APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)

Comments and questions about the APOD on the main view screen.
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Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)

Postby Chris Peterson » Tue Jul 11, 2017 8:08 pm

zendae1 wrote:As a follow up question, how close can two of these stars be, on average, before starting to interact with each other? Are they sufficiently far apart to not interact?

Depends what you mean by "interact". There is no distance limit on gravity, so the Sun and Alpha Centauri interact. Luckily, they're moving more or less together, so not much is changing. If another star passed within a light year of the Sun, it would greatly disrupt the Oort cloud (sending a barrage of comets into the inner system) and measurably perturb the orbits of the outer planets.

At the star density found in globular clusters (especially towards the center) it's unlikely that planetary systems can survive for long.
Chris

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Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)

Postby neufer » Tue Jul 11, 2017 9:06 pm

Chris Peterson wrote:
zendae1 wrote:As a follow up question, how close can two of these stars be, on average, before starting to interact with each other? Are they sufficiently far apart to not interact?

Depends what you mean by "interact". There is no distance limit on gravity, so the Sun and Alpha Centauri interact. Luckily, they're moving more or less together, so not much is changing. If another star passed within a light year of the Sun, it would greatly disrupt the Oort cloud (sending a barrage of comets into the inner system) and measurably perturb the orbits of the outer planets.

Whoa thar Rusty :!: Let's rethink that whole planets thing.

Voyager 1 is out at 139 AU (~0.002 lyr) where the Solar System escape velocity is ~3.6 km/s.

Voyager 1 is traveling at 17.043 km/s and is slowly
slowing down to 16.66 km/s = sqrt[(17.043)2-(3.6)2].

A star out at 1 lyr will generate only (0.002)2 of the solar acceleration
that is slowly slowing Voyager 1 by just 0.383 km/s [= (17.043 - 16.66)].

Furthermore...the star is accelerating the Sun by
practically the same amount in the same direction
:!:

Hence... the relative motion of Voyager 1 vs-a-vis the Sun
should be undetectable even with radar tracking.

Ergo: the relative motion of the planets vs-a-vis the Sun
would definitely be undetectable by passing stellar tidal forces.
Art Neuendorffer

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Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)

Postby Chris Peterson » Tue Jul 11, 2017 9:17 pm

neufer wrote:
Chris Peterson wrote:
zendae1 wrote:As a follow up question, how close can two of these stars be, on average, before starting to interact with each other? Are they sufficiently far apart to not interact?

Depends what you mean by "interact". There is no distance limit on gravity, so the Sun and Alpha Centauri interact. Luckily, they're moving more or less together, so not much is changing. If another star passed within a light year of the Sun, it would greatly disrupt the Oort cloud (sending a barrage of comets into the inner system) and measurably perturb the orbits of the outer planets.

Whoa thar Rusty :!: Let's rethink that whole planets thing.

Voyager 1 is out at 139 AU (~0.002 lyr) where the Solar System escape velocity is ~3.6 km/s.

Voyager 1 is traveling at 17.043 km/s and is slowly
slowing down to 16.66 km/s = sqrt[(17.043)2-(3.6)2].

A star out at 1 lyr will generate only (0.002)2 of the solar acceleration
that is slowly slowing Voyager 1 by just 0.383 km/s [= (17.043 - 16.66)].

Furthermore...the star is accelerating the Sun by
practically the same amount in the same direction
:!:

Hence... the relative motion of Voyager 1 vs-a-vis the Sun
should be undetectable even with radar tracking.

Ergo: the relative motion of the planets vs-a-vis the Sun
would definitely be undetectable by passing stellar tidal forces.

I think you underestimate the number of decimal places of precision we deal with in modeling planetary orbits. We can detect relativistic effects in orbital parameters of planets. I'm pretty sure that a rapidly moving star a light year away will measurably- even significantly alter the orbits of our planets. That is, the positions of the planets after the million year passage will be distinctly different than if the star had not passed at all.
Chris

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Maxedwell

Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)

Postby Maxedwell » Tue Jul 11, 2017 10:36 pm

Bystander:
Thanks so much for the artist rendition of "view from a planet inside of a globular cluster." Beautiful; especially when you view it full scale! How exciting that would be!
Reminds me somewhat of Olber's Paradox which we learned about in Astronomy 101 in college....
And, I do remember that Asimov story about the world that experiences "night" for the first time in the lives of the living inhabitants. An interesting read.
Also, nice quote from "2001..."
Great discussion and I'm humbled to even be allowed in this thread!

heehaw

Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)

Postby heehaw » Tue Jul 11, 2017 11:02 pm

"...try to reproduce as best I can the "true colours" of the objects" - reminds me of the tour of a deep underground cave with the guide shining UV light on some of the rocks to bring out dramatic colors. One tourist objected that that was fake! Not true colors! So the guide said, OK, true colors !! and he turned off all the lights!

