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beskeptical
2004-Sep-30, 05:57 AM
OK, I probably just missed it as a nursing science major taking astronomy for fun. Maybe the rest of you just understood it naturally.

The elements up to iron are made in the stellar furnace. That I knew.

As the hydrogen fuel is used up......

That I knew and I mostly thought of hydrogen becoming helium, but I knew about the other elements.

As the hydrogen is used up, the star can no longer 'hold itself up' and it collapses from the weight of its own mass.....

I had this picture of the energy from the fusion reaction causing the molecules of elements to move so fast they didn't allow gravity to stop them.

So here's where I stick my neck out and admit how little I either did or still do understand this, and, this is the part the prof. left out.

As fusion occurs, the hydrogen is being 'consumed', and some of it is becoming iron. And the iron is key to the collapse. It isn't just that the energy from the fusion is holding the star up, it is also the eventual mass of iron that contributes to the collapse.

And, since iron is only formed in the largest stars, that's why smaller stars don't go nova or supernova. Which also means in the mid-size stars it is not iron, but a simpler element that weighs it down in the end.

So, if I have this right now, I'd like to say to those of you that teach astronomy, you are leaving out the part where the formation of the iron contributes to the collapse. You are only telling us less sophisticated folks that the fuel is used up and the star collapses.

That doesn't mean you didn't mention the whole iron thing. But by making the final statement, "The fuel is used up and the star collapses", you may not realize some of the students don't automatically put two and two together and think about what the used up fuel becomes is part of the action.

Unless of course, I am the only idiot who missed that part, which is possible.

Charlie in Dayton
2004-Sep-30, 06:16 AM
And just slightly OT...

Haven't seen anywhere I might be able to buy this 4 hour masterpiece on DVD...if anyone has a link or an address or the like...this'n's worth checking under the couch cushions...

The Bad Astronomer
2004-Sep-30, 06:28 AM
Beskep, does this page (http://www.badastronomy.com/bitesize/sn87a_explosion.html) help? There are more in that series, too, that might help all this make sense to you.

The Bad Astronomer
2004-Sep-30, 06:28 AM
CiD, um, what are you talking about? I think it's more than slightly OT! Did you post in the wrong forum?

Cylinder
2004-Sep-30, 06:43 AM
He's probably misread beskeptical's post - thinking she was referring to the PBS' Origins episode that aired this evening. They gave a detailed treatment of nuclear synthesis in the final hour.

[Edit] I'll post a link to the Origins DVD in the existing thread (http://www.badastronomy.com/phpBB/viewtopic.php?t=16530).

Ut
2004-Sep-30, 06:47 AM
I'm not sure what you mean when you say the iron's mass that contributes to the collapse. A star at the end of its life is generally less massive than it is starting out... Are you referring to the amount of iron? The core will collapse more or less when the net energy output drops to zero.

beskeptical
2004-Sep-30, 06:49 AM
Beskep, does this page (http://www.badastronomy.com/bitesize/sn87a_explosion.html) help? There are more in that series, too, that might help all this make sense to you.Great help, thanks BA. I see both explanations, the gas pressure and the iron collapse are oversimplified.

My image of the outward pressure vs the inward gravitational force was correct. But I see there is a lot more to the collapse that is skimmed over in the statement, "the fuel gets used up and the star collapses".

Maybe those details just weren't known as well years back, (oh dear, now I sound old). I forgot to take that into consideration.

beskeptical
2004-Sep-30, 06:50 AM
He's probably misread beskeptical's post - thinking she was referring to the PBS' Origins episode that aired this evening. They gave a detailed treatment of nuclear synthesis in the final hour.That is what prompted me to post this.

beskeptical
2004-Sep-30, 06:54 AM
I'm not sure what you mean when you say the iron's mass that contributes to the collapse. A star at the end of its life is generally less massive than it is starting out... Are you referring to the amount of iron? The core will collapse more or less when the net energy output drops to zero.Well now that's another good point, the mass isn't changing, the arrangement of it is. The BA's link makes the whole thing more clear. I don't think I got the exact right idea from the Origins program's description of the collapse either.

beskeptical
2004-Sep-30, 06:56 AM
And just slightly OT...

