# Is the Sun really burning and getting smaller?

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Is the Sun really burning and getting smaller? If so, because there is no air in space, what can be keeping it burning and what materials get burnt? If no air is required, what can speed up or can stop the burning process?

The only other detail I can add to what has already been said above is about the actual fusion process inside he Sun - its engine.

4H -> He + neutrinos + gamma-ray photons

Mass of 4H = 6.693e-27 Kg Mass of He = 6.645e-27 Kg

Mass lost per single fusion reaction = 0.048e-27 Kg

This generates 4.3e-12 J

If 1Kg of H is converted into He, a mass of 0.007 kg (0.7% of total initial mass) is lost in the process, generated 6.3e14 J. This is equivalent to 20e6 kg of coal.

We know that the Sun's luminosity is 3.9e26 J/s which when divided by the energy/kg defined above gives us the mass of H converted per second: 600e12 Kg.

The Sun is not burning, but undergoing nuclear fusion at the core of the star. There, under great pressure, and high temperature, Hydrogen is converted into Helium and energy (light).

If you recall Einstein's equation $E = mc^2$, energy is equivalent to mass, meaning a small amount of mass can be converted into a large amount of energy.

The actual process of converting the Hydrogen to Helium is complicated.

I won't bother answering how the Sun "burns" its fuel - that is already answered in this question. Your second question is whether the Sun is getting smaller; the answer depends on what you mean by "smaller". We'll analyze this in two ways:

If we define the radius of the Sun by distance between the core and photosphere, then the Sun is not getting smaller. In fact, it is growing over time, as demonstrated in this graph:

Image courtesy of Ribas (2009), colored by user RJHall on Wikipedia under the Creative Commons Attribution-Share Alike 3.0 Unported license

The height of the photosphere is increasing ever so slightly, even in the Sun's main sequence stage; specifically, Table 3 of Sackmann et al. (1993) states that at the Sun's zero-age main sequence, it was $0.9R_odot$ in radius, and at its terminal age main sequence it will be $1.6R_odot$. That means that, over the course of about 10.9 billion years, the Sun's radius will have increased by $0.7R_odot$ - about 70% its current radius.

If we do the math $left (0.7R_odot over 10.9 ext{Gyr} ight )$, we could find that the Sun's average increase in radius (from beginning to end of main sequence) is about 4.5 centimeters each year (1.77 inches). This calculation shouldn't be taken too seriously, since the growth rate is not linear, but it demonstrates how slowly the Sun has been growing.

The Sun will continue to grow at a very slow rate until it leaves its main sequence stage. Once it enters the red giant branch, it will grow many times its current size, to approximately 1 AU.

Mass

The Sun is losing mass for two reasons: the nuclear fusion in its core converts some mass into energy, and high energy protons and electrons are constantly ejected from the Sun's corona as solar winds. Wood et al. (2002) shows that the Sun's mass-loss rate $dot{M}$ is (approximately) inversely proportional to the square of its age $t$:

$$dot{M} propto t^{-2.00 pm 0.52}$$

As of now, $dot{M} = 10^{-14}frac{M_odot}{mathrm{yr}}$. After the Sun leaves the main sequence, it will lose its mass at a much faster rate. As stated in Sackermann et al., the rate at which it a post-main sequence star loses mass can be generalized with Reimers' law:

$$dot{M} = -eta (4 imes 10^{-13}) frac{LR}{M}$$

where $eta$ is a value depending on the star (derived to be ~0.6 for the Sun), $M$, $L$ and $R$ are in solar units, and $dot{M}$ is in solar masses per year. This is simplified, but gives us an idea of how the Sun's phase affects its mass-loss rate.

By the time the Sun enters the red giant branch, it will have only 72.5% its current mass, and its luminosity and radius will reach $2300L_odot$ and $170R_odot$, respectively. Plugging those values in, we find that the rate of mass loss will be $1.3 imes 10^{-7} frac{M_odot}{mathrm{yr}}$. It will lose mass even faster in the asymptotic giant branch, when its mass will become ~54.1% of what it is now, meaning $dot{M}$ will be $2.5 imes 10^{-7} frac{M_odot}{mathrm{yr}}$.

## As the sun burns, is it losing mass, volume, neither or both?

The answer is more complicated than the possibilities you provided.

Do you mean the sun specifically, or stars generally? Because the change in volume is going to vary wildly based on the type of star and which evolutionary phase it is in.

