The Breakdown of Classical Gravity? X. Hernandez, M. A. Jim´enez, C. Allen
I'm uncertain if the paper has been properly refereed, but it is an interesting read.
Dark Matter was invented by cosmologists to explain "missing matter" in the universe. Various large scale cosmological phenomena, such as the speed of galaxy rotations, do not sit well with the laws of Newtonian dynamics unless the galaxies contain a lot of unseen mass. General relativity doesn't help either. As cosmologists can neither see nor directly prove the existence of this mass, it was christened "Dark Matter".
Hopefully we all remember our school physics lessons. The force of gravity decreases with the square of distance. Proponents of MOND (MOdified Newtonian Dynamics) claim that this rule changes at great distances (i.e. at very small accelerations). For our Sun, this means a distance of 7000AU (1 AU = distance from Earth to Sun). Unfortunately It is troublesome to go and measure the force of gravity in this location so a less direct proof is needed.
A lot of the star systems in our galaxy are binaries. Two stars orbiting around each other (remember Luke!) at great distance. The insight of the authors in this paper was to realise that the motions of stars in very wide binary star systems could be used as test cases to see if the law of gravity does need modifying at large distances / small accelerations.
We can't simply observe a wide binary (where the two stars are separated by 7000AU) around its entire orbit (usually several centuries), so we have to make an inference based on the observed proper motion of the stars. Their argument thus becomes a little weakened at this point, but no less convincing. They also make various attempts to "break" their argument by using different data and subsets of the data, but the hypothesis is not disproved.
They conclude that the motion of wide binary star systems points strongly to a need to modify Newtonian gravity. As the distance between the two bodies increases the force moves from the Newtonian inverse square rule of GM/r2 to a form of GaM/r. This means the orbital velocity of wide binaries should cease to decrease with separation and gradually settle on a constant velocity which is only dependent on the entire mass of the star system.
Perhaps we can view gravity as having two separate components, with only one operating on the inverse square rule. Or we can view it in the same way as light. There is a minimium unit of light enery (the "quantum") so why can't we have a minimum unit of gravitational energy?
If it can be proven that the equations of classical gravity need modification at low accelerations, then there is no need to invoke Dark Matter to explain the universe.
A lot of mainstream cosmologists strongly resist the idea of MOND solutions whereas I quite like them. Perhaps I need to do a nice rant against dark matter!
Use the 2.2m ESO telescope at La Silla, the observers have stacked 26 x 180s exposures of the distant comet Hale Bopp. The distance is around 30.7 Astronomical units, or 2,850,000,000 miles in more local terms.
Comet Halley had been observered at around 27 AU distance, but these new observations beat that record.
Objects the size of Hale-bopp (around 30km) are very faint that this extreme distance, researchers give a magnitude estimate of around magnitude 24.5
Comet Hale Bopp has been extremely active, but these recent telescope observations show that the comet activity is now in rapid decline. Some scattering from a light coma was detected, but the comet is now almost in deep freeze.
We ought to be able to continue tracking Hale-Bopp with the worlds largest telescopes until around the year 2020 when the magnitude falls below mag 30 and pickign the comet out of the background galaxies becomes impossible.
Although I enjoy a naked eye observation of a comet as much as the next person, I've never been particular interested in imaging them. However, this type of observation is in a different class of astrophotography. Simply finding the damn thing must be a challange. The researchers noted that the comet appeared slightly out of the predicted position in the sky, but comet revealed its nature by a slow movement against the background stars.
Hale-bopp wil be back in our skys in the year 4385, so don't wait up!
Paper reference :A&A 531, A11 (2011)
Frozen to death? Detection of comet Hale-Bopp at 30.7 AU
Some interesting observations on secondary mirror supports in Newtonian telescopes, and designs for these supports which minimise the diffraction spikes in images of stars.
I've often seen single arm supports and curly supports on commercial scopes, but I've not seen this approach before - although I'm sure it has been tried. I am also sure somebody will point one out to me.
I have reproduced one diagram from the paper here to illustrate their approach.
The reaction of most astrophotographers is one of horror - why should you wish to remove the attractive spikes. Some astrophotographers even add diffraction spikes to the images during processing.
For scientific observations, the large diffraction spikes are very annoying. I've done spectrographs on my telescope and it is very frustrating when a diffraction spike from a star cross the spectrum of a dim object you are trying to measure
From an imaging perspective, I think the interesting "point" is in figure 9c and 9d. As well as removing the gross spike artifacts, the intensity profile of the star appears improved in (d), perhaps giving a better fwhm, i.e. tighter stars and detail. That is of huge interest to astrophotographers.
I am moderately interested in starting a mini-project to try this on my own scope, but I've a few other jobs to get finished first!!