Question on Astronomy

[MobiusPizza]

I've been wondering all the time; When I see picture of the galaxies, some are rounded, some are spirals. But since it took few millions years for light to reach us from that galaxy; not to mention the galaxy may be few thousand light years wide; how do we make sure the galaxy's shape is not desorted due to the fact that the light from edge of the galaxy take even longer to reach us (Basic trigonometry)

Also; why would astronomers calculate the speed of galaxy with doppler effect when we know that the data we see now is few millions years old? Perhaps the galaxy had slowed down/is much further away now than we thought? Do astronomers take this into effect?

 

[brentkiosk]

Do a search on "Hubble Constant" and you will probably find as much information as you could want on determining astronomical distances.

 

[silverpig]

Well the truth is that light from the edges of a disk galaxy does take longer to get to us than from the central part when viewed edge on, but because of the speed of light, the relative slowness of stellar motion, the angle viewed (galaxy rotating in our plane of view), then you don't really see anything different.

When viewing face on (so the galaxy looks like a circle) all the light takes pretty much the same time to get to us, so we see the galaxy as it did all at the same time.

As to your last question, well, there's nothing we can really do about it. It is impossible to know what a galaxy is doing right now because the information simply can't get to us faster than light speed. It's kind of a moot point really. We can still learn a lot just by looking at galaxies as they appear now.

 

[unipidity]

Though we can also learn a lot about galaxy evolution by observing things like the luminosity of the central bulge and arms in different populations of galaxies- some at very high redshift and others much closer.

 

[MobiusPizza]

Ah thanks for the input.

The astronomers are claiming dark matter is the only explanation to the seem lack of mass holding galaxy together. What if maybe today at this time the galaxy is already blew apart? Maybe? Maybe not? We simply don't know bececause the galaxy we look at now is billions years ago. Even the whole galaxy doesn't exist now we can't be sure

 

[Calin]

That dark matter thing is just a figment of a imagination - it is the constant that must be added to, substracted from, multiplied by or demultiplied by the result to obtain the values the theory predicts.
The dark matter (which not everyones agree it is there) is just a way to fix the results so they match the theory. It might be the simplest way, but it could be the wrong way.

 

[MobiusPizza]

I do believe there s dark matter. God knows how much hydrogen clouds are sitting there as steller gas clouds; And how much neutrinos, neutrons and other light subatomic particles are banging around

It's just that whether there is enough of it to account for the theory discrepency

 

[MoD TaRkIn]

Generally, due to the observing distance and the size of the galaxy, the difference between the relative emission times for the centre and outer edge of a galaxy is not that long, add to that the fact that galaxy evolution is very slow, and you end up with more distortion resulting from viewing through earths atmosphere that resulting from galaxy size.

As for dark matter, it must and does exist, the evidence is irrefutable. Do a google search on 'galaxy rotation curves' and you'll see. If galaxys (including our own) were composed of only the mattrer we can see (which is mostly hot and luminous), we would expect the rotation speed of matter to decreace with distance form the centrr of the galaxy. But it does not, it instead remains (to a good approximation) constant, depending on the galaxy obviously. This implys that there is significaly more gravitating matter in the galaxy than we can see. it does make sense if you think about it. not all matter is going to be hot and luminous, there will be a large mumber of dead stars, brown dwarfs, large hydrogen coulds, all of which are too dim for us to see. True, there are lots of suggestion for dark matter candidates, some indeed which are rediculous. But like it or not, there MUST be somehting there, and its nothing to do woth theory, the evidence for dark matter is almost entirley observational.

You could go further onto dark energy and a non zero lamda, but that is a little more unlikely, although not that much.