As for the sun, the light we see is a result of hydrogen fusion. Keeping a long story short, a Helium atom (the end result of the fusion process) weighs less than the sum total of 2 protons and 2 neutrons. The difference in mass is transformed into energy in the process. Some of that energy is in the form of photons. So as the sun emits light, it is technically losing mass.

On top of that, the sun actually produces a Solar Wind, in which it expels roughly 2-3 . 10 -14 M_☉ (solar masses) per year.

The sun also loses mass through solar flares and coronal mass ejections, which amounts to roughly one order of magnitude less than the solar wind.

Other stars, such as Wolf-Rayet stars will lose a lot more mass due to winds. A quick google search gave me this article, which estimates that very massive stars (60 M_☉) lose around 10 -5 M_☉ per year. There exist even more massive stars (> 100 M_☉), which will lose even more mass by winds.

As for volume, youɽ basically have to read a whole book on stellar evolution by mass to see what happens to which stars in which phase, but suffice to say it's complicated. During the main sequence, most stars will be relatively stable, but many stars undergo big changes in volume over their lifetimes, some even pulsate quite extremely (e.g.: Cepheids and RR-Lyrae stars).

The sun will be relatively stable for now, then, once hydrogen core burning stops, it will expand and become a red giant. Once Hydrogen shell burning starts, it will pulsate. Once Helium core burning starts it will be relatively stable, until the AGB phase starts, during which it will pulsate some more because there will be an interchange between hydrogen shell burning, helium core burning, both and neither. Once that is done, the outer layers will be gone and a white dwarf remains (which is a size on the order of the size of of Earth).

## Is there any theoretical reason why the sun/moon relative sizes and distances are nearly the exact same?

It seems insanely unlikely that the sun and the moon apparent sizes are nearly identical in the sky. Is there any even theoretical reason that this happens, or is it truly pure chance?

Nope. It's a really weird coincidence, and is probably extremely unusual. It's not even going to be like this forever - the Moon is slowly receding, and appearing to get smaller. Eventually, the Moon will be noticeably smaller than the Sun in the sky, and we won't have total eclipses anymore. So we're doubly lucky in that we just happen to be around at the right time as well as the right place.

Plenty of other planets have eclipses from their moons, especially in the outer solar system where the Sun appears very small. But to have extremely brief total eclipses in a narrow strip is very unusual. You're more likely to either only have partial (i.e. annular) eclipses (or even just transits), or to have really long eclipses over a large chunk of the planet.

Considering the extreme coincidence, could there be a bit of the anthropic principle at play?

The main reason you attribute particular sig ificance to that is just a cognitive bias. If the moon was half the angular size of the sun, or twice, you would also find that significant and be amazed at how much of a coincidence it is.

It is made even worse by this bit :

At what % of difference do you stop considering "nearly" identical. Wouldn't you have been just as amazed if it was "almost" or "approximately" or just "somewhat" identical?

It's an unfulfilling answer, but it's more due the nature of the question. Especially when you take i'to account that the distance between the moon and the earth isn't exactly constant. What are the odds that you would be alive at a time where the moon is as it is? Vanishing my thin. The odds that someone is, though, are much, much higher. Same idea between the odds of you winning the lottery and someone winning the lottery.

The odds of any specific event are always increasingly small as the specificity increase. And the meaning attached to that probability just goes downhill with it too, and the reason usually end up being either "coincidence" or "because of the starting conditions of the universe", depending on how determinist you feel.

Of course, we're arguing/observing from the wrong end of things. We are here. That being the case we look back at natural history and are amazed at all the unlikely coincidences that produced the conditions that led to our existence.

But seen from the point of view of the beginning, random things happened that just randomly/coincidentally led to our existence.

WE are just coincidental results, perhaps.

From the point of view of the RESULT, everything that lead to it seems amazing, planned, intended, designed, and controlled. If different beings than us had happened to be the result, they would likely feel the same way.

I am not, by the way, denying any faith or belief in a creator, here, just pointing out that when one reasons from RESULTS it can skew one's perspective.

I disagree, I don't think twice or half would be anywhere near as interesting. And no obviously the less identical it is the less interesting it is, but I think we can appreciate the probability distribution for the apparent sizes of these 2 things an having them be so close being very improbable and interesting in a way that other outcomes would not be.

I hate coincidence as an explanation for anything, but I guess, probability being what it is, occasional coincidences do happen. Or, as Terry Pratchett put it, "Scientists have calculated that the chances of something so patently absurd [the Discworld] actually existing are millions to one. But magicians have calculated that million-to-one chances crop up nine times out of ten."

