How thick is our spiral arm?

[Airbrush]

The Milky Way is 100,000 ly across. The central bulge is thicker than our region of the galaxy. How thick is our galaxy in our region of our spiral arm?

 

I had been looking for info about the Kepler mission search for Earth-sized planets. I finally heard in a Science Channel program that Kepler's field of view includes stars that range in distance from us between 300 and 3,000 ly. Does that mean our spiral arm is roughly 6,000 ly thick? If we were located exactly on the plane of the galaxy, it makes sense that Kepler's field of vision should include all the stars furthest from us while gazing "upward", and not the nearby stars. Kepler is following the Earth around the Sun and looking "upward", in the direction of north if aligned with our solar systems "north", looking at the same patch of stars continuously, that extend to the "upper" edge of our spiral arm.

Edited June 25, 2013 by Airbrush

[StringJunky]

It's about 1000 LY thick.

 

https://en.wikipedia.org/wiki/Milky_Way#Size_and_composition

[Airbrush]

Thanks for the answer StringJunky. 1,000 ly is the AVERAGE thickness of our galaxy. Our region must be even less thick. Now I am confused. This does not make sense. How can Kepler be watching stars 3,000 ly away, when our galaxy is not even close to being that thick in our spiral arm? Is it possible that the plane of our solar system is not parallel with the plane of the Milky Way?

 

"...The stellar disk of the Milky Way Galaxy is approximately 100,000 ly (30 kpc) in diameter, and is, on average, about 1,000 ly (0.3 kpc) thick...."

 

https://en.wikipedia.org/wiki/Milky_Way#Size_and_composition

Edited June 25, 2013 by Airbrush

[pwagen]

I'm no expert, but might it not be kind of tricky to say how thick the galaxy is "in our region"? First of all because there might be stray stars here and there, above and below the galaxy's main mass. Do they count? If so, if a single star is 4000 LY above, and the second furthest away is 2000 LY up, is the galaxy 4000 LY in that direction?

 

Also, how do you define our region? Where do you draw the limit? I'm not sure the answer is clear cut.

 

There might very well be a more accurate answer than 1000 LY, but you might have trouble finding it.

[StringJunky]

Thanks for the answer StringJunky. 1,000 ly is the AVERAGE thickness of our galaxy. Our region must be even less thick. Now I am confused. This does not make sense. How can Kepler be watching stars 3,000 ly away, when our galaxy is not even close to being that thick in our spiral arm?

 

"...The stellar disk of the Milky Way Galaxy is approximately 100,000 ly (30 kpc) in diameter, and is, on average, about 1,000 ly (0.3 kpc) thick...."

 

https://en.wikipedia.org/wiki/Milky_Way#Size_and_composition

 

Like Pwagen's just said...distant galaxies look look nice and discrete from our observation point but actually measuring from inside one which is, don't forget, truly massive and probably actually quite fuzzy to determine..maybe the observers set an arbitrary boundary to define the furthest extent.

[pwagen]

 

maybe the observers set an arbitrary boundary to define the furthest extent.

However, there's nothing stopping anyone from doing the same. Airbrush, I can imagine you can find somewhat detailed maps of our surrounding area. You should then be able to define an area of the galactic disc to measure the height of. I would suggest looking at a circle with about the same diameter as the width of the Orion arm near us. If nothing else, you should be able to get a feel for whether the average thickness of 1,000 LY is accurate in our corner of the galaxy.

[EdEarl]

This tool may give you some info you seek.

http://www.worldwidetelescope.org/Home.aspx

[StringJunky]

It would actually appear to be arbitrary if I understand this diagram from the Wiki link. I would think there was a decreasing density distribution as you move away from the disk in the vertical plane,:

 

 

Further reading suggests the spiral arms just contain the youngest stars:

 

The spiral-arm population is the youngest in the disk; it appears to trace the spiral pattern of the Milky Way. This population is concentrated very close to the disk plane, with a scale height of ~100 pc. Representative objects include H I and molecular clouds, H II regions, protostars, stars of types O & B, supergiants and classical cepheids. The metallicity of this population is somewhat higher than that of the Sun (MB81).

 

http://www.ifa.hawaii.edu/~barnes/ast626_95/pcmw.html

Edited June 25, 2013 by StringJunky

[Airbrush]

Thanks for all the help above.

 

I searched "plane of solar system plane of milky way" and here is what I found:

 

QUESTION: "Are the orientations of our solar system and others in our galactic disc "in-line" with the disc or are they oriented in all different directions? What determines their orientation?

ANSWER: "They're oriented in all different directions. The size of a solar system is so much smaller than the size of the Galaxy, that the Galaxy's structure has no impact on the orientation of a solar system. What determines their orientations is the direction of the angular momentum that the system had when it formed, and that's pretty much random. Our own solar system is tipped by about 63 degrees with respect to the plane of the galaxy. "

 

http://curious.astro.cornell.edu/question.php?number=633

 

That explains it. Our solar system is not at all aligned with the Milky Way. So Kelper's field of vision is pointed in the northward direction relative to Earth, but that is looking at an angle 63 degrees into the Milky Way. If the average thickness of the Milky Way is 1,000 ly, then our region must be a few hundred ly thick.

