Explanation: What's that below the Moon? Jupiter -- and its largest moons. Many skygazers across planet Earth enjoyed the close conjunction of Earth's Moon passing nearly in front of Jupiter in mid-June. The featured image is a single exposure of the event taken from Morón de la Frontera, Spain. The sunlit lunar crescent on the left is overexposed, while the Moon's night side, on the right, is only faintly illuminated by Earthshine. Lined up diagonally below the Moon, left to right, are Jupiter's bright Galilean satellites: Callisto, Ganymede, Io (hard to see as it is very near to Jupiter), and Europa. In fact, Callisto, Ganymede, and Io are larger than Earth's Moon, while Europa is only slightly smaller. NASA's robotic spacecraft Juno is currently orbiting Jupiter and made a close pass near Io only a week ago. If you look up in the night sky tonight, you will again see two of the brightest objects angularly close together -- because tonight is another Moon-Jupiter conjunction.
Is the annotation for Jupiter's moons correct? I entered the date (June 14, 2023) and the location on Stellarium.
What I see is that the position of Europa and Ganymede are not correct in today's APOD annotation.
Btw, there is a star not annotated in today's APOD. It has no name in Stellarium:
I've been trying to find out it's name, any help?
Re: APOD: Moon Meets Jupiter (2023 Aug 08)
Posted: Tue Aug 08, 2023 12:18 pm
by Rauf
I think I've found the name of this star, between Ganymede and Jupiter ( I still think that's Ganymede to the right of Jupiter, not Europa, and there's been a mistake)
photo_2023-08-08_15-36-46.jpg (9.47 KiB) Viewed 6714 times
Rauf wrote: ↑Tue Aug 08, 2023 9:11 am
Is the annotation for Jupiter's moons correct? I entered the date (June 14, 2023) and the location on Stellarium.
...
Where did you get that date from? The date on the Instagram post with the pic is June 27, 2023. Though obviously the image could have been taken prior to that. I'm also not sure if a two week time difference would matter much.
Rauf wrote: ↑Tue Aug 08, 2023 9:11 am
Is the annotation for Jupiter's moons correct? I entered the date (June 14, 2023) and the location on Stellarium.
...
Where did you get that date from? The date on the Instagram post with the pic is June 27, 2023. Though obviously the image could have been taken prior to that. I'm also not sure if a two week time difference would matter much.
Rauf wrote: ↑Tue Aug 08, 2023 9:11 am
Is the annotation for Jupiter's moons correct? I entered the date (June 14, 2023) and the location on Stellarium.
...
Where did you get that date from? The date on the Instagram post with the pic is June 27, 2023. Though obviously the image could have been taken prior to that. I'm also not sure if a two week time difference would matter much.
Yes, the conjunction was June 14. Rauf is correct; the annotation has Europa and Ganymede switched.
✓
Re: APOD: Moon Meets Jupiter (2023 Aug 08)
Posted: Tue Aug 08, 2023 4:49 pm
by Ann
Rauf wrote: ↑Tue Aug 08, 2023 12:18 pm
I think I've found the name of this star, between Ganymede and Jupiter ( I still think that's Ganymede to the right of Jupiter, not Europa, and there's been a mistake)
I checked HD 13873. It appears to be a G0-type star, very slightly hotter and bluer than the Sun, but considerably brighter - 13.5 times brighter than the Sun to be reasonably exact.
Just imagine having 13 stars like the Sun in the sky!!! Or, okay, one star that is worth 13 Suns, at least when it comes to luminosity. Absolutely not when it comes to mass!
Ann
Re: APOD: Moon Meets Jupiter (2023 Aug 08)
Posted: Tue Aug 08, 2023 5:47 pm
by orin stepanek
Moon & Jupiter conjunction: good night to go skygazing!
Rauf wrote: ↑Tue Aug 08, 2023 12:18 pm
I think I've found the name of this star, between Ganymede and Jupiter ( I still think that's Ganymede to the right of Jupiter, not Europa, and there's been a mistake)
I checked HD 13873. It appears to be a G0-type star, very slightly hotter and bluer than the Sun, but considerably brighter - 13.5 times brighter than the Sun to be reasonably exact.
