reach us 330 years ago, but we must include the transit time of the signal which in this case from the star’s perspective would have been 10,000 years ago.

    Let us also say we beam a signal to the moon and reflect it on a plate there, and wait for its return. It’s pretty obvious the signal has a period of time between sending and receiving. It doesn’t come back instantly from our perspective just because it’s traveling at the speed of light.

    Unless we’re taking into account something else about the way light behaves … I think it’s pretty clear this event happened 10,330 years ago from the perspective of the star and Earth, but that the light from the event began to reach Earth 330 years ago following a long journey.

  33. Bjoern

    @Tom Marking:
    I agree wholeheartedly with you that one should use the metric system! :-)

    With respect to the “interstellar dust” you mentioned, but don’t think that it’s likely:
    1) If we take the “coal sack” as a typical example, a dust cloud has a diameter of about 50 light years. At a distance of about 10 000 light years, that would give a angular diameter of less than one degree – so yes, it is possible that a dust cloud obscured the one SN, but not the other which happened at a distance of about 9 degrees, as you calculated above.
    2) Looking at the Wikipedia article, they have another possible explanation; I’ll simply quote it:
    “Possible explanations lean toward the idea that the source star was unusually massive and had previously ejected much of its outer layers. These outer layers would have cloaked the star and reabsorbed much of the light released as the inner star collapsed.”
    Sounds sensible to me, but I think one should be able to check this hypothesis, e. g. looking closely at the spectra or at the structure of the expanding cloud. I don’t know if/how this is still studied…

  34. Stadred

    Can I add my vote for that longer post on relativity? I’m personally concerned with perceived problems with causality when you go faster than light (If that were even possible) where effect arises before cause. I read that as “go back in time” but I think I just didn’t understand something… That, or 88MPH is all I need…..

  35. Is it my eyes (which aren’t all that good anyway) or isn’t the neutron star exactly at the center of the expanding nebula?

  36. «bønez_brigade»

    Phil, I’m looking forward to your total explanation of this 330 vs 10,330. In the meantime, I’m siding with MarkH and Bjoern (et al). The frame of reference they’re using is more logical (to the layperson, at least). Is it bad that I’m majoring in astronomy & physics and not immediately seeing things in the same “light” as you?

  37. Supernovas sound scary. Someone should write a book.

    I’m confused about the 330 year thing now but as a lay person the way I would see it is that from our frame of reference she went boom 330 years ago but from an unfortunate Cas Aian point of view the clocks stopped 10k years ago. So because of the universal speed limit we can’t know about anything that happened 10k years ago until now so for all intents and purposes it did happen now. Or something.

  38. RamblinDude

    Phil did qualify his statement with “Literally, as far as we are concerned,” and yes, that makes sense, but isn’t that conditional on what our concerns are?

    For a lay person like me, it would help to avoid confusion if statements such as “About 330 years ago” were qualified with “from our perspective,” but I’m willing to concede that we may need to be retrained in our thinking to accommodate the shortcuts of astro-speak.

    I’m just wondering, though, using the same reasoning, couldn’t we just say that the earth is the center of the universe? Just to make things easier? (I’m not trying to be obnoxious; I’m serious. Really. Stop looking at me like that. . . .)

  39. Cindy

    For those of you who are confused about the 330 years vs 10,330 years, you have to think more like an astronomer who is analyzing the expansion of the supernova remnant. We have only seen 330 years of the remnant expanding. We can’t see any more than that because the light signal of the supernova only reached us 330 years ago. So the roughly 10,000 years when the light signal was traveling to us doesn’t count when we’re analyzing the expansion.

    Another example is the topic of Phil’s Ph.D. thesis: SN1987A. The signal of the supernova didn’t reach us until January 1987, so as far as we’re concerned, the supernova remnant has only been expanding for 22 years. You can’t talk about the SN1987A supernova remnant being older than 22 years, because before January 1987 it wasn’t a supernova but rather a blue supergiant.

    Phil could have been a little more clear about this.

