Space, the Final Frontier...

(The Andromeda Galaxy yesterday)

Ever paused to consider exactly HOW we know the Andromeda Galaxy is the nearest galaxy to us?

The answer to that is actually in debate in some circles.

In fact, the whole "distance measurement to other heavenly bodies" is an entire subject up for debate amongst certain people.

It may be useful therefore, to provide a brief history of how we got to this position.

Way back when we discovered the "wanderers" (planets) we used Newton's Laws to determine how far they were from us. This done, we then used those same laws to conclude how far the nearest stars were from the difference between their apparent "brightness" and the brightness we should expect them to be by virtue of their "spectra" (plural of "spectrum), because different sized stars gave off different kinds of light, once again, according to Newton's Laws.

Then comes the Bone of Contention.

The Cepheid Variables.

These particular stars pulsated. And this pulsation period (how often they went off and on) was related in a very simple way to their brightness. SO! NOW we had a tool to measure far bigger distances.

Then someone with an enormous telescope noticed Cepheids in Andromeda, and from the rate at which they switched "on" and "off" came to the conclusion that Andromeda was a massive distance from us, 2.5 million light years (a light year is the distance light travels in a whole year) from us.

Maybe a few other galaxies were discovered using the "Cepheid variable yardstick", but from then on, the galaxies were so distant that these stars just could not be seen.

Then came another breakthrough. Edwin Hubble realised that the Universe was expanding, and that all galaxies were receding from each other. It was at first thought that they were all receding from EARTH but a bit of quick fourth-dimensional thinking dispelled that. The idea was that the Universe could be envisaged as sitting on the surface of a balloon, which was being inflated. Any point on the balloon could be seen to be receding from every other point, so there was no "centre" to the expansion at all.

Anyway, once this "expansion" hypothesis had been accepted, a new yardstick came into play - the Redshift.

It is the same effect heard when police sirens approach you, pass you, then recede from you. A definite change in pitch is heard to the notes emitted from the siren. This is called the "Doppler" effect. The same thing happens to light, only with light, it is what you SEE that it affected. And what you SEE, as something goes away from you, is a shift of the spectrum towards the red end. Or, if it is coming at you, the blue end.


This "redshift" was calibrated with the known galaxies whose distance we knew from the Cepheids, and then extrapolated to include far more distant galaxies. Thus was born the "Hubble Constant" This is a measure of how fast the Universe is expanding, mostly based on those Cepheid Variables we can actually see.

It is currently about 80 km per second for each meagaparsec further out the galaxy is (a "parsec is about 6.2 light years). In other words, if we find a galaxy that is moving away from us at 80 km per second (determined by the redshift) then if we find one that is receding at 160 km per second, it is roughly twice as far away.

The Hubble Constant started out at more than double what we think it is now. To this day, no-one knows its true value simply from lack of data.

Anyway, to the main point. All these distances rely on correlation between brightness, the Cepheids, and redshift.

The method with the least data is that with the Cepheids. In other words, we are still not quite sure exactly how far things are "up".

Then there are the Quasars. These are extremely bright objects that some of us thought were stars at first (they were called "Quasi-Stellar" radio sources, the "quasi" taken to mean "likened to", from which sprouted the name), but according to their redshifts, were GI-normous distances away.

Nowadays, the current view is that they are the nuclei of very distant galaxies that are very active. It was once thought that, since they are about 13 billion light years away, and that the age of the Universe is about 14.5 billion years, that what we might be seeing was the Big Bang itself.

There have even been theories based upon the fact that quasars are no more than local star (in our own galaxy) whose redshift is due to something else.

The entire point being, the measurement of distances in space is sketchy to say the least, a point glossed over in many books on astronomy.

Recently, a new yardstick has emerged - Supernovae (what happens to a star when its nuclear fuel runs out? It goes bang!). There are very definite equations describing what would happen to a star and how bright it should be when this happens.

So, as before, the brightness of a supernova BANG will be an indication of how far away it would be. Theoretically, at least. Unfortunately, these are so rare, we still await salient data to test this out.

The conclusion?

E.T. may be nearer than at first thought!