The star system in question, HR 6819, is "bright" according to astronomers -- who can achieve very high signal-to-noise ratios even when breaking the light up with a high-resolution spectrograph. However, it is "faint" in ordinary human terms: at magnitude V = 5.4 or so, it would be barely visible if viewed from a rural site by someone who had been sitting outside in the dark for 10 or 20 minutes. It would be very difficult to pick out from other stars in the sky.

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Unfortunately for your idea, or any idea that relies upon ordinary baryonic matter to account for the dark matter in galaxies, measurements of the abundances of light elements such as hydrogen, deuterium, helium and lithium, together with models for the nucleosynthesis of these elements in the very early stages of the universe, set limits on the amount of baryonic matter in the universe. Those limits preclude amounts of baryonic matter large enough to produce the gravitational effects which are the basis for dark matter.

Good idea, but strong evidence argues against it.

You can find a freely available copy of the paper at this location:

      https://arxiv.org/pdf/1802.000...

Your summary of the paper is correct.

The authors of the paper use measurements of the host star's optical spectrum to infer that it doesn't produce a lot of UV emission, and note that it doesn't have frequent optical flares. That's good news for the habitability of the planet around it, as they point out.

However, they apparently did not note that Ross 128 is a relatively strong X-ray source, as measurements by the ROSAT X-ray satellite show. A colleague of mine worked out the X-ray luminosity of the host star, and it turns out to be not unlike that of the Sun, or even larger. That means that the X-ray flux striking the planet -- which is very close to this host star -- is likely high enough to remove the atmosphere of the planet. No atmosphere means not so interesting a planet, alas.

You can see the figures here for free --- and they provide much of the meat of the study.

http://www.nature.com/nature/j...

http://spiff.rit.edu/richmond/...

Astronomers have known for years that the ordinary matter we see every day -- made up of protons, electrons, and neutrons -- can only make up a small fraction of the mass-energy density needed to explain the large-scale structure of the universe. This ordinary, or "baryonic" matter, makes up around 4% of the critical amount. Another 23% or so is "dark matter", which isn't made of protons, electrons or neutrons, but does exert gravitational forces like baryonic matter; and the remaining 73% or so is the very mysterious "dark energy", which acts sort of like anti-gravity.

When most scientists see the phrase "missing matter", they think of the "dark matter" portion of the universe -- the 23%.

But this new result gives us information on a portion of the 4%, the ordinary baryonic matter. We think it should make up 4% of the critical density because of the relative abundances of hydrogen, helium, and lithium which were produced soon after the Big Bang ... but when we add up the stuff that we can see with telescopes -- stars and gas -- we find only about 1% of the critical amount. So, about 3% of the baryons were hiding somewhere.

This new study looked at radio waves from an event in a very distant galaxy. Those radio waves had to traverse a very long distance to reach us. As they flew through space, IF that space had even very thin traces of gas, waves of some frequencies would travel just a bit faster than others. That dispersion in frequency acts to spread out the arrival of the radio waves by the time they reach the Earth. The astronomers mentioned here observed a small spread in arrival times and used to to figure out how much gas the waves must have encountered in between the galaxies. The result: just the right amount of gas to account for all those hidden baryons.

So, yes, this study found missing baryons. It did not produce any direct measurements of dark matter or dark energy. On the other hand, if we can pinpoint other fast radio bursts in the future and study their host galaxies, we may learn something about those other entities, too.

... is here:

http://advances.sciencemag.org...

The authors have not placed a copy on the arXiv preprint server ... strange.

"Mission of Gravity", by Hal Clement.

My apologies. I should have marked the position of the variable star. I've just modified the web page so that the initial picture indicates the target -- click on that initial picture to see the movie. Thanks for pointing that out.

since you're doing such extensive image processing anyway, why not correct for the blooming of bright stars and make them all the same size and shape?

Well, in part, because I'm an astronomer, not a cinematographer, so my ability to make nice movies is rather limited. I could claim that there's some pedagogical value in seeing the ugly nature of the real scientific images, but, actually, that would just be covering up for the fact that I'm lazy.

Good idea. We astronomers try to eliminate such possibilities by measuring OTHER stars nearby and comparing their variations to those of the target. In this case, nearby stars didn't vary over the night, so we can rule out clouds in the Earth's atmosphere, which would have affected them all.

Now, it's possible that a cloud near the star itself could have something to do with this variation .... but the timescale for motions of such big objects is almost always far longer than a few hours. So, it's more likely that the variations are due to changes in the luminosity of the accretion disk around the black hole than to the motions of a big obscuring cloud in this case.

I've been using our university's observatory to take images of V404 Cyg for the past week. On Jun 23/24, the star underwent a particularly crazy series of variations: over a period of six hours, it fell to just 5 percent of its initial brightness, then recovered almost to its starting point.

I made an animated GIF showing the star's changes over this period. You can see it on my observing log for the the night:

http://spiff.rit.edu/richmond/...

That page also includes my full dataset, and pointers to additional reading.

The star is currently bright enough -- mag 11-14 -- to be studied easily with small telescopes. Anyone interested in joining the effort should start with the American Association of Variable Star Observers (AAVSO) -- go to their campaign page at

http://www.aavso.org/aavso-ale...

... thanks to arXiv:

      http://arxiv.org/abs/1501.0410...

This event is VERY interesting and unusual because the microlensing event was observed from two very different places: on Earth, and from the Spitzer Space Telescope, which is many millions of km away from the Earth. Gravitational lensing occurs when a background star and a lensing star line up exactly in the same direction, as seen from an observer. Because Spitzer was so far away, it saw the lensing star line up with the background star first; then, as the lensing star moved in its orbit around the center of the Milky Way, the lensing star eventually lined up with the background star as seen from Earth, about 18 days later.

This lag in time between two widely separated observers seeing a lensing event will help us to figure out exactly how the two stars involved in the event were moving, and where they are, and other properties. Since most telescopes are located on Earth, in basically the same place, we almost never get this extra information.

Rah, rah, Spitzer! Rah, rah, OGLE!

I'm co-teaching a graduate course on exoplanets, and we talked about this paper in one of our meetings last week. Here's the link to our discussion of "spectroscopy of exoplanet atmospheres:"

      http://spiff.rit.edu/classes/e...

You can read all our materials at

      http://spiff.rit.edu/classes/e...

Enjoy!

The summary has a link to a paywalled article (silly Ethan). The full article is freely available to all on the arXiv preprint server:

      http://arxiv.org/abs/1408.1706

I'm peripherally involved with the supernova field, though I study only the nearby examples. There has been for years the understanding that IF a difference should arise between the nearby events that we can study well, and the distant events which appear dimly and vaguely, AND if we did not realize that such a difference existed, THEN we could reach incorrect conclusions.

Scientists in the field have worried about this for years. It's not a sudden new realization.

It's very pleasant to see that a space telescope -- SWIFT -- which was built to study one type of object (gamma ray bursts) has turned out to provide vital information on a different type (supernovae). Since it is in space, it can detect ultraviolet light, and so show us that some nearby supernovae emit different amounts of ultraviolet light, even though they appear similar in the optical region. This UV difference hints at differences in chemical composition between supernovae, which may indeed be significant when we try to study very distant events with other telescopes.

Fortunately, light from those distant events is redshifted into the optical regime, so we can use very large ground-based telescopes to see the same UV light and compare it to the nearby events.

It's a very interesting field to follow: things change on timescales of 3-5 years. And yes, we are more aware of the uncertainties in the business than some news articles might imply.