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Parallax, the limit.


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I don't think there is a simple answer as it depends on how accurate you want it to be.  

"For parallaxes, uncertainties are typically around 0.04 mas for sources brighter than ~14 mag, around 0.1 mas for sources with a G magnitude around 17, and around 0.7 mas at the faint end, around 20 mag. "

Have a look at this this which is where the quote is from.

Regards Andrew

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16 hours ago, andrew s said:

"For parallaxes, uncertainties are typically around 0.04 mas for sources brighter than ~14 mag, around 0.1 mas for sources with a G magnitude around 17, and around 0.7 mas at the faint end, around 20 mag. "

So the most distant object that could be measured to an accuracy of say 20% is  5000 parsec.  (0.2 mas)

We then need to know what objects exist that are luminous enough to appear mag 14 at that distance

You can  use the distance modulus equation

https://astro.unl.edu/naap/distance/distance_modulus.html

to estimate the absolute magnitude (ie the luminosity) of an object with a brightness of mag 14 at 5kpc and compare it to the luminosity of the sun for example   

Cheers

Robin

Edited by robin_astro
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22 hours ago, andrew s said:

I don't think there is a simple answer as it depends on how accurate you want it to be.  

"For parallaxes, uncertainties are typically around 0.04 mas for sources brighter than ~14 mag, around 0.1 mas for sources with a G magnitude around 17, and around 0.7 mas at the faint end, around 20 mag. "

Have a look at this this which is where the quote is from.

Regards Andrew

I might seem a bit simple but how can an object viewed from two different points using parallax be uncertain?  I thought the whole point of parallax was exactly that it was certain because it was fact. Hence my question, at what point does parallax become questionable. Is there a light year distance that parallax is no longer viable?
 

I had for some foolhardy idea that parallax was the first order of estimation to distant objects. Because it is physical, ie seen by the eye from two points at maximum distance from each other. I presumed that parallax would be free from ambiguity as the basic idea has been around for a few hundred years.

If we have a huge degree of uncertainty with regards to parallax then where does that leave us with Cephid Variables?

I understand that Cephid Variables are the ‘Standard Candle’ with regards to distance and luminosity but if we cannot answer the first question (parallax) then how is the second idea and onward valid?

Are there objects in space with confirmed parallax that also have cephid variable data to compare, I would have have thought Andromeda a likely candidate as it was studied by Edwin Hubble. 
 

Marvin

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@Marvin Jenkins all measurements are uncertain. The major part of the paper I linked to is about them. For a start we only know the Astronomical Unit with a certain precision and the coordinate system based on distant QSO has a finite accuracy before we get to the instrumental errors.

As the first step of the cosmic distance ladder the parallax distances are much more accurate than the remaining ones.

Regards Andrew 

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4 hours ago, Marvin Jenkins said:

 

I understand that Cephid Variables are the ‘Standard Candle’ with regards to distance and luminosity but if we cannot answer the first question (parallax) then how is the second idea and onward valid?

Are there objects in space with confirmed parallax that also have cephid variable data to compare, I would have have thought Andromeda a likely candidate as it was studied by Edwin Hubble. 
 

 

The parallax of objects outside our Galaxy is too small to measure. There are many Cepheids in our Galaxy  close enough to accurately measure the distance to using parallax though.  (The Gaia uncertainty of 0.04 mas is only a small percentage of distance for nearby objects). These are used to establish the luminosity/period relationship.  You can then measure the periods and apparent brightness of Cepheids in nearby galaxies. From this you can calculate how far away they are and step up another rung on the distance ladder

Robin

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The archetype Cepheid variable delta Cephei for example (~900 light years away) has a parallax of 3.77 mas so the distance uncertainty from Gaia would be 0.04/3.77 = ~1% .  If we found a Cepheid with the same period as Delta Cephei in another Galaxy we would know it has the same luminosity so by measuring its apparent brightness we can work out how far it (and hence the Galaxy) is 

Robin

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