<p>I think it's a fun question, if impossible. The only way to turn Jupiter into a star that's even remotely practical is to add to it's mass. Ignoring brown dwarfs that are very limited in energy output, to get a red dwarf going, you'd need to add at least 75-80 or so Jupiter masses (a bit more than 24,000 earth masses). You'd want to add a fair percentage of hydrogen, but some rocky debris wouldn't hurt the mix.</p>
<p>Anyway, assuming the impossible is done, there are several things to consider. The greater gravity (75-80 times) would significantly alter all the planets' orbits. Predicting exactly how is hard, but that much more mass, and the planets' orbits - certainly all the inner ones - would wobble a lot more, and some might get pulled completely out of their orbit, likely thrown out of the solar system.</p>
<p>You might think that the planets nearer to Jupiter would be the most affected, but it really has more to do with tidal synch than anything else. Any of the 4 inner planets could get tugged into a new orbit. You'd also likely see the earth's orbit elongate in resonance with Jupiter, perhaps increasing the ice age/ice melt cycle. Precise answers are hard, and none of these things would happen over 1 orbit, but over time, certainly. Orbital changes to all the inner planets and perhaps Saturn as well would be inevitable if Jupiter becomes a red dwarf. Imagine if Saturn was pulled closer to the earth, into an orbit between Mars and Jupiter, or Mercury was pulled out past the earth.  Odds are it wouldn't hit us, but we might want to keep an eye on it.</p>
<p><a href="http://en.wikipedia.org/wiki/Stability_of_the_Solar_System#Mercury.E2.80.93Jupiter_1:1_resonance" rel="nofollow noreferrer">http://en.wikipedia.org/wiki/Stability_of_the_Solar_System#Mercury.E2.80.93Jupiter_1:1_resonance</a></p>
<p>The second thing to consider is magnetism and solar flares.   Young stars tend to spin very fast due to conservation of angular momentum when the stars form, and this creates enormous magnetic fields and huge solar flares, much bigger than we get from the sun. It's strange to think that a tiny red dwarf, 4 times as far from our sun as the sun would create solar flares to worry about, but it's possible.  Whether it would need high angular momentum for this to happen, I'm not sure, but we could see larger solar flares from star-Jupiter than from the sun.</p>
<p><a href="http://en.wikipedia.org/wiki/Flare_star" rel="nofollow noreferrer">http://en.wikipedia.org/wiki/Flare_star</a></p>
<p>Brightness, heat, and visibility were covered above, but I'll touch on that again. Brightness of -6.25 would be 5-6 times brighter than Venus and you'd see it at night, Venus isn't seen in peak darkness, so it would be significantly brighter than any other star/planet in the sky, but significantly less bright than the moon, like, you couldn't make your way with just that star's light the way you can see things around you in moonlight. But when I run the numbers, I think it would be quite a bit brighter than that.</p>
<p>Mass to luminosity is to the power of 3.5 - quick estimate, so, let's say the red dwarf has a mass of 80 Jupiters.  That's 0.076 Suns. 0.076^3.5 = about 1/8,000, so 4.2 times as far away at closest point (square of that), 1/8000th as bright, we're looking at 1/140,000 times the light we get from the sun - not very much and likely less than that in its early stages and because the smaller stars tend to fall off, so let's estimate 1/200,000 - 1/300,000 the apparent brightness of the sun as a ballpark estimate. That's not enough to heat the earth at all, but that's still brighter, (a little bit) than the full moon which is about 1/400,000 the brightness of the sun. It would be enough light to see your way around, but I wouldn't want to try to read by it.  It would also be distinctly reddish light.  Not the white light we're used to getting from the day or night sky.</p>
<p>Finally size - a red-dwarf star of 80 Jupiter masses would actually be slightly smaller than Jupiter due to the gravitation, so it would appear like a planet - not quite a point in the sky, but almost a point, but a bit brighter than the full moon and red. That's likely bright enough to see during the day too. I don't think it would be hard to look at or hurt your eyes, but it would shine like a tiny bright red flashlight in the distance.</p>
<p><a href="http://www.space.com/21420-smallest-star-size-red-dwarf.html" rel="nofollow noreferrer">http://www.space.com/21420-smallest-star-size-red-dwarf.html</a></p>
<p>I don't think I like star-Jupiter.Let's not plan on doing this.   :-)</p>

