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HaleBopper
sage
Reged: 01/14/08
Posts: 237
Loc: Great White North
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I just read this article from S&T website
http://www.skyandtelescope.com/news/home/24314749.html
and it really brings up a nagging question I have had for a while. Can type Ia SN be truly considered as "standard candles" to support the accelerating universe observations?
It's stated in the atricle that the faint SN has a smaller amount of radioactive Ni. It is possible that this has an effect on the overall brightness? I ask this because, in the early universe, the "metal" content was lower. Might this have an effect on the "explosion physics?" A lower metallicty would be expected for the distant type Ia SN that led to the conclusion of an accelerating universe.
The Sep 2006 issue of Astronomy, brings this up, but does not go into details.
Any thoughts, comments?
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8" SCT CG5 mount
Canon Digital Rebel 400
Kodak Easyshare 2.0 Megapixels
5, 8, 13, 17, 21, 24mm Orion Stratus
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Taqyon
sage
Reged: 06/17/08
Posts: 241
Loc: Cape Town, South Africa
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Interesting - that would throw a few things on it's head to say the least.
However, I think quite a few type 1a supernovas' distance have been confirmed using other means - making it safe to assume that ALL 1a's properties are the same.
Perhaps the spectroscopic properties of the nova is taken as a factor along with the amount of energy released to arrive at the distance figure.
Any pro's out there?
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Hein du Plessis
10" Black Diamond Skywatcher Collapsible Dobsonian
Celestron NexImage Webcam
7mm SWA 58° Plössl
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Edited by Taqyon (07/11/08 05:14 AM)
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HaleBopper
sage
Reged: 01/14/08
Posts: 237
Loc: Great White North
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Quote:
However, I think quite a few type 1a supernovas' distance have been confirmed using other means - making it safe to assume that ALL 1a's properties are the same.
It's not the distance I was getting at. Assuming that the distance is 100% correct, is it the fact that we still don't know everything about SN's in general that the very distant SN's were dimmer than expected? Hence the conclusion that the universe is accelerating? In other words, might these type Ia SN's be behaving as they should in the early universe, (since we are looking far back in time)?
I happen to accept that the universe is accelerating, but I thought I would get people's opionions on this. Food for though so to speak...
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8" SCT CG5 mount
Canon Digital Rebel 400
Kodak Easyshare 2.0 Megapixels
5, 8, 13, 17, 21, 24mm Orion Stratus
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Taqyon
sage
Reged: 06/17/08
Posts: 241
Loc: Cape Town, South Africa
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Surely the red shift is used in stead of the brightness to determine the rate of departure?
So, you accellerating universe principal is not affected by the fact that ancient supernovae had less metals and therefore not as bright.
Please have patience if I misunderstood, I'm not as knowledgable as most people here, but I love a good converstation!
Edited by Taqyon (07/11/08 06:13 AM)
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HaleBopper
sage
Reged: 01/14/08
Posts: 237
Loc: Great White North
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I had no idea I was being impatient. Either way, I apologize if I came across as such. I'm no expert either. I have a degree in chemistry, not physics 
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8" SCT CG5 mount
Canon Digital Rebel 400
Kodak Easyshare 2.0 Megapixels
5, 8, 13, 17, 21, 24mm Orion Stratus
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Taqyon
sage
Reged: 06/17/08
Posts: 241
Loc: Cape Town, South Africa
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Not at all! I was just prempting it in case I came accross as unknowledgable but stubborn 
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Hein du Plessis
10" Black Diamond Skywatcher Collapsible Dobsonian
Celestron NexImage Webcam
7mm SWA 58° Plössl
My Jupiter Pictures|My Company
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Inkswitch
super member
Reged: 11/09/03
Posts: 147
Loc: columbus OH USA
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I believe the key to type Ia supernovae is the Chandraskar (spelling?  ) limit. As the white dwarf pulls matter from its companion it settles on the surface and when the pressure reaches the Chandraskdar limit it explodes. I do not believe it matters what matter is being pulled over, what matters is the temperature/pressure on the surface of the white dwarf. Since all type Ia`s go off at the same limit it produces a light curve that is always the same, hence we know its actual brightness and can calculate its distance.
Disclaimer:
I do not have a degree in astro physics, this is my understanding of the matter from reading texts by various writers on the subject. YMMV.
