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How many arms does the Milky Way have?

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tommyk
1089444.  Wed Aug 13, 2014 12:03 pm Reply with quote

I don't know if this would count as general ignorance - I always thought the Milky Way had two spiral arms, but I don't know how many people would have thought that. Anyway:


How many spiral arms does the milky way have? If you said two - wrong, though you’re in good company.
Radio observations on nebulae in the Milky way 60 years ago suggested that there were 4 major spiral arms.
NASA’s Spitzer space telescope in 2008 made infrared observations of the light emitted by stars in our galaxy, indicating around 100 million stars, but only 2 spiral arms.

A new study, completed in 2013, surveyed 1750 massive stars with radio telescopes, revealing, 4 spiral arms. This discrepancy is probably due to the Spitzer program seeing only cooler, less massive stars, whilst the recent program observed much more massive stars. These massive stars have much shorter lifetimes and are thus more likely to be found in the arms in which they were formed, whereas the smaller longer lived stars rotate around the MW for much longer. The gravitational pull from the two stellar arms causes the majority of stars to congregate in those two large spiral arms, but not in the other two smaller ones,yet star formation does occur in all four of the arms.

http://adsabs.harvard.edu/abs/2014MNRAS.437.1791U

 
dr.bob
1089613.  Thu Aug 14, 2014 8:50 am Reply with quote

The simple fact of the matter is that "nobody knows" (as was covered on a popular TV panel show a while back).

You're right to say that the Spitzer survey was biased towards one type of star. However, the survey reported in the link is also biased as they only measured young, massive stars. Neither approach gives you a good idea of how the galaxy would look if you were able to take a step back and look at it from the outside.

There's also a problem with measurement error. Astronomers use "standard candles" to measure distances on the cosmological scale, but these have significant limitations in their accuracy. Thus, pretty much all measurements of interstellar distances have fairly significant error bars. These errors add to the uncertainty.

The biggest problem, of course, is that, because we're actually inside the Milky Way, it's quite hard to see it all because a lot of stuff gets in the way. I recently went to a talk about the Milky Way where the speaker admitted that we're not even 100% sure if the galaxy has a bulge in the middle of it.

 
tetsabb
1089700.  Thu Aug 14, 2014 7:24 pm Reply with quote

dr bob wrote
Quote:
a lot of stuff

I love it when you talk technical!
:-)

 
dr.bob
1089727.  Fri Aug 15, 2014 4:41 am Reply with quote

Stop me if I'm blinding you with science :)

 
AlmondFacialBar
1089729.  Fri Aug 15, 2014 4:48 am Reply with quote

dr.bob wrote:
I recently went to a talk about the Milky Way where the speaker admitted that we're not even 100% sure if the galaxy has a bulge in the middle of it.


...16000 lightyears thick, but out by us it's just 3000 lightyears wide.

Or was that one of the bits Brian Cox made them retract in the light of new evidence? ;-)

:-)

AlmondFacialBar

 
tommyk
1090664.  Thu Aug 21, 2014 12:55 pm Reply with quote

dr.bob wrote:
You're right to say that the Spitzer survey was biased towards one type of star. However, the survey reported in the link is also biased as they only measured young, massive stars. Neither approach gives you a good idea of how the galaxy would look if you were able to take a step back and look at it from the outside.


So are we saying that it's either 2 or 4, or that its premature to put any kind of number on it at all? Looking from the outside would give us the best view, but it seems like it almost depends on how we define an arm? The Spitzer study operates in IR and so detects cooler stars. The radio survey can detect young,hot,massive things. It appears as though all the older stars are mainly in two spiral arms, but the newly forming stars occur in two extra 'arms'.

Were we outside the Milky way we would be able to see the two main arms, as detected by Spitzer, but with a radio telescope we would detect two more.

Where this leaves us I don't know, but its certainly fun! :)

 
spursystarman
1218407.  Wed Dec 28, 2016 2:30 pm Reply with quote

dr.bob wrote:
The simple fact of the matter is that "nobody knows" (as was covered on a popular TV panel show a while back).

You're right to say that the Spitzer survey was biased towards one type of star. However, the survey reported in the link is also biased as they only measured young, massive stars. Neither approach gives you a good idea of how the galaxy would look if you were able to take a step back and look at it from the outside.

