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midtskogen
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07 Jun 2017 01:07

NIL DIFFICILE VOLENTI
 
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Watsisname
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07 Jun 2017 01:42

That effect occurs at the microarcsecond scale or less, which requires VLBI and is mostly in the context of observing extragalactic sources at that level of precision.  But for more general stellar parallax surveys this isn't important -- Hipparcos had milliarcsecond precision, while Gaia is closer but still too coarse to observe this.  Gaia can observe microlensing events of stars in our galaxy, but they just appear as a change in the light curve.
 
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07 Jun 2017 03:21

Watsisname wrote:
Source of the post It would be like measuring a mass to be -0.1 +/- 0.2 grams.  You know a negative mass makes no sense, but what this result actually means is that it is indistinguishable from zero.

Thank you for answering me. This is exactly what I thought. But for some reason I think there's something more. Why don't the catalogues truncate the uncertainty? I mean, in you example of the -0.1 +/- 0.2 grams it sould say instead: the answer is in the interval from 0.0 to 0.1 grams because there is no uncertainty at all in reality in the -0.1 to 0.0 interval because we know objects have positive masses.

By the way. This seems to appear in the vast majority of astrometric catalogues. For example in the integrated Gaia-realease with the Tycho-2 and Hipparcos catalogue there are stars that have even -24.82 +/- 0.63 mas, like TYC-2 829-566-2. This places all the range of possible values in the very far negative parallaxes.

By the way, I heard that it could have to do with what reference for infinity you have placed. For example, if you have distant background stars you can measure parallax of closest stars comparing to them and have a pretty accurate idea of how much they have moved in the sky, but if you take distant stars from the same field of view it can get negative since they would move lees than you reference for the zero parallax. Is this idea absolute garbage or has some truth in it? I feel is not very satisfactory

I have another possible explanation: maybe due to high proper motion during a year the angular measurements for the position of the star may change in the opposite direction, creating the feeling they have negative parallaxes. If you did both measurements at the exact same time you would get a positive parallax but since you have spent 6 months from one point of the earth's orbit to its antipotal the star could have moved in that time, and if it moved quickly and in the correct direction you could see the star changing it's apparent postion as if it had a negative parallax. Is this a good alternative idea?

Edit: Wow, it loooks like someone thinked about it in a forum

I'm very interested in understanding this issue, because there are a lot of stars with negative parallaxes.
 
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Watsisname
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07 Jun 2017 14:23

They are not truncated because this would lose information which is valuable for performing data reduction, error analysis, and examining when other (astrophysical) effects are taking place.  It is somewhat of an inconvenience for someone wanting to use parallaxes to determine distances, and in that case the best method is to truncate them or use alternative methods (e.g. spectroscopic parallax rather than geometric).

Negative stellar parallaxes are quite common, but the small ones (few milliarcsec) are almost solely due to uncertainty.  This is actually true even when the stated parallax is more than a few times more negative than the +/- range.  When we report measurements, we may say the result is "x +/- y", but what that actually means is "68% of the measurements fall within y of x".  The 68% might seem mysterious, but it arises from a statistical analysis of all the measurements to determine how they are distributed.  If the errors are small, random, and uncorrelated, they may form a Normal Distribution. Then we say that the best estimate is the mean, and the uncertainty is the standard deviation, which is related to the width of the distribution and 68% lie within one standard deviation of the mean.  This is also the "1 sigma" range.

So if I state a result of 10.0 +/- 0.1, which came from performing 1000 measurements, then if they are normally distributed you would expect about 680 of those measurements to be between 9.9 and 10.1 (the 1 sigma range).  But you would also expect to see about 45 measurements below 9.8 or above 10.2 (outside the 2 sigma range), and 2 or 3 measurements less than 9.7 or greater than 10.3 (outside the 3 sigma range).

With the stellar parallaxes from Hipparcos, the distribution of the small parallaxes of single stars is consistent with this form of error:
(From "Validation of the new Hipparcos reduction")

Image
"Figure 16: Top: the histogram, for all single stars with five-parameter solutions, of the formal error on the parallaxes. Bottom: the distribution of parallaxes less than 1 mas s-1 as a function of formal error on the parallax determination for the new solution. The diagonal lines show the one, two and three sigma levels as based on the formal errors."

In the case of very negative parallaxes other causes are usually at play.  For example, the orbital motion of stars in binary or multi-star systems can be important.  For relatively nearby or fast-moving stars, their proper motions can be important.

By the way, I heard that it could have to do with what reference for infinity you have placed.


This is mainly for older catalogs where parallaxes were measured with respect to a "background" of other stars in the field.  This can cause negative parallaxes to appear which are essentially the negative of the valid (positive) parallax you would expect, if the star you are measuring is actually the background of a field of more nearby cluster stars.  But these days the parallax is measured with respect to an extragalactic reference frame, so this typically isn't important.