Just imagine having 13 stars like the Sun in the sky!!! Or, okay, one star that is worth 13 Suns, at least when it comes to luminosity. Absolutely not when it comes to mass!
...
Ann
So, what do you mean there? That this star, though 13 times brighter would only be, what, about the fourth root of 13 times as massive?
Rauf wrote: ↑Tue Aug 08, 2023 12:18 pm
I think I've found the name of this star, between Ganymede and Jupiter ( I still think that's Ganymede to the right of Jupiter, not Europa, and there's been a mistake)
I checked HD 13873. It appears to be a G0-type star, very slightly hotter and bluer than the Sun, but considerably brighter - 13.5 times brighter than the Sun to be reasonably exact.
Just imagine having 13 stars like the Sun in the sky!!! Or, okay, one star that is worth 13 Suns, at least when it comes to luminosity. Absolutely not when it comes to mass!
...
Ann
So, what do you mean there? That this star, though 13 times brighter would only be, what, about the fourth root of 13 times as massive?
Don't ask me about math, Johnny!
But consider this: Sirius, which is 23 times brighter than the Sun in yellow-green light (and a lot brighter than that in ultraviolet light) is only about twice as massive as the Sun.
That would make HD 13873 how massive? Some 1.7 times as massive as the Sun?
Maybe?
Edit: I asked Google. What is the fourth root of 13? It's 1.89882892212, said Google.
And you could be right. Maybe HD 13783 really is 1.89882892212 the mass of the Sun.
Ann
Re: APOD: Moon Meets Jupiter (2023 Aug 08)
Posted: Tue Aug 08, 2023 8:47 pm
by Ann
Okay, this got me thinking. What kind of star would you actually get from 13 solar masses?
Of course, it depends on how evolved or non-evolved the star is, but here goes anyway:
A blue, not terrifically evolved 13 solar mass star could be like Gamma Cassiopeiae.
The spectrum of this massive star matches a stellar classification of B0.5 IVe. A luminosity class of IV identifies it as a subgiant star that has reached a stage of its evolution where it is exhausting the supply of hydrogen in its core region and transforming into a giant star. The 'e' suffix is used for stars that show emission lines of hydrogen in the spectrum, caused in this case by the circumstellar disk. This places it among a category known as Be stars; in fact, the first such star ever to be so designated.[16] It has 17 times the Sun's mass and is radiating as much energy as 34,000 Suns.
17 times the Sun's mass? Okay, but in the "information box" at right, under "Details", it says that the primary star of Gamma Cas has a mass of 13 solar.
Right! But what will a highly evolved red star of 13 solar masses look like? How about... Antares?
Antares is a red supergiant star with a stellar classification of M1.5Iab-Ib, and is indicated to be a spectral standard for that class.[6] Due to the nature of the star, the derived parallax measurements have large errors, so that the true distance of Antares is approximately 550 light-years (170 parsecs) from the Sun.[3]
The brightness of Antares at visual wavelengths is about 10,000 times that of the Sun, but because the star radiates a considerable part of its energy in the infrared part of the spectrum, the true bolometric luminosity is around 100,000 times that of the Sun. There is a large margin of error assigned to values for the bolometric luminosity, typically 30% or more.
... The mass of the star has been calculated to be about 12 M☉,[11] or 11 to 14.3 M☉.[10] Comparison of the effective temperature and luminosity of Antares to theoretical evolutionary tracks for massive stars suggest a progenitor mass of 17 M☉ and an age of 12 million years (MYr),[11] or an initial mass of 15 M☉ and an age of 11 to 15 MYr.
Well, that's good enough, I think. A 13 solar mass star would give you something like blazingly hot, 34,000 L☉ Gamma Cas, or cool red supergiant star Antares, with a luminosity of some 100,000 L☉.