Coincidence explains most things. Similar situations happen over and over again, so the odds are deceiving. What are the odds that I'm going to walk by a building and have a piano fall on my head? Pretty low. What are the odds that at least one of the 7,500,000,000 people will have a piano fall on their head? Much much higher.

Coincidence. The Sun is about 400 times the diameter of the Moon, and happens to be about 400 times farther away.

Since the Moon's orbit is getting larger, this wasn't true for most of Earth's history, and won't be true in the future. We just happen to be living at a time when the Moon appears the same size as the Sun in the sky.

Given the anthropic principle covers so much else about the universe's convenience for us, this is really the only thing that is just straight up unexpected. There's no reason in particular we exist during a time when we can observe each type of soar eclipse. We're just lucky we do.

It's the one point Iɽ give to the religious camp that believes the world is specially created for us. Every other thing has a good explanation, but for this one we're just lucky.

Edit: to be clear, there are reasons we expect to have a large moon. It's a meteor shield and may make intelligent life far more likely by reducing mass extinction events. But that still gives a good range of values that would allow for only annular or only partial/total eclipses.

That coincidence means the antikythera mechanism was built by Archimedes to calculate eclipses (which were a big deal to them) - which led to modern computing ultimately

Maybe there's some explanation such as a large moon causes larger tides which are better for mixing chemicals to form life etc but without a larger sample size of planets with life I think it would be very hard to tell. There could be aliens asking why theirs is the only planet without a large moon and coming up with equally convincing reasoning.

Anyway I think people massively overestimate how coincidental the similarity in the apparent angles is. At it's closest the moon can cause a beyond total eclipse and at it's furthest it's only an annular eclipse. If you consider the gas giants to have a "surface" then we aren't the only planet to have a moon with these properties. Based on this admittedly tiny sample the chance of a moon having this property is about 1% which is hardly a mindblowing coincidence.

You could have different types of eclipse too, if you lived on one of the moons of a gas giant Iɽ have thought total eclipses would be more likely since there would be more drastic changes in apparent diameter of other moons though I don't know of anyone having tested this. If we lived around a binary star the same thing could also apply.

## We really might be alone out here. - What are the odds really?

I was doing some random thinking and I started to wonder at how miniscule the chance was that we would ever encounter another alien civilization and if we did the even smaller chance that we would encounter one with a technological level equivalent to our own.

My thought process was this. The universe is almost 14 billion years old. Our star is 4.5 billion years old. Homo Sapiens are somewhere around 400 thousand years old. Humanity has been able to leave our planet for 60 years.

Now we have some unknowns. Is interstellar travel ever going to be possible in any method beyond generational space ships, will we destroy ourselves or at the very least knock ourselves backward in our technological evolution, etc.

My point is the time frames keep getting smaller and smaller. If we assume we are nothing special, we did not evolve much faster or much slower than what might be normal then the odds of finding another race similar to ourselves on the tech tree is very low. We are most likely to find a species in the cave-man stage of their development (it constitutes 99.9% of our species time on planet) or considering how quickly we see technology advances (more advancement in last 100 years than the previous 4000 by a huge margin) a species that is centuries or millennia's more advanced or possibly even more likely long since dead from their own mistakes or natural disasters.

So to encounter intelligent alien life we would have to encounter them within the same 100 or so year range of technological development (assuming similar to our own and assuming advancement does not hit some major law of physics wall soon) is really small when measured against the age and time span of the universe. A suitable star that formed merely 10,000 years before or after the formation of our Sol would be unlikely to have life on it that existed at a stage contemporary to our own.

Beyond the HUGE assumption that we can use humanity and our own history as a relatively decent scale (basically not much faster nor much slower than average development) does anyone see any huge hole in my logic?

Also does anyone know how soon after the Big Bang when the first G-Type stars start forming with solar systems? They live about 10 billions years so if they could form soon after the Big Bang there could have been a whole generation of them that formed lived and died by now.

Just to clarify: it's vanishingly unlikely that we're in fact alone in the universe, but it is also extremely unlikely that we'll ever make contact with an extraterrestrial people, or that any two extraterrestrial civilizations will make contact with each other, given galactic timescales.

As for earlier suns, my understanding is that the first generation of stars after the Big Bang didn't form planets, at least not terrestrial ones. The elemental distribution wasn't right. Only after the first set of stars went supernova and new stars formed from the remains did we get star systems.

