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Whatever happened to Megrez?

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alanbthomas's picture
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Whatever happened to Megrez?

I went into the garden tonight at 21.55 to scan the sky.

I was startled to find that Megrez in Ursa Major (Delta Ursae Majoris), high in the sky,  had disappeared!  Was there something wrong with me? Or was there a temporary obscuration? Or what? Help!

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Megrez fading

This post piqued my interest so I did a little research:

My Research From ‘Wikipedia’ : This star has an excess emission of infrared radiation, indicating the presence of circumstellar matter. This forms a debris disk around an orbital radius of 16 Astronomical Units from the star. This radius is unusually small for the estimated age of the disk, which may be explained by drag from the Poynting–Robertson effect causing the dust to spiral inward.

From another internet site:

William Creed

Thursday, 2nd March 2017 5:58:14 AM

When I was a kid in around 1955 to 1969 that star was very visible in the night. Why has it seemed to fade over the 20 some years.

And another site:


Is it just me or is the star that connects the dipper with the handle fading out
There are no clouds its a cold , clear night
For those that can , step outside and observe
I have never come across this before 

Its barely there if at all(Feb 2013)

Finally Patrick Moore, in his book ‘Stargazing’ in 2001, at page 25 states:

“Old astronomers of more than 1,000 years ago stated that it was as bright as the other Ursa Major stars. Either they were wrong or (less probably) Megrez has actually faded about a magnitude since then.”

My question – Is it another case of dust obscuration – now becoming more rapid ????  Trouble is, at 57 degrees north it is just about inaccessible from here in Brisbane Australia...

Peter Anderson's picture
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Megrez fading

Seems I was getting excited over nothing. A colleague of mine emailed me some AAVSO information including the attached graph. Maybe in poorer sky conditions with the star near the limit of visibility,it appears to be fainter...  I just don't know.

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Megrez fading

The diagram above starts at 31st December 1954 and ends at 25th March 2020. (Bit difficult to read). It shows little apparent variation in magnitude.

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Megrez

I looked at it two or three nights ago while searching for the comet and thought to myself "That's dimmer than I remember" and then carried on without a second thought. There doesn't appear to be any AAVSO data for two or three nights ago.

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Megrez fading

Thanks for those comments. So far I am reassured that I am not going nuts!

Alan

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local conditions ?

I was taking spectra last night and the signal from the target suddenly disappeared. I went outside to check and as far as I could tell the sky was completely clear. I went back inside and watched as the signal came back up.  Half an hour later the same thing happened but this time I looked outside to find it was foggy, putting an end to observations

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local conditions

I had been out observing for a couple of hours earlier in the evening and was just having a final look around. So far as I can tell, the skies were perfectly clear. But I suppose a passing puff of cloud might be responsible. Or some other obstruction of course.

What surprised me was that Megrez was not simply faint but invisible, at least to the naked eye. That's something I have never seen (or not seen) before in over 60 years of skywatching.

I will be having a closer look tonight with optical aid.

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Megrez fading

I went out at 20.30 and again at 21.50. I could detect Megrez with the naked eye, but only just, a little more easily with averted vision. It certainly seems unusually faint - but is it?

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light curve for delta UMa

This is from KWS (Kamogata/Kiso/Kyoto Wide-field Survey).  delta UMa also has a NSV designation (NSV 5513)

Gary

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light curve for delta UMa

Hi Gary

I am not a variable star observer. How would you interpret these data?

Thanks

Alan

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delta UMa

Hi Alan,

Well this is the first time I've seen a plot of delta UMa myself.  VSX says the variability is of unspecified type, but this link suggests a possible EB    http://liber.onu.edu.ua/pdf/astro/all/OAP_9/000_pdf/bryukh.pdf

It's quite interesting that the amplitude of the fades vary from one fade to the next.  Be nice to see a much longer plot.   My eyes are possibly seeing a secondary wave on that light curve.

Gary

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not grey

Interesting that the variability in I is much less than in V. Reminiscent of the recent Betelgeuse fading

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Is KWS reliable ?

Is the short term variation  in KWS V mag real ?  eg

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light curve for delta UMa

I don't know what the Ic V and B designations for the plots refer to, but do I detect a downward trend in the red plot - and an upward in the green? (this refers to Gary's earlier post)

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Spectral type

According to SIMBAD it is of A2Vn C spectral type. I also found reference to a debris disk in Wikipedia linking to a paper by Wyatt et al 2007 https://arxiv.org/pdf/astro-ph/0703608.pdf.

So it is a main sequence A-type star and the paper quotes an age of 300 Myr, though I think that comes from another paper (I've not read the paper in detail, just skimmed a few bits). I am surprised a main sequence star could have a debris disk as I thought this was associated with young stars. If I've understood the paper there are collisions in the debris disk generating dust and an IR excess. Perhaps the variation in luminosity is due to variable obscuration by the dust?

