<p>According to <a href="https://en.wikipedia.org/wiki/UY_Scuti" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/UY_Scuti</a></p>
<blockquote>
<p>Apparent magnitude (V)    8.29 - 10.56</p>
</blockquote>
<blockquote>
<p>Absolute magnitude (MV)   −6.2</p>
</blockquote>
<p>UY Scuti, being a variable star, obviously has variable apparent magnitude; however why is the absolute magnitude a single value? Am I missing something i.e. does this have anything to do with the way UY Scuti is variable for instance being enshrouded in dust, or is it simply a Wikipedia error?</p>
<p>A quick check of other pages also shows some inconsistencies:<br />
<a href="https://en.wikipedia.org/wiki/W_Virginis" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/W_Virginis</a>  (same as UY Scuti)</p>
<blockquote>
<p>Absolute magnitude (MV)   −3.0</p>
</blockquote>
<p><a href="https://en.wikipedia.org/wiki/Mira" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/Mira</a> (absolute magnitude flagged as variable)</p>
<blockquote>
<p>Absolute magnitude (MV)   +0.99[7] (variable)</p>
</blockquote>

CC BY-SA 4.0
2022-10-01T04:49:13.257Z
UV Scuti is a pulsating variable star; this means it cyclically expands and contracts back to its original size. I would reckon that its absolute magnitude remains the same, as it always emits the same amount of light; however, when it expands, this light is emitted by a larger surface area, so it appears dimmer.
Pierre Paquette
CC BY-SA 4.0
2022-10-01T05:38:05.223Z
@pierre That doesn't seem right. Perhaps the surface brightness is constant, but if the star emits the same amount of light, it can't appear dimmer.
James K
CC BY-SA 4.0
2022-10-01T05:46:56.957Z
@PierrePaquette do you mean the rate of energy released via fusion is constant (or at least it varies slower than the period of pulsation)? Because pulsating variables expand due to its atmosphere becoming (more) opaque (less light actually escapes) hence radiation pressure increases, pushing the star's envelope further. Am I right?
Cheng
CC BY-SA 4.0
2022-10-01T06:36:44.207Z
I’m not sure about the mechanisms of pulsation (and the Wikipedia page doesn’t help; it seems very technical, and I don’t have time to read it all now), but a very crude analogy would be that you’re painting a larger sphere with the same amount of paint as a small sphere. The small sphere will have more paint per unit area (the star seems brighter when smaller), while the larger one will have less (the star seems dimmer when larger, as its light is more spread out).
Pierre Paquette
CC BY-SA 4.0
2022-10-01T06:39:15.437Z
@PierrePaquette Indeed, but because the stars are unresolvable, we measure the flux, and if the same amount of energy is emitted, the flux observed  at a distance is the same:

$$\frac{\text{Luminosity}}{4 \pi r^2}$$

Of course, the surface flux evaluated with $r = \text{radius of star} $ decreases....
Cheng
CC BY-SA 4.0
2022-10-01T06:44:18.087Z
True. I did a little more reading, and specially of the Wikipedia page about Cepheids and the one about https://en.wikipedia.org/wiki/Kappa%E2%80%93mechanism the κ-mechanism. The star’s opacity changes as it pulsates, so this will allow more or less light to filter through. This seems like a better explanation of why the star’s apparent brightness would change while the nuclear reactions in its core don’t change (hence it maintains the same energy production).
Pierre Paquette
CC BY-SA 4.0
2022-10-01T11:00:37.517Z
Absolute magnitude can be  computed from apparent magnitude and distance, so, if the apparent magnitude changes, so should the absolute magnitude. I think this is just a glitch. You can use the given range of apparent magnitudes and the distance to find what the corresponding absolute magnitude would be and see where -6.2 falls in that range.
Barry Carter
CC BY-SA 4.0
2022-10-01T18:22:38.657Z
I agree with @BarryCarter. This is just yet another case of wikipedia being wikipedia. It is not a reliable source of information.
David Hammen
CC BY-SA 4.0
2022-10-01T20:33:48.360Z
@barry It's not so easy because there is significant extinction due to dust.   The -6.2 value seems to be an estimate of the average luminosity made in about 1970, using IR observations (that are less affected by dust), adjusted to an M_v value using a special model.  There is considerable uncertainty in the value, but it is undoubtedly a huge and powerful supergiant star.
James K
<absolute-magnitude><variable-star>
(Wikipedia) UY Scuti: variable apparent magnitude, non-variable absolute magnitude?
Cheng
# (Wikipedia) UY Scuti: variable apparent magnitude, non-variable absolute magnitude?