We have gone from steam trains to being close to be able to trace civilization pollutants on exoplanet atmospheres in 150 years, and that's without seriously trying, if you consider the budgets for astrophysics.

Sending information between stars using precisely aimed laser is not unfeasible at all. The "chat" may have centuries of lag, but that only bothers us because we are stupidly impatient, that's all.

or that any two extraterrestrial civilizations will make contact with each other, given galactic timescales.

I'm not sure that's true. The galaxy is 10 billion years old, yet it would take only a million years or so for intelligent life to spread throughout the entire galaxy once they have the necessary technology. And it would only have to happen once for the galaxy to be full of intelligent life.

I agree. When I say alone in the universe I mean alone in that the odds of meeting another at a level equal enough to ourselves to be useful is so so small.

Imagine developing light speed travel that allowed you to travel at 10C. You go out, you find the right stars, find the planets in the right zone, and yet likely the best thing you find is the alien equivalent to Australopithecus Africanus.

Yeah, I was reading on early stars and how they were massive, fast burning, and did not live long. I was wondering how long after the Big Bang before G-Type stars like our Sol began to form. Sol formed a BB+10b (Big Bang plus 10 billion) is that early or were G-Types forming at BB+1b? I have not dug enough to find out of astronomers even have a rough theory of an answer. Barring outside influence a G-Type star would have had to form at BB+3.5b or earlier to have died by now.

You are getting all the wrong answers, and your conclusions are also off. There's a few things you MUST know to make any sort of mathematical conclusions.

The chance of it starting.

How fast intelligence can develop outside of earth.

If you can't answer those, you can not make any probabilistic conclusions about there being other life out there.

And since we have no idea what process starts life or what the odds are, we can not make any conclusions. For all we know life can start in the vacuum of space, and doesn't require planets or atmosphere at all. For all we know the most dominant lifeforms in the universe share ZERO similarities to life on earth. For all we know Life starting is so incredibly rare that we are the only ones. For all we know life is so easy to start, and so numerous, that the galaxy is filled with it.

Mathematically, the chance that we are unique is very low. That doesn't mean we aren't, but in terms of pure statistics. The number of stars that exist and the number of planets and the the number of those in the Goldilocks zone, which may or may not even be relevant for other types of life forms.

Also, consider the fact that when you mention how so much of our history was spent as "caveman" times, that's also heavily influenced by the type of animal we evolved from and their nature. What if we had evolved from ants, who work together towards a cook goal all the time, always. Or sloths, who aren't aggressive. Without wars weɽ have advanced a hell of a lot quicker.

Would we advance quicker without war? Homo Sapiens are between 300k to 500k years old. Yet as far as we can tell all the advances of relevance (outside of the creation of spoken language) have happened in the last 10,000 years. It seems unlikely that war with sticks and stones retarding us from advancing technologically for 300,000 plus years. I think it may be more along the line of certain aspects of technological evolution might just take that long. Tech evolution may be a snowball effect, it is just that the snowball starts out atomic sized.

Evolving like ants may retard science even more. An ultra efficient society may not need to advance, everyone would know their role and stay in it.

Evolving like sloths may also not help, why make more, do better, invent more, when there is no push to do so. Also if sloths were the dominant species one cannot say they would stay the way they are as their populations grew and scarcity of resource caused conflict between different groups that culturally evolved in isolation and thus had their own languages, social mores, ideologies, etc.

I'm going to push back a bit, not because I disagree, but because I want to question some of your assumptions and describe alternative pathways for theoretical extraterrestrial life.

My point is the time frames keep getting smaller and smaller. If we assume we are nothing special, we did not evolve much faster or much slower than what might be normal.

This is a reasonable assumption, but there is data suggesting alternatives, some of which I find more compelling. For example, a known driver of evolutionary divergence is catastrophes, like a massive volcanic eruption or asteroid impact. Presumably, there's a range of volcanic and asteroid activity that a biosphere could withstand, with the bare minimum vulcanism of a tectonically-active planet with a functioning magnetosphere, and a maximum frequency of such disasters that life could adapt to them without being permanently reduced or wiped out entirely. With no other examples of habitable planets with which to compare, we cannot accurately place Earth in this range. But it is reasonable to think that the Earth isn't at either extreme. To bring this back to the pace of evolution it's possible that the total evolutionary rate could be "accelerated" with a higher frequency of cataclysms, but perhaps there's a threshold where too high a frequency retards diversification. One could then imagine alien biospheres evolving at faster and slower rates, with implications on the evolution of sapience and technological civilizations.