  40. «bønez_brigade»

    @ Cindy,
    “We have only seen 330 years of the remnant expanding. We can’t see any more than that because the light signal of the supernova only reached us 330 years ago. So the roughly 10,000 years when the light signal was traveling to us doesn’t count when we’re analyzing the expansion.”

    With that logic, we might as well ignore _all_ of the stars currently visible, as they are only useful once they go asplodey. SN1987A was once a star that was visible. Is that part of its existence not meaningful anymore? This method of viewing supernovae as if they are not more than a ly away is going to make it extremely difficult to get the point across to creationists that the Universe ain’t 6-10,000 years old. This will only appear to them to bolster their position, as all that time between stellar birth and stellar death can then be viewed like their “a day is like a thousand years” nonsense. I don’t think the enormous distances in the Universe (or just in our galaxy, for that matter) should be ignored.

    If I’ve misunderstood your position, please clarify. I’d like to understand this, but the frame of reference chosen thus far seems “highly illogical” to my evolved (and somewhat-scientifically-trained) brain.

  41. Brian

    Phil, clearly you were correct — this will require a long post to clear up. I’m actually a bit surprised at how many people resist the idea.

    Perhaps an analogy with sound might help. Somebody at a distance of 1 km fires a gun. Three seconds later, you hear the bang. Now, it makes perfect sense to say that the gun was fired three seconds ago, because the speed of sound is not a fundamental aspect of space-time. But what if I said “The sound happened three seconds ago.” Does that still make sense? Wouldn’t you sort of want to object to that? I know I would: I would say something like: “Well, the sound happened three seconds ago over there, at the gun. But it also happened right now, over here.” Or maybe I would say something like: “It doesn’t really make sense to talk about exactly when a bang ‘happened’, because it’s actually smeared out across space and time.”

    But like Phil said, to really dive into it requires a long post about relativity.

  42. Adrian Lopez

    “With that logic, we might as well ignore _all_ of the stars currently visible, as they are only useful once they go asplodey.”

    I think what Cindy means is that travel time is irrelevant with regard to the age of the explosion. Cas A has been expanding for some 330 years now, Earth time, despite it being 10,000 light-years away from us.

    Having said that, I’m still not convinced we can’t also say the explosion occurred 10,330 years ago.

  43. Chip

    BTW – a long time ago on the PBS program “Violent Universe” someone made a film of the Crab Nebula by scaling and photographing a series of observatory images spanning many years. I still remember it as quite wonderful though only a few seconds long.

    We now have enough good images of the Crab to span every year for a full century!

  44. TropicalBound

    @Bjoern,
    So, if I understand the ‘Twins Paradox” correctly, what we’re saying is that the light from the SN left 10,000 years ago (as far as we’re concerned), but the photons themselves are not 10,000 years old. Correct?

  45. Bjoern

    @TropicalBound:
    Well, the age of the photon depends on in frame of reference you measure it. In our frame of reference, they are 10,000 years old. In their own frame of reference (which is essentially meaningless, but let’s pretend that one could talk about that actually), they would be 0 years old.

    @Brian:
    I don’t think your sound analogy is good. You seem to equate “the sound ‘happened’ at the time x” with “the SN happened at the time x”. But the sound is, as you point out, something which spreads through space – whereas the SN is something that happened at one precise location in space.

    In short, the analogy goes more like this:
    gun firing SN
    bang heared light of the SN observed

    And as you yourself said, it makes perfect sense to say that the gun was fired 3 seconds ago.

  46. Cindy

    Bonez_brigade,

    Sorry, I was typing that right before going to bed, so wasn’t fully clear.

    If you are an astronomer studying how a supernova remnant expands, then it’s the time since the supernova explosion is what is important. Knowing what star it was before the explosion is very important, but it’s very rare that astronomers have observed the star before the supernova. So we have only been able to observe CasA expanding for 330 years.

  47. Todd W.

    I just reread Phil’s post, and he’s right. 330 years ago relative to the supernova, the star exploded. So, as far as supposed absolute time from our reference point, 10,330 years ago (give or take a few years for changing positions), the star went boom. 10,000 years ago, the spread reached the expanse that we now observe. Got it…I think.

