I may add to this thread on and off but, for the moment...
Since the 1930s, astronomers have wrestled with the fact that galaxies are spinning much faster than their apparent densities should allow.
https://www.newscientist.com/article/mg ... al-matter/
How do people observe - or try to calculate without knowing its mass - how fast a galaxy is spinning? I can think of no way to do it. And why would its density have anything to do with its rate of spin?
What puzzles me
What puzzles me
Nullius in verba ... ☎||||||||||| ... To Fate I sue, of other means bereft, the only refuge for the wretched left.
When flower power came along I stood for Human Rights, marched around for peace and freedom, had some nooky every night - we took it serious.
Who has a spare two minutes to play in this month's FG Trivia game! ... My other OS is Slackware.
When flower power came along I stood for Human Rights, marched around for peace and freedom, had some nooky every night - we took it serious.
Who has a spare two minutes to play in this month's FG Trivia game! ... My other OS is Slackware.
What puzzles me
As I understand it, the principle tool they have used in the Doppler effect. They observe the differences in the red/blue shift in detected light and can tell a little bit about rotation speeds of the stars in the galaxy.
I think that they really need to establish a secondary point of view to really refine the observations, though.
The Hubble can help, but it is still, from an interstellar perspective, far to close to our own point of view. They need to establish something like Hubble in a solar orbit, perhaps out near the orbit of Pluto, and then compare the views from that observatory to the Hubble. That would refine the final observations.
I think that they really need to establish a secondary point of view to really refine the observations, though.
The Hubble can help, but it is still, from an interstellar perspective, far to close to our own point of view. They need to establish something like Hubble in a solar orbit, perhaps out near the orbit of Pluto, and then compare the views from that observatory to the Hubble. That would refine the final observations.
The home of the soul is the Open Road.
- DH Lawrence
- DH Lawrence
What puzzles me
So you find a spectral line related to the end of each limb and you see how much it's shifted by in each case, and there will be a net difference. Then you need to know the radius of the galaxy out as far as the place you took the spectra from (which requires you to already know the absolute distance to the galaxy), and half the net difference in the shift is the speed each arm is rotating, and you know from the radius what the circumference is of the galaxy at that place so you can work out the time it takes to fully rotate.
Okay. If it's a really close galaxy then you can then have a pretty good estimate of the radius, but for anything further away you'll be limited by measuring the angle the galaxy subtends, and how far along each radius your spectrum is being sampled at. The error range is going to accumulate very easily with those estimates, and all of that accumulated error range is going to apply to your eventual estimate of the speed of rotation.
I'll see if I can find some estimates for galaxies at different distances and whether they come with margins of error.
Okay. If it's a really close galaxy then you can then have a pretty good estimate of the radius, but for anything further away you'll be limited by measuring the angle the galaxy subtends, and how far along each radius your spectrum is being sampled at. The error range is going to accumulate very easily with those estimates, and all of that accumulated error range is going to apply to your eventual estimate of the speed of rotation.
I'll see if I can find some estimates for galaxies at different distances and whether they come with margins of error.
Nullius in verba ... ☎||||||||||| ... To Fate I sue, of other means bereft, the only refuge for the wretched left.
When flower power came along I stood for Human Rights, marched around for peace and freedom, had some nooky every night - we took it serious.
Who has a spare two minutes to play in this month's FG Trivia game! ... My other OS is Slackware.
When flower power came along I stood for Human Rights, marched around for peace and freedom, had some nooky every night - we took it serious.
Who has a spare two minutes to play in this month's FG Trivia game! ... My other OS is Slackware.
What puzzles me
I would have thought that x millions of miles per hour was a pretty high margin of error, when you're talking in those sorts of figures.
Using older methods pretty much a complete rotation would have to be observed in order to gauge their speed. Using older methods a Police Officer would have had to measure a car covering a set distance with a stopwatch in order to know what speed it was doing. These days they are monitored in real time using lasers. The advances in radio telescopes & mass spectrometers are so advanced that they can make such measurements over a far shorter time.
Using older methods pretty much a complete rotation would have to be observed in order to gauge their speed. Using older methods a Police Officer would have had to measure a car covering a set distance with a stopwatch in order to know what speed it was doing. These days they are monitored in real time using lasers. The advances in radio telescopes & mass spectrometers are so advanced that they can make such measurements over a far shorter time.