Other hypotheses include Greater oxygen concentration encouraging faster development of oxidative phosphorylation techniques to generate chemical energy, leading to quicker emergence of multicellularity Greater aquatic concentration of bioavailable nitrogen, which encourages protein metabolism, activity, and diversification in the marine microbiome, stimulating complexity Warmer average temperatures reducing metabolic demand on temperature regulating mechanisms in macroscopic animals, freeing up energy and nutrient resources for other tasks, stimulating diversification etc.

. then the odds of finding another race similar to ourselves on the tech tree is very low.

Instead of potentially limitless technological advancement progressing exponential, what if there was an upper horizontal asymptote, as depicted here? One might expect advanced alien civilizations to "graduate" past the inflexion point and aggregate near the upper asymptote. The question then becomes where Humanity is on this double-asymptote graph.

So to encounter intelligent alien life we would have to encounter them within the same 100 or so year range of technological development (assuming similar to our own and assuming advancement does not hit some major law of physics wall soon) is really small when measured against the age and time span of the universe. A suitable star that formed merely 10,000 years before or after the formation of our Sol would be unlikely to have life on it that existed at a stage contemporary to our own.

This is accurate. If would could theoretically travel among the stars and explore freely, sampling all the worlds we come across for life, most of the complex lifeforms that we see will be like plants and wild animals, perhaps with social species and complex behaviors, but sapient life is unlikely, and technological civilizations are less so. If the lifetime of the universe is like a 1-trillion second movie (almost 32,000 years!), our solar system has existed for the last 4.5 billion seconds, life has existed here for the last 3.9 billion seconds, but our technologically modern civilization has only existed for the last

150 seconds, if that. We started sending people into space around a minute ago. If our characters go to any other life-bearing world, the chances of us finding a similar technological civilization is extremely low. Weɽ be sharing the same 150-500 (or however long it takes for us to collapse/destroy ourselves) seconds of the trillion-second movie, which is statistically quite improbable. But not impossible. Things get interesting when you realize that, in the seemingly-infinite expanse of space, there is a virtual statistical certainty that a similar civilization exists out there somewhere. Maybe not in this galaxy, but somewhere in the literal hundreds of billions of other galaxies. And we haven't even gotten to all the super advanced civilizations hanging out around the upper asymptote that are zipping around doing their own business.

Beyond the HUGE assumption that we can use humanity and our own history as a relatively decent scale (basically not much faster nor much slower than average development) does anyone see any huge hole in my logic?

​Here's a compelling argument for why using humanity as a standard is practically useful, even with a sample size of one: We are a part of the universe, and our existence demonstrates that life is possible. If life is possible here, it's theoretically possible anywhere else with similar-enough conditions. It may be the case that life emerges wherever conditions are acceptable, and in the right evolutionary context, the right pressures could push one or more species to evolve sapience. If those conditions could be recreated, we could experiment (arguably unethically) and create test peoples who will create their own alien civilizations. What if that happened to chimpanzee ancestors 6 million years ago and that's what made us?

To focus on more data-based explanations, we (and all Earth life) are made of carbon because carbon is a fertile chemical that can form four covalent bonds, and unlike silicon, the next most fertile, carbon has an intermediate stability that makes it ideal for the dynamics biochemistry. Silicon forms very strong covalent bonds, which makes it difficult and energy-expensive to use in a process analogous to carbon-transforming processes like photosynthesis or the Krebbs cycle. In the context of a technological civilization, any alien species would require sufficient perception (eyes, ears, etc.) and body parts (opposable thumbs, fingers) to hyper-effectively manipulate the environment to create tools, shelter, etc. Pretending if humans never existed, aliens that came here might conclude that elephants and whales are intelligent, but don't have the capacity for technological civilization. These are just a few examples of fundamental traits we might expect to see in the aliens of some xenos civilization.

## The Moon is Wrong: Astronomy 101 | Bookmans

If you are anything like a typical Bookmans employee you stand out in your yard at 3am and wonder what is wrong with the moon. This behavior could also be linked to growing up in Tucson. Little desert kids spend lots of time looking up, or laying on the ground staring at the night sky, possibly because there is so much to look at. Arizona has long been a hub on astronomical research with the University of Arizona’s Flandrau Planetarium and Kitt Peak. In lieu of deeper psychological exploration which might expose our potentially embarrassing fascination with standing in our yards late at night we will just say it’s typical for Tucsonans. We do things like this.