CC BY-SA 4.0
2015-05-11T19:45:39.887Z
2022-11-25T10:40:35.737Z
2022-11-25T10:40:35.737Z
userLTK
<p>Ignoring the impossibility of Jupiter going solar:</p>

<p>Assume that Jupiter turns into  duplicate of the Sun in terms of energy output.  Energy transmitted to the earth follows an inverse-square law.  Since Jupiter is, at best, 4 times farther from the Earth than the Sun, Jupiter will supply the Earth with, at most, 1/16 the energy that the Sun supplies, for an increase of a bit more than 6%, at the most.  </p>

<p>By comparison, between aphelion and perihelion, the Sun-Earth distance increases from around 147 million kilometers to around 152 million kilometers.  This implies a seasonal energy input change of about 7%, that we experience now every year... </p>

CC BY-SA 3.0
2017-05-11T19:07:45.537Z
While this is a good answer I can be a lot more concerned about how the change in mass of jupiter can affect orbits of the planets since now it can be a binary system
jean
CC BY-SA 3.0
2014-08-07T20:41:09.753Z
And I'm pretty sure Lucifer's energy output was far less than the Sun's, so the increase would be even smaller.
Keith Thompson
CC BY-SA 3.0
2014-08-06T21:49:48.183Z
2014-08-06T21:49:48.183Z
DJohnM
<p>In reality, Jupiter doesn't have nearly enough mass to initiate stellar ignition or sustain it if we could somehow start it going.</p>

<p>Even the smallest star would require on the order of some 80 to 90 times the mass of Jupiter just to put out a faint red glow. </p>

<p>Even to become a brown dwarf proto-star, Jupiter would require a mass increase on the order of at least 10-fold or so. </p>

<p>Lucifer is simply not possible unless Jupiter collides with something to provide the extra mass it needs to go stellar and even then, it would be a red dwarf at best and quite faint, like a red-hot nail glowing in the dark.</p>

<p>But one can dream.</p>

<p>:)</p>

CC BY-SA 3.0
2014-08-10T16:08:02.983Z
Just a correction to make: a brown dwarf isn't a proto-star; it's a "failed star" - that is, it began as a proto-star but simply didn't have the mass necessary to enter the main sequence. I hate to be a nit-picker, though. +1 for a good, logical explanation.
HDE 226868
CC BY-SA 3.0
2014-08-07T03:01:04.577Z
2014-08-07T03:01:04.577Z
JayT
<ol>
<li>Sun-Earth distance: 1AU    </li>
<li>Earth-Jupiter distance (at the conjunction): 4AU</li>
</ol>

<p>So Lucifer will be four times farther than Sun when it is nearer (six times when it is farthest), and at the same time it is <a href="https://en.wikipedia.org/wiki/Jupiter_mass" rel="noreferrer">a thousand times smaller</a>. This is approx 40 times more light than full moon concentrated in a tiny point on sky.</p>