  48. @Brian “Phil, clearly you were correct — this will require a long post to clear up. I’m actually a bit surprised at how many people resist the idea.”

    Which Phil would that be? Would it be the Phil who created this post or the one who talked about the universe being 13.73 billion years old (I believe he even got Vox Populi to post on that thread)?

    http://blogs.discovermagazine.com/badastronomy/2008/03/05/the-universe-is-1373-12-billion-years-old

    As Bjoern has correctly pointed out, the two posts are inconsistent at the very least.

  49. Tom Marking, you and everyone else disagreeing with me are not thinking relativistically. You are using classical, Newtonian thinking. Instead of simply posting here and disagreeing with me, why not go out and look for the information? It’s out there. I don’t have time to argue it here, but I will when I get a chance.

  50. DaveS

    Of course it’s simply two ways of saying the same thing. What I’m objecting to is Phil basically saying “it’s 330 years old, not 10,330″, when either frame is equally as valid. I claim foul because Mr. Plait himself, in many blog posts, refers to “looking back in time” and “earlier stars” when seeing many millions of light-years away. If “now” is defined as what photons are hitting earth at the present time, and any other frame is invalid (by his own statement), then his own descriptions are bogus.

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    Bye from Italy (in Europe, near Jupiter :D )
    Michele

  52. Law Mom

    I’m still trying to wrap my head around this, too. I hope that this is just a semantic argument. The star exploded at some point in time, we found out about it 10,000 years later. Is “it” the explosion or the effect, or our perception, of the explosion? Using another gun analogy, if a gun goes off at noon, and the bullet hit a man at 12:00:01, was he shot at 12:00:00 or 12:00:01?

    I always hated philosophy.

  53. @Phil “Tom Marking, you and everyone else disagreeing with me are not thinking relativistically. You are using classical, Newtonian thinking.”

    No Phil, my only comment on the “controversy” has to do with your consistency. It appears that in some of your previous posts you were using classical Newtonian thinking as well. That’s fine and I don’t have a problem with shifting back and forth between the two viewpoints as long as you don’t deride others as being “wrong” and maintain that you are “right”.

    It appears to me that the “controversy” is caused by a confusion between 2 different events in Minkowski 4-space. The first event is at the earth (x = x-Earth, y = y-Earth, z = z-Earth, t = 1670 CE) which is when we observed the SN to happen (or would have if not for certain factors such as interstellar dust). This is a directly observable event in 4-space (i.e., observed by Homo sapiens).

    The second event is INFERRED – we have no direct knowledge of it but we can infer its existence from the distance to Cas A and the known speed of light. It is the actual supernova explosion at the source. Assuming the x-axis lies along the line of sight for Cas A we have: x = x-Earth – 11,000 ly, y = y-Earth, z = z-Earth, t = 9331 BCE).

    So the confusion is that Phil is talking only about event 1 and Bjoern, et al are talking about event 2. They are two different events, not the same event. So you need to be careful to specify which of these two events you are talking about.

  54. Adrian Lopez

    “Tom Marking, you and everyone else disagreeing with me are not thinking relativistically.”

    Thinking relativistically, sure… but from where?

    Let’s think forward instead of backwards. You are at Cas A, and it has just exploded. Earth is 10,000 light years away. In that case, when do the people on Earth get to see the explosion, and how old is the explosion from my point of view on Cas A once the people on Earth get to see it?

  55. @Adrian Lopez “when do the people on Earth get to see the explosion, and how old is the explosion from my point of view on Cas A once the people on Earth get to see it?”

    Again, from the point of view of the CasA-ans (to coin a phrase – they must be tough SOBs to have withstood a supernova explosion, but that’s another topic) the event you are speaking of (i.e., when do Earthlings get to see the explosion) is also INFERRED – they have no direct experience of it. But they can calculate it based on the distance and the speed of light. So again, 2 events = 2 points in Minkowski 4-space, NOT 1 event = 1 point in Minkowski 4-space.

