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Forum » SpaceEngine » General Discussions » Low luminosity?
Low luminosity?
SirSpunkyDate: Monday, 06.01.2014, 18:22 | Message # 1
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I'm by no means an astronomer so I probably missed something. But I noticed that the star luminosity of big stars as seen in Space Engine is much lower than on Wikipedia.

Some examples:
Bellatrix - Space Engine: 1022.8 Wikipedia: 6400
Murzim - Space Engine: 3159 Wikipedia: 26600
Naos - Space Engine: 20067 Wikipedia: 550000

Temperature and mass are much more similar, so I'm wondering if Space Engine is using another luminosity measure than Wikipedia.

As a side note, my understanding is that luminosity is the measure of the energy emitted by the star, so 100 solar luminosity means that it emits 100 times more energy than the sun. But is this really correct, considering that the luminosity is so high on some stars?


Edited by SirSpunky - Monday, 06.01.2014, 18:34
 
HarbingerDawnDate: Monday, 06.01.2014, 18:46 | Message # 2
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Quote SirSpunky ()
But is this really correct, considering that the luminosity is so high on some stars?

Yes. There are some stars which are millions of times more luminous than Sol, and can be naked-eye visible over a radius of hundreds of thousands of light-years. These stars would seem like another sun in the sky of all its neighboring star systems.

Quote SirSpunky ()
But I noticed that the star luminosity of big stars as seen in Space Engine is much lower than on Wikipedia.

Stars have their apparent magnitudes input, not their luminosities. This makes them look correct from Earth, but results in an underestimation of their luminosity since light is lost due to extinction from interstellar dust between the star and Earth.

Also, general astronomy questions can be asked here, and general SE questions can be asked here. A new thread is not necessary.





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apenpaapDate: Monday, 06.01.2014, 18:49 | Message # 3
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In previous versions, a number of these stars had their actual luminosity in the game, which resulted in distortions in Earth's skies. The brightest star was some supergiant with a HD number for a name. Eta Carina was between Canopus and Sirius in brightness. The Pistol Star and a star in the LMC were visible with the naked eye, etc.




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ermcgratDate: Wednesday, 08.01.2014, 04:52 | Message # 4
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Quote HarbingerDawn ()
Stars have their apparent magnitudes input, not their luminosities. This makes them look correct from Earth, but results in an underestimation of their luminosity since light is lost due to extinction from interstellar dust between the star and Earth.

Also, general astronomy questions can be asked here, and general SE questions can be asked here. A new thread is not necessary.


Hello, and greetings from a first time poster. Space Engine is the greatest program I have ever used.

I too was confused by the great discrepancy in luminosity, which eventually lead me to this thread. Your answer still confuses me. I understand the actual luminosity of the object may need to be lowered to prevent adverse affects in the software, but why not call it "apparent magnitude" then? As another example, the info for VY Canis Majoris states the luminosity is 1224.3, which is grossly misunderstated. It just seems very misleading to call the property "Luminosity" if that is not what it is.

Also, I read through the FAQ thread that you linked, and it does not cover this confusion over luminosity. SirSpunky posted this question in this subforum "General Discussion: Post here for general comments and questions about SpaceEngine", so I don't see why the discouragement from new threads. Is this not the purpose of this subforum? Just trying to understand, and thank you for all your help. I see you are a great contributor on this board, and I just want to understand how things work.
 
HarbingerDawnDate: Wednesday, 08.01.2014, 04:56 | Message # 5
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The effects of interstellar extinction are not currently taken into account. Therefore, if you give a star like Deneb or Eta Carinae its actual brightness it will appear too bright in Earth's sky. And because extinction is not modeled, the only way to get the star to be the correct brightness is to reduce the luminosity. Apparent magnitude is just a function of luminosity and distance. There is no way to separate the two.