It would be something way, way, way different than the combined luminosity of 13 stars like the Sun!
Ann
Re: APOD: Moon Meets Jupiter (2023 Aug 08)
Posted: Tue Aug 08, 2023 8:56 pm
by johnnydeep
Ann wrote: ↑Tue Aug 08, 2023 8:47 pm
Okay, this got me thinking. What kind of star would you actually get from 13 solar masses?
Of course, it depends on how evolved or non-evolved the star is, but here goes anyway:
A blue, not terrifically evolved 13 solar mass star could be like Gamma Cassiopeiae.
The spectrum of this massive star matches a stellar classification of B0.5 IVe. A luminosity class of IV identifies it as a subgiant star that has reached a stage of its evolution where it is exhausting the supply of hydrogen in its core region and transforming into a giant star. The 'e' suffix is used for stars that show emission lines of hydrogen in the spectrum, caused in this case by the circumstellar disk. This places it among a category known as Be stars; in fact, the first such star ever to be so designated.[16] It has 17 times the Sun's mass and is radiating as much energy as 34,000 Suns.
17 times the Sun's mass? Okay, but in the "information box" at right, under "Details", it says that the primary star of Gamma Cas has a mass of 13 solar.
Right! But what will a highly evolved red star of 13 solar masses look like? How about... Antares?
Antares is a red supergiant star with a stellar classification of M1.5Iab-Ib, and is indicated to be a spectral standard for that class.[6] Due to the nature of the star, the derived parallax measurements have large errors, so that the true distance of Antares is approximately 550 light-years (170 parsecs) from the Sun.[3]
The brightness of Antares at visual wavelengths is about 10,000 times that of the Sun, but because the star radiates a considerable part of its energy in the infrared part of the spectrum, the true bolometric luminosity is around 100,000 times that of the Sun. There is a large margin of error assigned to values for the bolometric luminosity, typically 30% or more.
... The mass of the star has been calculated to be about 12 M☉,[11] or 11 to 14.3 M☉.[10] Comparison of the effective temperature and luminosity of Antares to theoretical evolutionary tracks for massive stars suggest a progenitor mass of 17 M☉ and an age of 12 million years (MYr),[11] or an initial mass of 15 M☉ and an age of 11 to 15 MYr.
Well, that's good enough, I think. A 13 solar mass star would give you something like blazingly hot, 34,000 L☉ Gamma Cas, or cool red supergiant star Antares, with a luminosity of some 100,000 L☉.
It would be something way, way, way different than the combined luminosity of 13 stars like the Sun!
Ann
Yes. I think fusion energy production in stars rises as the fourth power of mass. But I'm sure there's a more accurate way of stating it.
Re: APOD: Moon Meets Jupiter (2023 Aug 08)
Posted: Wed Aug 09, 2023 4:41 am
by Ann
johnnydeep wrote: ↑Tue Aug 08, 2023 8:56 pm
Well, that's good enough, I think. A 13 solar mass star would give you something like blazingly hot, 34,000 L☉ Gamma Cas, or cool red supergiant star Antares, with a luminosity of some 100,000 L☉.
It would be something way, way, way different than the combined luminosity of 13 stars like the Sun!
Ann
Yes. I think fusion energy production in stars rises as the fourth power of mass. But I'm sure there's a more accurate way of stating it.
[/quote]
I'm sure there is a mathematical formula to state it. But I can't do math, so I must try to use words to paint a picture in my mind that I can understand.
It has to do with how stellar fusion works, and what part quantum tunneling plays in the process.
The greatest source of concentrated energy in the Universe today is starlight, where the largest single objects in the Universe emit tremendous amounts of power through the smallest of processes: the nuclear fusion of subatomic particles.
...
How does this happen? Deep inside the hearts of stars — including in our own Sun's core — light elements are fused together under extreme conditions into heavier ones. At temperatures over about 4 million kelvin and at densities more than ten times that of solid lead, hydrogen nuclei (single protons) can fuse together in a chain reaction to form helium nuclei (two protons and two neutrons), releasing a tremendous amount of energy in the process.