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-Lawen
12.5" Meade Starfinder dob.
6" Orion dob.
8X56 Orion mini-giant
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HaleBopper
sage
Reged: 01/14/08
Posts: 237
Loc: Great White North
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I know the theory. I'm referring to "metal" levels already present before going SN.
For example, if our sun were a white dwarf companion, and with its "higher metal" content, went SN, would it be more luminous than another "metal poor" white dwarf SN in the early universe?
Edited by HaleBopper (07/11/08 02:52 PM)
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Taqyon
sage
Reged: 06/17/08
Posts: 241
Loc: Cape Town, South Africa
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How much less metal? Our sun is relatively rich in metals as stars go, yet it's less than a fraction of a percent of its composition. If this is the case, would 0.2% vs 0.1% really make a difference?
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Hein du Plessis
10" Black Diamond Skywatcher Collapsible Dobsonian
Celestron NexImage Webcam
7mm SWA 58° Plössl
My Jupiter Pictures|My Company
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HaleBopper
sage
Reged: 01/14/08
Posts: 237
Loc: Great White North
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I don't know, that's my question. Our sun is metal rich yes, as stars go, so if it were to undergo at type Ia SN, would it appear more luminous than a star that is less metal rich? Can a small percentage make a difference? I don't know. I don't know if the "metal" content makes a difference at all to the overall luminosity. I was just curious what other people thought. The article in the Sep 2006 Astronomy and now the recent article in S&T made me wonder about all over again...
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8" SCT CG5 mount
Canon Digital Rebel 400
Kodak Easyshare 2.0 Megapixels
5, 8, 13, 17, 21, 24mm Orion Stratus
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VanJan
super member
Reged: 07/09/08
Posts: 111
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S&T news article had me running for reference, as well. From what I read from various sources, the radioactive Ni is a product of the explosion, not an initial element in the pre-supernova white dwarf itself. Qualifier: I am just an interested layperson regarding astro-physics, so please take anything I opine with a very healthy dose of base metals.
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StupendousMan
member
Reged: 08/21/05
Posts: 77
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Quote:
S&T news article had me running for reference, as well. From what I read from various sources, the radioactive Ni is a product of the explosion, not an initial element in the pre-supernova white dwarf itself. Qualifier: I am just an interested layperson regarding astro-physics, so please take anything I opine with a very healthy dose of base metals.
The nickel in a SN Ia is produced during the runaway thermonuclear reactions which cause the explosion; the chemical makeup of the star when it formed out of the interstellar medium has very little effect on that. Remember, the (very likely) precursor of a Type Ia is a white dwarf, in which carbon and oxygen produced by fusion reactions in the star dominate the composition.
The original chemical mix of the star might have small effects on the appearance of the supernova, due to its absorption and scattering of light as the energy produced by the explosion fights its way out through the outer layers of the star. Astronomers who make models of supernovae do include these small effects in their models, and observers of supernovae are aware of the possibility that small changes in initial composition might cause systematic changes in the appearance of Type Ia SNe at different redshifts.
It's a complicated topic, and we don't know all the answers, but we are aware of the questions and working on them.
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astrotrf
sage
Reged: 09/30/07
Posts: 287
Loc: Rodeo, NM
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I'm sorry to come into this topic late; can't think how I missed it before ... but as there doesn't seem to be a resolution in this thread, I thought I'd jump in.
The whole point of the distant type Ia SNe observations was to verify that the expansion of the Universe is constant over time; that's the result everyone was expecting to see.
In order to do this, you cannot use redshift to measure the distances to the SNe, because using redshift to calculate the distance presupposes that the expansion rate has always been constant. So there had to be a method, independent of redshift, to determine the distance.
Type Ia SNe seemed a good choice, because it is thought that they all have the same brightness. Therefore, the brightnesses of these SNe were measured, their distances were calculated from the brightness, and this led to the result that the expansion rate of the Universe is not constant, but is, in fact, accelerating (and has been doing so for several billion years).
As the OP suggested, this observation is entirely dependent on type Ia SNe in fact being reliable standard candles. Were this to prove not to be the case, the interpretation of these observations would be thrown into doubt.
However, there are other observations that also support the idea of an accelerating universe, not the least of which is the WMAP satellite's measurement of the cosmic microwave background.
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Terry (astrotrf)
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