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ermcgratDate: Wednesday, 08.01.2014, 05:06 | Message # 6
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Understood. I guess I'm just nitpicking. If I could somehow rename the property "Luminosity" to "Apparent Brightness" I'd be totally content. I guess I will just have to accept that that statistic is not to be trusted for conventional purposes of comparison smile
 
HarbingerDawnDate: Wednesday, 08.01.2014, 05:31 | Message # 7
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Quote ermcgrat ()
I guess I will just have to accept that that statistic is not to be trusted for conventional purposes of comparison

Yes it is, just not for some specific stars near Earth.

Quote ermcgrat ()
If I could somehow rename the property "Luminosity" to "Apparent Brightness" I'd be totally content.

But that's not what luminosity is. The thing is that you can't change the relationship between luminosity and apparent magnitude at a given distance. If you decrease one, you also decrease the other. And since, in this case, catalog stars have their apparent magnitudes from Earth listed (because this is how astronomers measure things, and the only way they can measure things), and because a lot of the light from the star is blocked by dust before it reaches Earth, then it means that their luminosities are underestimated.





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SirSpunkyDate: Wednesday, 08.01.2014, 21:11 | Message # 8
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Thanks for answering, I can definitely understand why Space Engine chose to use apparent magnitude for drawing stars.

I read some more about this on Wikipedia, and I thought their explanation was pretty good:

"In astronomy, luminosity is the amount of electromagnetic energy a body radiates per unit of time. It is most frequently measured in two forms: visual (visible light only) and bolometric (total radiant energy), although luminosities at other wavelengths are increasingly being used as instruments become available to measure them. A bolometer is the instrument used to measure radiant energy over a wide band by absorption and measurement of heating. When not qualified, the term "luminosity" means bolometric luminosity, which is measured either in the SI units, watts, or in terms of solar luminosities."

I guess the most similar concept to visual luminosity would be absolute magnitude, which reports visible brightness at a fixed distance, although it uses a different unit.

To avoid confusion, maybe the label in Space Engine should be "Luminosity (visual)" or something similar. Of course, it would be great if it could also include "Luminosity (bolometric)" as a separate label, but I assume that would be more work because it would have to be calculated.


Edited by SirSpunky - Wednesday, 08.01.2014, 21:13
 
HarbingerDawnDate: Thursday, 09.01.2014, 06:40 | Message # 9
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You don't understand, it has nothing to do with visual vs. bolometric. It has to do with how luminosity is calculated as a function of distance and apparent magnitude. If starlight reaches the observer unobstructed, then that observer can accurately calculate the luminosity by measuring the brightness of the star and its distance. If some of the starlight is blocked, however, for example by interstellar dust, then the luminosity will be underestimated. This is not visual luminosity, it is simply a miscalculation. You then have to study the star to determine its properties, which will tell you what its luminosity should be, and then you can figure out how much light was extincted by interstellar dust. SpaceEngine can not simulate dust extinction right now. So any star in the catalog which is input by its apparent magnitude (which should be all of them since that's how astronomers catalog objects) and which in reality is affected by interstellar extinction will have too low a luminosity in SE.

The only way to fix that would be to manually re-catalog all those stars using their true luminosities, which would be a lot of work, and then Earth's sky would not be accurate because of the incorrect apparent brightness of some stars.

If someone wants to make an addon which re-catalogs some of those stars to use their true luminosities for people who don't care about the accuracy of Earth's sky, then that would be great. But I hope you can understand why the default catalog in SpaceEngine is the way it is. It is a consequence of being a real astronomical catalog made from observational data.





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SirSpunkyDate: Thursday, 09.01.2014, 09:19 | Message # 10
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Oh, ok, now that's more confusing than I thought. But I think I'm starting to understand better what you're trying to say.

So because the luminosity in Space Engine does not subtract for interstellar extinction (like space dust blocking the light), it's an underestimation of the true visual luminosity. Is that correct?

Just to clarify, I'm not talking about changing the way stars are rendered, I don't want to change that, only what numbers are displayed in the upper-left corner.

If the luminosity number in Space Engine does not really correspond to either of the two established definitions of "luminosity", I think you'll continue to get this question by others. Although it might not be highest priority, I think it might be a good idea to try to either simulate true luminosity statistics (either visual or bolometric), or change the name of the label like ermcgrat suggested above, or just hide the label completely. Again, I don't mean changing any way how stars are rendered, only what numbers to display.

