Surface vs Apparent Brightness

In summary, the sun's luminosity is due to the nuclear reactions that take place in the core of the star. The temperature and density of a star increase towards the core due to the pressure that is exerted there.
  • #1
Huej
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I'm confused what the difference between the two are...I thought surface brightness was luminosity, but apparently it's not: L=Surface brightess x Area...But I came across a similar equation that seems to assume surface brightness is the same as apparent brightness. Please help!
Edit: Also, why does temperature increase towards the core of a star but also increase in density? Shouldn't they be inversely related?
 
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  • #2
Huej said:
I'm confused what the difference between the two are...I thought surface brightness was luminosity, but apparently it's not: L=Surface brightess x Area...But I came across a similar equation that seems to assume surface brightness is the same as apparent brightness. Please help!

https://www.e-education.psu.edu/astro801/content/l4_p4.html

have you read this article ?
does it help ?

Huej said:
Edit: Also, why does temperature increase towards the core of a star but also increase in density? Shouldn't they be inversely related?

because only in the core is where the nuclear reactions are occurring and heat is transported to the surface by 2 methods
firstly by a radiative zone then finally to the surface by a convective zone ...

The Sun.jpg
Huej said:
Edit:... Shouldn't they be inversely related?

why would you think that ?Dave
 
  • #3
Huej said:
Also, why does temperature increase towards the core of a star but also increase in density? Shouldn't they be inversely related?
Because the pressure increases as well. Remember a volume in the interior has to support the weight of all of the mass above it.
 
  • #4
davenn said:
https://www.e-education.psu.edu/astro801/content/l4_p4.html

have you read this article ?
does it help ?
because only in the core is where the nuclear reactions are occurring and heat is transported to the surface by 2 methods
firstly by a radiative zone then finally to the surface by a convective zone ...

View attachment 91652

why would you think that ?Dave
The ideal gas law would show an inverse relationship.
 
  • #5
The sun is not an ideal gas.
 
  • #6
I don't think the answer to the OP's question is that the sun is not an ideal gas. The sun is actually quite well described by the ideal gas laws. The answer is that the ideal gas laws do not imply an inverse relationship between density and temperature unless the pressure is constant, as vela said a couple of posts ago. The ideal gas law is usually written P V = n k T, so if we define the density as ρ = n/V, then we can write it as P = k ρ T. In the sun, P, T, and ρ all increase monotonically as we move from the outside to the interior, and they increase in such a way that the ideal gas laws are satisfied at all radii. There is no contradiction here to the ideal gas laws.

As far as the original question on surface brightness and apparent brightness, try reading the Wikipedia article on luminosity and magnitude here. Luminosity refers to the total amount of radiation a star emits, whereas brightness depends on how far away it is. A 100 Watt light bulb has a specific luminosity, but its brightness gets less as you move away from it. If you have specific questions on this, come back and ask.
 
  • #7
Huej said:
I'm confused what the difference between the two are...I thought surface brightness was luminosity, but apparently it's not: L=Surface brightess x Area...But I came across a similar equation that seems to assume surface brightness is the same as apparent brightness. Please help!
Edit: Also, why does temperature increase towards the core of a star but also increase in density? Shouldn't they be inversely related?

Well, a star is a big collection of hydrogen gas, initially. What keeps all this hydrogen together to form the star is gravity. Since the gravity in the ball of hydrogen never shuts off, the ball of hydrogen wants to get smaller and smaller, and this means that the hydrogen in the center is going to get hotter and denser as the ball gets smaller. After a certain point, the hydrogen at the very core of the ball of gas will 'ignite' in a series of nuclear fusion reactions, the chief result of which is that the temperature at the core of the star gets really hot, like millions of degrees hot, and it stays hot, as long as there is hydrogen in the core to keep feeding the fusion reactions.

Now, this hot ball of fusing hydrogen gas wants to expand, because of the high temperature, but if it expands beyond a certain point, the fusion reactions will stop, gravity takes over again, the core gets smaller and hotter, and the fusion reactions start again, making the core want to expand. After a certain time, some of the energy produced by the fusing hydrogen at the core of the star eventually reaches the outer, less dense layers, and is radiated into space. In other words, a system is set up, whereby the star is continually kept from collapsing onto itself by the high temperatures at the core wanting to expand the star and thus serving to counterbalance the attractive nature of gravity.

https://en.wikipedia.org/wiki/Stellar_structure

The study of the mechanism of how stars work, how they form, and how they age and eventually die is covered by stellar astrophysics.

http://ads.harvard.edu/books/1989fsa..book/
 
  • #8
Thanks for the additional posts guys
 
  • #9
Huej said:
The ideal gas law would show an inverse relationship.
That's only if you ignore gravity, adiabatic compression, and fusion. That is essentially ignoring everything that happens inside the Sun.

Any point in the interior of the Sun must be close to being in an equilibrium state, where the outward forces due to pressure more or less balance the inward forces due to gravitation. (Justification: The Sun has remained more or less unchanged for about for 4.6 billion years.) This leads to the hydrostatic equilibrium condition: The pressure at any point inside the Sun is just enough to counteract the weight of all of the stuff above that point. As the temperature of a gas increases as it is compressed adiabatically, the temperatures in the interior of the Sun should be (and are) much higher than the surface temperature.

This compression would ultimately lead to a gravitational collapse if it weren't for fusion in the Sun's core. Suppose the fusion rate drops at some point in the Sun's core. This would in turn make the temperature drop, which in turn would pressure drop, which would in turn result in in-falling material, which would in turn increase the pressure, which would in turn increase the temperature, which would in turn increase the fusion rate. Fusion represents a built-in negative feedback regulator.

As an aside, the lack of such a negative feedback regulatory mechanism is what makes very massive stars turn into neutron stars or black holes at the end of their lives.
 
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Likes davenn

What is the difference between surface brightness and apparent brightness?

Surface brightness refers to the amount of light per unit area coming from a specific object or region in the sky. It is measured in units of luminosity per square arcsecond. Apparent brightness, on the other hand, is the amount of light per unit area received by an observer on Earth. It is measured in units of energy per unit time per unit area (such as watts per square meter).

How are surface brightness and apparent brightness related?

Surface brightness and apparent brightness are related by the distance between the object and the observer. The farther away an object is, the dimmer its apparent brightness will appear, even if its surface brightness remains constant. This is because the same amount of light is spread out over a larger area as the distance increases.

Why is surface brightness important in astronomy?

Surface brightness is important in astronomy because it allows us to compare the brightness of objects at different distances. By measuring the surface brightness of an object, we can determine its intrinsic luminosity and thus better understand its physical properties, such as size, mass, and temperature.

How is surface brightness measured?

Surface brightness is typically measured using a combination of imaging and photometry techniques. Astronomers use telescopes and cameras to capture images of objects in the sky, and then use software to measure the amount of light in each pixel. By summing up the light in all the pixels within a certain area, the surface brightness can be determined.

What are some factors that can affect surface brightness?

The surface brightness of an object can be affected by factors such as distance, age, and composition. Objects that are farther away will have lower surface brightness, and objects that are older or made up of dimmer materials will also have lower surface brightness. Additionally, dust and gas can also affect surface brightness by absorbing or scattering light.

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