Folks from Michigan go fishing, we stand around getting a kink in our necks. We therefore notice things, things that aren’t right – like the moon. Just before the new year, around Dec. 18th, some of you might have also been standing in your yard noticing that the moon was wrong. Totally wrong. It was getting smaller like usual but it was disappearing from the top down. Initially it looked like cloud cover was simply obscuring the top quarter of the Moon but NOPE. It kept doing this strange thing night after night. It actually was waning (your first technical term it means getting smaller, yeah I had to look it up to be sure) from a horizontal position. Wrong. No, no, the moon is supposed to get smaller – or larger – from left to right or right to left, depending upon the phase and where you are on the planet. We will try to cover all of this but it gets messy.
The point is that we are all familiar with the crescent moon. It is a reassuring image, we understand it (kind of). We at least have seen it before but this top down disappearing moon is a problem. It’s a good thing we have a bookstore like Bookmans to help explain it. Here is the deal, it happens. We aren’t crazy, at least not because of the moon, and others did see it too.
It is called the sideways moon (2nd technical term). Here is your big technical explanation: “the ecliptic is tilted, but it never flips over itself, so the only time the sun and moon are at the same azmuth is during a new moon. 6 hours away just means they are 90 degrees apart in azmuth and that is only loosely connected to when they will rise or set. What it does mean is that if the sun is due west right now, the moon is due south right now and 6 hours from now, the sun will be due north and the moon due west. But depending on the tilt of the ecliptic for your season and location, the sun, the moon, both, or neither may be up at either of those times.” Reference https://www.physicsforums.com/threads/sideways-moon.203670/ .
Shockingly enough that makes sense, even to us. Then again we know a lot about these things because we live in Arizona and look at the sky. It is a tremendous relief to know that the Earth had not slipped off it’s axis or that the moon was spinning wildly out of control ready to crash into the sun or something. This was a close call however and a warning to all of us, we better get to Bookmans and pick up some books on astronomy. It seems that simply staring into the night sky isn’t always enough. We took our own advise and found some books that made us feel safer, and smarter!
40 Nights to Knowing the Sky by Fred Schaaf is subtitled A Night by Night Skywatching Primer. This work is perfect, it includes charts, graphs, photos and plain explanations to why we shouldn’t panic when we see a sideways moon. It also includes information about astrology, the brightest stars, the ecliptic and the zodiac, and how to NOT observe a solar eclipse. That last bit is probably especially important for us, just saying. Next is Spring Forward, the Annual Madness of Daylight Saving Time by Michael Downing. We really love this book because we don’t do Daylight Savings Time so we can sit back and laugh while the rest of the country tries to figure out their clocks and watches. Yeah, who’s crazy now? Last is Astronomy for Dummies by Stephen P. Maran, 2nd ed. The best part of this work are the color photos included. We also appreciate how straightforward it is and easy to follow. So don’t worry, don’t panic, we can explain everything to you. We have a large Astronomy section that will plainly cover all these strange things you seen in our desert sky. We can even explain those aliens, yes we saw them too, they are out there. That is a different section though, just ask us, we’ll talk.

what do you think ?
have you googled something like .
1) the surface temperature of the Sun or
2) the core temperature of the Sun ?

there's 2 things for you to do an report back with your findings

Red dwarf stars are main sequence stars but they have such low mass that they’re much ' cooler' than stars like our Sun.

Then Simple question here : Sun is cool or hot??

The sun has coronal holes which are not convective. The corona further complicates the hot vs cool question.

The temperature of a star is a real number. There is no obvious gap in the hertzsprung-Russell diagram. The transition from not having a core to having a tiny core is very difficult to see. Somewhere in the M3 to M4 range. CNO burning of hydrogen is taking place in the Sun but at a rate too slow to cause core convection. You could find an overlap where a star has some core convection and some surface convection.

Are they divided by star types or by telescope/detector type? It would also be reasonable for Astronomers to group by the age of the stars they are studying. It is hard to see M-dwarfs that are far away.

Would a study of the Orion Nebula go to the "hot star" conference or the fit in with the "cool" astronomers?

A paper link. Also a video from NASA.

They also found the temperatures reversed. The more massive brown dwarf is cooler. This could be caused by spots.

If you are interested in cold spots on hot stars do you go to the cool conference or the hot conference?

I assume you are referring to the hottest part of the sun, which lies above the surface of the sun. As we get to the surface, temperatures drop significantly. Observations suggest that below the surface temperatures drop even more. Indeed, looking into holes indicates lower temperatures.