What midtskogen found was a paper showing negative parallaxes also arise at the microarcsecond scale, caused by weak lensing from the small variations in gravitational field of our galaxy.  As precision of parallax measurements approaches that scale then this becomes a fundamental limitation.
 
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spaceguy
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08 Jun 2017 20:34

What's this star in the Orion nebula called?
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Watsisname
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09 Jun 2017 01:24

HIP 26258, and the nebula containing it is M43.  It's a variable O-type star, very hot, and is ionizing the gas around it.
 
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09 Jun 2017 19:54

Watsisname wrote:
HIP 26258, and the nebula containing it is M43.  It's a variable O-type star, very hot, and is ionizing the gas around it.

Thanks!! I tried looking for this star online but there's barely any information on it from googling. Is there a reason for this? It seems like a pretty significant star in the nebula and is pretty apparent.
 
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Watsisname
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09 Jun 2017 21:05

Sure thing!  As to the dearth of information, I cannot say... I mean there's a little bit but not a whole lot.  I guess it's just not as well known as some other stars or star groups, like the Trapezium.  But it is a pretty significant star in that nebula.

Here's a fun little page about it.

In general a good way to figure out what star you're looking at is with star chart or planetarium software.  I used Stellarium.  The star is also in Space Engine. :)
 
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Watsisname
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11 Jun 2017 18:28

A fun challenge question:

During the 2017 eclipse, the bright star Regulus will be only 1.32° from the center of the eclipsed Sun.  In the spirit of Einstein's famous test of general relativity, by how much should that angle be changed due to the gravitational lensing of the Sun?
 
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spaceguy
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11 Jun 2017 20:07

How can stars with 4-7 solar masses have B-O spectra if they're not massive enough to achieve such?
I was under the impression that the more massive the star, the higher the rate of nuclear fusion and thus the hotter they are. I thought this was only possible in 15 and above solar masses?
 
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Watsisname
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11 Jun 2017 20:41

B's start at about 2 solar masses.  O's at 16.
 
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spaceguy
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11 Jun 2017 21:03

Watsisname wrote:
B's start at about 2 solar masses.  O's at 16.

How can a two solar mass star be so hot? :O

(God I hope that doesn't come off as stupid. Aren't B stars typically blue?)
 
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11 Jun 2017 22:25

Heh. :p  Well, is it that hot?  I think it depends how you look at it.  Consider that a 2 solar mass star isn't even twice as hot as the Sun.  A 16 solar mass star is ~5 times hotter than the Sun.  When described that way, maybe it doesn't seem so surprising.

To really see why stars work the way they do, you'd need to study the maths of stellar structure, with which you can derive things like the core temperature, pressure, luminosity, and other properties based on the star's mass.  There are a few simple relationships that come out of that, like a mass-luminosity relationship across the main sequence that fits observations pretty well. You could also find a relationship with surface temperature by using the luminosity and radius.

As for the color, the blueness is subtle for the lower mass B's, and it gets more pronounced as you get up toward the O's.  There's actually a pretty big range of surface temperatures in the B-types, from 10,000 to 30,000K, compared to 7500-10,000K for the A's.

Vega is an A0, or the hottest A type star, and it's a little bit blue as well. :)
 
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Salvo
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11 Jun 2017 23:43

I wonder why stellar classes are not proportional nor exponential but their range is like: 3700 - 1500 - 800 - 1500 - 2500 - 20000

Actually they're based on color, but still some classes are very "close" (for example G and F or A and B somehow). Do some of you know if there was an exact criteria or the ranges have been decided arbitrarily?  :)

If we consider our sun as a "pure white light emitter" (even if for atmosphere-scattering it looks intense yellow-red) we can easily see differences in color since lower mass G's have a yellowish spectrum average while late F's would have a slightly blueish color, but it looks a very "egocentric" vision, aliens might have something to argue about.  8-)
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spaceguy
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11 Jun 2017 23:57

Watsisname wrote:
Heh. :p  Well, is it that hot?  I think it depends how you look at it.  Consider that a 2 solar mass star isn't even twice as hot as the Sun.  A 16 solar mass star is ~5 times hotter than the Sun.  When described that way, maybe it doesn't seem so surprising.

To really see why stars work the way they do, you'd need to study the maths of stellar structure, with which you can derive things like the core temperature, pressure, luminosity, and other properties based on the star's mass.  There are a few simple relationships that come out of that, like a mass-luminosity relationship across the main sequence that fits observations pretty well. You could also find a relationship with surface temperature by using the luminosity and radius.

As for the color, the blueness is subtle for the lower mass B's, and it gets more pronounced as you get up toward the O's.  There's actually a pretty big range of surface temperatures in the B-types, from 10,000 to 30,000K, compared to 7500-10,000K for the A's.

Vega is an A0, or the hottest A type star, and it's a little bit blue as well. :)

Ty. I should research stars more. :P

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