Starship Asterisk*

http://en.wikipedia.org/wiki/Dark_Horse_(astronomy) wrote:
<<The Dark Horse Nebula or Great Dark Horse is a large dark nebula, which as seen from Earth, obscures part of the upper central bulge of the Milky Way. The Dark Horse lies in the southern constellation Ophiuchus (the Serpent Bearer), near its borders with the more famous constellations Scorpius and Sagittarius. This region of dark nebulae is called Dark Horse because it resembles the side silhouette of a horse and appears dark as compared with the background glow of stars and star clouds. It is also known as "Great" because it is one of the largest (in apparent size) groups of dark nebulae in the sky.



The rear of The Great Dark Horse (its rump and hind legs), is also known as the Pipe Nebula, which itself carries the designation B77, B78, and B59. (The 'B' numbers named after the astronomer Edward Emerson Barnard, who catalogued dark nebulae.) The ability to see The Great Dark Horse with the naked eyes is an indication that the skies are very dark.>>

• Rev.6:5 I heard the third beast say, Come and see.
  And I beheld, and lo a black horse;
  and he that sat on him had a pair of balances in his hand.

neufer wrote:

neufer wrote:

[url=http://en.wikipedia.org/wiki/The_Treachery_of_Images]Wiki: The Treachery of Images[/url] wrote: The picture shows a pipe that looks as though it might come from a tobacco store advertisement. Magritte painted below the pipe: "Ceci n'est pas une pipe" (This is not a pipe), which seems false but is actually true. The painting is not a pipe, but rather an image of a pipe. As Magritte himself commented: "The famous pipe. How people reproached me for it! And yet, could you stuff my pipe? No, it's just a representation, is it not? So if I had written on my picture 'This is a pipe,' I'd have been lying!" (cited in Harry Torczyner, Magritte: Ideas and Images, p. 71.)

bystander wrote:

neufer wrote:

Ceci n'est pas une cheval

neufer wrote:Cela est un cheval.

bystander wrote:

neufer wrote:Cela est un cheval.

Non, ce n'est pas un cheval.
Il ressemble plus à un orignal,
mais ce n'est pas un orignal, soit.

http://en.wikipedia.org/wiki/File:Moose ... -750px.JPG
http://en.wikipedia.org/wiki/File:Moose_standing.jpg

DavidLeodis wrote:Yes NoelC, it is a wonderful view. It must be amazing to be able to see such a clear sky view.

Realising that the image seemed very famiIiar I've just done a search of the APOD archive and found the APOD of June 23 2006 http://antwrp.gsfc.nasa.gov/apod/ap060623.html. That the image in the two APODs is so very similar should not perhaps be a surprise, but I still think it's awesome that there is no really obvious change despite that the Universe is said to be expanding rapidly. 8)

neufer wrote:The Galactic Center is located about .0076 Mpc away.

Hubble expansion constant = 74.2 ± 3.6 (km/s)/Mpc.

Galactic Center is retreating by 1261 miles per hour due to galactic expansion.

Chris Peterson wrote:

neufer wrote:The Galactic Center is located about .0076 Mpc away.

Hubble expansion constant = 74.2 ± 3.6 (km/s)/Mpc.

Galactic Center is retreating by 1261 miles per hour due to galactic expansion.

I'm a little confused. Galactic expansion? What's that? I'm guessing (since the calculation works) that you mean Universal expansion. However, the Milky Way is gravitationally bound, and is not itself getting any larger due to the expansion of the Universe. We don't see the galactic center retreating.

neufer wrote:Well, if it really is universal expansion (i.e., the expansion of space time itself) then it ought to take place within galaxies as well, although not nearly enough (a few kilometers per second) that one would notice it compared to other galactic motions.

However, if what we are talking about is simply the recoil of galaxies from an initial explosion (with slowing due to matter and expansion due to dark energy) then I agree that that is a somewhat different matter. But if that is the case, I don't understand why it has taken so long for photons from the cosmic microwave background to reach us. (But, then, there are a lot of things that I don't understand.)

Chris Peterson wrote:

neufer wrote:Well, if it really is universal expansion (i.e., the expansion of space time itself) then it ought to take place within galaxies as well, although not nearly enough (a few kilometers per second) that one would notice it compared to other galactic motions.

What you calculated from the Hubble constant was the expansion of the Universe. But we don't see that expansion within the galaxy (and we are easily capable of measuring a few kilometers per second using Doppler shift) because the gravitational field of the galaxy is much too strong to allow the local space to expand.

• The Andromeda Galaxy is observed to be advancing at 301 ± 1 km/s.
  
  The Andromeda Galaxy is located about 0.778 Mpc away.
  
  Hubble expansion constant ~ 74.2 (km/s)/Mpc.

Chris Peterson wrote:

neufer wrote:However, if what we are talking about is simply the recoil of galaxies from an initial explosion (with slowing due to matter and expansion due to dark energy) then I agree that that is a somewhat different matter. But if that is the case, I don't understand why it has taken so long for photons from the cosmic microwave background to reach us. (But, then, there are a lot of things that I don't understand.)

The photons from the edge of the observable Universe have been reaching us for 13.7 billion years (or so). In the beginning, they were much more energetic. Over the billions of years, as space expanded, they have had their wavelength stretched more and more by expanding space. What we're seeing now was emitted 13.7 billion years ago, but is only now reaching us because the space between the emitting material and us has expanded so far (47+ billion light years). In another 13.7 billion years we'll still see the photon background, but it will be even colder.

neufer wrote:Perhaps...but let us take the case of the Andromeda Galaxy whose Doppler shift
(for some bizarre reason) IS only capable of being measured to a few kilometers per second:

• The Andromeda Galaxy is observed to be advancing at 301 ± 1 km/s.
  
  The Andromeda Galaxy is located about 0.778 Mpc away.
  
  Hubble expansion constant ~ 74.2 (km/s)/Mpc.

Now I would argue that the Andromeda Galaxy is retreating by about 58 km/s (= 74.2 x 0.778) due to universal expansion but it is actually observed to be advancing at 301 ± 1 km/s thanks to a combination of a strong initial velocity and (to a much lesser extent) by its attraction to the Milky Way.

I'm assuming that you would argue that there IS no expansion within the Local (Galactic) Group because the gravitational field of the Local (Galactic) Group is too strong to allow the local space to expand. Is that your argument? (At what point is universal expansion finally allowed?).

Chris Peterson wrote:I think your assessment is correct, or nearly so. The fields found between the bound members of galaxy clusters aren't strong enough to completely counter expansion. Eventually most galaxy clusters will come apart (except where some of their components first merge).

astrolabe wrote:Hello neufer,

A while back an image was posted that showed the red and blue shifting in the CMB indicating to me our movement in the Universe relative to it. The question that follows is this: Are we advancing toward Andromeda in relation to the CMB or is it the other way around?

astrolabe wrote:Approximately 46 billion ly. across now by some estimates and which has taken about 13.7 billion lyrs. for it to get to that diameter.

NoelC wrote:What if the universe is not expanding at all?

What if the passage of time isn't constant?

Maybe it's still the size of a pea.

From an observer inside if the passage of time changed it might seem that things are getting farther apart as photons would take longer to get from one place to another.

Very few folks (certainly not I) can get their heads around this kind of relativity.

frankly, not very good philosophy