CC BY-SA 3.0
2014-08-06T19:31:58.150Z
I'm not sure this answers the question. And @Envite, how does your link prove anything?
HDE 226868
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2014-08-07T07:42:13.787Z
@HDE226868 The link is the reference for the mass relation between Sun and Jupiter
Envite
CC BY-SA 3.0
2014-08-07T13:50:38.537Z
Right, @Envite, but mass and size aren't necessarily correlated. And Jupiter still doesn't have anywhere near enough mass to begin fusion.
HDE 226868
CC BY-SA 3.0
2014-08-07T13:53:36.050Z
Look, I feel that the whole exercise is futile. If Jupiter turned into a star - even a red dwarf - we would have a lot of problems with gravity. The solar system would become unstable, and there's a chance that some of the planets would be flung out of the solar system. We can't calculate the energy output because we can only guess at what type of star Jupiter would become, and we can't come up with any definite answer. There are dozens of possibilities; not a single one of them has any more merit than any of the others. Does the book specify?
HDE 226868
CC BY-SA 3.0
2014-08-07T14:34:44.843Z
@HDE226868 Aboslutely False. We will not have any problems with gravity if Jupiter "magically" (as expressed by the OP) becomes a star **with its own mass**.
Envite
CC BY-SA 3.0
2014-08-07T14:51:30.760Z
In order to turn Jupiter into a star, Wikipedia says the monoliths turned themselves into von Neumann machines to increase Jupiter's density. (As a side note, that's not how von Neumann machines work.) Increasing its density would involve compressing it or adding more mass. @Envite, How could you compress a gas giant? You couldn't; you would have to add mass to increase density.
HDE 226868
CC BY-SA 3.0
2014-08-07T17:41:57.870Z
Actually, I should edit that. The monoliths apparently turned Jupiter into a star by increasing its density. That said, Jupiter actually has a very similar density to that of the Sun, so density isn't the issue; that alone should show that the star-forming technique described in the novel is impossible. You would have to add more mass; An object with the mass of Jupiter could not become a star.
HDE 226868
CC BY-SA 3.0
2014-08-07T20:40:09.120Z
@HDE226868: I believe the monoliths *compressed* Jupiter; they didn't appear to add mass (that's certainly what it looked like in the movie). The Sun supports fusion because its core is sufficiently dense, because it has all that mass compressing it gravitationally. Assuming Jupiter's core is mostly hydrogen, it should also support fusion given enough density/pressure, whether that pressure is applied by gravity or by "sufficiently advanced technology" (a.k.a. magic). I don't know whether continuing intervention would be needed to keep it from re-expanding.
Keith Thompson
CC BY-SA 3.0
2014-08-07T20:45:30.153Z
@KeithThompson, Jupiter has a rocky core. Also, to compress a gas giant, you would have to make sure it was surrounded in a large, closed space, with absolutely no gaps. Just a wee bit unlikely. But, more importantly, the only hydrogen in Jupiter's core is in layers on the outside of it. It is predominately made of other materials. That said, if there was enough hydrogen in Jupiter's core, then I suppose you could be right. Interesting.
HDE 226868
CC BY-SA 3.0
2014-08-11T08:42:23.007Z
Please, double check the question. It is not about if it is possible or not. It is about what would happen if it happens (by unspecified methods). Discussions about possible or impossible are **out of the question**.
Envite
CC BY-SA 3.0
2014-08-11T13:02:25.653Z
Sorry, @Envite. My objection stemmed from the fact that Lucifer's properties and type would be hard to determine. The mass-luminosity relation could, of course, be used if Lucifer was a main-sequence star, but I had doubts as to whether it could be considered a main-sequence dwarf and not in a separate category of its own (in which case the relation could be non-applicable).
HDE 226868
CC BY-SA 3.0
2014-08-06T15:10:00.223Z
2014-08-06T15:10:00.223Z
Envite
<p>Before I start, I'll admit that I've criticized the question based on its improbability; however, I've been persuaded otherwise. I'm going to try to do the calculations based on completely different formulas than I think have been used; I hope you'll stay with me as I work it out.</p>

<p>Let's imagine that Lucifer becomes a main-sequence star - in fact, let's call it a low-mass red dwarf. Main-sequence stars follow the mass-luminosity relation:</p>

<p>$$\frac{L}{L_\odot} = \left(\frac{M}{M_\odot}\right)^a$$</p>

<p>Where $L$ and $M$ are the star's luminosity and mass, and $L_\odot$ and $M_\odot$ and the luminosity and mass of the Sun. For stars with $M &lt; 0.43M_\odot$, $a$ takes the value of 2.3. Now we can plug in Jupiter's mass ($1.8986 \times 10 ^{27}$ kg) into the formula, as well as the Sun's mass ($1.98855 \times 10 ^ {30}$ kg) and luminosity ($3.846 \times 10 ^ {26}$ watts), and we get </p>

<p>$$\frac{L}{3.846 \times 10 ^ {26}} = \left(\frac{1.8986 \times 10 ^ {27}}{1.98855 \times 10 ^ {30}}\right)^{2.3}$$</p>

<p>This becomes $$L = \left(\frac{1.8986 \times 10 ^ {27}}{1.98855 \times 10 ^ {30}}\right)^{2.3} \times 3.846 \times 10 ^ {26}$$</p>

<p>which then becomes</p>

<p>$$L = 4.35 \times 10 ^ {19}$$ watts.</p>

<p>Now we can work out the apparent brightness of Lucifer, as seen from Earth. For that, we need the formula</p>