All told, by looking at the power output of the Sun, we measure that it emits a continuous 4 × 1026 Watts, which means that inside the Sun's core, a whopping 4 × 1038 protons fuse into helium-4 every second.
If you consider that there are some 1057 particles in the entire Sun, of which a little less than 10% are in the core, this might not sound so far-fetched. After all:
These particles are moving around with tremendous energies:
*each proton has a speed of around 500 km/s in the center of the Sun's core.
*The density is tremendous, and so particle collisions happen extremely frequently: each proton collides with another proton billions of times each second.
*And so it would only take a tiny fraction of these proton-proton interactions resulting in fusion into deuterium -- about 1-in-1028 -- to produce the necessary energy of the Sun.
So we do our calculations, we calculate how the protons in the Sun's core have their energy distributed, and we come up with a number for these proton-proton collisions with sufficient energy to undergo nuclear fusion.
That number is exactly zero. The electric repulsion between the two positively charged particles is too great for even a single pair of protons to overcome it and fuse together with the energies in the Sun's core.
I have quoted so much already, so I'd better try to summarize how Ethan Siegel explains the fact that nuclear fusion in the Sun can happen at all. It has to do with the fact that in the quantum world, particles are not just particles, but they have wave properties, too. And because of their wave properties, their positions can overlap.
So, while the electromagnetic force makes protons repel each other extremely strongly, the protons' wave properties will on extremely rare occasions make their positions overlap so that the strong nuclear force can "grab hold" of the two of them and make them stick together (and simultaneously turning one of them into a neutron). From there, more reactions can make the two proton-neutron pairs join to become a helium core.
And emitting a gamma ray photon in the process. This gamma ray photon will eventually - after spending perhaps a million years making the journey from the core of the Sun to its photosphere - leave the Sun as a large number of much less energetic photons. Or, in other words, the gamma ray photon that was created as a byproduct of core nuclear fusion will leave the Sun's visible "surface" as light and heat.
The quantum tunneling that makes two protons overlap happens rarely indeed:
And this also explains why the energy production in stars rises as the fourth power of mass. Because the stellar core clearly gets that much hotter and denser and that much more conducive to quantum tunneling as a function of increasing stellar mass.
Rauf wrote: ↑Tue Aug 08, 2023 12:18 pm
I think I've found the name of this star, between Ganymede and Jupiter ( I still think that's Ganymede to the right of Jupiter, not Europa, and there's been a mistake)
I checked HD 13873. It appears to be a G0-type star, very slightly hotter and bluer than the Sun, but considerably brighter - 13.5 times brighter than the Sun to be reasonably exact.
Just imagine having 13 stars like the Sun in the sky!!! Or, okay, one star that is worth 13 Suns, at least when it comes to luminosity. Absolutely not when it comes to mass!
13 Suns in the sky.png
Ann
Where I live, daily temperatures rise as high as 48° Celsius with our own Sun, so thank you, I don't wanna imagine HD 13873 in place of the sun
Re: APOD: Moon Meets Jupiter (2023 Aug 08)
Posted: Wed Aug 09, 2023 12:44 pm
by johnnydeep
Ann said (after presenting an explanation of the process of fusion found on Forbes.com):
And this also explains why the energy production in stars rises as the fourth power of mass. Because the stellar core clearly gets that much hotter and denser and that much more conducive to quantum tunneling as a function of increasing stellar mass.
Well, why it's the fourth power specifically is still not clear to me. Clearly, the fusion frequency has to be a function of core density and temperature, but why does it end up being the fourth power of mass? Double the mass and 16 times more fusions per second happen? Well, let's see: double the mass at the same core density and temp would only get you double the fusions. That would only be equivalent to having to two identical stars. So where is the extra factor of 8 coming from? Double the density, and double the temperature and double the ... ???