(Asked by: username_0 on 2022-10-01)

According to <https://en.wikipedia.org/wiki/UY_Scuti>

> Apparent magnitude (V) 8.29 - 10.56

> Absolute magnitude (MV) −6.2

UY Scuti, being a variable star, obviously has variable apparent magnitude; however why is the absolute magnitude a single value? Am I missing something i.e. does this have anything to do with the way UY Scuti is variable for instance being enshrouded in dust, or is it simply a Wikipedia error?

A quick check of other pages also shows some inconsistencies:  
<https://en.wikipedia.org/wiki/W_Virginis> (same as UY Scuti)

> Absolute magnitude (MV) −3.0

<https://en.wikipedia.org/wiki/Mira> (absolute magnitude flagged as variable)

> Absolute magnitude (MV) +0.99[7] (variable)

**Tags:** absolute-magnitude, variable-star
> **Comments:**
> **username_1**: UV Scuti is a pulsating variable star; this means it cyclically expands and contracts back to its original size. I would reckon that its absolute magnitude remains the same, as it always emits the same amount of light; however, when it expands, this light is emitted by a larger surface area, so it appears dimmer.  
> **username_2**: @pierre That doesn't seem right. Perhaps the surface brightness is constant, but if the star emits the same amount of light, it can't appear dimmer.  
> **username_0**: @PierrePaquette do you mean the rate of energy released via fusion is constant (or at least it varies slower than the period of pulsation)? Because pulsating variables expand due to its atmosphere becoming (more) opaque (less light actually escapes) hence radiation pressure increases, pushing the star's envelope further. Am I right?  
> **username_1**: I’m not sure about the mechanisms of pulsation (and the Wikipedia page doesn’t help; it seems very technical, and I don’t have time to read it all now), but a very crude analogy would be that you’re painting a larger sphere with the same amount of paint as a small sphere. The small sphere will have more paint per unit area (the star seems brighter when smaller), while the larger one will have less (the star seems dimmer when larger, as its light is more spread out).  
> **username_0**: @PierrePaquette Indeed, but because the stars are unresolvable, we measure the flux, and if the same amount of energy is emitted, the flux observed  at a distance is the same:  $$\frac{\text{Luminosity}}{4 \pi r^2}$$  Of course, the surface flux evaluated with $r = \text{radius of star} $ decreases....  
> **username_1**: True. I did a little more reading, and specially of the Wikipedia page about Cepheids and the one about https://en.wikipedia.org/wiki/Kappa%E2%80%93mechanism the κ-mechanism. The star’s opacity changes as it pulsates, so this will allow more or less light to filter through. This seems like a better explanation of why the star’s apparent brightness would change while the nuclear reactions in its core don’t change (hence it maintains the same energy production).  
> **username_3**: Absolute magnitude can be  computed from apparent magnitude and distance, so, if the apparent magnitude changes, so should the absolute magnitude. I think this is just a glitch. You can use the given range of apparent magnitudes and the distance to find what the corresponding absolute magnitude would be and see where -6.2 falls in that range.  
> **username_4**: I agree with @BarryCarter. This is just yet another case of wikipedia being wikipedia. It is not a reliable source of information.  
> **username_2**: @barry It's not so easy because there is significant extinction due to dust.   The -6.2 value seems to be an estimate of the average luminosity made in about 1970, using IR observations (that are less affected by dust), adjusted to an M_v value using a special model.  There is considerable uncertainty in the value, but it is undoubtedly a huge and powerful supergiant star.  

---

## 0 answers