    I hope they are not having this same argument on the CasA blog. :)

  56. k9_kaos

    “Literally, as far as we are concerned, that star really did blow up 330 years ago, not 10,330.”

    Phil,
    Does this mean that the star blew up 330 years ago in our reference frame?

    I’m familiar with relativity (I’ve read Einstein’s little book) and I understand that there are some weird effects of relativity, for example, two events that are simultaneous in one reference frame may not be simultaneous in another (particularly if the two reference frames are moving relative to each other). Not to mention Lorenz contraction and time dialation.

    However, I don’t remember reading about the effect you described. I’m sure there is a lot more I have to learn. I look forward to reading a more detailed explanation from you.

  57. Bjoern

    @Phil:
    Well, IMO, I *am* thinking relativistically. Have you looked at what I actually wrote in my comments? I took great care in specifying which frame of reference I talked about, and provided the actual spacetime coordinates of the relevant events – as did Tom Marking in his last comment.

    Even in the theory of Special Relativity, *not* everything is relative – one can make some definite statements, and the statement that the SN explosion happened 10,330 years ago *in our frame of reference* is definitely right. (note that I am *not* talking about when that explosion became visible to us – I’m talking about when it actually happened)

  58. k9_kaos “I look forward to reading a more detailed explanation from you.”

    Detailed explanation? It must be all in the book. (“Death from the Skies” is available from amazon.com for the LOW, LOW price of $16.25 … hint, hint)

    *crickets*

    *crickets*

    *crickets*

  59. Peter

    Why is anyone talking about relativity affecting the various discussions of “when” the SN event happened in our frame of reference? Cas A is close enough and in the same galactic arm as we are so the separation velocity between us is so low that relativistic effects are of piddling importance. Even the radial velocity of the expanding layers of the Sn are low enough that the doppler shifting in radaition frequency is very low.

    This is not a relativistic issue at all. The only issue with discussing time here is the transit time of the radiation from there to here, otherwise simulanity is not affected – good old Newtonian calculation based on the distance and velocity c give pretty accurate numbers for the transit time.

    The real issue is for what we are observing, do we talk about it t with t=0 defined as the actual time in our reference frame the SN occured – ie 10330 years ago and then convert every observation back to that origin or do we ignore the transit time and talk about t=0 as when the first radiation from the SN explosion would have first been observable here? Obviously if we want to easily talk about the evolution of the event – ie what it looked like one day after, or 1 year after the explosion then it is far easier to ignore transit time.

    For SN’s this makes sense – the interesting events are fast compared to the transit time for most SN’s we observe, For the far reaches of the galaxy though, when we talk about highly redshifted objects obviously we are interested in how far back into time we are looking and time here is dicussed as if it stretched back to the big bang.

    The problem comes for intermediate objects – that evolve on the same sort of time scales as the transit time- ie a planteary nebula which is 10000 light years away and has been expanding for 20,000 years – what we see appears to be 10,000 years of expansion. What do we say is the age of nebula, how do we answer when did it explode? It is 20000 years too late for the poor aliens whose planet orbitted the star, but what we see is a nebula 10000 years after the original explosion. The bald statement “it happened x years ago” is less than precise unless we have a solid convention in discussing this sort of event. For astronomers studying “local” events in our and nearby galaxies there is a solid convention which is to ignore the transit time and discuss the events only in terms of what we observe – we see a 10000 year old nebula so that is how old it is.

    Again this is NOT a relativistic issue.

  60. Miroslav

    I would love to agree with Phil but I am not sure if I can! Can someone explain the recent discovery and warning that Earth can be wiped out as a result of a star going supernova at about 3.000 light years away? Has it already happened (3.000) years ago or is it about to happen? According to Phil it is about to happen, but I find it difficult to understand! Of course we have to take into account the fact that bodies are moving away from each others, so CASA A was much closer to Earth 10.000 years ago. Am I correct?

  61. Lee

    For us, it happened 330 years ago. For some poor soul very close to Cas A it happened 10,330 years ago. For someone roughly in between us it happened about 5330 years ago. Location, location, location :)

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