Starship Asterisk*

tytower wrote:Who is it that is making these absolute statements as if they know?

If this is the ejected material of one side of a black hole spewing new stars forth and we are watching directly over a pole then the gas could be a very big part of the new star formation process.

pcstarship wrote:Compare today's image of the Seven Sisters with this one, the second-ever post on APOD:

http://antwrp.gsfc.nasa.gov/apod/ap950620.html

It's amazing how imaging technology has advanced since 1995. How good do you think this cluster will this look in 2023?

geckzilla wrote:I don't much like the post processing done on this image today. Too much color bleeding, for lack of a better way to describe it. I think it'd be fun to have the original and try myself at ramping up the colors. I don't know if I could do any better but it'd be nice to try.

http://antwrp.gsfc.nasa.gov/apod/ap060715.html wrote:
<<In the well known Pleiades star cluster, starlight is slowly destroying this wandering cloud of gas and dust. The star Merope lies just off the upper left edge of this picture from the Hubble Space Telescope. In the past 100,000 years, part of the cloud has by chance moved so close to this star - only 3,500 times the Earth-Sun distance - that the starlight itself is having a very dramatic effect. Pressure of the star's light significantly repels the dust in the reflection nebula, and smaller dust particles are repelled more strongly. As a result, parts of the dust cloud have become stratified, pointing toward Merope. The closest particles are the most massive and the least affected by the radiation pressure. A longer-term result will be the general destruction of the dust by the energetic starlight.>>

• Merope's radiation pressure at 3,500 AU
  
  ~ the Sun's radiation pressure at 200 AU

harry wrote:The question is:
If the universe is expanding. What makes stars cluster?

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

<<An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud, and are still loosely gravitationally bound to each other. In contrast, globular clusters are very tightly bound by gravity. Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring. They are usually less than a few hundred million years old: they become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic center, as well as losing cluster members through internal close encounters. Young open clusters may still be contained within the molecular cloud from which they formed, illuminating it to create an H II region. Over time, radiation pressure from the cluster will disperse the molecular cloud. Typically, about 10% of the mass of a gas cloud will coalesce into stars before radiation pressure drives the rest away.>>

http://en.wikipedia.org/wiki/Pleiades_(star_cluster) wrote:
<<The Pleiades cluster core radius is about 8 light years and tidal radius is about 43 light years. The cluster's relative motion will eventually lead it to be located, as seen from Earth many millennia in the future, passing below the feet of what is currently the constellation of Orion. Also, like most open clusters, the Pleiades will not stay gravitationally bound forever, as some component stars will be ejected after close encounters and others will be stripped by tidal gravitational fields. Calculations suggest that the cluster will take about 250 million years to disperse, with gravitational interactions with giant molecular clouds and the spiral arms of our galaxy also hastening its demise.>>

Universal expansion is separating the Pleiades cluster at about 1 mph.

harry wrote:

Neufer wrote:Universal expansion is separating the Pleiades cluster at about 1 mph.

Where on earth do you get that evidence?

neufer wrote:Hubble Constant = 74.2 (km/s)/Mpc = 267,000 (km/hr)/Mpc = 0.267 (km/hr)/pc ~ 1 mph / (6 pc)

The Pleiades cluster ~ 6 parsecs in diameter.

<<The Pleiades cluster core radius is about 8 light years and tidal radius is about 43 light years.>>

Chris Peterson wrote: Areas with mass concentrations don't expand, which includes galaxies. The Pleiades cluster is, of course, deeply embedded in the Milky Way; space is not expanding between its components.

canuck100 wrote:I have read this elsewhere. I believe that superclusters are the largest structures in the universe. Are they also the largest structures whose components are not expanding with respect to each other?

We investigate the dynamical state of superclusters in Lambda cold dark matter ($\Lambda$CDM) cosmological models, where the density parameter $\Omega_0=0.2-0.4$ and $\sigma_8$ (the rms fluctuation on the $8h^{-1}$Mpc scale) is $0.7-0.9$. To study the nonlinear regime, we use N-body simulations. We define superclusters as maxima of the density field smoothed on the scale $R=10h^{-1}$Mpc. Smaller superclusters defined by the density field smoothed on the scale $R=5h^{-1}$Mpc are also investigated. We find the relations between the radially averaged peculiar velocity and the density contrast in the superclusters for different cosmological models. These relations can be used to estimate the dynamical state of a supercluster on the basis of its density contrast. In the simulations studied, all the superclusters defined with the $10h^{-1}$Mpc smoothing are expanding by the present epoch. Only a small fraction of the superclusters defined with $R=5h^{-1}$Mpc has already reached their turnaround radius and these superclusters have started to collapse. In the model with $\Omega_0=0.3$ and $\sigma_8=0.9$, the number density of objects which have started to collapse is $5 \times 10^{-6}h^3$Mpc$^{-3}$. The results for superclusters in the N-body simulations are compared with the spherical collapse model. We find that the radial peculiar velocities in N-body simulations are systematically smaller than those predicted by the spherical collapse model ($\sim 25$% for the $R=5h^{-1}$Mpc superclusters).

Once thought to be the largest structures in nature, superclusters are now understood to be subordinate to enormous walls or sheets, usually called "filaments", sometimes called "super cluster complexes", "walls" or "sheets", that can span a billion light-years in length, more than 5% of the observable universe.

According to some astronomers, no clusters of super clusters (“hyperclusters”) are known; the existence of structures larger than superclusters is debated (see Galaxy filament). Interspersed among super clusters are large voids of space in which few galaxies exist. Even though superclusters are the largest structures confirmed, the total number of superclusters leaves possibilities for structural distribution; the total number of super clusters in the universe is believed to be close to 10 million.

canuck100 wrote:I googled "Are superclusters expanding" and found an abstract from a 2002 paper at http://arxiv.org/abs/astro-ph/0204121.

I don't get the details of their abstract ...
... but what I take from it is that according to their simulations, most superclusters are expanding as they state that their model suggests that 'Only a small fraction of the superclusters defined with . . . has already reached their turnaround radius and these superclusters have started to collapse.' I note that this is a simulation that suggests a result, not an observation that confirms a result.

Abstract: In this paper we discuss the properties of the quasi-steady state cosmological model (QSSC) developed in 1993 in its role as a cyclic model of the universe driven by a negative energy scalar field. We discuss the origin of such a scalar field in the primary creation process first described by F. Hoyle and J. V. Narlikar forty years ago. It is shown that the creation processes which takes place in the nuclei of galaxies are closely linked to the high energy and explosive phenomena, which are commonly observed in galaxies at all redshifts.
The cyclic nature of the universe provides a natural link between the places of origin of the microwave background radiation (arising in hydrogen burning in stars), and the origin of the lightest nuclei (H, D, He$^3$ and He$^4$). It also allows us to relate the large scale cyclic properties of the universe to events taking place in the nuclei of galaxies. Observational evidence shows that ejection of matter and energy from these centers in the form of compact objects, gas and relativistic particles is responsible for the population of quasi-stellar objects (QSOs) and gamma-ray burst sources in the universe.
In the later parts of the paper we briefly discuss the major unsolved problems of this integrated cosmological and cosmogonical scheme. These are the understanding of the origin of the intrinsic redshifts, and the periodicities in the redshift distribution of the QSOs.