Starship Asterisk*

FrogSplash wrote:And people keep telling me that galaxies colliding is not catastrophic. This picture proves different.

Chris Peterson wrote:

FrogSplash wrote:
And people keep telling me that galaxies colliding is not catastrophic. This picture proves different.

It's catastrophic for the galaxies as a whole, in terms of their structure. But there are no star collisions, and for most of the stars it isn't a catastrophic thing at all.

• ____ King Lear Act 4, Scene 1
  
  GLOUCESTER: As flies to wanton boys, are we to the gods.
  ____ They kill us for their sport.

http://www.spacetelescope.org/images/opo9734u/ wrote:

In NGC 7252, where we had discovered ~40 globular clusters
with the Hubble before its refurbishment, we have recently
discovered ~500 globulars with the refurbished telescope.
Each of bluish pointlike sources is a likely globular.
Credit: B. Whitmore (STScI)

idahogie wrote:To me, this is the most interesting thing about today's post: "Since the sideways velocity of the Andromeda Galaxy (M31) is presently unknown,..."

Can anyone explain that to me in more detail? I assume that our relative motion on the axis between the Milky Way and Andromeda is known. But why is the "sideways velocity" unknown? Plus, it always seems to be presented as a given that the MW and Andromeda will one day collide. Now it's completely unknown whether that will happen? What changed?

idahogie wrote:To me, this is the most interesting thing about today's post: "Since the sideways velocity of the Andromeda Galaxy (M31) is presently unknown,..."

Can anyone explain that to me in more detail? I assume that our relative motion on the axis between the Milky Way and Andromeda is known. But why is the "sideways velocity" unknown? Plus, it always seems to be presented as a given that the MW and Andromeda will one day collide. Now it's completely unknown whether that will happen? What changed?

neufer wrote:it depends whether you are a glass half full or half empty type person.

Chris Peterson wrote:
It is easy to measure radial velocity, by looking at the Doppler shift of light.
But tangential velocity can only be inferred from indirect methods.

• __ Hamlet, Prince of Denmark Act 2, Scene 1
  
  LORD POLONIUS: And thus do we of wisdom and of reach,
  __ With windlasses and with assays of bias,
  __ By indirections find directions out:
  __ So by my former lecture and advice,
  __ Shall you my son.

Chris Peterson wrote:
We simply haven't observed long enough to see any actual tangential movement.

http://en.wikipedia.org/wiki/Hipparcos wrote:
<<[Over five years of operation The High Precision Parallax Collecting Satellite (Hipparcos)] objective was to provide the positions, parallaxes, and annual proper motions for some 100,000 stars with an unprecedented accuracy of 2 mas (milliarcseconds), a target in practice eventually surpassed by a factor of two.

Since 1997, several thousand scientific papers have been published making use of the Hipparcos and Tycho Catalogues. A detailed review of the Hipparcos scientific literature between 1997–2007 was published in 2009. Some examples of notable results include (listed chronologically):

* studies of Galactic rotation from Cepheid variables[14]
* the nature of Delta Scuti variables[15]
* studies of local stellar kinematics[16]
* testing the white dwarf mass-radius relation[17]
* the structure and dynamics of the Hyades cluster[18]
* kinematics of Wolf-Rayet stars and O-type runaway stars[19]
* subdwarf parallaxes: metal-rich clusters and the thick disk[20]
* fine structure of the red giant clump and associated distance determinations[21]
* unexpected stellar velocity distribution in the warped Galactic disk[22]
* refining the Oort and Galactic constants[23]
* Galactic disk dark matter, terrestrial impact cratering and the law of large numbers[24]
* vertical motion and expansion of the Gould Belt[25]
* the use of gamma ray bursts as direction and time markers in SETI strategies[26]
* evidence of a galaxy merger in the early formation history of the Milky Way[27]
* study of nearby OB associations[28]
* close approaches of stars to the Solar System[29]
* studies of binary star orbits and masses[30]
* the HD 209458 planetary transits[31]
* formation of the stellar Galactic halo and thick disk[32]
* the local density of matter in the Galaxy and the Oort limit[33]
* ice age epochs and the Sun's path through the Galaxy[34]
* local kinematics of K and M giants and the concept of superclusters[35]
* an improved reference frame for long-term Earth rotation studies[36]
* the local stellar velocity field in the Galaxy[37]>>

Chris Peterson wrote:
What this means is that we don't know with any certainty the orbital parameters of the galaxies in the Local Group. We do know that the galaxies are gravitationally bound, which means that all will eventually collide or be ejected from the cluster. But the Milky Way and Andromeda are far too massive to experience ejection from perturbations, so it is certain they will eventually collide. When that will happen is less clear, however.

strangerbarry wrote:Maybe this is a really dumb idea. To estimate the sideways motion of the andromeda galaxy, would it be possible to measure the red shift of light reflected off distant intergalactic dust located at 90 degrees to our own line of sight to Andromeda ?

ZenGrouch wrote:This is probably a stupid rookie question, but I have to wonder how galaxies manage to collide, when the universe is expanding? Were they propelled in different directions, allowing this interaction, prior to the time they were formed, as the hydrogen atoms were drawn together?

strangerbarry wrote:[snip] where the cookie expands but the chips don't.