<p>$$m = m_\odot - 2.5 \log \left(\frac {L}{L_\odot}\left(\frac {d_\odot}{d}\right) ^ 2\right)$$</p>

<p>where $m$ is the apparent magnitude of the star, $m_\odot$ is the apparent magnitude of the Sun, $d_\odot$ is the distance to the Sun, and $d$ is the distance to the star. Now, $m = -26.73$ and $d(s)$ is 1 (in astronomical units). $d$ varies. Jupiter is about 5.2 AU from the Sun, so at its closest distance to Earth, it would be ~4.2 AU away. We plug these numbers into the formula, and find</p>

<p>$$m = -6.25$$</p>

<p>which is a lot less brighter than the Sun. Now, when Jupiter is farthest away from the Sun, it is ~6.2 AU away. We plug <em>that</em> into the formula, and find</p>

<p>$$m = -5.40$$</p>

<p>which is dimmer still - although, of course, Jupiter would be completely blocked by the Sun. Still, for finding the apparent magnitude of Jupiter at some distance from Earth, we can change the above formula to </p>

<p>$$m = -26.73 - 2.5 \log \left(\frac {4.35 \times 10 ^ {19}}{3.846 \times 10 6 {26}}\left(\frac {1}{d}\right) ^ 2\right)$$</p>

<p>By comparison, the Moon can have an average apparent magnitude of -12.74 at full moon - much brighter than Lucifer. The apparent magnitude of both bodies can, of course, change - Jupiter by transits of its moon, for example - but these are the optimal values.</p>

<p>While the above calculations really don't answer most parts of your question, I hope it helps a bit. And please, correct me if I made a mistake somewhere. LaTeX is by no means my native language, and I could have gotten something wrong.</p>

<p>I hope this helps.</p>

<p><strong>Edit</strong></p>

<p>The combined brightness of Lucifer and the Sun would depend on the angle of the Sun's rays and Lucifer's rays. Remember how we have different seasons because of the tilt of the Earth's axis? Well, the added heat would have to do with the tilt of Earth's and Lucifer's axes relative to one another. I can't give you a numerical result, but I can add that I hope it wouldn't be too much hotter than it is now, as I'm writing this!</p>

<p><strong>Second Edit</strong></p>

<p>Like I said in a comment somewhere on this page, the mass-luminosity relation really only works for main-sequence stars. If Lucifer was not on the main sequence. . . Well, then none of my calculations would be right.</p>

CC BY-SA 3.0
2017-07-08T15:22:47.853Z
@HDE 226868 Just for fun did you think anymore about what it would take to make Europa habitable for the aliens (I know, it depends on the alien). Jupiter couldn't get "too hot" obviously. I love A. C. Clarke, but he did need to ignore science for the sake of the story sometimes (ie. humans wouldn't survive in Jupiter's orbit due to the magnetic field).
Jack R. Woods
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2014-08-11T17:55:40.800Z
It's an interesting answer!  It sounds as thought there would be very little effect in regards to extra light or temperature.
Maelish
CC BY-SA 3.0
2014-08-11T20:42:20.633Z
In answer to the edit you made to your comment: Yep. Not a big difference. At least, not on Earth. An interesting follow-up would be to see if it could indeed cause conditions on Europa to change in favor of life.
HDE 226868
CC BY-SA 3.0
2014-08-11T17:23:36.460Z
2017-05-11T13:57:13.627Z
2017-05-11T13:57:13.627Z
HDE 226868
<p>In Clarke's book 2010, the monolith and its brethren turned Jupiter into the small star nicknamed <code>Lucifer</code>.  Ignoring the reality that we won't have any magical monoliths appearing in our future, what would the effects be on Earth if Jupiter was turned into a star?</p>

<p>At it's closest and furthest:  </p>

<p>How bright would the "back-side" of the earth be with light from Lucifer?  </p>

<p>How much heat would the small star generate on earth?  </p>

<p>How many days or months would we actually have night when we circled away behind the sun?</p>

<p>How much brighter would the sun-side of earth be when Lucifer and the sun both shine on the same side of the planet?</p>

