The Distance to SN 1987A
If you live in the Southern Hemisphere you can look up on any given starry night and just over from the Southern Cross you’ll see a fuzzy patch. It’s called the Large Magellanic Cloud (LMC) and is actually another galaxy quite close to our own Milky Way. To get a sense of size and distance our Milky Way is about 100,000 light-years across and the LMC is about 160,000 light-years away from us but only about 1/10th of the size. Right on our doorstep so to speak.
160,000 light-years is still a long way away though. A light-year is the distance light can travel in a year which equates to about 9,460,730,000,000 kilometres and this LMC is 160,000 of those.
There are different types of stars and they all go through different stages of their lives. Some stars burn up all their fuel and end up collapsing in on themselves at the same time as shedding an ‘outer shell’ and they go nuts burning new materials - these are called Supernovae, you’ve probably heard of them.
One day in 1987 a star in the LMC (previously catalogued as Sk-69 202 but now called SN 1987A) did just this and it was a spectacular sight. Observatories around the world kept an eye on it as it wrecked havoc in slow motion over the next few years.
One thing they noticed was that when the star in the middle brightened the main ring would brighten eight months later and when the star dimmed the ring would, again, dim eight months later. This means that the light that comes directly from the star to our eyes travels a shorter distance than the light that has to go out to the ring and bounce off toward us. Which also means that the ring has a radius of 0.658 light-years (or a diameter of 6,200,000,000,000 km if we’re feeling clever).
Knowing the size of an object is very useful if you want to be able to know the distance to that object. If you know that person is 2m tall and they look very small (this is really measured in angles) you know that they are a long way away.
Astronomers were able to measure the total angle that SN 1987A took up (a radius of 0.808 arcseconds - very small but still useful) and were able to calculate that it is 168,000±3.5% light-years away from us. Which means that what we saw in the skies in 1987 actually happened 168,000 years ago.
April 11th, 2008 at 11:14 pm
Ole Pythagoras, huh?
That’s really cool stuff there.
The only way I’d heard of calculating such distances other than this was by measuring the angle difference (still very small) when measured at extreme points in our orbit around the sun… That’s still using Pythagorean Theorem, but the diameter of the earth’s orbit (~93mil x 2 = ~~186,000,000 miles) is waaaay less than .658 light years, so it makes the calculations have more “±-ness” to them… This, it seems, would be a far more accurate measurement.
Very cool.
I wonder what’s on the other side of the wall at the edge of the universe???
-d-
April 11th, 2008 at 11:24 pm
Also…
I wonder about the possibility of some slowing-down (and/or speeding-up?) of light-travel… Know what I mean? That light has to pass ‘through’ a lot of ’space’ to get to us, and maybe (I really, really don’t know…) the ’speed of light’ can vary at different places?
Weird to even think about that…
I’m loving it, though…
What do you think?
-d-
April 14th, 2008 at 12:04 pm
Good question Dale, I haven’t had the time to actually find out why science treats light as a constant but I will do over the next week or so. I understand that light is constant in a vacuum but that it can slow down in different materials though so it sounds like a perfectly valid question to me. It could be that cosmologists are assuming that light wasn’t overly-obstructed on its way here otherwise we wouldn’t be seeing what we see. Perhaps you could have a look around too?
If, though, you are asking with the intention of somehow-possibly-maybe trying to get it down to 6000 years or so there are a couple of problems that will need to be dealt with:
1. If light was going much much faster in the earlier stages of the universe then it would have to have slowed down exponentially (or abruptly) and has now levelled out because we don’t notice any changes in our measurements. If the speed of light was, say, 56 times faster for the first half of the journey and normal speed for the last half (to bring it down to 6000 years you see) then the eight-month time delays we observe now would have meant that back then the rings must have been 56 times larger and this then means that SN1987A must be 56 times further away (or is that 28 times because of the whole half-journey thing?) from us which leaves you in an even stickier situation because if it’s further away you have to increase the speed of light even more in an attempt to keep it to 6000 years and so on in a never-ending loop.
2. If you find some tricky way to get 168,000 down to 6000 (which I would love to see an explanation for) you’re still left with the fact that the LMC is one of our closest neighbouring galaxies and there are, by conservative estimates, hundreds of billions of galaxies out there. Our brains don’t “do” millions let alone billions and it’s hard to comprehend this but if you hold a grain of sand out at arm’s length you are covering at least 10,000 galaxies and each galaxy typically contains billions and billions of stars.
I realise that you’re not a YEC Dale but thought I should settle the matter in case someone read your comments and thought that the whole 6000-year thing is still up for discussion. It’s not. It’s delusional. (Of course there’s the YEC theory that the universe was created with all the light photons already in motion (and pretend fossils in the ground and so on) to give the illusion that everything is really old - and if you’re willing to posit a god in the first place I can see no reason why a little divine deception wouldn’t be out of the question).
April 14th, 2008 at 5:10 pm
Not my intention.
If anything, the distortion would make the cosmic light-travel speed slower than the constant vacuum speed…
April 17th, 2008 at 7:15 am
Ooops, just a delusional guy wandering through…
All sarcasm aside, I am pretty firmly YEC, but I love reading the various stuff about it. I, for one, don’t understand how distances to stars are calculated in the first place ???
Neat things out there.
April 17th, 2008 at 7:21 am
Hi Bernard and welcome. There are lots of ways that are used to measure the distances to other stars and galaxies. Would you be interested to learn of the different ways of measuring (a trip that I’m happy to take with you) and, more importantly, do you think that any evidence will convince you that the earth and the universe is far, far older than 6000 years?
April 17th, 2008 at 7:49 am
Damian - I smile when I read your last sentence. We’re all out to convince someone, aren’t we?
