# Ability to measure distances with parallax versus size

1. Feb 2, 2016

### Yosty22

I would like to preface this by stating that I am not very well-versed in cosmology or astrophysics, but I've been thinking: I understand the idea of parallax, both in the sense of human eyes measuring distances to nearby objects and in the sense of telescopes on earth (or in orbit) measuring the distance to nearby stars. From my understanding, it is all about angle. In the case of human eyes, a very nearby distance has photons entering the eye at a very steep angle with respect to the normal while photons from far away objects enter the eye at much more shallow angle with respect to the normal. This change in angle allows us to estimate distances to distant objects.

In the case of measuring the distance to nearby stars, the idea of using two eyes is exactly identical, except these "two eyes" is just the earth being at different places in space as it orbits the sun - opposite sides of the sun every 6 months. However, since the Earth's orbit is fairly small, especially compared to some of these astronomical distances, this must only be sensitive for the more nearby stars.

My question is this: Once you begin looking at objects more distant than these further stars we can measure distances to with parallax, does size matter? That is, for example, our nearest galaxy is located much further away than the nearest stars, but the galaxy is also much larger. Does the added size (or area, maybe?) of the object we are trying to measure distances to with parallax make it easier for this technique to work?

2. Feb 2, 2016

### Chalnoth

For measuring parallax, size is largely irrelevant. How bright the object is matters somewhat, as if the object is too dim it's harder to see.

However, sometimes we can obtain an estimate of how big a far-away object really is, and by observing how big it appears we can estimate how far away it is. One interesting example is a supernova called SN1987A. Before this star exploded, it had some rather violent events that expelled a large ring of gas around the star. This ring of gas was illuminated by light from the supernova about two thirds of a year after the supernova exploded, indicating that it was about two thirds of a light year away. By observing how big this ring of gas appeared in the Hubble telescope (0.808 arc seconds, or 0.000224 degrees across), we can calculate how far away the supernova was (168,000 light years).

3. Feb 3, 2016

### Staff: Mentor

Nope, parallax is essentially all about the distance to the object and the size of the orbit the telescope is placed in. In addition, the distance to an extended object (like a nebula) can actually be more difficult to measure using parallax than a point-source (stars).

4. Feb 3, 2016

### Yosty22

Thanks for the reply. So building off of this, if we want to use parallax to measure more and more distant objects (say stars), the best way to get resolution is to put telescopes in space with larger and larger orbits around the sun/earth? If so, it makes it rather evident why we don't use parallax for the extremely distant objects as the amount of time it would take to make a measurement like this would be immense.

5. Feb 3, 2016

### Chalnoth

This is a difficult proposition. It's really hard to put an observing satellite in an orbit much further from the Sun than the Earth is (we frequently put objects at the L2 Lagrange point, but that's only 1% further from the Sun than the Earth). Orbits further out will be out of sync with the Earth, so we'll only be able to get signals from the satellite for part of the year. Plus once the satellite is out there, it's really hard to do any sort of maintenance on it. And then we can only really gain a factor of a few in terms of resolving power.

It's actually much easier to instead focus on increasing the angular accuracy of the telescope. So instead of increasing the size of the lever arm, we just make a more accurate determination of the precise angle.

In the end, though, no amount of improvement to the accuracy of parallax measurements will help with measuring distances on cosmological scales. At best we might expect to be able to get a few hundred thousand light years out of it (I think current state of the art is still less than a hundred thousand light years for parallax). Those kinds of distances won't even let you measure distances outside our own galaxy. For measuring distances to other galaxies, we need other methods. Parallax measurements are often used as a mechanism to calibrate some other distance measures. One common measure for medium distances is cepheids. These are high-mass stars that periodically change in size and temperature, and there is a relationship between how rapidly they change and their average brightness.