Celestia Forums

Is there any evidence for local 'rifts' (places where stellar density is lower than average*) or 'clusters' (places where stellar density is greater than average) within about 1000pc of Sol? And how far would the edge of the spiral arm that we're in be from Sol? Would there be a noticeable drop in stellar density beyond that, or does it mean that there are less bright stars?

I can't really spot any gaps/clumps in Celestia, even using the 2 million stars dataset... (beyond things like the Hyades or Pleiades anyway)


*: I'm assuming that the stellar density around Sol - which appears to be about 140 stars in a 7 parsec radius - is "average".

Oh, Evil One,

Are we using the same version of Celestia???

There are quite a few "OB Associations" known to be in our vicinity. Just check the catalogs on VizieR. Also, there's an extensive DSC Catalog of OpenClusters on my "catalogs" web page.

My "bright stars" addon seems to display some very clumpy behavior. Some clumps seem to correspond to known open clusters, but some don't.

Also, although I haven't had a chance to follow up on either of them yet, I seem to see a lack of OB stars in the very near vicinity of the Sun as well as a very strange asymmetry in the distrubution of M class stars at large distances. I fear that either of these effects could be the result of selection criteria in the list of stars included in the Hipparcos Input Catalog, though, which does have me concerned. I haven't actually investigated that yet.

As for finding the "edge" of the arm, I'm not sure that's the right term. The current models of galactic development show the arms to be density waves in the interstellar medium, becoming locations of increased stellar production (including higher populations of OB stars), not actually of higher overall stellar density. The regions between the arms are filled with low luminosity stars: the density waves have moved on and the short-lived bright stars have all burnt out.

selden wrote:There are quite a few "OB Associations" known to be in our vicinity. Just check the catalogs on VizieR. Also, there's an extensive DSC Catalog of OpenClusters on my "catalogs" web page.

My "bright stars" addon seems to display some very clumpy behavior. Some clumps seem to correspond to known open clusters, but some don't.

I was wondering more about the distribution of less massive stars, actually - ie would one expect to find pretty much the same number of K amd M V star in the vicinity of a G V star that is say 1000 lightyears away, as one finds around the sun? Or would that vary?

But I know the OB stars are not distributed too evenly through space.

Also, although I haven't had a chance to follow up on either of them yet, I seem to see a lack of OB stars in the very near vicinity of the Sun as well as a very strange asymmetry in the distrubution of M class stars at large distances. I fear that either of these effects could be the result of selection criteria in the list of stars included in the Hipparcos Input Catalog, though, which does have me concerned. I haven't actually investigated that yet.

I was wondering about the lack of OB stars near Sol too. What's the assymetry you're seeing in the M (V?) stars though? I'd have thought we can't see the dim ones that are more than a few dozen parsecs away - we seem to have trouble enough finding dim ones within 6 pc of Sol! So I'd assume that there appear to be many more M V stars near Sol than there are further away. Is that what you're seeing?

As for finding the "edge" of the arm, I'm not sure that's the right term. The current models of galactic development show the arms to be density waves in the interstellar medium, becoming locations of increased stellar production (including higher populations of OB stars), not actually of higher overall stellar density. The regions between the arms are filled with low luminosity stars: the density waves have moved on and the short-lived bright stars have all burnt out.

As I suspected - so it's not the gaps between the arms are not the sci-fi staple of a big void with barely any stars at all - it's just a region that has no massive, bright stars.

Oh Evil One,

Evil wrote:I was wondering about the lack of OB stars near Sol too. What's the assymetry you're seeing in the M (V?) stars though? I'd have thought we can't see the dim ones that are more than a few dozen parsecs away - we seem to have trouble enough finding dim ones within 6 pc of Sol! So I'd assume that there appear to be many more M V stars near Sol than there are further away. Is that what you're seeing?

There's certainly a collection of M stars in a 100LY sphere around the sun, but I was referring to the giants that are more like 500-1000LY away.

Here's a picture of the situation as seen from the galactic center looking toward the sun:

(as usual, this links to a much larger picture)

And here's a URL for this viewpoint:
cel://Follow/mwc/2004-03-12T23:55:26.63117?x=AAAAQHqzxr7U/Xug/////w&y=AAAAwJpMcP0XfBdf/////w&z=AAAAAMgf+nEpkhp6Bg&ow=-0.862879&ox=0.027315&oy=-0.049069&oz=0.502281&select=Sol&fov=11.712729&ts=1.000000<d=0&rf=14263&lm=144

This is using my Addon of OB&M stars which has their AbsMag set to -13.

I see more red in the upper right and lower left quadrants than I see in the upper left and lower right.

I also see a distinct tilt in the distribution of the OB stars compared to the plane of the galactic coordinate system.

So now my question is if these effects are real or if they might be due to how they selected the stars that were measured.

Reading the documentation for the Hipparcos star catalog at http://astro.estec.esa.nl/Hipparcos/CATALOGUE_VOL1/catalog_vol1.html, I found a plot of "stars per square degree" in galactic coordinates. It seems to show an asymmetry that's compatible with the distribution of M stars that I see. http://astro.estec.esa.nl/Hipparcos/CATALOGUE_VOL1/MISSING_FIGS_3.2/f3_2_001.pdf

(I still haven't located the documentation for the Hipparcos Input Catalog, which is essential, of course.)

Volume 1, Section 3.3, in the Hipparcos docs at http://astro.estec.esa.nl/Hipparcos/CATALOGUE_VOL1/sect3_03.pdf points out that the selection of stars in the Hipparcos Input catalog is biased and cannot be considered a "survey", while the selection criteria for theTycho catalog is much less biased "although the limiting magnitude depends somewhat on the locaion on the sky."

An image showing Tycho's number of observed stars per square degree in galactic coordinates at http://astro.estec.esa.nl/Hipparcos/CATALOGUE_VOL1/MISSING_FIGS_3.3/f3_3_001.pdf also shows an asymmetry that might be correlated with the spatial distribution of M stars pictured above, although the differences are hard to see in their colored image.

If I squint, there also seems to be an asymmetry in the distribution of stars in the galactic plane, but the tilt seems to me to be opposite to the tilt seen when looking toward the sun from the galactic center.

selden wrote:Oh Evil One,
I also see a distinct tilt in the distribution of the OB stars compared to the plane of the galactic coordinate system.

Are those density waves that are supposed to represent the spiral arms supposed to be a bit tilted relative to the galactic plane? Could that be what you're seeing?

I expect you will have heard of Gould's belt; if not a good description can be found here-
http://ottawa.rasc.ca/observers/1999/an9912p8.html
and
http://ottawa.rasc.ca/observers/2000/an0001p3.html

it seems we are inside a huge drift of OB stars, which share a common ancestry to a greater or lesser extent...

(edit- it is tilted too)

this is a very old proposition, made by Gould in the 19C;
do you think it is still valid, and can it be seen in Celestia?

Thanks for the links! I'd heard about Gould's Belt but had completely forgotten about it.

I'd expect that some of its features should be visible in Hipparcos. The smaller tilt that I noticed is at right angles to it, though. Additional investigations will be needed.