Sorry for my lack of astronomy knowledge, I just want to help make this awesome program even better. I think increasing the understanding and quality of the statistical numbers shown to the player is not a bad thing, especially if this is to become a real game in the future. This program is by its nature very inspiring to people and makes them want to learn what those numbers are really about, which I think is a good thing.
 
HarbingerDawnDate: Thursday, 09.01.2014, 10:49 | Message # 11
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Quote SirSpunky ()
So because the luminosity in Space Engine does not subtract for interstellar extinction (like space dust blocking the light), it's an underestimation of the true visual luminosity. Is that correct?

Not exactly. You've over-complicating it. It's as simple as this:

SpaceEngine uses the Hipparcos star catalog, which contains information on the distances and apparent brightnesses of nearly 120k stars in this part of the galaxy. The luminosity you see in SE is simply a calculation derived from the star's apparent magnitude and distance from Sol, because that is all the information there is available. Apparent brightness is how astronomers measure stars in the sky, and it's how they are input in SE for known stars.

Quote SirSpunky ()
If the luminosity number in Space Engine does not really correspond to either of the two established definitions of "luminosity", I think you'll continue to get this question by others.

But it does. Just because the value is erroneous for some stars does not mean it's a different definition of luminosity. It just means the value is wrong. In this case, the value is wrong by necessity. Luminosity is linked to apparent magnitude and distance, it is hard coded that way. If you change one, you change another. So for stars whose light is blocked by interstellar dust, there's no way to have all three numbers be accurate in SE. The only way to do so would be to re-code the engine to allow an overriding luminosity value to be entered, independent of the apparent magnitude and distance, and which would not affect the rendering of the star. This may solve your particular problem, but it also introduces additional confusion.

Until some method to simulate interstellar extinction is implemented in SE (which could take a very long time as it's pretty complicated), then there is no perfect system, and inaccuracies and confusion will always exist.

Unless of course, you understand why they exist while you're visiting stars in SE, in which case you can take that into account and not be confused by them.





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JCandeiasDate: Thursday, 09.01.2014, 16:27 | Message # 12
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Quote HarbingerDawn ()
Until some method to simulate interstellar extinction is implemented in SE (which could take a very long time as it's pretty complicated), then there is no perfect system, and inaccuracies and confusion will always exist.


Well, that's not necessarily so. It would be possible to create a "TrueLuminosity" tag, affecting the local environment only (the local system and up to a few parsecs, for instance), and the data that pops up in informative lists. It could be used to add true luminosities of stars that have them studied and known with some precision, which is, I believe, far less than the 120 thousand of the Hipparcos catalog. To investigate and add true luminosities would still be a whole lot of work, though, and something I'd prefer Vladimir didn't waste time with. But with the tag implemented, this work could be left to the community, through addons that, upon checking, could later be included in official releases.





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HarbingerDawnDate: Thursday, 09.01.2014, 17:28 | Message # 13
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However, this could create additional confusion, as the star would not behave like most stars, nor would its listed luminosity match the apparent brightness of the star as seen from most places. This discrepancy would probably be just as confusing and/or misinformative as the current system, hence why I discounted it.




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JCandeiasDate: Thursday, 09.01.2014, 18:26 | Message # 14
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May be, but I think it's worth thinking about it. The confusion is already there, since people will be cross-checking SE's values with those coming from other sources and finding discrepancies. I think it's far more likely that people do that than calculating for themselves luminosities from apparent magnitudes and finding discrepancies there.

And, in any case, it's a question of putting up a few discrepant numbers versus total incorrectness in local environment. I much prefer the first option, and I also think that, due to the patchy nature of interstellar gas and dust, the only way for SE to take light attenuation into account would be with a very detailed 3D model of galactic dust, which is something I don't think would be feasible with current hardware specs. Assuming such a model even exists.

And even with such a model implemented, the same potential for discrepant numbers remains.

Besides, SE could, and maybe should, have a table explaining briefly but clearly what the various astronomical/aeronautical terms mean, where those discrepancies could be explained.





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