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Re: Some technical details on the image

Postby BillT » Wed Jul 12, 2017 1:05 am

MikeODay wrote:A deep look at Omega Centauri ( NGC 5139 )

Capture:
9 sets of sub-images with exposure duration for each set doubling ( 1s to 240s ) all at ISO800.

Processing:.
Calibration: master bias, master flat and no darks.
Integration in 9 sets.
HDR combination.
Pixinsight

May 2017


Hi Mike,

Great image. I'm curious as why why you didn't use darks?

Bill

LMMT

Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)

Postby LMMT » Wed Jul 12, 2017 7:58 am

A very bight night sky I imagine, especially towards the core!

As a first approximation perhaps one could assume a sphere with radius 75 light years and total of 10 million stars evenly spread through the sphere giving an average cubic volume per star of a little under 0.6cu light years ( if there is such a unit :ssmile: ) or say an average separation of 0.6 light years.

Wikipedia states

"Globular clusters can contain a high density of stars; on average about 0.4 stars per cubic parsec, increasing to 100 or 1000 stars per cubic parsec in the core of the cluster.[28] The typical distance between stars in a globular cluster is about 1 light year,[29] but at its core, the separation is comparable to the size of the Solar System (100 to 1000 times closer than stars near the Solar System)"

Very interesting, thank you!!

Lizardly

Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)

Postby Lizardly » Wed Jul 12, 2017 8:23 am

Yes, "may be the remnant core of a small galaxy merging with the Milky Way."

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Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)

Postby MikeODay » Wed Jul 12, 2017 10:49 am

heehaw wrote:"...try to reproduce as best I can the "true colours" of the objects" - reminds me of the tour of a deep underground cave with the guide shining UV light on some of the rocks to bring out dramatic colors. One tourist objected that that was fake! Not true colors! So the guide said, OK, true colors !! and he turned off all the lights!


Made me smile :)

Yes the quotes were on purpose. I am sure someone on this thread would know or be able to estimate it, but I imagine a person would need very different eyes in order to be able to see unaided the very faint stars and huge dynamic range ( according to Pixinsight 32bits ) in this image of Omega Centauri.

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Re: Some technical details on the image

Postby MikeODay » Wed Jul 12, 2017 11:19 am

BillT wrote:
MikeODay wrote:A deep look at Omega Centauri ( NGC 5139 )

Capture:
9 sets of sub-images with exposure duration for each set doubling ( 1s to 240s ) all at ISO800.

Processing:.
Calibration: master bias, master flat and no darks.
Integration in 9 sets.
HDR combination.
Pixinsight

May 2017


Hi Mike,

Great image. I'm curious as why why you didn't use darks?

Bill


Thanks Bill

I have been into Astrophotography for 3 years now and I have gone through a number of stages with darks.

First I recorded and calibrated with darks because that was the advice on the various sites. However, nothing that I did ( including dithering ) would remove the "rain like" effect it introduced in the dark parts of the image.

Then for around 18 months I used in camera dark subtraction ( ie. long exposure noise reduction ). Again I did this because of the online advice that I needed to subtract darks and it seemed to work in that it did not add artefacts. The problem of course was the imaging time wasted in capturing one dark for every exposure.