CC BY-SA 3.0
2016-09-10T21:44:27.353Z
One who looks for a good physics expert’s opinion about that, look [here](http://physics.stackexchange.com/a/34624). Note: Ī̲ don’t advertise that site as a whole, only this particular posting.
Incnis Mrsi
CC BY-SA 3.0
2017-10-13T22:57:45.700Z
Jupiter can burn as brightly as you want it to depending on how much mass you add to it. If you somehow put a very massive core at the center of Jupiter, the total mass of the system would determine how much fusion can take place. It can probably range from a supernova if you put in a neutron star just below the Chandrasekhar limit inside to a very weak red dwarf if you just add enough mass to make fusion start.
A. C. A. C.
CC BY-SA 4.0
2018-06-14T00:22:11.530Z
How do you know we won't have any "magical" monoliths appearing in the future? It's as good of a scenario of first contact as any.
Jack R. Woods
CC BY-SA 3.0
2014-08-06T17:06:20.383Z
While this is an interesting question, I don't know if there's a proper way to answer it. Jupiter's mass is far less that that of the smallest brown dwarfs, also dubbed "failed stars". Brown dwarfs don't have enough mass to sustain hydrogen fusion, and don't emit a whole lot of light. I don't think that there's any way that you could realistically do the calculations for a Jupiter-star scenario, because of the impossibility of it beginning hydrogen fusion. Still, it's an interesting idea.
HDE 226868
CC BY-SA 3.0
2014-08-11T18:15:25.340Z
Okay, I relent. +1 for an interesting idea.
HDE 226868
<star><the-sun><light><jupiter><heat>
What would the effects be on Earth if Jupiter was turned into a star?
Maelish
# What would the effects be on Earth if Jupiter was turned into a star?

(Asked by: username_0 on 2014-08-06)

In Clarke's book 2010, the monolith and its brethren turned Jupiter into the small star nicknamed `Lucifer`. Ignoring the reality that we won't have any magical monoliths appearing in our future, what would the effects be on Earth if Jupiter was turned into a star?

At it's closest and furthest:

How bright would the "back-side" of the earth be with light from Lucifer?

How much heat would the small star generate on earth?

How many days or months would we actually have night when we circled away behind the sun?

How much brighter would the sun-side of earth be when Lucifer and the sun both shine on the same side of the planet?

**Tags:** star, the-sun, light, jupiter, heat
> **Comments:**
> **username_4**: While this is an interesting question, I don't know if there's a proper way to answer it. Jupiter's mass is far less that that of the smallest brown dwarfs, also dubbed "failed stars". Brown dwarfs don't have enough mass to sustain hydrogen fusion, and don't emit a whole lot of light. I don't think that there's any way that you could realistically do the calculations for a Jupiter-star scenario, because of the impossibility of it beginning hydrogen fusion. Still, it's an interesting idea.  
> **username_4**: Okay, I relent. +1 for an interesting idea.  
> **username_1**: One who looks for a good physics expert’s opinion about that, look [here](http://physics.stackexchange.com/a/34624). Note: Ī̲ don’t advertise that site as a whole, only this particular posting.  
> **username_2**: Jupiter can burn as brightly as you want it to depending on how much mass you add to it. If you somehow put a very massive core at the center of Jupiter, the total mass of the system would determine how much fusion can take place. It can probably range from a supernova if you put in a neutron star just below the Chandrasekhar limit inside to a very weak red dwarf if you just add enough mass to make fusion start.  
> **username_3**: How do you know we won't have any "magical" monoliths appearing in the future? It's as good of a scenario of first contact as any.  

---

## 5 answers

### Answer by username_4 on 2014-08-11. Score: 14 | Accepted answer

Before I start, I'll admit that I've criticized the question based on its improbability; however, I've been persuaded otherwise. I'm going to try to do the calculations based on completely different formulas than I think have been used; I hope you'll stay with me as I work it out.

