# Hubble Law

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#1
Why can't the distance determined by trigonometric parallax? Parallax should be able to determine distance to nearby galaxy. Thankss a lot!!
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5 months ago
#2
(Original post by ialfighter2020)
Why can't the distance determined by trigonometric parallax? Parallax should be able to determine distance to nearby galaxy. Thankss a lot!!
Impossible to measure the parallax angle produced by this method with acceptable accuracy:

The Earths orbital radius around the sun is just over 8 light minutes. The distance to the nearest galaxy (Andromeda) is 2.5x106 light years distance. In angular terms, that equates to a shift of arcsin(495 / 7.9x1013) = 3.6x10-10 degrees.

Scaling that for context: if the Earth's orbit could fit into a 10p coin, the nearest galaxy would be 2 million miles away.
Last edited by uberteknik; 5 months ago
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#3
(Original post by uberteknik)
Impossible to measure the parallax angle produced by this method with acceptable accuracy:

The Earths orbital radius around the sun is around around 8 light minutes. The distance to the nearest galaxy (Andromeda) is 2.5x106 light years distance. In angular terms, that equates to a shift of arcsin(495 / 7.9x1013) = 3.6x10-10 degrees.

Scaling that for context: if the Earth's orbit could fit into a 10p coin, the nearest galaxy would be 2 million miles away.
Does it mean that trigonometric parallax can only be used to measure distance to stats in the milk way? Other methods, e.g. standard candle, should be adopted to measure distance to star in other galaxy?
0
5 months ago
#4
(Original post by ialfighter2020)
Does it mean that trigonometric parallax can only be used to measure distance to stats in the milk way? Other methods, e.g. standard candle, should be adopted to measure distance to star in other galaxy?
Yes, and even then, for stars relatively local. Parallax is a direct-distance measurement and the limiting accuracy for this method is measuring the arc made at the apex of an isosceles triangle. The largest base of that triangle, for an observer on earth, is with reference to the earth's orbit around the sun and using that together with the best instruments, can catalogue stars out to a distance of say 15,000 ly or so.

For distances greater than a few thousand light years, several other (inevitably less accurate) methods are used. For each new method, accuracy falls away with increasing distance and the next best accuracy method takes over.

The succession of methods is called the cosmic distance ladder of which standard-candles is the next. This method measures distance by calibrating the brightness of local stars (distance measured by parallax) exhibiting certain types of physical behaviour - such as Lyrae and Cepheid variable stars. The brightness is then used as a reference to compare the relative brightness (apparent magnitude) of these same types of Cepheid variable stars found further away in our own galaxy and with similar stars found in other galaxies. Having measured the apparent magnitude, the distance to that star (hence distance to the galaxy) can be calculated from the known experimentally confirmed relationship between distance and brightness measured in the laboratory.
Last edited by uberteknik; 5 months ago
0
#5
(Original post by uberteknik)
Yes, and even then, for stars relatively local. Parallax is a direct-distance measurement and the limiting accuracy for this method is measuring the arc made at the apex of an isosceles triangle. The largest base of that triangle, for an observer on earth, is with reference to the earth's orbit around the sun and using that together with the best instruments, can catalogue stars out to a distance of say 15,000 ly or so.

For distances greater than a few thousand light years, several other (inevitably less accurate) methods are used. For each new method, accuracy falls away with increasing distance and the next best accuracy method takes over.

The succession of methods is called the cosmic distance ladder of which standard-candles is the next. This method measures distance by calibrating the brightness of local stars (distance measured by parallax) exhibiting certain types of physical behaviour - such as Lyrae and Cepheid variable stars. The brightness is then used as a reference to compare the relative brightness (apparent magnitude) of these same types of Cepheid variable stars found further away in our own galaxy and with similar stars found in other galaxies. Having measured the apparent magnitude, the distance to that star (hence distance to the galaxy) can be calculated from the known experimentally confirmed relationship between distance and brightness measured in the laboratory.
Thankss a lot for your step by step and detailed explanation!!
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