Starship Asterisk*

RogueRob wrote:
Is that a blue star toward the upper left that has a faint, narrow ring?

Saeed Khan wrote:Recently I heard about a theory that our Solar System not only rotates around the Milky Way Galaxy, it also moves up and down while doing so. Whenever it passes through the Central Disk of Dark Matter, the Dark Matter dislodges asteroids of the asteroid belts. Some of these then hit our planet, causing extinction of most species on Earth!

Chris Peterson wrote:

Saeed Khan wrote:
Recently I heard about a theory that our Solar System not only rotates around the Milky Way Galaxy, it also moves up and down while doing so. Whenever it passes through the Central Disk of Dark Matter, the Dark Matter dislodges asteroids of the asteroid belts. Some of these then hit our planet, causing extinction of most species on Earth!

This hypothesis is entirely unsupported by evidence.

• The necessary evidence, one way or the other, should be coming from Gaia:

http://www.dailygalaxy.com/my_weblog/2014/05/our-solar-system-orbit-through-milky-ways-dark-matter-disk-does-it-trigger-comet-impacts-and-mass-ex.html wrote:
Our Solar-System's Scary Orbit Through Milky Way's Dark Matter Disk
Daily Galaxy, May 05, 2014

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<<Our Sun orbits around the Galactic center, taking approximately 250 million years to make a complete revolution. [Also] the Solar System weaves up and down, crossing the plane of the Milky Way approximately every 32 million years. This bobbing motion, which extends about 250 light years above and below the plane, is determined by the concentration of gas and stars in the disk of our Galaxy. This ordinary “baryonic” matter is concentrated within about 1000 light years of the plane. Because the density drops off in the vertical direction, there is a gravitational gradient, or tide, that may perturb the orbits of comets in the Oort cloud, causing some comets to fly into the inner Solar System and periodically raise the chances of collision with Earth. However, the problem with this idea is that the estimated galactic tide is too weak to cause many waves in the Oort cloud.

In their new study [Dark Matter as a Trigger for Periodic Comet Impacts, Lisa Randall and Matthew Reece,Phys. Rev. Lett. 112, 161301 (2014), Published April 21, 2014], Randall and Reece suggest that the galactic tide could be made stronger with a thin disk of dark matter. Dark disks are a possible outcome of dark matter physics, as the authors and their colleagues recently showed. Here, the researchers consider a specific model, in which our Galaxy hosts a dark disk with a thickness of 30 light years and a surface density of around 1 solar mass per square light year. Although one has to stretch the observational constraints to make room, their thin disk of dark matter is consistent with astronomical data on our Galaxy.

Randall and Reece’s dark disk model is not made of an ordinary type of dark matter. The most likely candidate of dark matter—known as weakly interacting massive particles (WIMPs)—is expected to form a spherical halo around the Milky Way, instead of being concentrated in the disk. This WIMP dark matter scenario has been remarkably successful in explaining the large-scale distribution of matter in the Universe. But, there is a long-standing problem on small-scales—the theory generally predicts overly dense cores in the centers of galaxies and clusters of galaxies, and it predicts a larger number of dwarf galaxy satellites around the Milky Way than are observed.

Alternatively, this small-scale conflict could be evidence of more complex physics in the dark matter sector itself. One solution is to invoke strong electromagnetic-like interactions among dark matter particles, which could lead to the emission of “dark photons”. These self-interactions can redistribute momentum through elastic scattering, thereby altering the predicted distribution of dark matter in the innermost regions of galaxies and clusters of galaxies as well as the number of dwarf galaxies in the Milky Way. Although self-interacting dark matter could resolve the tension between theory and observations at small-scales, large-scale measurements of galaxies and clusters of galaxies only allow a small fraction (less than 5%) of the dark matter to be self-interacting. Recently, Randall, Reece, and their collaborators showed that if a portion of the dark matter is self-interacting, then these particles will collapse into a dark galactic disk that overlaps with the ordinary baryonic disk .

Did a thin disk of dark matter trigger extinction events like the one that snuffed out the dinosaurs? The evidence is still far from compelling. First, the periodicity in Earth’s cratering rate is not clearly established, because a patchy crater record makes it difficult to see a firm pattern. It is also unclear what role comets may have played in the mass extinctions. The prevailing view is that the Chicxulub crater, which has been linked to the dinosaur extinction 66 million years ago, was created by a giant asteroid, instead of a comet. Randall and Reece were careful in acknowledging at the outset that “statistical evidence is not overwhelming” and listing various limitations for using a patchy crater record. But the geological data is unlikely to improve in the near future, unfortunately.

On the other hand, advances in astronomical data are expected with the European Space Agency’s Gaia space mission, which is currently studying the Milky Way in unprecedented detail. Gaia will observe millions of stars and measure their precise distances and velocities. These measurements should enable astronomers to map out the surface-density of the dense galactic disk as a function of height. Close to the plane, astronomers could then directly see whether there is a “disk within the disk” that has much more mass than we could account for with the ordinary baryonic matter. Evidence of such a dark disk would allow better predictive modeling of the effects on comets and on the life of our planet.>>

pnelson wrote:
If you take this older APOD image of NGC 1055, rotate it 90 degrees anti-clockwise, and compare it to today's image, they are clearly 180 degrees out of synch. I'm curious to know if that is the result of the instrumentation used to observe the galaxy, or a result of the image processing methods. Which is the correct orientation?

Alex675 wrote:There is a strange-shaped backgound galaxy, top right of the image.

pnelson wrote:@neufer: Well yes, an image that has been rotated by 180 degrees is, by definition, it's mirror image.

I appreciate you re-phrasing my point into more common lay terms and expanding upon it, but that doesn't answer my question. One of these georgeous images is a scientifically accurate repesentation of what an earth-bound observer might see, the other is just a pretty picture. Which is which, and why was the inaccurate image not notated as such?

pnelson wrote:@neufer: Well yes, an image that has been rotated by 180 degrees is, by definition, it's mirror image. I appreciate you re-phrasing my point into more common lay terms and expanding upon it, but that doesn't answer my question. One of these georgeous images is a scientifically accurate repesentation of what an earth-bound observer might see, the other is just a pretty picture. Which is which, and why was the inaccurate image not notated as such?

pnelson wrote:
I noticed that the two APOD images appeared to be at odds with each other, so my intent was to determine which image most accurately represents what an earthbound observer might see. @neufer, there are three images represented in this discussion, two from APOD, and the wikimedia image that you uploaded. Which are you saying is correct, and if you don't mind, how are you able to make that determination? And thanks for helping too.

Tom W. wrote:The bright yellow star under the right edge of the Galaxy in this image, seen up close appears to have a ring of material seen nearly edge-on. If true, could this be pre-planatary material?

rstevenson wrote:

Tom W. wrote:The bright yellow star under the right edge of the Galaxy in this image, seen up close appears to have a ring of material seen nearly edge-on. If true, could this be pre-planatary material?

Could you do a screen grab and point out on it which star you're referring to?

Rob