Let's imagine that Lucifer becomes a main-sequence star - in fact, let's call it a low-mass red dwarf. Main-sequence stars follow the mass-luminosity relation:

$$\frac{L}{L\_\odot} = \left(\frac{M}{M\_\odot}\right)^a$$

Where $L$ and $M$ are the star's luminosity and mass, and $L\_\odot$ and $M\_\odot$ and the luminosity and mass of the Sun. For stars with $M < 0.43M\_\odot$, $a$ takes the value of 2.3. Now we can plug in Jupiter's mass ($1.8986 \times 10 ^{27}$ kg) into the formula, as well as the Sun's mass ($1.98855 \times 10 ^ {30}$ kg) and luminosity ($3.846 \times 10 ^ {26}$ watts), and we get

$$\frac{L}{3.846 \times 10 ^ {26}} = \left(\frac{1.8986 \times 10 ^ {27}}{1.98855 \times 10 ^ {30}}\right)^{2.3}$$

This becomes $$L = \left(\frac{1.8986 \times 10 ^ {27}}{1.98855 \times 10 ^ {30}}\right)^{2.3} \times 3.846 \times 10 ^ {26}$$

which then becomes

$$L = 4.35 \times 10 ^ {19}$$ watts.

Now we can work out the apparent brightness of Lucifer, as seen from Earth. For that, we need the formula

$$m = m\_\odot - 2.5 \log \left(\frac {L}{L\_\odot}\left(\frac {d\_\odot}{d}\right) ^ 2\right)$$

where $m$ is the apparent magnitude of the star, $m\_\odot$ is the apparent magnitude of the Sun, $d\_\odot$ is the distance to the Sun, and $d$ is the distance to the star. Now, $m = -26.73$ and $d(s)$ is 1 (in astronomical units). $d$ varies. Jupiter is about 5.2 AU from the Sun, so at its closest distance to Earth, it would be ~4.2 AU away. We plug these numbers into the formula, and find

$$m = -6.25$$

which is a lot less brighter than the Sun. Now, when Jupiter is farthest away from the Sun, it is ~6.2 AU away. We plug *that* into the formula, and find

$$m = -5.40$$

which is dimmer still - although, of course, Jupiter would be completely blocked by the Sun. Still, for finding the apparent magnitude of Jupiter at some distance from Earth, we can change the above formula to

$$m = -26.73 - 2.5 \log \left(\frac {4.35 \times 10 ^ {19}}{3.846 \times 10 6 {26}}\left(\frac {1}{d}\right) ^ 2\right)$$

By comparison, the Moon can have an average apparent magnitude of -12.74 at full moon - much brighter than Lucifer. The apparent magnitude of both bodies can, of course, change - Jupiter by transits of its moon, for example - but these are the optimal values.

While the above calculations really don't answer most parts of your question, I hope it helps a bit. And please, correct me if I made a mistake somewhere. LaTeX is by no means my native language, and I could have gotten something wrong.

I hope this helps.

**Edit**

The combined brightness of Lucifer and the Sun would depend on the angle of the Sun's rays and Lucifer's rays. Remember how we have different seasons because of the tilt of the Earth's axis? Well, the added heat would have to do with the tilt of Earth's and Lucifer's axes relative to one another. I can't give you a numerical result, but I can add that I hope it wouldn't be too much hotter than it is now, as I'm writing this!

**Second Edit**

Like I said in a comment somewhere on this page, the mass-luminosity relation really only works for main-sequence stars. If Lucifer was not on the main sequence. . . Well, then none of my calculations would be right.

> **Comments:**
> **username_0**: It's an interesting answer!  It sounds as thought there would be very little effect in regards to extra light or temperature.  
> **username_4**: In answer to the edit you made to your comment: Yep. Not a big difference. At least, not on Earth. An interesting follow-up would be to see if it could indeed cause conditions on Europa to change in favor of life.  
> **username_3**: @HDE 226868 Just for fun did you think anymore about what it would take to make Europa habitable for the aliens (I know, it depends on the alien). Jupiter couldn't get "too hot" obviously. I love A. C. Clarke, but he did need to ignore science for the sake of the story sometimes (ie. humans wouldn't survive in Jupiter's orbit due to the magnetic field).  

---

### Answer by username_5 on 2015-05-11. Score: 8

I think it's a fun question, if impossible. The only way to turn Jupiter into a star that's even remotely practical is to add to it's mass. Ignoring brown dwarfs that are very limited in energy output, to get a red dwarf going, you'd need to add at least 75-80 or so Jupiter masses (a bit more than 24,000 earth masses). You'd want to add a fair percentage of hydrogen, but some rocky debris wouldn't hurt the mix.