Re: APOD: Moon Meets Jupiter (2023 Aug 08)
Posted: Wed Aug 09, 2023 1:57 pm
by Ann
johnnydeep wrote: ↑Wed Aug 09, 2023 12:44 pm
Ann said (after presenting an explanation of the process of fusion found on Forbes.com):
And this also explains why the energy production in stars rises as the fourth power of mass. Because the stellar core clearly gets that much hotter and denser and that much more conducive to quantum tunneling as a function of increasing stellar mass.
Well, why it's the fourth power specifically is still not clear to me. Clearly, the fusion frequency has to be a function of core density and temperature, but why does it end up being the fourth power of mass? Double the mass and 16 times more fusions per second happen? Well, let's see: double the mass at the same core density and temp would only get you double the fusions. That would only be equivalent to having to two identical stars. So where is the extra factor of 8 coming from? Double the density, and double the temperature and double the ... ???
My complete amateur guess is that it has all to do with the efficiency of the quantum tunneling. If the mass of a star is higher and the density is higher, then it is likely that the probability of quantum tunneling goes up by a factor of the fourth power of the extra mass.
Why would it be so? Well, why not?
The quantum world is not like the macro world. Why does the number 1/137 turn up everywhere in quantum physics? No one knows. So why wouldn't hydrogen fusion in stars increase or decrease as a function of the fourth power of the stellar mass?
Click to play embedded YouTube video.
Ann
Re: APOD: Moon Meets Jupiter (2023 Aug 08)
Posted: Wed Aug 09, 2023 2:15 pm
by Chris Peterson
johnnydeep wrote: ↑Wed Aug 09, 2023 12:44 pm
Ann said (after presenting an explanation of the process of fusion found on Forbes.com):
And this also explains why the energy production in stars rises as the fourth power of mass. Because the stellar core clearly gets that much hotter and denser and that much more conducive to quantum tunneling as a function of increasing stellar mass.
Well, why it's the fourth power specifically is still not clear to me. Clearly, the fusion frequency has to be a function of core density and temperature, but why does it end up being the fourth power of mass? Double the mass and 16 times more fusions per second happen? Well, let's see: double the mass at the same core density and temp would only get you double the fusions. That would only be equivalent to having to two identical stars. So where is the extra factor of 8 coming from? Double the density, and double the temperature and double the ... ???
I don't believe that this "fourth power relationship" exists. The rate of fusion depends on temperature and on density, but neither of these has a fixed relationship to stellar mass. There is an empirical relationship between mass and luminosity for stars, and it is defined by a power law. But the exponent varies considerably depending on where the star lies in its evolution. For most main sequence stars the exponent is about 3.5 (which may be where this fourth-power idea is coming from), but it ranges from 1 to 4 across the range of stellar masses. This is because there's a lot more going on than just mass. Giant stars, for instance, can have very low densities despite very large masses. Some stars transport energy by convection, some don't, which has a huge impact on internal temperature.
johnnydeep wrote: ↑Wed Aug 09, 2023 12:44 pm
Ann said (after presenting an explanation of the process of fusion found on Forbes.com):
And this also explains why the energy production in stars rises as the fourth power of mass. Because the stellar core clearly gets that much hotter and denser and that much more conducive to quantum tunneling as a function of increasing stellar mass.
Well, why it's the fourth power specifically is still not clear to me. Clearly, the fusion frequency has to be a function of core density and temperature, but why does it end up being the fourth power of mass? Double the mass and 16 times more fusions per second happen? Well, let's see: double the mass at the same core density and temp would only get you double the fusions. That would only be equivalent to having to two identical stars. So where is the extra factor of 8 coming from? Double the density, and double the temperature and double the ... ???
I don't believe that this "fourth power relationship" exists. The rate of fusion depends on temperature and on density, but neither of these has a fixed relationship to stellar mass. There is an empirical relationship between mass and luminosity for stars, and it is defined by a power law. But the exponent varies considerably depending on where the star lies in its evolution. For most main sequence stars the exponent is about 3.5 (which may be where this fourth-power idea is coming from), but it ranges from 1 to 4 across the range of stellar masses. This is because there's a lot more going on than just mass. Giant stars, for instance, can have very low densities despite very large masses. Some stars transport energy by convection, some don't, which has a huge impact on internal temperature.