I would LOVE to take as long a trip as you care to provide along the ways of measurement, but my faith in the God that perhaps we disagree on carries me very far on the journey as to earth age. Argumentative discussion, for or against creationism, really doesn’t appeal to me, because I’m not as well versed in the particulars as I should be to engage that. I also maintain that it is not my job to change your thoughts. That is beyond my power. I will, however, do my best to actually BE a Christian and still not be afraid of those who may not be.
I will absorb being called delusional and even ignorant in the interest of “how do they measure stars”…
Thanks,
Bernard
April 17th, 2008 at 8:08 am
Bernard,
This doesn’t have to be about God or anything. Just how we might be able to measure distances. The reason I asked the question I did was that it’ll be futile to try to arrive at the truth of the matter if you’ve already decided that there is a 6000-year boundary that is non-negotiable.
If you feel that I have some anti-Christian agenda that influences my approach to measuring distances then there is another possible option: Perhaps I could call on my good friend Dale who visits here from time to time and who is himself a Christian. Because you both share the same faith but disagree on how we can measure things he might be kind enough to explain. Dale?
April 17th, 2008 at 8:24 am
Damian - Please don’t feel constrained to answer within the bounds of the 6000 years. I fully realize that there are complications with the YEC theory that we are hard pressed to answer without resorting to “created with the appearance of age.” My purpose in asking is not, I promise, to trick you or lure you into some protracted argument regarding your methods. I am sincerely interested in how the distance to and the size of these galaxies, constellations, and stars is measured. Feel free to answer as you would like, and I would love to have Dale’s response as well. I understand your original question and I don’t resent it in the least. I’m here for education, not argument.
Carry on
April 17th, 2008 at 8:46 am
Hehe, I started writing (and researching) and quickly realised that this was going to be more than just a comment and was going to start a whole new topic. But then I came across this website that does a superb job of explaining the different methods. Perhaps later when I have more time I’ll attempt a layman’s overview of the main methods and how they overlap and confirm each other. In the meantime have a look at the website and at this Google search which links to lots of other websites. (And there’s this excellent article that explains a brilliant way to measure some distances using trigonometry!)
April 17th, 2008 at 12:22 pm
I’m not sure you need my help, Damian!
That first page is complex enough to keep me reading for quite a while…
-d-
April 28th, 2008 at 9:28 am
Dale, as promised I took some time out to look into your question about the constancy of the speed of light. This weekend I went back over some relevant chapters in a couple of books of mine (A Brief History of Time by Stephen Hawkings and e=mc² by David Bodanis).
Here’s my interpretation of the explanations I found:
The speed of light in a vacuum is a constant like absolute zero is a constant. Absolute zero is −273.15°C and while it’s possible in theory to just subtract another number (ie -274) it’s not possible to do so when talking about temperature. Absolute zero is the point at which molecules no longer move. You can’t go colder than that.
The speed of light in a vacuum has a maximum speed of about 300,000km/second but it can be slowed down. Its maximum speed has to do with some kind of flip-flopping between magnetism powering up electricity and electricity powering up magnetism that photons do. Like temperature, the speed of light is just a description of a physical constraint that is observed.
Because light can be slowed down an interesting question would be “how do we know that SN1987A isn’t actually older than what we deduce by using the maximum speed of light?”. I don’t know the answer to that one and I don’t know who or where to ask.
A side note about e=mc² if you (like me) struggle to get your head around it:
It’s basically that “energy is another form of mass”. Sort of like how “inches are another form of centimetres” would be expressed as i=cm*2.54.
Except that it’s “lots of energy is a form of not much mass”. You can turn energy into mass and you can turn mass into energy. When you lose a tiny bit of mass (like when two hydrogen atoms are flung toward each other and merge to become a single helium atom in the hearts of stars) you let off a lot of energy. This is what keeps stars burning; our star is losing mass by converting 700 million tons of hydrogen into 695 million tons of helium every second (which gives it 5 million tons to convert to pure energy).
To see why the speed of light was used in the equation read this (I struggle to understand it but it sounds like they’re saying that this is more to do with time than light because energy is expressed with respect to time.)
April 28th, 2008 at 4:46 pm
Wow. Very cool.
Indeed, it would make sense (at least to my mind) that light could be slowed down, thus making it possible that our friend SN1987A is further away than we thought, but not closer…
beautiful stuff…
April 28th, 2008 at 5:06 pm
Beautiful indeed. I’ve only recently begun to grasp the fact that we’re made of exploded stars. I mean, I’d heard it before and thought it a little hippy. But we really are made of the elements that those massive foundries we call “stars” crushed together and they blew off in massive explosions when then went on to form things like other stars and the planets and even our squidgy brains.
I recently watched (and recorded, if you want a copy) a doco series on Sky called “The Universe” where they go through, among other things, the various stages of a star’s life and how they pound atoms together to make more and more exotic elements like carbon and oxygen and gold and uranium.
I find it mind-boggling. So much so that (I’m kind of ashamed to say) I even get a bit emotional about it. Weird eh? There’s probably some psychological reason for it that’s got something to do with big numbers and a kind of ‘mental vertigo’ but, hey, at least it’s better than getting worked up about Shortland Street eh?
April 29th, 2008 at 11:14 am
AMEN! Words cannot express how much I do NOT care about Shortland Street!!!
The various adjectives you used are apt: ‘beautiful’, ‘massive’, ‘exotic’, ‘mind-boggling’, ‘emotional’, ‘weird’, etc.
The funny (there’s another adjective) thing about it all is that these feelings (however subjective and un-scientific) are ubiquitous. Wonder doesn’t fit in a test tube, but it’s real.
April 29th, 2008 at 11:16 am
Oh yes, and I would say as well; I don’t think you should be ashamed at all to be emotional about such things. Reason and emotion don’t have to be enemies.