Galactic Unit

The Astronomical Unit is defined in Wikipedia as, 'a unit of length, roughly the distance from Earth to the Sun.' Perhaps there should be the Galactic Unit, which I would define as, 'a unit of length, roughly the distance from the Sun to the center of the Milky Way Galaxy.' Wikipedia calls the Astronomical Unit a 'convenient yardstick'. The Galactic Unit would be equally convenient. Has anyone seen hide or tail of similar proposals? DE

 

Log in or Sign up to hide all adverts.

How would you determine the exact centre of the Milky Way? If you can't do that your proposed measure would be useless.

 

Log in or Sign up to hide all adverts.

Lightyears and parsecs seem to work just fine.

 

Log in or Sign up to hide all adverts.

There are lots of things that fall nicely around the range of AUs - namely all the planets in our SS, which range from .25 to 40AU, as well as all the planets in the 2000 or so other SSs.

What would one usefully measure with galactic AUs that is not already covered by light years and parsecs? It's not like we commonly deal with the orbits of stars around the galactic center.

 

I think david just really, really wants to coin a phrase or a word.

 

Kristoffer, your just jealous. You're what I call a forum piranha. Almost every conversation thread tree has at least one or two. A dime a dozen. It is best to simply ignore piranhas and get down to the meat of the discussion. The star systems at 1 GU in a galaxy might have certain common features that are different from star systems at 0.5 GU. Hence the value of such a system. DE

 

There are already several existing units for galactic measurements - parsecs, and light years.

The reason AUs make sense in a solar system is because planets are discrete items at discrete distances. Integer units and the occasional fraction work fine.

In a galaxy, there is no such deliniation. There are myriad stars at every possible distance. In fact, the galaxy has a structure, and it makes more sense to talk about, say, the Orion Spiral Arm, than it does to say "all stars at X distance from the centre". Orbital distance, in what is essentially a static object, is essentially uninteresting.

I have to agree with Kristoffer; you're looking for a solution for which there is no problem. The only reason to promote it is to be able to coin a phrase.

 

The other reason the AU makes sense for the Solar system it is also tied to the Earth's orbital period upon which we base a common time unit, the year. Thus if we measure another object's distance from the Sun in AU, it is very quick and easy to use Kepler's laws to find its orbital period as measured in years.

Now while there is an equivalent "galactic year" for our solar system's trip around the galaxy, it is really too long (225-250 million years) for practical purposes of observation, and in addition, due to the dark matter halo the stars in the galaxy, the stars do not follow Kepler's laws.

 

I looked around for this 'forum piranha' stuff. It really fits well on many members here.

On the OP Janus has clarified the point nicely. But when we talk of Cosmological distances, AU is too small.

 

We already have the term Galactic year, which is different for each galaxy (being the time it takes for one complete galactic rotation), so we should be careful about how we use Galactic, especially if we are only referring to the Milky Way.

 

Galaxies don't rotate as if they a solid object, the rotational periods for its individual stars varies with the distance from the center. It makes no sense to talk about a rotation period for a galaxy as a whole. The Galactic year refers to how long it takes for our Sun to make one trip around the galaxy.

 

I'm no astronomer but I don't understand how that can be so. Doesn't Kepler's 3rd Law say the orbital period is proportional to the 3/2 power of the distance from the central attracting body? If so, then as you go from the centre of a galaxy outward, the outer parts will pretty rapidly lag further and further behind the inner ones, won't they?

 

It was the fact that the orbital speeds of the stars in the disk of galaxies remained nearly constant as you moved away from the center of the galaxy ( rather than falling off by the distance to the power of 3/2) that led to the hypothesis of dark matter. A star orbiting twice as far from the center as another but orbiting at the same speed will take twice as long to complete an orbit. (whereas a planet orbiting twice as far from the Sun would take 2.83 times longer)
Here's a typical galaxy rotation curve compared to the Kepler prediction. Note that the y axis is labeled in km/sec, a measure of linear velocity not angular velocity.

Please Register or Log in to view the hidden image!

 

Thanks for this. I'm beginning to understand, now.

Please Register or Log in to view the hidden image!

 

That's the whole reason behind the Dark matter hypothesis. Kepler's law works as stated if you assume a large central mass which is the same for every orbiting body. This holds for the solar system because the vast majority of its total mass resides in the Sun, this is not true with bodies such as galaxies.
Look back at the diagram I gave in my last post note how even the Keplerian prediction has the orbital speeds increasing at first as you move from the center. This is because as you move away from the center, there are more and more stars closer to the center than you are and they contribute to the mass you are orbiting, this increase in mass has a greater effect on increasing the orbital speed than your increasing distance has on decreasing it. Once you move out of the central bulge and into the disk, the rate that interior mass decreases (assuming you only count the visible matter) and the orbital speeds should begin to decrease closer to power of 3/2 rate. The fact that observations show that they do not seems to indicate that there is more mass than what we can see and that this mass is not distributed in the same way as the visible parts of the galaxy. (for instance, it extends above and below the visible disk and all that apparent) It is less densely distributed than the visible matter but also has a greater volume. It is the combined gravitational effect of the visible matter of the galaxy and this less dense dark matter cloud in which it is imbedded, that results in the observed orbital speeds in the graph.

 

YES! I realised, as I was driving across Richmond Park today to see my old dad in the nursing home, after posting my earlier message, that I had been a bit of a jackass.

Please Register or Log in to view the hidden image!

Since the mass of a galaxy is distributed, one would presumably need something like Newton's shell theorem (but for an oblate sphere or disc?) to work out how much effective mass there was, gravitationally acting at any given radius from the galactic centre. Sorry for being obtuse. The graph you provided makes it all much clearer.

 

Correction, the period would increase by the distance to the power of 3/2, the orbital speed would decrease by the inverse of the square-root of the distance

 

DaveC, when you accuse me of trying to coin a phrase, you are essentially accusing me of trying to come up with good ideas. What is your point? DE

 

Yes I realised that was what you meant. But thanks for confirming.

(This thread is proving a nice opportunity for me to increase my understanding of astronomy - just what these forums should be for.

Please Register or Log in to view the hidden image!

)