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Comment Found the missing _baryons_ (Score 5, Informative) 38

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.

Comment Re:Watch a movie of V404 Cyg in the optical (Score 2) 58

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.

Comment Re:Watch a movie of V404 Cyg in the optical (Score 4, Interesting) 58

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.

Comment Watch a movie of V404 Cyg in the optical (Score 4, Informative) 58

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:

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

Comment Link to the full article, freely available ... (Score 4, Informative) 21

... thanks to arXiv:

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!

Comment We just covered this paper in our class last week (Score 2) 43

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:"

You can read all our materials at


Comment Link to the full article, freely available (Score 5, Informative) 199

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

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.

Comment Re:Um, they're going to be awful this year (Score 2) 31

When the Moon is full, it rises at sunset and sets at sunrise. Each day, the Moon rises (and sets) about one hour later. So, 2 or 3 days after the full Moon, the Moon will rise 2 or 3 hours after sunset, and set 2 or 3 hours after sunrise.

Which means that, after midnight, the Moon will be high in the sky, ruining the view of the Perseids. It will not "set several hours before dawn."

In short, the response above is wrong.

Comment False alarm -- just a normal background source (Score 5, Informative) 129

The team which announced the event has now figured out that it wasn't interesting after all:

NUMBER: 16336
SUBJECT: Swift trigger 600114 is not an outbursting X-ray source
DATE: 14/05/28 07:57:12 GMT
FROM: Kim Page at U.of Leicester

K.L. Page, P.A. Evans (U. Leicester), D.N. Burrows (PSU), V. D'Elia (ASDC) and A. Maselli (INAF-IASFPA) report on behalf of the Swift-XRT team:

We have re-analysed the prompt XRT data on Swift trigger 600114 (GCN Circ. 16332), taking advantage of the event data.

The initial count rate given in GCN Circ. 16332 was based on raw data from the full field of view, without X-ray event detection, and therefore may have been affected by other sources in M31, as well as background hot pixels. Analysis of the event data (not fully available at the time of the initial circular) shows the count rate of the X-ray source identified in GCN Circ. 16332 to have been 0.065 +/- 0.012 count s^-1, consistent with the previous observations of this source [see the 1SXPS catalogue (Evans et al. 2014):

We therefore do not believe this source to be in outburst. Instead, it was a serendipitous constant source in the field of view of a BAT subthreshold trigger.

This circular is an official product of the Swift-XRT team.

Better luck next time.

Comment For reference, here's one of the current systems (Score 4, Informative) 38

If you're interested in the current state of the art, read this article from the Publications of the Astronomical Society of the Pacific (April 2013). It describes the hardware and software used by the Pan-STARRS team to detect asteroids automatically in data taken with their 1.8-meter telescope on Hawaii and its 1.4-gigapixel CCD camera.

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