North Star Closer to Earth Than Thought

reply to post by XPLodER

in this case either the star was less luminous, or a different size? and if we cant accurately be sure of either (one has to be in question) then the "expected" luminosity size equation is wrong.

That is the whole reason Cephids can be used. Their absolute luminosity is directly related to their variability.

so its a change in size/luminosity? or star type?

The article goes beyond my depth but a correlation between the period and the spectrum of Polaris is used to calculate a more refined value for its luminosity, this allows a more accurate distance calculation.

That is the whole reason Cephids can be used. Their absolute luminosity is directly related to their variability.

hold on if that were the case, then how could the luminosity be in question? we know that their is a relationship between luminosity and variability, then how did we get the luminosity so wrong?
it must be a different spectral class to be "wrong" is this a case of over estimated luminosity?

The article goes beyond my depth but a correlation between the period and the spectrum of Polaris is used to calculate a more refined value for its luminosity, this allows a more accurate distance calculation.

would this mean that other cephids are suspect of being "of a differnt value" when comparing absolute / luminosity?




you know well my bias towards looking at things as being "closer" than we thought,
so dont take this the wrong way, but if simple mistakes like spectral class or manitude can be wrong,
how do we trust the H/R diagram?

xploder

reply to post by XPLodER

we know that their is a relationship between luminosity and variability, then how did we get the luminosity so wrong?

Because there are other factors to consider in refining the value. Polaris is a somewhat unusual Cephid (as well as having two companions). The period and amplitude of its variability, vary. This is one reason why this new study will be put to further scrutiny before being accepted as providing the true distance of Polaris.

The luminosity was not "so wrong". Older, less refined values, based strictly on luminosity, yield a distance of 360 light years (about 10% greater). astro.wku.edu... (table 4)

arxiv.org...

reply to post by XPLodER


Polaris is actually a triple star system, of which the Cepheid is the brighter one, but as the stars all revolve around the centre of gravity of the system, thus making the parallax measurement of the main component very difficult.

Furthermore, the period and intensity of Polaris's pulsations is changing, as it moves along the stellar evolution line. Not a lot, about 3 seconds per year, but enough to provide more uncertainity as to exactly what its position is on the Luminosity/period curve. The reason why Polaris is so important on that table is that it is the closest Cepheid, and there are only 273 Cepheids whose distance can be accurately measured by parallax.

As you can see from the following graph, it is not a simple straight line, so there will always be errors.



In the next few years, the figure will change again, as Gaia, Hipparcos's replacement is due to be launched in 2013. It won't be able to measure Polaris directly, as it is too bright for the instruments, but by evaluating other Cepheids, they should be able to refine exactly where Polaris should fit on the Cepheid table, and I can almost guarantee you, a lot of things we currently know about Cepheids will change.

Like an old professor of mine kept on saying, "The more you learn, the more you realize how much more there is to learn".

iopscience.iop.org...
sci.esa.int...

Originally posted by Hellhound604
reply to post by XPLodER

 

Polaris is actually a triple star system, of which the Cepheid is the brighter one, but as the stars all revolve around the centre of gravity of the system, thus making the parallax measurement of the main component very difficult.

aahhaa so the problem is the dynamic interaction between these three stars
thank you a parralax problem is involved

Furthermore, the period and intensity of Polaris's pulsations is changing, as it moves along the stellar evolution line. Not a lot, about 3 seconds per year, but enough to provide more uncertainity as to exactly what its position is on the Luminosity/period curve. The reason why Polaris is so important on that table is that it is the closest Cepheid, and there are only 273 Cepheids whose distance can be accurately measured by parallax.

so if this is a problem of stellar evolution is it that we have the wrong spectral class? or its evolution is different from expected values?

As you can see from the following graph, it is not a simple straight line, so there will always be errors.

In the next few years, the figure will change again, as Gaia, Hipparcos's replacement is due to be launched in 2013. It won't be able to measure Polaris directly, as it is too bright for the instruments, but by evaluating other Cepheids, they should be able to refine exactly where Polaris should fit on the Cepheid table, and I can almost guarantee you, a lot of things we currently know about Cepheids will change.

i look forward to Gaia and the new data sets we will have to evaluate after launch,
and i to beleive that our understanding of cephids will evolve

Like an old professor of mine kept on saying, "The more you learn, the more you realize how much more there is to learn".

iopscience.iop.org...
sci.esa.int...

the wisest thing a man may know is all that he knows is nothing compared to what he has yet to learn



thanks

xploder

I still think the North star is far far away, this new information didn't change my perception of it at all.

reply to post by XPLodER


No, the spectral class is correct, but Cepheids are not stable stars, and there is still a great deal of uncertainity about them. In short, Cepheids are dying stars. As they move along in their stellar evolution, their characteristic Luminosity/Frequency changes.