Then I read in detail the various blog posts on http://www.clarkvision.com/articles

My take away from those articles was ( in relation to DSLRs ):
1. Dark subratction does nothing to remove random noise - in fact it adds the random noise from the darks into the integrated sum of the lights
2. Dark subtraction is useful if your camera has significant pattern noise ( mostly older cameras ) or it has a significant problem with hot pixels and you don't want to go to the effort of producing an "hot pixel map"
3. Modern cameras with "dark current suppression" have very little patten noise
4. If the light pollution in your area is significant and you expose so that contribution of light pollution is much larger than the random noise ( and patten noise for that matter ) then you don't have to worry about the noise as it will be overwhelmed by light pollution ( that is, cooling and dark subtraction is only useful if your skies are very dark or for some reason you need to take very short exposures such that the noise is visible through the light pollution)

In my case:
1. My camera is relatively new, has no discernible pattern noise and very few hot pixels ( Nikon D5300 )
2. My skies have moderate light pollution ( pale green zone ). - 240sec exposure at ISO 800 results in the average light pollution level of 0.025 on a 0..1 scale in a linear integrated set of images ( equivalent to the jpeg histogram peak occurring around 30% from the left )

So, I stoped using darks. As a result my images have greatlhy improved. I put this down to:
1. I can now take double the number of lights in the same time ( or in effect double the integration time ); and
2. I am no longer adding random noise via the dark images.

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Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)

Postby neufer » Wed Jul 12, 2017 1:14 pm

Chris Peterson wrote:
neufer wrote:
Chris Peterson wrote:
If another star passed within a light year of the Sun, it would greatly disrupt the Oort cloud (sending a barrage of comets into the inner system) and measurably perturb the orbits of the outer planets.

Whoa thar Rusty :!: Let's rethink that whole planets thing.

Voyager 1 is out at 139 AU (~0.002 lyr) where the Solar System escape velocity is ~3.6 km/s.

Voyager 1 is traveling at 17.043 km/s and is slowly
slowing down to 16.66 km/s = sqrt[(17.043)2-(3.6)2].

A star out at 1 lyr will generate only (0.002)2 of the solar acceleration
that is slowly slowing Voyager 1 by just 0.383 km/s [= (17.043 - 16.66)].

Furthermore...the star is accelerating the Sun by
practically the same amount in the same direction
:!:

Hence... the relative motion of Voyager 1 vs-a-vis the Sun
should be undetectable even with radar tracking.

Ergo: the relative motion of the planets vs-a-vis the Sun
would definitely be undetectable by passing stellar tidal forces.

I think you underestimate the number of decimal places of precision we deal with in modeling planetary orbits. We can detect relativistic effects in orbital parameters of planets. I'm pretty sure that a rapidly moving star a light year away will measurably- even significantly alter the orbits of our planets. That is, the positions of the planets after the million year passage will be distinctly different than if the star had not passed at all.

The gravitational tidal forces of a star out at 1 light year are equivalent
to the gravitational tidal forces of "a Pluto" out at 118 AU.

Can't be done :!:
Art Neuendorffer

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Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)

Postby Chris Peterson » Wed Jul 12, 2017 1:34 pm

neufer wrote:
Chris Peterson wrote:
neufer wrote:Whoa thar Rusty :!: Let's rethink that whole planets thing.

Voyager 1 is out at 139 AU (~0.002 lyr) where the Solar System escape velocity is ~3.6 km/s.

Voyager 1 is traveling at 17.043 km/s and is slowly
slowing down to 16.66 km/s = sqrt[(17.043)2-(3.6)2].

A star out at 1 lyr will generate only (0.002)2 of the solar acceleration
that is slowly slowing Voyager 1 by just 0.383 km/s [= (17.043 - 16.66)].

Furthermore...the star is accelerating the Sun by
practically the same amount in the same direction
:!:

Hence... the relative motion of Voyager 1 vs-a-vis the Sun
should be undetectable even with radar tracking.

Ergo: the relative motion of the planets vs-a-vis the Sun
would definitely be undetectable by passing stellar tidal forces.

I think you underestimate the number of decimal places of precision we deal with in modeling planetary orbits. We can detect relativistic effects in orbital parameters of planets. I'm pretty sure that a rapidly moving star a light year away will measurably- even significantly alter the orbits of our planets. That is, the positions of the planets after the million year passage will be distinctly different than if the star had not passed at all.

The gravitational tidal forces of a star out at 1 light year are equivalent
to the gravitational tidal forces of "a Pluto" out at 118 AU.