Chris Peterson wrote:

ZenGrouch wrote:This is probably a stupid rookie question, but I have to wonder how galaxies manage to collide, when the universe is expanding? Were they propelled in different directions, allowing this interaction, prior to the time they were formed, as the hydrogen atoms were drawn together?

It's a rookie question, but it isn't a stupid one <g>.

The force of gravity is stronger than the "force" of expanding space. Galaxies in clusters have enough gravity to hold space together locally, so the region doesn't expand. It is orbital dynamics, as governed by gravity, that hold galaxy clusters together. For the same reason, dense bodies like stars and planets aren't expanding with the Universe, and we aren't getting farther from the Sun.

Presumably, the member galaxies in a cluster all formed together, or nearly so, from material local to their region of the Universe, and have remained bound (sometimes loosely) since.

strangerbarry wrote:
I believe you're on the right track. There are "local" movements of nearby galaxies in different directions. As well gravitational attraction among local groups of galaxies can create localized areas where gravity acts to overcome cosmic expansion. The expansion applies on cosmic spatial scales with local areas of contraction. Think of a large chocolate chip cookie being baked in an oven where the cookie expands but the chips don't.

http://en.wikipedia.org/wiki/Toll_House_cookies wrote:

strangerbarr Universe

---

WMAP Universe

---

<<The chocolate chip cookie was accidentally developed by Ruth Wakefield in 1930. She owned the Toll House Inn, in Whitman, Massachusetts, a very popular restaurant that featured home cooking in the 1930s. Wakefield is said to have been making chocolate cookies and on running out of regular baker's chocolate, substituted broken pieces of semi-sweet chocolate from Nestlé thinking that it would melt and mix into the batter. It clearly did not and the chocolate chip cookie was born. A different history of the cookie derives from George Boucher, who was at one time head chef at the Toll House Inn. Boucher said that the vibrations from a large Hobart electric mixer dislodged bars of Nestlé's chocolate stored on the shelf above the mixer so they fell into the sugar cookie dough it was mixing, then broke them up and mixed the pieces into it. Boucher claimed to have overcome Wakefield's impulse to discard the dough as too badly ruined to waste effort baking them, leading to the discovery of the popular combination.>>

http://en.wikipedia.org/wiki/Mixmaster_universe wrote:
<<Misner's Mixmaster Universe is a solution to Einstein's field equations of general relativity studied by Charles Misner in an effort to better understand the dynamics of the early universe. He hoped to solve the horizon problem in a natural way by showing that the early universe underwent an oscillatory, chaotic epoch.

The model is similar to the closed Friedmann-Lemaitre-Robertson-Walker universe, in that spatial slices are positively curved and are topologically three-spheres S³. However, in the FRW universe, the S³ can only expand or contract: the only dynamical parameter is overall size of the S³, parameterized by the scale factor a(t). In the Mixmaster universe, the S³ can expand or contract, but also distort anisotropically. Values of three shape parameters describe distortions of the S³ that preserve its volume and also maintain a constant Ricci curvature scalar. Therefore, as the three parameters assume different values, homogeneity but not isotropy is preserved.

Misner showed that the physical universe would expand in some directions and contract in others, with the directions of expansion and contraction changing repeatedly. Because the potential is roughly triangular, Misner suggested that the evolution is chaotic. Misner hoped that the chaos would churn up and smooth out the early universe. Since the directions of expansion and contraction varied, presumably given enough time the horizon problem would get solved in every direction.

While an interesting example of gravitational chaos, it is widely recognized that the cosmological problems the Mixmaster universe attempts to solve are more elegantly tackled by cosmic inflation. The metric Misner studied is also known as the Bianchi type IX metric.>>

ZenGrouch wrote:For argument's sake, if this weren't true, and space/time were expanding in our little piece of space, would we even be aware of it, or be able to measure it?

ZenGrouch wrote:

Chris Peterson wrote:
The force of gravity is stronger than the "force" of expanding space. Galaxies in clusters have enough gravity to hold space together locally, so the region doesn't expand. It is orbital dynamics, as governed by gravity, that hold galaxy clusters together. For the same reason, dense bodies like stars and planets aren't expanding with the Universe, and we aren't getting farther from the Sun.

Thanks for the explanation.

Unfortunately, your answer raised another 'rookie' question in my mind...

"For the same reason, dense bodies like stars and planets aren't expanding with the Universe, and we aren't getting farther from the Sun."

For argument's sake, if this weren't true, and space/time were expanding in our little piece of space, would we even be aware of it, or be able to measure it?

and perhaps we are even able to measure it

[list]Hubble expansion H₀ = 70.6 (km/sec)/Mpc

H₀ = 2,228,000,000,000 (m/yr) / [206,265,000,000 AU]

H₀ = 10.8 (m/yr)/AU

H₀ = 27.6 (mm/yr)/(mean lunar distance)[/list][/color]

neufer wrote:Laser reflectors on the moon indicate that the Moon is spiraling away from Earth at a rate of 38 mm per year.
Perhaps 28 mm per year is due to Hubble expansion and only 10 mm per year is due to tidal forcing.

Chris Peterson wrote:The expansion of the Universe as we recognize it is, in fact, an observable phenomenon, so I don't see why we wouldn't observe it locally if it were occurring.