Only in the 1950's was it discovered that there more to Cepheids than what originally thought. Now Cepheids are divided into the following groups:

Classical Cepheids or Population I Cepheids. Their absolute visual magnitudes are closely tied to their pulsation periods, allowing them to serve as prime distance indicators. They are Yellow supergiants 4-20 times more massive than the sun, and up to about 100 000 times more luminous.
Population II Cepheids. they are lower metal, lower mass versions of classical Cepheids with respectively short, intermediate, and longer periods that range to 100 days. They are also less massive than our sun.
RR Lyrae stars, low mass pulsating stars with helium-fusing cores. They have variation amplitudes of a few tenths of a magnitude and periods under a day. Their uniform absolute magnitudes make them good distance indicators.
Delta Scuti stars, class A dwarfs, subgiants, giants, with variations of a few hundredths to tenths of a magnitude and multiple periods under a day.
Gamma Doradus stars, class F versions of the above, but with longer periods and a somewhat different pulsation mechanism.


If you look at the following HR-diagram, you'll see a light orange transfer region which marks the region where Cepheid variable stars lie.


Remember, the HR-diagram shows stellar evolution. a specific star will follow a specific line on that diagram from birth to death.

Originally posted by Lucid Lunacy
reply to post by MagicWand67

 

So if I was able to extend my life by about 4 times, and had a spaceship capable of light speed, I could reach the North Star if I dedicated my whole life to getting there.

Space is so big

Space might not be as big as they say. Some satellite have traveled farther then they should have in the time that it took them, suggesting that space is not as big as we are told.

reply to post by Infi8nity

Some satellite have traveled farther then they should have in the time that it took them

Which ones?

Originally posted by Hellhound604
reply to post by XPLodER

 

No, the spectral class is correct, but Cepheids are not stable stars, and there is still a great deal of uncertainity about them. In short, Cepheids are dying stars. As they move along in their stellar evolution, their characteristic Luminosity/Frequency changes.

so frequency is a factor of evolutionary state?

Only in the 1950's was it discovered that there more to Cepheids than what originally thought. Now Cepheids are divided into the following groups:

Classical Cepheids or Population I Cepheids. Their absolute visual magnitudes are closely tied to their pulsation periods, allowing them to serve as prime distance indicators. They are Yellow supergiants 4-20 times more massive than the sun, and up to about 100 000 times more luminous.
Population II Cepheids. they are lower metal, lower mass versions of classical Cepheids with respectively short, intermediate, and longer periods that range to 100 days. They are also less massive than our sun.
RR Lyrae stars, low mass pulsating stars with helium-fusing cores. They have variation amplitudes of a few tenths of a magnitude and periods under a day. Their uniform absolute magnitudes make them good distance indicators.
Delta Scuti stars, class A dwarfs, subgiants, giants, with variations of a few hundredths to tenths of a magnitude and multiple periods under a day.
Gamma Doradus stars, class F versions of the above, but with longer periods and a somewhat different pulsation mechanism.
so the "size" is a direct factor in period of variability ? or just age?

If you look at the following HR-diagram, you'll see a light orange transfer region which marks the region where Cepheid variable stars lie.

Remember, the HR-diagram shows stellar evolution. a specific star will follow a specific line on that diagram from birth to death.

thats the first H/R i have seen with cephids
thanks man


i wounder if they are as predictable as the diagram suggests?
if they are progressing along their life cycle wound they "throw off" some of their mass?

and do you know what causes the variability?

xploder

Originally posted by Phage
reply to post by Infi8nity

 

Some satellite have traveled farther then they should have in the time that it took them

Which ones?

lol i thought that might get your attension
i too want to know the answer to that one

not voyager? cant be?

xploder

Cepheids are semi-predictable. They do loose mass, and the amount of fusionable helium gets less as fusion progresses to heavier materials in the core. I will briefly try to describe what we know about Cepheids down below:

For stars to be stable, heat must flow from layer to layer at a constant rate, equal to the energy production in the core, and the loss of heat at the surface. This requires a balance between temperature and density in a given region, and in the layers above and below that region. If, for example, a region is too low in density to effectively block the flow of radiation, so that heat flows through the region faster than it can be replaced, the region will cool and contract, causing an increase in its density, which allows it to more effectively block the outward flow of radiation. Conversely, if a region has too high a density, and heat cannot pass through the region fast enough, the gas in that region will heat and expand, causing a decrease in its density, which allows the heat to escape more easily. Fluctuations in these densities cause vibrations to pass through stars , but they do not usually produce substantial changes in the star's structure, because any variation from a stable structure is kept small, by a continual adjustment of density and temperature.
Cepheids are relatively massive, young stars which are in the last stages of their life, and passing through a yellow-giant stage. When the surface temperature is in a temperature range similar to that of the Sun, there is a region not far below the surface where single-ionized helium ions are close to the temperature at which they become double-ionized.
Suppose that in the region where temperatures are slightly above the double ionization temperature, some fluctuation in conditions causes temperatures to become a little too high, so that the gas ought to expand and cool off. The outer layers of the star start to expand and cool, as energy stored in the kinetic energy of the gas is transformed into gravitational potential energy (the star gets larger). Under normal circumstances, as the star expands, the temperature drops; but in Cepheids, as the temperature in the region where helium is doubly ionized reaches the double ionization temperature, the temperature stops going down, even though the star continues to get larger. Heat is still being converted into gravitational potential energy, but temperature stops dropping, because the recombination of electrons with the doubly ionized helium atoms releases large amounts of energy, and keeps the temperature at a constant value.
The expansion continues, without any change in temperature in the region where helium is recombining, until all the helium has recombined; but by then, the star is much larger and brighter than it was, so it starts to cool, and has to contract again. Everything now runs in reverse, with gravitational potential energy being converted into kinetic energy as the star gets smaller and hotter; but just as before, when the temperature reaches the double ionization temperature, things get "stuck", as the singly ionized helium ions lose their remaining electron, and gravitational energy which ought to go into heating up the gas is used to ionize the atoms, instead.

In other words, the ionization of helium prevents the temperature from rising until all the helium is doubly ionized, while the star contracts; while the recombination of the ions prevents the temperature from dropping until all the helium is singly ionized, while the star expands. The transfer of heat to ionization during contraction pushes the star to too small a size, while the transfer of heat from recombination during expansion pushes the star to too large a size, and thus the star is oscillating, and that is your Cepheid variable.

In multiple star systems, more variables comes into play, as some of the outer layers of the star are drawn towards the companion stars.

Interesting indeed and "could" have even more interesting ramifications......but IMO until someone actually gets on a craft, flies to a star at light speed, comes back, and asks everyone how long he's been gone, these calculations are based on theories and dare I say it "assumptions" that could later be proven incorrect

For example, space itself could be pulsating, some objects get further away, others get a little closer and we just haven't detected it yet...who knows, there may be variables in play that we just have no idea of.

reply to post by MagicWand67


This is awesome, I love when science get's things wrong. It gives us an opportunity to learn, and rethink what we " Know. " I don't believe in universal constants. It's incredibly pious to assume we know so very much, about the teeny tiny portion of the universe we can observe.

I can't help but wonder this though. How do they know their earlier estimate was wrong? Our star moves, through the galaxy, as do all the others in the universe. Is it not possible both these stars have moved away from each other? I can't remember the exact number but our star is barreling through the galaxy at some 11,000 miles a second. Can you even imagine how fast that is? So how fast is polaris moving? We can only guess, and it's pretty hard to judge distance and exact speed given we are moving at such a speed ourselves. We can only compare it's speed to ours. Is it we got the estimate wrong and it is closer than we thought, or were we right both times, and it is now 100 light years closer. We may never know in our lifetimes, but I always enjoy reading the steps we take to understanding these things. Thank you OP Thoroughly enjoyed this.

Yeah science constantly claims proof of previous speculation through new brekthrough. Too often giving an impression of having everything in our cosmos sorted. New information showing they know squat gives me a warm fuzzy feeling, the more science truly learns the more it learns how little it knows. s+f.

Originally posted by lestweforget
Yeah science constantly claims proof of previous speculation through new brekthrough. Too often giving an impression of having everything in our cosmos sorted. New information showing they know squat gives me a warm fuzzy feeling, the more science truly learns the more it learns how little it knows. s+f.

I don't really know where you get your idea from that scientists think they know everything. In fact, the most common answer any scientist will give you, is "We don't know, but we think that ...". Maybe you should meet some actual scientists ....

"Slumber, watcher, till the spheres,
Six and twenty thousand years
Have revolv'd, and I return
To the spot where now I burn.
Other stars anon shall rise
To the axis of the skies;
Stars that soothe and stars that bless
With a sweet forgetfulness:
Only when my round is o'er
Shall the past disturb thy door."

I assume that Polaris must have been right at the limit of the distance measuring capabilities of Hipparcos? An error of 100 light years out of 400 is rather large, although I appreciate that the parallax angles involved here are extremely small, and therefore even a slight discrepancy in the measurements could result in a significantly different result.