There's also a problem with measurement error. Astronomers use "standard candles" to measure distances on the cosmological scale, but these have significant limitations in their accuracy. Thus, pretty much all measurements of interstellar distances have fairly significant error bars. These errors add to the uncertainty.

The biggest problem, of course, is that, because we're actually inside the Milky Way, it's quite hard to see it all because a lot of stuff gets in the way. I recently went to a talk about the Milky Way where the speaker admitted that we're not even 100% sure if the galaxy has a bulge in the middle of it.

The standard candles that astronomers used are Cepheid Variables, a type of star whose light-changes are proportional to their real luminosities. Knowing a star's real luminosity and comparing that with its apparent luminosity allows us to derive their distances. But this technique is typically only used to determine distances to other galaxies, rather than internally to our own. Since the Sun is close to the galactic plane, the central bulge of the galaxy obscures what is on the opposite side, so the number of arms remains uncertain.

 
spursystarman
1218408.  Wed Dec 28, 2016 2:36 pm Reply with quote

tommyk wrote:
dr.bob wrote:
You're right to say that the Spitzer survey was biased towards one type of star. However, the survey reported in the link is also biased as they only measured young, massive stars. Neither approach gives you a good idea of how the galaxy would look if you were able to take a step back and look at it from the outside.


So are we saying that it's either 2 or 4, or that its premature to put any kind of number on it at all? Looking from the outside would give us the best view, but it seems like it almost depends on how we define an arm? The Spitzer study operates in IR and so detects cooler stars. The radio survey can detect young,hot,massive things. It appears as though all the older stars are mainly in two spiral arms, but the newly forming stars occur in two extra 'arms'.

Were we outside the Milky way we would be able to see the two main arms, as detected by Spitzer, but with a radio telescope we would detect two more.

Where this leaves us I don't know, but its certainly fun! :)

One of the main emitters of IR radiation is star formation, which typically takes place in or close to the arms. The protostars and the molecular clouds from which they form radiate little or no light, but there is what is called 'Infra-red excess'. I'm currently looking at a forming star (GM Cephei) which is located in a huge starforming cloud called IC1396 where Spitzer recently detected about 30 forming stars in a small area, few or none of which were actually visible in visual light.

 
dr.bob
1219259.  Wed Jan 04, 2017 6:54 am Reply with quote

spursystarman wrote:
The standard candles that astronomers used are Cepheid Variables, a type of star whose light-changes are proportional to their real luminosities. Knowing a star's real luminosity and comparing that with its apparent luminosity allows us to derive their distances. But this technique is typically only used to determine distances to other galaxies, rather than internally to our own.


Cepheid Variables can absolutely be used to measure distances within our own galaxy. After all, the name derives from the Delta Cephei star system which was one of the first examples of its type discovered, and that lives a mere 887 light years away.

Cepheid Variables can be used to measure extra-galactic distances, but only really out as far as the local group. For properly cosmological studies, other standard candles, like Type Ia Supernovae are much more useful.

 
spursystarman
1219372.  Wed Jan 04, 2017 12:24 pm Reply with quote

Sure, but that's why I said 'typically - because that's how they were traditionally used by Edwin Hubble, Leavitt et al (Leavitt found several cepheids in the Magellanic clouds for instance). And as you say, SN are used these days because of their predictable absolute magnitudes.
While it is true that one could use cepheids to determine distances in our own galaxy, you could only determine the distance of that variable star itself since there would be no larger unit for that variable to be in.
However a subclass of cepheids (RR Lyrae stars) obey the same period-luminosity law and can be used to determine distances to globular clusters around our Galaxy.

 
dr.bob
1219504.  Thu Jan 05, 2017 9:21 am Reply with quote

spursystarman wrote:
While it is true that one could use cepheids to determine distances in our own galaxy, you could only determine the distance of that variable star itself since there would be no larger unit for that variable to be in.


Whilst that's often the case, NGC 6067 and NGC 6087 are both open clusters that contain Cepheid variable stars.

 
Snowywebb
1288020.  Mon Jun 25, 2018 10:08 pm Reply with quote

If nobody (still) knows how many arms there are in the Milky Way galaxy, how can we be sure it is a spiral shape?

 
dr.bob
1288086.  Tue Jun 26, 2018 9:33 am Reply with quote

We can map the prevalence of gas in the Milky Way by studying things like the 21cm emission lines from molecular Hydrogen. By examining this, we see definite signs of a spiral structure, though the error bars are too big to define precisely how many spiral arms there are.

 

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