According to emmited heat, the consensus seems to be that our sun may be average or slightly below. It is about at the median. This is also complicated by the definition of a star.

There appears to be no consistent temperature throughout the sun and it is very difficult to get emissions deep in the sun. So, the answer is difficult. For most other stars I think we only measure emission temperatures.

## Tiny Planet Mercury Is Shrinking Fast

The surface of Mercury is shrinking faster than previously thought, photos from a NASA spacecraft orbiting the tiny planet reveal.

The first comprehensive survey of the surface of Mercury by NASA's MESSENGER spacecraft shows that planet's crust has contracted as it cooled by as much as 4.4 miles (7 kilometers), significantly more than previous estimates. The findings clear up a long-standing clash between scientists' understanding of the heat production and loss and the contraction of Mercury.

"These new results resolved a decades-long paradox between thermal history models and estimates of Mercury's contractions," said study lead author Paul Byrne of the Carnegie Institution for Science in a statement. [Latest Photos of Mercury from NASA's MESSENGER Probe]

The incredible shrinking planet

The surface of Mercury is made up of just one continental plate covering the entire planet. Its enormous iron core, estimated to be about 2,500 miles (4,040 km) across, leaves only 260 miles (420 km) for a mantle and crust — an extremely thin skin for the solar system's smallest planet. The Earth's mantle, for comparison, is about 1,800 miles (2,900 km) thick, while the crust above it averages 25 miles (40 km) in thickness.

And as if that weren't enough for little Mercury, the tiny planet is shrinking.

Over the billions of years since its formation at the birth of the solar system, the planet has slowly cooled, a process all planets suffer if they lack an internal source of heat renewal. As the liquid iron core solidifies, it cools, and the overall volume of Mercury shrinks.

When NASA's Mariner 10 mission circled the planet in the 1970s, it captured images of surface features created by the shrinkage. The contracting planet pushed the crust up and over itself, forming scarps that can extend miles below the planet's surface. At the same time, the shrinking surface caused the crust to wrinkle up on itself, forming so-called "wrinkle ridges."

Byrne and his team used NASA's MESSENGER spacecraft to identify 5,934 ridges and scarps created by the contracting planet, ranging from 5 to 560 miles (9 to 900 km) in length. This created a substantially larger sample than those collected Mariner 10, which only imaged 45 percent of the surface. MESSENGER was able to map the entire surface.

NASA's MESSENGER probe (the name is short for MErcury Surface, Space ENvironment, GEochemistry, and Ranging) launched in 2004 and is currently in the middle of an extended mission around Mercury.

From Mariner 10 to MESSENGER

The scarps and wrinkle ridges identified by Mariner 10 allowed scientists to estimate that the planet had lost approximately 1 to 2 km, in global radius, a finding that contrasted with their understanding of the heat loss the planet suffered over time. Byrne's findings of a contraction of up to 4.4 miles (7 km) fits far more cleanly with present models.

"The discrepancy between theory and observation, a major puzzle for four decades, has finally been resolved," MESSENGER principle investigator Sean Solomon said in the same statement.

"It is wonderfully affirming to see that our theoretical understanding is at last matched by geological evidence."

Byrne's paper was published online today (Mar 16) in the journal Nature Geoscience.

## Shared Flashcard Set

Which of the following practices is not considered to be plagiarism?

B. Combining Wikipedia answers with APOD descriptions

D. Selecting answers from a web source and giving the reference to that source

Approximately, how many astronomical units (AU) are there in one light year (ly)?

The number 7.14 x 10^6 is equivalent to:

Why does the Moon appear to move relative to the stars as observed from Earth?

A. It is due to Moon rotating on its axis

B. It is due to the Earth rotating on its axis

C. It is due to Moon revolving around the Earth

D. It is due to the Earth rotating around the Sun

What types of distances are typically listed in parsec?

B. The length of speed-skating races

C. Distances in the solar system

D. The diameter of the universe

Which is Kepler&rsquos ﬁrst Law

A. The orbits of planets are ellipses with the Sun at one focus

B. Force equals mass times acceleration

C. The orbits of planets are circles with the Sun at the center

D. What goes up has to come down

What does Kepler&rsquos second law indicate about the orbital speed of a planet?

A. The orbital speed of each planet is constant

B. The orbital speed of a planet varies in no predictable way

C. A planet moves at its slowest when it is closest to the sun

D. A planet moves at its fastest when it is closest to the sun

On Earth, if we drop a feather and a hammer at the same moment from the same height, we see the hammer hit the ground ﬁrst. On the moon both strike the ground at the same time. Why?