Anyway, assuming the impossible is done, there are several things to consider. The greater gravity (75-80 times) would significantly alter all the planets' orbits. Predicting exactly how is hard, but that much more mass, and the planets' orbits - certainly all the inner ones - would wobble a lot more, and some might get pulled completely out of their orbit, likely thrown out of the solar system.

You might think that the planets nearer to Jupiter would be the most affected, but it really has more to do with tidal synch than anything else. Any of the 4 inner planets could get tugged into a new orbit. You'd also likely see the earth's orbit elongate in resonance with Jupiter, perhaps increasing the ice age/ice melt cycle. Precise answers are hard, and none of these things would happen over 1 orbit, but over time, certainly. Orbital changes to all the inner planets and perhaps Saturn as well would be inevitable if Jupiter becomes a red dwarf. Imagine if Saturn was pulled closer to the earth, into an orbit between Mars and Jupiter, or Mercury was pulled out past the earth. Odds are it wouldn't hit us, but we might want to keep an eye on it.

<http://en.wikipedia.org/wiki/Stability_of_the_Solar_System#Mercury.E2.80.93Jupiter_1:1_resonance>

The second thing to consider is magnetism and solar flares. Young stars tend to spin very fast due to conservation of angular momentum when the stars form, and this creates enormous magnetic fields and huge solar flares, much bigger than we get from the sun. It's strange to think that a tiny red dwarf, 4 times as far from our sun as the sun would create solar flares to worry about, but it's possible. Whether it would need high angular momentum for this to happen, I'm not sure, but we could see larger solar flares from star-Jupiter than from the sun.

<http://en.wikipedia.org/wiki/Flare_star>

Brightness, heat, and visibility were covered above, but I'll touch on that again. Brightness of -6.25 would be 5-6 times brighter than Venus and you'd see it at night, Venus isn't seen in peak darkness, so it would be significantly brighter than any other star/planet in the sky, but significantly less bright than the moon, like, you couldn't make your way with just that star's light the way you can see things around you in moonlight. But when I run the numbers, I think it would be quite a bit brighter than that.

Mass to luminosity is to the power of 3.5 - quick estimate, so, let's say the red dwarf has a mass of 80 Jupiters. That's 0.076 Suns. 0.076^3.5 = about 1/8,000, so 4.2 times as far away at closest point (square of that), 1/8000th as bright, we're looking at 1/140,000 times the light we get from the sun - not very much and likely less than that in its early stages and because the smaller stars tend to fall off, so let's estimate 1/200,000 - 1/300,000 the apparent brightness of the sun as a ballpark estimate. That's not enough to heat the earth at all, but that's still brighter, (a little bit) than the full moon which is about 1/400,000 the brightness of the sun. It would be enough light to see your way around, but I wouldn't want to try to read by it. It would also be distinctly reddish light. Not the white light we're used to getting from the day or night sky.

Finally size - a red-dwarf star of 80 Jupiter masses would actually be slightly smaller than Jupiter due to the gravitation, so it would appear like a planet - not quite a point in the sky, but almost a point, but a bit brighter than the full moon and red. That's likely bright enough to see during the day too. I don't think it would be hard to look at or hurt your eyes, but it would shine like a tiny bright red flashlight in the distance.

<http://www.space.com/21420-smallest-star-size-red-dwarf.html>

I don't think I like star-Jupiter.Let's not plan on doing this. :-)


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### Answer by username_6 on 2014-08-06. Score: 7

Ignoring the impossibility of Jupiter going solar:

Assume that Jupiter turns into duplicate of the Sun in terms of energy output. Energy transmitted to the earth follows an inverse-square law. Since Jupiter is, at best, 4 times farther from the Earth than the Sun, Jupiter will supply the Earth with, at most, 1/16 the energy that the Sun supplies, for an increase of a bit more than 6%, at the most.

By comparison, between aphelion and perihelion, the Sun-Earth distance increases from around 147 million kilometers to around 152 million kilometers. This implies a seasonal energy input change of about 7%, that we experience now every year...

> **Comments:**
> **username_8**: And I'm pretty sure Lucifer's energy output was far less than the Sun's, so the increase would be even smaller.  
> **username_7**: While this is a good answer I can be a lot more concerned about how the change in mass of jupiter can affect orbits of the planets since now it can be a binary system  

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### Answer by username_9 on 2014-08-07. Score: 6

In reality, Jupiter doesn't have nearly enough mass to initiate stellar ignition or sustain it if we could somehow start it going.