Haven't seen anywhere I might be able to buy this 4 hour masterpiece on DVD...if anyone has a link or an address or the like...this'n's worth checking under the couch cushions...Just go to the NOVA website, I think. They sell them at the "Channel 9" store here in Seattle. Channel 9 is the PBS station.

here (http://shop.wgbh.org/webapp/wcs/stores/servlet/CatalogSearchResultView?storeId=11051&catalogId=10 051&langId=-1&pageSize=20&searchText=origins), at the bottom of the page.

Looks like it isn't available until 11/15/04.

ToSeek
2004-Sep-30, 03:12 PM
I'm not sure what you mean when you say the iron's mass that contributes to the collapse. A star at the end of its life is generally less massive than it is starting out... Are you referring to the amount of iron? The core will collapse more or less when the net energy output drops to zero.

Iron is key in the "death of stars" not because of its mass but because fusing iron takes energy rather than releasing it, and it's the first element in the periodic table (and the fusion chain) of which that is true. So a star that's turning itself into iron is running out of ways to create the energy it needs to survive and therefore isn't going to last much longer.

chiaroscuro25
2004-Sep-30, 03:36 PM
A point I'm surprised no one has brought up: This talk of an iron core and collapse is relevant to a supernova, not a nova. Despite their similar names, they are quite different. (When they were originally named, observers just saw a new star in both cases, so they named them similarly, not knowing the underlying mechanism was so different.)

Normandy6644
2004-Sep-30, 03:40 PM
A point I'm surprised no one has brought up: This talk of an iron core and collapse is relevant to a supernova, not a nova. Despite their similar names, they are quite different. (When they were originally named, observers just saw a new star in both cases, so they named them similarly, not knowing the underlying mechanism was so different.)

Right, because a nova is just an increase in brightness over a period of time due to gravitational and radiation instability, I think. Still reading my Introduction to the Theory of Stellar Structure and Evolution book, so i'll know more about it soon enough. :D

chiaroscuro25
2004-Sep-30, 03:52 PM
A nova occurs when a white dwarf in a binary star system is able to steal mass from the other star, temporarily restarting fusion. This site (http://www.daviddarling.info/encyclopedia/N/nova.html) has quite a few more details.

Ut
2004-Sep-30, 05:52 PM
A point I'm surprised no one has brought up: This talk of an iron core and collapse is relevant to a supernova, not a nova. Despite their similar names, they are quite different. (When they were originally named, observers just saw a new star in both cases, so they named them similarly, not knowing the underlying mechanism was so different.)

Yeah, but I think it was pretty clear what bsk was talking about from the context. Obviously, we're all just too lazy to point out the faux pas.

beskeptical
2004-Sep-30, 08:16 PM
Oh boy, now I have to go read a whole lot more.

Size of star = whether or not it will end up a dwarf, a quasar or a black hole. I thought the last two were both from resulting collapse. So do they both result in supernovae? Clearly I had the wrong idea about novae.

And this is what I said about mid-size stars that don't have enough mass to make iron, "Which also means in the mid-size stars it is not iron, but a simpler element that weighs it down in the end."

So, essentially, I don't have a clue now, but I assume such stars also run out of energy since they run out of enough hydrogen to keep fusion going. But there is no iron that is using more energy than it is returning to the reaction.

Normandy6644
2004-Sep-30, 08:23 PM
Oh boy, now I have to go read a whole lot more.

Size of star = whether or not it will end up a dwarf, a quasar or a black hole. I thought the last two were both from resulting collapse. So do they both result in supernovae? Clearly I had the wrong idea about novae.