Can't be done :!:

A Pluto at 118 AU most certainly alters the orbits of all the inner planets over a million years!
Chris

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Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)

Postby neufer » Wed Jul 12, 2017 1:57 pm

Chris Peterson wrote:
neufer wrote:
Chris Peterson wrote:
I think you underestimate the number of decimal places of precision we deal with in modeling planetary orbits. We can detect relativistic effects in orbital parameters of planets.

The gravitational tidal forces of a star out at 1 light year are equivalent
to the gravitational tidal forces of "a Pluto" out at 118 AU.

Can't be done :!:

A Pluto at 118 AU most certainly alters the orbits of all the inner planets over a million years!

I assumed that you meant detectable with current technology over no more than a few hundred years.

Passing stars disrupting the Oort Cloud is a far different matter
than passing stars producing any measurable effect on the Solar System.

When we have placed a laser ranging device on Pluto itself and
generated a full orbit's worth of data we can reconsider the issue.
Art Neuendorffer

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Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)

Postby Chris Peterson » Wed Jul 12, 2017 2:11 pm

neufer wrote:
Chris Peterson wrote:
neufer wrote:The gravitational tidal forces of a star out at 1 light year are equivalent
to the gravitational tidal forces of "a Pluto" out at 118 AU.

Can't be done :!:

A Pluto at 118 AU most certainly alters the orbits of all the inner planets over a million years!

I assumed that you meant detectable with current technology over no more than a few hundred years.

Then you didn't read what I wrote carefully enough. The question had to do with the consequences to the Solar System of having nearby stars moving by (as in a globular cluster). Such interactions will occur on time scales of hundreds of thousands of years, so that's the time scale of the perturbations.

Of course, if a solar mass star simply materialized one light year from the Sun, we would be able to detect its effect on planetary orbits within a pretty short time- a few years at most, I would think. (Just as we'd detect the effect of Pluto disappearing instantly.)
Chris

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Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)

Postby neufer » Wed Jul 12, 2017 3:19 pm

Chris Peterson wrote:
neufer wrote:
Chris Peterson wrote:
A Pluto at 118 AU most certainly alters the orbits of all the inner planets over a million years!

I assumed that you meant detectable with current technology over no more than a few hundred years.

Then you didn't read what I wrote carefully enough. The question had to do with the consequences to the Solar System of having nearby stars moving by (as in a globular cluster). Such interactions will occur on time scales of hundreds of thousands of years, so that's the time scale of the perturbations.

The only cumulative interaction that will occur over many planetary orbits is orbital precession. The orbital precession effect on Mercury due to a star at one light year is comparable to that due to Pasiphae and is much much less than can be measured with current technology. (Ditto with the other planets.)

https://en.wikipedia.org/wiki/Apsidal_precession wrote:
<<In celestial mechanics, perihelion precession, apsidal precession or orbital precession is the precession (rotation) of the orbit of a celestial body. More precisely, it is the gradual rotation of the line joining the apsides of an orbit, which are the points of closest and farthest approach. Perihelion is the closest point to the Sun.

There are a variety of factors which can lead to periastron precession, such as general relativity, stellar quadrupole moments, mutual star–planet tidal deformations, and perturbations from other planets.

    ωtotal = ωtide + ωGeneral Relativity + ωquadrupole + ωperturbations
For Mercury, the precession due to perturbations from all the other planets in the Solar System is 532″ per century. The perihelion precession rate due to general relativistic effects is 43″ per century.
Chris Peterson wrote:
Of course, if a solar mass star simply materialized one light year from the Sun, we would be able to detect its effect on planetary orbits within a pretty short time- a few years at most, I would think. (Just as we'd detect the effect of Pluto disappearing instantly.)

Even if the Sun were to suddenly disappear it would probably take us at least a couple of hours to notice that the Earth had stopped moving in a curved orbit (or that the ocean tides were readjusting).
Art Neuendorffer

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Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)

Postby MikeODay » Sat Jul 15, 2017 11:34 am

I have just uploaded a re-processed version of this APOD to my Flickr page:

https://flic.kr/p/WupNxf

This one goes deeper ( bringing out more stars ).

( Sorry, I would post it here rather than a link but I don't know how to upload an image. )

Last edited by bystander on Sat Jul 15, 2017 1:24 pm, edited 1 time in total.
Reason: added image

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Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)

Postby MikeODay » Sat Jul 15, 2017 4:07 pm

Thankyou bystander for adding the image - much appreciated.


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