We discussed this on our latest 'Real Science Friday' radio show (archived here).

The Crab Nebula is a remnant of a star that is beleived to have exploded in ~1054. It is a SuperNova remnant, or SNR. OVer time SNRs expand in size. The problem for Old agers is that we do not see any "old" SNRs! SNRs should reach a diameter of about 300 light years after 120,000 years, so we should be able to see many this size if the universe is billions of years old. We don't see any this size.

‘Why have the large number of expected remnants not been detected?’ - Clark and Caswell, 1976. Monthly Notices of the Royal Astronomical Society, 174:267

“Another surprise is how rare Crab Nebula type SNRs are” - Dr Malcolm Longair, Royal Observatory, U.K.

BTW, on our April 6th show we plan on answering emails and taking calls. If you have any questions you would like to submit for the radio show, or if you plan on calling in, please let me know. The show airs live in Denver at 3:pm on the 55,000-watt AM 670 KLTT.

Fred

Cool an astronomy topic, right up my ally.



Trouble with this argument is you need to factor in time and distance. The longer the time since the supernova the grater the dispersal of the remnants of the star, and the fainter it will become. The fragments of the supernova do not produce their own luminescence (there is one small exception) but rely on reflection. The light from the SN has long since passed us by.

Distance – the further away the SN the less detail there is able to be resolved by the telescopes. Indeed distant SN can only be detected by the light, the nebula being invisible (it’s just not bright enough).



1976! …… 1976! They were still using wet film back then!
Have a look at the Hubble website, there more new (and more detailed in structure) nebula than you can poke a stick at.



Well SN are reasonably rare events and logically even more so to any specific point in the universe (a good thing too).



well yes there are a few:

1. Do you think, time since event, is important regarding the visibility of the SNR?
2. Do you think, distance from ourselves, is important regarding the visibility of the SNR?
3. Do you think the fact that, the SNR is visible via reflected (or excited by radiation) is important, re the visibility of the SNR?
4. Do you think there have been no new SNR discovered since 1976.
5. is the 1976 quote relevant, given the modern instruments are so much better than the naked eye or long exposure film techniques of the 70’s. Modern observations make use of CCD, flexible mirrors, computer algorithms to remove atmospheric interference, the combining of two telescopes plus computers to remove atmospheric interference, placing telescopes above the atmosphere.

Even though supernova are rare, there are a lot of stars out there. There have been great strides in detection over the last ten years.

Absolutely.

I did a quick search and found various estimates, a typical answer is that no more that half a dozen(ish) are observed in our Milky Way every century.

Chance, we are looking for serious questions on the radio show, not disingenuous smart-alecky ones that also happen to double as strawman arguments. SNRs are as rare today as they were in 1976. From the most recent radio survey we now have ~265 SNRs in the galaxy (Brogan 2006). From the abstract:

“These results suggest that further deep low frequency surveys of the inner Galaxy will solve the discrepancy between the expected number of Galactic SNRs and the significantly smaller number of currently known SNRs.” - Brogan, C.L. et al. 2006. Discovery of 35 new supernova remnants in the inner galaxy. Astrophysical Journal 639:L25–L29.

These authors admit a huge discrepancy in their old earth model, and invoke the usual “dog ate my homework” excuse that if we only do “further deep low frequency surveys” we will close this “significant” discrepancy. Care to make any wagers on this?

The following quote you ridiculed happens to be as true in ’76 as it is today:

‘Why have the large number of expected remnants not been detected?’ - Clark and Caswell, 1976. Monthly Notices of the Royal Astronomical Society, 174:267

Fred

I ask those questions in all seriousness, perhaps you are reading more into the text than was intended. A better turn of phrase would have been “have you considered the effects of:

a. time affect how bright the SNR will appear, the more time that passes the fainter the nova as it’s material disperses in space.

b. Distance affects how bright the SNR will appear in telescopes, too far away and it wont be detectable at all.

c. reflected light is the cause of the illumination, the progenitor may not be able to illuminate the nova, depending on what it has become, or because it has relative motion to the nova and has moved away from it.

to the illumination of a SNR.




But most importantly Fred, and in all seriousness, I believe you have misunderstood the article. I hope you delay your broadcast, to reflect on the explanation I will provide below. Please read.





As methods of detection get better and as the sky is systematically searched more and more SNR’s will be discovered. SNR discovered should be proportional to the number of supernova, and inversely proportional to the time since detonation, and distance from us.



No they don’t Fred, you have misinterpreted the article, here is the abstract in full (my bold)

Take a look at the qualifiers in the article:

a resolution of 42” a limit of the telescope/instruments.

The area of sky covered is not 100%, they quote using the galactic coordinate system between 4.5deg < 22deg l (this is galactic longitude) @ 1.2 b (this is galactic latitude). The search is restricted to the galactic plane (where you are more likely to find them). This is a very small segment of the sky.
Explanation of the Galactic coordinate system http://en.wikipedia.org/wiki/Galactic_coordinate_system

Ability to see them is dependant on “surface brightness”. Meaning faint SNR are not able to be resolved.

