Parallax Measurement

Classical Mechanics Level 1

These images show the same part of the night sky as seen in January and July 2018. Most of the stars have remained fixed, though two seem to have moved relative to the others.

Which star is closer to us, red or blue?

Assume that the red and blue stars have not moved significantly relative to the Sun in the time between the two images were taken, and that the colors are used simply to label the stars (they do not indicate frequency shifts).

Red Blue Cannot be determined

16 solutions

Chew-Seong Cheong
Sep 2, 2018

Consider the figure above, as the Earth revolves round the Sun from January to July observers on Earth will find that the blue star, which is nearer, extends a larger angle or parallax than red star, which is further away.

But what if the Red Star is moving significantly faster than the Blue one? Wouldn't it then be possible for the Red Star to be closer without the assumption that they're moving at the same speed relative to Earth?

Matthew Garvey - 2 years, 9 months ago

Unlike planets, stars are relatively stationary in the universe.

Chew-Seong Cheong - 2 years, 9 months ago

The stars are so far away (in light-years). Any movement of any star is not significant at all when obverse from Earth. The fastest moving star S2 is moving at 18 million miles an hour or 2.5% the speed of light. The nearest star from the Sun, Proxima Centuari is 4.22 light-year from Earth. Therefore, if Proxima Centuari is moving at 2.5% the speed of light for a year perpendicular to our view, the angle extended is $\frac {0.025}{4.22}\approx 0.33^\circ$ .

Chew-Seong Cheong - 2 years, 9 months ago

We do not know if the star is moving faster. Astronomers have recently discovered rogue stars, ejected from a galaxy at phenomenal speeds.

Thor Stambaugh - 2 years, 9 months ago

The stars are so far away (in lightyeas). The movement of any star is not significant at all when observed from Earth.

Chew-Seong Cheong - 2 years, 9 months ago

@Chew-Seong Cheong – So what? For all we know, the image could be showing us a $0.1^\circ$ view...

C . - 2 years, 9 months ago

What if the red star even moved one turn and back to where it was within 6 months, but blue star was not? If the red star is further than blue star, doesn't it move faster than blue star due to Newton's gravity laws of motion and Hubble's law?

Diệu Linh Linh - 2 years, 9 months ago

Chew-Seong Cheong - 2 years, 9 months ago

So they can't make sure about the result, can they?

Diệu Linh Linh - 2 years, 9 months ago

@Diệu Linh Linh – My answer is correct. From Parallax on Wikipedia : "Due to foreshortening, nearby objects show a larger parallax than farther objects when observed from different positions, so parallax can be used to determine distances."

Chew-Seong Cheong - 2 years, 9 months ago

How do you determine parallax or the angle ?

Naresh Ambulgekar - 2 years, 9 months ago

You may want to read this up -- Parallax

Chew-Seong Cheong - 2 years, 9 months ago

Where is the observer ? At the north pole or south pole ?

Naresh Ambulgekar - 2 years, 9 months ago

Anywhere on Earth and at night.

Chew-Seong Cheong - 2 years, 9 months ago

Why don't the other stars move?

Do the red and blue stars appear read and blue or are they colored for reference only?

Chris Arsenault - 2 years, 9 months ago

The stars are so far away (in light-years). Any movement of any star is not significant at all when obversed from Earth. The fastest moving star S2 is moving at 18 million miles an hour or 2.5% the speed of light. The nearest star from the Sun, Proxima Centuari is 4.22 light-year from Earth. Therefore, if Proxima Centuari is moving at 2.5% the speed of light for a year perpendicular to our view, the angle extended is $\frac {0.025}{4.22} \approx 0.33^\circ$ .

The colors used are for reference only.

Chew-Seong Cheong - 2 years, 9 months ago

I didn't ask for your boilerplate excuse seen elsewhere in this discussion, I asked why only 2 stars moved.

And the "colors used are for reference only" should be explained in the problem, not here. I will report as this is causing some confusion.

Chris Arsenault - 2 years, 9 months ago

@Chris Arsenault – Sorry, I didn't read your question properly. The other stars do not move mean they are much further away.

Chew-Seong Cheong - 2 years, 9 months ago

WE KNOW THAT STARS OF RED COLOUR ARE MORE FAR AWAY FROM US,SINCE THE RED COLOUR HAS GREATER WAVELENGTH THEN BLUE COLOUR,SO WE CAN ALSO CONCUDE BY THIS FACT THAT THE RED STAR IS FARTHER AWAY FROM US THAN THE BLUE STAR✌️✌️✌️✌️ ISN'T THAT CORRECT??

Saptarshi Bhaumik - 2 years, 9 months ago

Actually, red shift means "moving away", it says nothing about distance...

And can you please stop shouting? -.-

C . - 2 years, 9 months ago

Color of stars depend on the temperature of the star and not the distance from us. Red shift only indicates the direction of the shift and it does not means that stars further away turn red.

Chew-Seong Cheong - 2 years, 9 months ago

Ram Mohith
Sep 2, 2018

As we know that more the distance of a certain body from Earth less is the relative distance as observed from Earth. To sum up :

If a heavenly body is every much far from Earth it relative motion as observed from Earth is very negligible.
If a heavenly body is closer from Earth it's relative motion can be observed from Earth.

From the given two pictures as The Bue Star has more displacement relative to Earth is is the closer star to us

This is using the view of perspective. The closer something is, the faster and more it seems to move. The us since the blue moved across the sky more, January to July, that means that the blue is closer; again using the view of perspective. of course, that is assuming that the blue is not actually moving faster than the red. After all, we do not have that information.

Zachrey Mueller - 2 years, 9 months ago

Agreed without knowing if the speed is constant it cannot be determined...

Byron Nelson - 2 years, 9 months ago

Wayne Moyle
Sep 6, 2018

the red coloured star must be further away due to red shift compared to blue

Actually, red shift means "moving away", it says nothing about distance...

C . - 2 years, 9 months ago

Mohammad Farhat
Sep 3, 2018

One solution is that blue star looks bigger than the red star but this is only true if the two stars have the same size.

Mitchell Aninyang
Sep 9, 2018

It is proven through theoretical physics that the blue color appears closer than the red color. You can tell that by mere looking at it. Its like an optical illusion.

Ammar Atef
Sep 2, 2018

The more far the star, the less movement speed we can notice

Alexander Campbell
Sep 9, 2018

Just find the bigger star!

Joel Vaz
Sep 9, 2018

The farther the stars... The slower they'll move due to a huge orbit around the sun. The nearer, they'll move faster.

Nandish Shah
Sep 9, 2018

According to me its just basic Science its like forming a mirage of the stars we see that the stars visibilty concept through our naked eyes too it will create a simple formulation as if we see tiny things it may be big but the distance is more as compared to which we see big in respect to that I can define the problem through simple notation as observence.

Gourav Ganguly
Sep 8, 2018

The blue star moves a greater angle on the sky than the red. The further away a star is, the smaller angle it has with the Earth. Ergo, red is further away, since it moves the least angle of the two.

Bibhor Singh
Sep 8, 2018

Hey, I did not went on that much thinking. I simply watched the two images. Blue one had shining light streaks on its outer surface whereas red star looked hazy in both the images. And we know the star is closer to us when it shines the most.

Sarthak Ram Nimbarmunde
Sep 8, 2018

As the color of star is blue,this indicates its distance from earth must be less than that of red star by Doppler effect. As red light is seen from the star the light have to travel many light years , this cause increase in its wavelength (red shift). For blue appearing star implies it must be closer to earth than red appearing star with difference many light years