The initial mass of a star determines whether it becomes a white dwarf (low mass, less than 1.4 time the mass of the sun), neutron star, or black hole. Neutron stars and black holes both come from Type II supernovae (core collapse), the difference being initial mass again. I think a black results if the mass was greater than like 8 solar masses. White dwarfs happen when a star sheds its outer layers and becomes a planetary nebula, leaving the white dwarf behind.

Now Type Ia supernovae are different. They are not core collapse, but rather (most likely) white dwarf stars in a binary system accreting mass from its compansion star. If the mass accretion pushes the white dwarf above 1.4 solar masses (the Chandrasekhar Limit), it will explode in a Type Ia supernova. The difference can be noted experimentally by the absence of hydrogen in the spectral lines in a Type Ia supernova (it's a bit more subtle than this, but that's the main difference). Hope that helps a bit. :D

beskeptical
2004-Sep-30, 08:27 PM
Much help, and boy are you fast.

The Bad Astronomer
2004-Sep-30, 08:39 PM
Careful! A quasar (http://www.astr.ua.edu/keel/agn/) is a type of galaxy, not a star.

The "Imagine" website might be a little basic for you, but it does talk about stellar evolution (http://imagine.gsfc.nasa.gov/docs/teachers/lifecycles/SC_index.html).

Ut
2004-Sep-30, 09:51 PM
It takes energy to make energy. Basically, smaller stars simply aren't massive enough to compress its core enough for fusion to take place beyond, say, carbon. They're just not hot enough to make the nuclei move fast enough to overcome electrostatic repulsion.

A dwarf star is simply a star that isn't a giant. You're leaving out some pretty important classifations here. SUPERnova != nova. WHITE dwarf != dwarf. The sun's a dwarf star. Betelgeuse is a giant star. A white dwarf is the collapsed core of a low mass star, a neutron star is the collapsed star of a high mass star, and a black hole is the collapsed core of an ultra high mass star.

Van Rijn
2004-Oct-01, 12:05 AM
The initial mass of a star determines whether it becomes a white dwarf (low mass, less than 1.4 time the mass of the sun), neutron star, or black hole. Neutron stars and black holes both come from Type II supernovae (core collapse), the difference being initial mass again. I think a black results if the mass was greater than like 8 solar masses. White dwarfs happen when a star sheds its outer layers and becomes a planetary nebula, leaving the white dwarf behind.


A little clarification here - the maximum mass of a white dwarf is about 1.4 solar masses (Chandrasekhar limit). Beyond that you have a neutron star, up to about 3 solar masses, where you get a black hole.

In the red giant phase a star can lose a great deal of mass. I've seen various estimates of the maximum initial mass of a star that could form a white dwarf (and don't forget that a companion star can make things more complicated) but a common figure is about 8 solar masses. Eventually fusion can no longer keep up with gravity and you get a white dwarf.

I THINK I read that Sirius B (white dwarf) was thought to have an initial mass around 6 solar masses. In any event, it evolved much quicker than Sirius A (2+ solar masses), which itself burns much hotter than our sun, but will still become a white dwarf.

beskeptical
2004-Oct-01, 06:32 AM
Careful! A quasar (http://www.astr.ua.edu/keel/agn/) is a type of galaxy, not a star.

The "Imagine" website might be a little basic for you, but it does talk about stellar evolution (http://imagine.gsfc.nasa.gov/docs/teachers/lifecycles/SC_index.html).Whoops, I meant Neutron star. It was late. :oops:

beskeptical
2004-Oct-01, 06:38 AM
It takes energy to make energy. Basically, smaller stars simply aren't massive enough to compress its core enough for fusion to take place beyond, say, carbon. They're just not hot enough to make the nuclei move fast enough to overcome electrostatic repulsion.

A dwarf star is simply a star that isn't a giant. You're leaving out some pretty important classifations here. SUPERnova != nova. WHITE dwarf != dwarf. The sun's a dwarf star. Betelgeuse is a giant star. A white dwarf is the collapsed core of a low mass star, a neutron star is the collapsed star of a high mass star, and a black hole is the collapsed core of an ultra high mass star.Ya, I think we're getting that all straightened out.

