So how far away is that star/quasar, really?

When an astronomer tells you that a star is 50 million light years away, does that mean the apparent distance, i.e. how far it looks to us now, as opposed to how far it really is, taking into account how much it may have moved during the 50 million years that have passed in the meantime?

The implication is that when we say the universe is 13+ billion years old, by looking at a quasar some 13 billion LYs out, and the damn thing is also speeding away at some appreciable fraction of light speed, its true distance would be considerably farther than 13B LYs. If two of them are 180 degrees apart, you’ve got a distance approaching 50 billion LY, which flies in the face of the 13 billion year age factor.

I suspect that relativity of time has something to do with the answer, but my brain is too small.

Bad Astronomer can tell you more, but here’s an article for your perusal on the topic.

There are three systems of measurement used, depending on distance.

First, the nearest stars can be determined by use of parallax – the shift in their positions over six months, during which the earth moves to the opposite end of a baseline of 186,000,000 miles as it orbits the sun. A hypothetical star at 3.2 light years distance would have shifted by one second of arc (1/3600 of a degree) – that distance therefore being one parsec. A parsec is the inverse of the parallex shift in seconds – a star at 6.4 LY would appear to move half a second of parallax, and therefore be two parsecs; one at 12.8 LY, a quarter second, and hence four parsecs; and so on.

Obviously, this measurement only works up to a limited distance – those angles become exceedingly tiny, even with the best of equipment. However, they provide a fairly good cross-section of the types of star.

Now, we turn to the H-R diagram. On this, a star on the main sequence will be of a given intrinsic brightness (absolute magnitude) to radiate at a given spectrum. Stars off the main sequence will have certain specific characteristics, such as broadened absorption bands from the distended atmospheres of red giants, etc.

Hence by identifying a star as of a given brightness and “type,” we can by the process of comparing visual magnitude with absolute magnitude calculate how far away that star is. A star that would be -3 at ten parsecs distance (its absolute magnitude) which is +1 in visual magnitude is obviously significantly farther away than ten parsecs.

A variation on this technique involves the Cepheids, which are blue supergiant stars that pulse in a period intrinsically related to their absolute brightness and size, Ergo, if a cluster of stars or galaxy contains a Cepheid with visual magnitude of +12 that has a given variability period, it is possible to figure out what a Cepheid of that period’s absolute magnitude is, and hence how far away it – and the cluster of which it is a part – is. This extends the brightness-distance measurement to the nearest galaxies – say 2-5 million light years out.

Beyond this, the red shift measurement is done on the basis of the expanding universe, and is only used where the other measurements will not work. A given galaxy known by Cepheid measurements to be, say 3 million LY out, will radiate in a spectrum with specific lines marking absorption by given elements, transitions of excited ions, etc. Those lines are at specific points on the spectrum. If a faint galaxy shows a spectrum where such a line pattern is shifted down the spectrum to a different wavelength, then one can assume that the spectroscopic Doppler effect of movement at relativistic speeds is occurring, and objects at extreme distances are assumed to be moving at such speeds due to the expansion of the universe due to the shifts in their spectral patterns.

Astronomers actually measure quasar distances in term of their redshift: the amount that lines in the spectrum are shifted, due to the motion of the quasar away from us. To get from that to a distance in light-years, you need a relation between speed and distance. For nearby galaxies, speed and distance are proportional and the constant of proportionality is called the “Hubble constant”. For more distant objects, you need to choose a model of the expanding universe in order to translate redshift into “distance”. This distance isn’t the distance to where the quasar is now, rather, it is the distance the light has traveled to get to us.

What is the most distant star that we can reliably measure using the parallax method with current technology?


It’s tough to say which star is the most distant we can measure via parallax, when the data gets squidgier the further out you go.

I’m a long time reader of Astronomy magazine, so I’m reasonably familiar with the paralax, Cepheid, and redshift measurements. I think you’re right, FriendBob, that it is the appearance figure that is usually provided, but it’s never been clear to me about distances to fast moving “edge of the universe” objects.

To us, the 13 b ly distant object looks as it did 13 b years ago - (as we do from its viewpoint). If the quasar is moving at 99% of light speed, during that 13 b years, it is now considerably farther out. From our viewpoint, not much time has passed for it, but there it is, 13 b ly away. It certainly must be a LOT farther away now in actual mileage.

Now turn around and look at one in the opposite direction. It’s just as far in the opposite direction. Since they can’t be 50 b ly apart after 13 b years, time is obviously moving at a different pace for all three (them and us), but still must they not be farther apart than the 13b year estimate? Einstein and his fast train thought experiments seem to address some of this, but…[/brain explodes]

As to the actual question in the OP, yes, the distances given to stars, galaxies, quasars, etc. are their apparent distance as perceived by us now. Keep in mind, that for very far off objects, we may not necessarily know what has happened to them since.

Take quasars. There are no quasars “near” us. That means, they have all “gone away”. (Burned up, changed into something else, etc.) Trying to calculate the distance to an object that currently may no longer exist (in a philosophical sense) doesn’t seem to be the best use of a person’s time.

Yeah, but that place where the ‘thing’ was is still out there, and we should be able to roughly estimate its distance?