[BrightQuark]

I saw this thread yesterday and did a quick search out of interest and found a discussion elsewhere, on the same topic, with respect to the "thickness" issue, one poster gave a measurement of between 900 lt yrs, and 10,000 light yrs, working through the spiral arm, I assume, very uneven in short, however, unfortunately he posted nothing to back up where this information was taken from..... and then I happened on the Wiki article that you've already copied in above on the MW, with some of the comments half-way down concerning the difficulty of calculations from within the galaxy made across the galactic plane.

Edited June 26, 2013 by BrightQuark

[Spyman]

Kepler's search volume, in the context of the Milky Way galaxy.
http://en.wikipedia.org/wiki/Kepler_(spacecraft)#Spacecraft_orbit_and_orientation

 

When we look at other spiral galaxies from an edge on view, we can see that the disc has a very uniform thickness from the central bulge and out to its edge, like with this image of the spiral galaxy NGC 4565:

http://en.wikipedia.org/wiki/NGC_4565

Edited June 26, 2013 by Spyman

[Airbrush]

Thanks Spyman, that illustration of Kepler's visual field exactly answers my original question. Its' view extends for many more thousands of light years beyond its' effective range of 3,000 ly but the more distant stars must be too distant for Kepler to take readings.

[David Levy]

Dear Spyman.

Look at those pictures of spiral galaxies; Try to focus and look carefully…

What do you see?

How come that the center is full of light? How come that the super massive black hole is not black? Why it has a disc shape? Why all the stars are moving in one direction? Why the stars at the far end of the galaxy are moving at ultra speed? Why???

There is only one answer for that!

By wiki " The bar may be surrounded by a ring called the 5-kpc ring that contains a large fraction of the molecular hydrogen present in the galaxy, as well as most of the Milky Way's star formation activity."

The Supper massive black hole has the power to create new Atoms. This creation process generates the ultra lightning that we see in the center. Those new Atoms are the foundation of the new stars forming process. Those new stars are drifting outwards from the center and create the famous shape of the spiral arms.

So simple. So clear. No need for dark energy. No need for quantum. No need for expansion. So simple and easy…

Edited June 28, 2013 by David Levy

[Spyman]

David Levy

 

The conservation laws of nature says that matter/energy neither can be created nor destroyed, supermassive black holes are not perpetual motion machines that spawn matter from nothing. Also even though supermassive black holes holds a impressive amount of mass/energy they are still very small compared to their housing galaxy.

 

Astronomers have made good observations of the orbital velocities of stars in both other galaxies and the Sun's neighbourhood by measuring their redshift, if stars in general would be drifting outwards, we would not only already know about it but have measured that speed with high precision too.

 

Dark energy and space expansion have nothing to do with structures within galaxies, they represent a feature that is dominant between superclusters of whole groups of galaxies. Quantum physics deals with phenomena at microscopic scales which also are irrelevant on galaxtic scales.

 

Your idea might be simple and easy but it is also terribly wrong.

Edited June 28, 2013 by Spyman

[BrightQuark]

Would the centre of the galaxy be anywhere near as bright as in that artist's impression, if the gas clouds moved aside and we could see it. I've heard somewhere that it would show up in the sky like another moon?

Edited June 28, 2013 by BrightQuark

[imatfaal]

!

Moderator Note

 

David Levy

 

Do not continue with your off-topic hijacking of a simple question with your speculation. Any further posts on baryogenesis etc within the galactic hub will be removed.

 

Do not respond to this moderation within the thread. Report this post if you wish to register a complaint.

 

 

 

Dear Spyman.

Look at those pictures of spiral galaxies; Try to focus and look carefully…

What do you see?

How come that the center is full of light? How come that the super massive black hole is not black? Why it has a disc shape? Why all the stars are moving in one direction? Why the stars at the far end of the galaxy are moving at ultra speed? Why???

There is only one answer for that!

By wiki " The bar may be surrounded by a ring called the 5-kpc ring that contains a large fraction of the molecular hydrogen present in the galaxy, as well as most of the Milky Way's star formation activity."

The Supper massive black hole has the power to create new Atoms. This creation process generates the ultra lightning that we see in the center. Those new Atoms are the foundation of the new stars forming process. Those new stars are drifting outwards from the center and create the famous shape of the spiral arms.

So simple. So clear. No need for dark energy. No need for quantum. No need for expansion. So simple and easy…

[StringJunky]

Would the centre of the galaxy be anywhere near as bright as in that artist's impression, if the gas clouds moved aside and we could see it. I've heard somewhere that it would show up in the sky like another moon?

 

The light is that combined from many stars clustered around the centre that has an inactive super massive black hole, like the Milky Way How bright is the sun? how bright is many suns, such that they appear to be one viewed from a great distance? i think the answer is yes. An active SMBH which is feeding on gas and stars would be even more luminous.

 

An IR picture from the Spitzer space telescope speaks a thousand words:

 

 

https://en.wikipedia.org/wiki/Galactic_Center

Edited June 28, 2013 by StringJunky