Even the smallest star would require on the order of some 80 to 90 times the mass of Jupiter just to put out a faint red glow.

Even to become a brown dwarf proto-star, Jupiter would require a mass increase on the order of at least 10-fold or so.

Lucifer is simply not possible unless Jupiter collides with something to provide the extra mass it needs to go stellar and even then, it would be a red dwarf at best and quite faint, like a red-hot nail glowing in the dark.

But one can dream.

:)

> **Comments:**
> **username_4**: Just a correction to make: a brown dwarf isn't a proto-star; it's a "failed star" - that is, it began as a proto-star but simply didn't have the mass necessary to enter the main sequence. I hate to be a nit-picker, though. +1 for a good, logical explanation.  

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### Answer by username_10 on 2014-08-06. Score: 5

1. Sun-Earth distance: 1AU
2. Earth-Jupiter distance (at the conjunction): 4AU

So Lucifer will be four times farther than Sun when it is nearer (six times when it is farthest), and at the same time it is [a thousand times smaller](https://en.wikipedia.org/wiki/Jupiter_mass). This is approx 40 times more light than full moon concentrated in a tiny point on sky.

> **Comments:**
> **username_4**: I'm not sure this answers the question. And @Envite, how does your link prove anything?  
> **username_10**: @HDE226868 The link is the reference for the mass relation between Sun and Jupiter  
> **username_4**: Right, @Envite, but mass and size aren't necessarily correlated. And Jupiter still doesn't have anywhere near enough mass to begin fusion.  
> **username_4**: Look, I feel that the whole exercise is futile. If Jupiter turned into a star - even a red dwarf - we would have a lot of problems with gravity. The solar system would become unstable, and there's a chance that some of the planets would be flung out of the solar system. We can't calculate the energy output because we can only guess at what type of star Jupiter would become, and we can't come up with any definite answer. There are dozens of possibilities; not a single one of them has any more merit than any of the others. Does the book specify?  
> **username_10**: @HDE226868 Aboslutely False. We will not have any problems with gravity if Jupiter "magically" (as expressed by the OP) becomes a star **with its own mass**.  
> **username_4**: In order to turn Jupiter into a star, Wikipedia says the monoliths turned themselves into von Neumann machines to increase Jupiter's density. (As a side note, that's not how von Neumann machines work.) Increasing its density would involve compressing it or adding more mass. @Envite, How could you compress a gas giant? You couldn't; you would have to add mass to increase density.  
> **username_4**: Actually, I should edit that. The monoliths apparently turned Jupiter into a star by increasing its density. That said, Jupiter actually has a very similar density to that of the Sun, so density isn't the issue; that alone should show that the star-forming technique described in the novel is impossible. You would have to add more mass; An object with the mass of Jupiter could not become a star.  
> **username_8**: @HDE226868: I believe the monoliths *compressed* Jupiter; they didn't appear to add mass (that's certainly what it looked like in the movie). The Sun supports fusion because its core is sufficiently dense, because it has all that mass compressing it gravitationally. Assuming Jupiter's core is mostly hydrogen, it should also support fusion given enough density/pressure, whether that pressure is applied by gravity or by "sufficiently advanced technology" (a.k.a. magic). I don't know whether continuing intervention would be needed to keep it from re-expanding.  
> **username_4**: @KeithThompson, Jupiter has a rocky core. Also, to compress a gas giant, you would have to make sure it was surrounded in a large, closed space, with absolutely no gaps. Just a wee bit unlikely. But, more importantly, the only hydrogen in Jupiter's core is in layers on the outside of it. It is predominately made of other materials. That said, if there was enough hydrogen in Jupiter's core, then I suppose you could be right. Interesting.  
> **username_10**: Please, double check the question. It is not about if it is possible or not. It is about what would happen if it happens (by unspecified methods). Discussions about possible or impossible are **out of the question**.  
> **username_4**: Sorry, @Envite. My objection stemmed from the fact that Lucifer's properties and type would be hard to determine. The mass-luminosity relation could, of course, be used if Lucifer was a main-sequence star, but I had doubts as to whether it could be considered a main-sequence dwarf and not in a separate category of its own (in which case the relation could be non-applicable).  

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