What puzzles me
What has been measured with precision and margins of error - https://en.wikipedia.org/wiki/Galaxy_rotation_curve discusses it - is "that most stars in spiral galaxies orbit at roughly the same speed", Which makes determining the radius irrelevant, and it was the raduis which seemed an unlikely candidate for precise measurement.
Nullius in verba ... ☎||||||||||| ... To Fate I sue, of other means bereft, the only refuge for the wretched left.
When flower power came along I stood for Human Rights, marched around for peace and freedom, had some nooky every night - we took it serious.
Who has a spare two minutes to play in this month's FG Trivia game! ... My other OS is Slackware.
When flower power came along I stood for Human Rights, marched around for peace and freedom, had some nooky every night - we took it serious.
Who has a spare two minutes to play in this month's FG Trivia game! ... My other OS is Slackware.
What puzzles me
Perhaps the reason is that the speed is close to that of the speed of light & therefore not able to travel any faster?
What puzzles me
There's a graph at https://en.wikipedia.org/wiki/Galaxy_rotation_curve
That's around 0.04% of the speed of light.
That's around 0.04% of the speed of light.
Nullius in verba ... ☎||||||||||| ... To Fate I sue, of other means bereft, the only refuge for the wretched left.
When flower power came along I stood for Human Rights, marched around for peace and freedom, had some nooky every night - we took it serious.
Who has a spare two minutes to play in this month's FG Trivia game! ... My other OS is Slackware.
When flower power came along I stood for Human Rights, marched around for peace and freedom, had some nooky every night - we took it serious.
Who has a spare two minutes to play in this month's FG Trivia game! ... My other OS is Slackware.
What puzzles me
spot;1499751 wrote: There's a graph at https://en.wikipedia.org/wiki/Galaxy_rotation_curve
That's around 0.04% of the speed of light.
Wouldn't that speed be related to the acceleration of gravity? 32 ft/sec. ^2?
Here's an interesting thought, because the Universe is not only expanding but accelerating in it's expansion, our race, by the sheer luck of being one of the earlier stars to be born, will be one of the lucky races to have stars and galaxies in the sky. In a billion years or so, the galaxies will be so far apart, and the light will have to travel so far, that they will be invisible to each other.
Can you imagine how hard it would be for future races to figure out the nature of the Universe? They will think theirs is the only galaxy in all of Creation. (But of course, a galaxy is probably a big enough playground for any race.
That's around 0.04% of the speed of light.
Wouldn't that speed be related to the acceleration of gravity? 32 ft/sec. ^2?
Here's an interesting thought, because the Universe is not only expanding but accelerating in it's expansion, our race, by the sheer luck of being one of the earlier stars to be born, will be one of the lucky races to have stars and galaxies in the sky. In a billion years or so, the galaxies will be so far apart, and the light will have to travel so far, that they will be invisible to each other.
Can you imagine how hard it would be for future races to figure out the nature of the Universe? They will think theirs is the only galaxy in all of Creation. (But of course, a galaxy is probably a big enough playground for any race.
What puzzles me
Saint_;1499822 wrote: Wouldn't that speed be related to the acceleration of gravity? 32 ft/sec. ^2?
Here's an interesting thought, because the Universe is not only expanding but accelerating in it's expansion, our race, by the sheer luck of being one of the earlier stars to be born, will be one of the lucky races to have stars and galaxies in the sky. In a billion years or so, the galaxies will be so far apart, and the light will have to travel so far, that they will be invisible to each other.
Can you imagine how hard it would be for future races to figure out the nature of the Universe? They will think theirs is the only galaxy in all of Creation. (But of course, a galaxy is probably a big enough playground for any race.
Given the amount of time required for the light to have got here for us to see it, that may have already happened, we won't find out for a few million years.
Here's an interesting thought, because the Universe is not only expanding but accelerating in it's expansion, our race, by the sheer luck of being one of the earlier stars to be born, will be one of the lucky races to have stars and galaxies in the sky. In a billion years or so, the galaxies will be so far apart, and the light will have to travel so far, that they will be invisible to each other.
Can you imagine how hard it would be for future races to figure out the nature of the Universe? They will think theirs is the only galaxy in all of Creation. (But of course, a galaxy is probably a big enough playground for any race.
Given the amount of time required for the light to have got here for us to see it, that may have already happened, we won't find out for a few million years.
The home of the soul is the Open Road.
- DH Lawrence
- DH Lawrence