Thanks, that makes sense. So, it's just not as simple a relationship as say, volume increasing as the third power of radius, or gravitational force increasing as the reciprocal of the square of distance, or even the Stefan-Boltzmann Law, which is where I think I might have gotten the idea from:
The thermal energy radiated by a blackbody radiator per second per unit area is proportional to the fourth power of the absolute temperature and is given by...
johnnydeep wrote: ↑Wed Aug 09, 2023 12:44 pm
Ann said (after presenting an explanation of the process of fusion found on Forbes.com):
And this also explains why the energy production in stars rises as the fourth power of mass. Because the stellar core clearly gets that much hotter and denser and that much more conducive to quantum tunneling as a function of increasing stellar mass.
Well, why it's the fourth power specifically is still not clear to me. Clearly, the fusion frequency has to be a function of core density and temperature, but why does it end up being the fourth power of mass? Double the mass and 16 times more fusions per second happen? Well, let's see: double the mass at the same core density and temp would only get you double the fusions. That would only be equivalent to having to two identical stars. So where is the extra factor of 8 coming from? Double the density, and double the temperature and double the ... ???
My complete amateur guess is that it has all to do with the efficiency of the quantum tunneling. If the mass of a star is higher and the density is higher, then it is likely that the probability of quantum tunneling goes up by a factor of the fourth power of the extra mass.
Why would it be so? Well, why not?
The quantum world is not like the macro world. Why does the number 1/137 turn up everywhere in quantum physics? No one knows. So why wouldn't hydrogen fusion in stars increase or decrease as a function of the fourth power of the stellar mass?
...
Ann
Well, I could also ask, why not the fifth power, or the eighth, right?
johnnydeep wrote: ↑Wed Aug 09, 2023 12:44 pm
Ann said (after presenting an explanation of the process of fusion found on Forbes.com):
Well, why it's the fourth power specifically is still not clear to me. Clearly, the fusion frequency has to be a function of core density and temperature, but why does it end up being the fourth power of mass? Double the mass and 16 times more fusions per second happen? Well, let's see: double the mass at the same core density and temp would only get you double the fusions. That would only be equivalent to having to two identical stars. So where is the extra factor of 8 coming from? Double the density, and double the temperature and double the ... ???
My complete amateur guess is that it has all to do with the efficiency of the quantum tunneling. If the mass of a star is higher and the density is higher, then it is likely that the probability of quantum tunneling goes up by a factor of the fourth power of the extra mass.
Why would it be so? Well, why not?
The quantum world is not like the macro world. Why does the number 1/137 turn up everywhere in quantum physics? No one knows. So why wouldn't hydrogen fusion in stars increase or decrease as a function of the fourth power of the stellar mass?
...
Ann
Well, I could also ask, why not the fifth power, or the eighth, right?
My complete amateur guess is that it has all to do with the efficiency of the quantum tunneling. If the mass of a star is higher and the density is higher, then it is likely that the probability of quantum tunneling goes up by a factor of the fourth power of the extra mass.
Why would it be so? Well, why not?
The quantum world is not like the macro world. Why does the number 1/137 turn up everywhere in quantum physics? No one knows. So why wouldn't hydrogen fusion in stars increase or decrease as a function of the fourth power of the stellar mass?
...
Ann
Well, I could also ask, why not the fifth power, or the eighth, right?
I don't consider a fourth power law (more like a 3.5 power law as Chris explained) to be at the same fundamental level as that 1/137 number, or that c is 299792 km/s, etc. Those facts DO seem arbitrary, at least until we discover a still more fundamental explanation. But the equation that describes a star's output as a function of mass, age, composition, etc., no doubt exists and has a clear mathematical explanation, at least for those well versed in astrophysics, which sadly, I am not.