I did have the wrong idea on the nova, I misspoke on the neutron star and I figured white dwarf was implied.

I don't think of our Sun as a dwarf, is that term used for the Sun? And, a brown dwarf, correct me if I'm wrong, (since I have been quite a bit here), but isn't that somewhere between a gas giant planet and a small star?

Ut
2004-Oct-01, 12:17 PM
Most, if not all, stars on the main sequence are classified as dwarf stars. It distinguishes them from giant starts. For instance, a red dwarf is specifically signified to be a dwarf star to distinguish it from a red giant. If there were yellow or white giants, we'd have to specifically distinguish them from sun-like stars by calling the sun a dwarf. It's a classification that pretty much means it's not a giant.

I'm not sure of the technical definition of a brown dwarf. Anything below ~0.08 solar masses is not a star, but it's all sort of fuzzy from there on down. Some of these low-mass protostars do fuse duterium in their cores for a shot period, but can't fuse hydrogen. If I had my way, those would be the brown dwarfs.

White dwarfs are not dwarf stars, since they're not stars at all. That's a most unfortunate naming convention.

beskeptical
2004-Oct-01, 06:46 PM
OK, makes sense.

tjm220
2004-Oct-01, 08:50 PM
Most, if not all, stars on the main sequence are classified as dwarf stars. It distinguishes them from giant starts. For instance, a red dwarf is specifically signified to be a dwarf star to distinguish it from a red giant. If there were yellow or white giants, we'd have to specifically distinguish them from sun-like stars by calling the sun a dwarf. It's a classification that pretty much means it's not a giant.

I'm not sure of the technical definition of a brown dwarf. Anything below ~0.08 solar masses is not a star, but it's all sort of fuzzy from there on down. Some of these low-mass protostars do fuse duterium in their cores for a shot period, but can't fuse hydrogen. If I had my way, those would be the brown dwarfs.

White dwarfs are not dwarf stars, since they're not stars at all. That's a most unfortunate naming convention.

Good article in a Scientific American special (http://www.sciamdigital.com/browse.cfm?ITEMIDCHAR=22976A87-2B35-221B-69DF48EE32D9AA96&methodnameCHAR=&interfacenameCHAR =browse.cfm&ISSUEID_CHAR=225AF1C1-2B35-221B-637AAA58A8F177B9&ArticleTypeSubInclude_BIT=1&seque ncenameCHAR=itemP) on stars. Brown dwarfs have an article all to themselves though I only got a brief moment to browse it.

The Bad Astronomer
2004-Oct-01, 09:00 PM
The website attached to this board has several articles about BDs. Check the Bitesize pages, and the Planet X stuff.

AK
2004-Oct-02, 09:45 AM
A little clarification here - the maximum mass of a white dwarf is about 1.4 solar masses (Chandrasekhar limit). Beyond that you have a neutron star, up to about 3 solar masses, where you get a black hole.


Sirius A (2+ solar masses), which itself burns much hotter than our sun, but will still become a white dwarf.

Aren't you contradicting yourself here? You say that stars that mass greater than 1.4 solar masses will result in a neutron star, but then you say Sirius A with 2+ solar masses will not.

Or am I missing something here? #-o

Ut
2004-Oct-02, 03:13 PM
No. The star's core must be > 1.4 solar masses for it to collapse into a neutron star. Sirius as a whole is ~ 2 solar masses. It's core is far, far smaller.

AK
2004-Oct-03, 06:51 PM
Gotcha.

tracer
2004-Oct-05, 02:54 AM
SUPERnova != nova. WHITE dwarf != dwarf.
And, just as importantly for this discussion,

NOVA != red giant.

You wouldn't believe how many people get these two kinds of objects confused. "No no no," I've had to say a bazillion times, "The sun is not going to turn into a nova at the end of its main sequence lifetime, it's going to turn into a red giant. And the process of expanding from main-sequence star to red giant will take tens of thousands of years."