They expect to find more as they search that section of the sky, i.e. statistically there should be some figure, it just a matter of detecting them.

What you have not realised is the survey is yet to complete it’s task, the claim of “not finding” is not one of some inexplicable anomaly of science, but one of a systematic search that is expected to end up with figure X when complete, but at the moment is only part way through.

I did not misrepresent the article, chance. You should read the article, not just the abstract. As the article mentions, the “missing” SNRs only detectable by low frequency surveys is only a part of the equation. The remaining piece is the missing “old, large” remnants, which is what I am saying do not exist since the universe is not much older than 6K years[1]. My question to you is, has the significant number of missing SNRs been resolved? Yes or No? If yes, then I misrepresented the article, if no then I didn’t. As often happens in evolution “science” we are told it will happen. In the Brogan article we see the oft-used evolutionary word “suggest”. Does “suggest” mean the problem has been solved? My point is that you implied the quote in my OP about missing remnants has been proven wrong, and I have shown you otherwise. You may think that evidence shows good reason why we are missing these remnants and that’s a fair debate, but the problem itself still exists (BTW, the article notes that by extrapolating their results they can only get to half the expected number of missing remnants). The problem has not been solved. There are only “suggestions” on why we observe fewer SNRs than we expect. Suggestions are not solutions.

The more important point getting back to my OP is the fact that we do not find “old” SNRs as we would expect if the universe is 20bil years old. A new article coming out in the March CRSQ that is not yet available to non-members examines all the known SNRs - using accepted Interstellar Medium (ISM) densities their size all fall within the range of being reasonably 6000K years old or less. Even if you assume an Interstellar medium (ISM) density more favorable to an old universe model, you only get a handful of SNRs older than 120K (but less than 200K). The ISM density used to achieve these >6K dates is believed to only occupy constitute 20% of the universe. Regardless, if 200K is maximum size using favorable assumptions for old ages, this does not bode well for the old age model of a univsers 20 bya. Given the relatively short expanse of space such an SNR would occupy, evolutionists can’t excuse this by saying older SNRs would no longer be visible.

SNRs clearly fit the young universe model. Hopefully this particular CRSQ article will be selected for the public section.

Fred

[1] – “This deficit is likely the result of selection effects acting against the discovery of old, faint, large remnants, as well as young, small remnants in previous low resolution and/or poor sensitivity Galactic radio surveys” - Brogan, C.L. et al. 2006. Discovery of 35 new supernova remnants in the inner galaxy. Astrophysical Journal 639:L25–L29.

I did not accuse you of misrepresentation, I said that I think you have misunderstood it.


(my paragraphing)

Re.a Low frequency is used because as SNR fade in the visible light spectrum you have to use other methods to detect them, (radio).

Re.b the missing, as you put it, would be better understood as ‘undetected’ because they are faint, not because they don’t exist.




I would have to think the answer is yes, for the section of sky that was searched, they found an additional 35 SNR, if that was extrapolated to the remainder of the sky, you would certainly get a lot more. The proportion of SNR should be along the galactic plane, which is why they were searching in that area.

It’s quite possible that the entire area of sky will not be systematically searched, depending on the research being conducted a statistical answer may be all they are attempting to do. I.e. choose a representational section of the sky, say 0.5% then extrapolate that out for the remainder, it’s not an uncommon practice, and makes use of the valuable ‘telescope time’ (there are a lot of people lining up to use these telescopes) and if you were only granted 6 hrs of time, you have to make the best of what you got.

From the full article –
http://www.citebase.org/abstract?id=oai%3A...ro-ph%2F0601451
(i have to type this in longhand as it’s a .pdf, so apologies for typos etc)


(my paragraphing)

Re.a This is explaining that they expect that X number of SNR should exist, given what is known about supernova in general. Note they are not saying “we can see all, and the SNR are missing”.

Re.b here the reasons are given why there is a deficit, i.e. they are faint (several causes), older surveys had poorer tools to observe with (very much so), this survey uses the Very Large Array, that’s one super serious radio telescope/s http://en.wikipedia.org/wiki/Very_Large_Array






Can’t find that bit, but I suspect (without reading) it is because the VLA is in the northern hemisphere and a large section of the southern hemisphere is imposable to view.

The problem has not been solved. There are only “suggestions” on why we observe fewer SNRs than we expect. Suggestions are not solutions.



Not to be totally unexpected, SNR are temporary features, they disperse and fade away. the wiki quotes a maximum life of a SNR to be a million years
The ‘visible’ life would be considerable shorter than that. Perhaps it’s just an unfortunate coincidence that the visible life coincides with a 6000 year figure, but IMO point 1 in the wiki article states that the majority of the ejecta expansion is over by “a few hundred years”, how long it takes to fade after that I cant find.






re the Inter Stellar Medium ISM – (the partials that occupy the space between the stars). http://en.wikipedia.org/wiki/Interstellar_medium
You seem to be using this term to describe a percentage of stars (or SNRs).

Or what you may be inferring is that the ISM prevents the ‘visibility’ past a certain age (this would be the actual explanation) i.e. dispersal, cooling and slowing down of the ejecta.