Interesting how such a well known and prominent star can throw up surprises to us, well spotted Alan!

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Megrez

Thanks Andy!

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short term variability

B,V Ic correspond to the band in which the brightness was measured. (The survey instrument uses a CCD camera and wide field lens).  V corresponds roughly to what the eye sees Ic is the near infrared. There are small long term average trends from season to season but the  variation from day to day/week to week of up to 0.5 magnitudes seen in the V data would be  obvious by eye if real. The very tight error bars on the points would suggest they are real but I am somewhat sceptical.

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short term variability

Many thanks, Robin, for the clarification on the data series. I never thought variable stars were that interesting - but I'm beginning to wonder now!

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Variability

I too am sceptical about those V-band variations in the KWS, Robin. In spite of the tight errors bars.

The interesting thing is there whilst there are many photometric surveys of faints stars, there are very few surveys of bright stars like Megrez.

I think we should all take a leaf out of Alan's book (me included!) and look up at the sky with our naked eyes to check if there is something out of the ordinary. After all, it was only a few weeks ago that Betelgeuse was observed to be dimming.

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Megrez

Thanks Jeremy!

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Megrez.

Hello all.

Just to add that a look upwards last night from my fairly light polluted location revealed Megrez to be 'missing'. If I looked a little longer I could just about make it out. This has never been the case in the past and the familiar pattern has always been obvious.

Chris.

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What is the reason for the

What is the reason for the scepticism about the V-band measurements? Is the scatter not similar to AAVSO visual data, albeit for a different frequency of observation? Either way, a few days of continued observation should resolve the matter, should it not?

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Megrez

Naked eye observation at 20.26. Megrez not visible.

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suspect KWS V band

I just had a look at the last 6 months data of a similar magnitude star with variability  that I am familiar with  and which has good photometric data in the AAVSO database.

Here is KWS

and here is AAVSO with the same Y scale range

I looks like KWS is unreliable at least with bright stars in V

Robin

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Suspect KWS V-band

Thanks Robin, I see your point now! Cheers.

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Megrez

Naked eye observation 21.59. Megrez barely visible.

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Megrez

Naked eye observation 22.50. Megrez barely visible with averted vision. The Plough is looking a bit like Trafalgar Square without Nelson. Of course it may be something or nothing - but it doesn't look right.

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Megrez.

Just went outside (11pm ish) and was lucky enough to catch a fair sized gap in the clouds here in Berkshire. Megrez appeared as I would expect, easily seen. It was certainly much harder to see last night. Perhaps haze? 

Chris.

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Megrez

I don't know what's going on, but it makes a change from coronavirus!

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all sky camera

Cloudy here but I grabbed a frame off an all sky camera (University of Hertfordshire   http://observatory.herts.ac.uk/allsky/ )  and compared it with the same date and time a year ago.  See attached. A crude comparison but the relative brightness of Megrez looks roughly the same to the camera at least

Cheers

Robin

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Megrez

That's interesting, though the camera is more sensitive than the eye. I took a few shots myself and M is clearly visible.

Here's a (possibly dodgy) hypothesis:

1. The current 'lockdown' has reduced the volume of pollutants in the atmosphere. 

2. This has increased the effects of light pollution.

3. This will be especially significant in urban areas (such as here, between Manchester and Liverpool).

4. Ergo a) stars around the mag of Megrez will be pushed closer to the limits of naked-eye visibility. The effect on Megrez itself is especially noticeable because of its prominent location. b) this effect will be more visible from urban (heavily light polluted) sites than rural ones.

In principle, these implications appear to be amenable to test fairly easily. 

However, this reasoning seems to imply (counter-intuitively) that atmospheric pollution reduces light pollution which improves seeing!

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Atmosphere

Certainly the transparency over the last few nights has not been 100% in this part of Cheshire. Something like a very thin veil that means the sky lacks that sparkle. this shows up in my CCD images as a high background. Even this morning the sky, though blue, lacks that deep blue.
This reduces the naked eye limiting mag, although Megrez has always been clearly visible to me.

But I suspect this is a meteorological situation independent of lockdown 

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visual light curves for Megrez

I can't comment on V light curves, but I am always wary of visual estimates of the magnitude of Megrez. The comparison stars typically used (alpha Dra, chi UMa, psi UMa, lambda Dra) are quite a distance away on the sky from Megrez and can for much of the year be at significantly different altitudes and thus affected differently by any haze present. This inevitably affects the reliability of visual estimates made under such conditions and increases the visual light curve scatter. (This issue also affects other 'isolated' stars such as epsilon Peg, for which there have been claims of large brightness changes over the years). It is also the case that visual magnitudes made when the star or its comparisons are close to the limit of visibility show more scatter than when the stars involved are easily seen.