A. The surface gravity of Earth is stronger than the gravity of the moon

B. There is no air resistance eﬀect on the moon

C. In strong gravity ﬁelds, heavier objects fall faster

If the semi-major axis of a planet is 4 AU, what is its orbital period?

The mass of &alpha Cen A is about 8 times larger than the mass of it&rsquos distant, faint binary companion,

Proxima Centauri. Which describes best the location of the center-of-mass of the &alpha-Proxima system?

A. Half-way between the two stars

B. 8 times closer to &alpha Cen than to the Proxima

C. Because &alpha Cen A has an almost equaly massive, close-in companion &alpha Cen B, there is no such thing as a center of mass in this system

D. 8 times closer to the Proxima than to &alpha Cen

Consider the energy output of the Sun. How is this energy produced, year after year after year?

A. The Sun is made up of coal, that slowly burns up to emit the energy we observe today

B. Inside the Sun, a process called nuclear fusion occurs that powers the Sun

C. The Sun is slowly getting smaller and is converting its gravitational potential energy into light

D. Because the Sun is very hot and ionized, there is a semi-continuous electric discharge happening

(a.k.a., lightning) that generates the light that we see

A spy satellite orbiting Earth that is designed to &ldquoresolve&rdquo objects the size of people needs to have a

A. that fully reﬂects X-rays so as to be able to penetrate the Earth&rsquos atmosphere

B. as large as 10 meters because it is a really diﬃcult job to see a person at such a large distance

C. of medium size, say one foot, because this task is neither very diﬃcult nor very easy

D. that needs not be very big, say three inches, since the spy satellite is orbiting so close to the Earth

According to Newton&rsquos laws, how does the amount of gravitational force exerted on Earth by the Sun

compare to the amount of gravitational force exerted on the Sun by Earth?

A. The amount of force exerted on Earth by the Sun is greater by the ratio of the Sun&rsquos mass to

B. The amount of force exerted on the Sun by Earth is negligible

C. The amount of force exerted on the Sun by Earth is greater by the ratio of the Sun&rsquos mass to

D. The amount of force exerted on the Sun by Earth is the same as the amount of force

exerted on Earth by the Sun

Suppose that Planet Q exists such that it is an identical planet to Earth in mass and size, yet orbits

the Sun at a distance of 3 AU. How does the amount of gravitational force exerted on Planet Q by the

Sun compare to the amount of gravitational force exerted on Earth by the Sun?

A. The amount of force on Planet Q is 1/3 the force on Earth

B. The amount of force on Planet Q is 3 times the force on Earth

C. The amount of force on Planet Q is 1/9 the force on Earth

D. The amount of force on Planet Q is 9 times the force on Earth

Which of the following would cause the gravitational force between the Moon and the Earth to decrease

A. Double mass of the Earth

B. Halve the mass of the Earth

C. Quadruple the mass of the Moon

D. Halve the radius of the Moon

The visible part of the electromagnetic spectrum can be divided into seven color bands: red, orange,

yellow, green, blue, indigo, and violet (from long to short wavelength). A single photon of which of

these colors has the greatest amount of energy?

The entire electromagnetic spectrum can be divided into seven bands: radio, microwave, infrared,

visible, ultraviolet, X ray, and gamma ray (from longest to shortest wavelength). To which of these

two bands is Earth&rsquos atmosphere the transparent?

B. ultraviolet and infrared

C. visible and ultraviolet

Which power of a telescope might be expressed as &rdquo0.5 seconds of arc&rdquo?

Which power of a telescope is the most important?

What advantage do the builders of large telescopes today have over the previous generation of telescope

A. Large mirrors can now be made thinner and lighter than before

B. Tracking celestial objects today is computer controlled and can take advantage of simple/cheap

C. High-speed computing today can be used to reduce the eﬀect of Earth&rsquos atmosphere

The primary mirror of telescope A has a diameter of 20 cm, and telescope B has a diameter of 100 cm.

How do the light gathering powers of these two telescopes compare?

A. Telescope A has 5 times the light gathering power of telescope B

B. Telescope B has 5 times the light gathering power of telescope A

C. Telescope A has 25 times the light gathering power of telescope B

D. Telescope B has 25 times the light gathering power of telescope A

An astronomer proposes to install an adaptive optics system on the successor to the Hubble Space

Telescope, the &ldquoJames Web Space Telescope.&rdquo This idea is:

A. Wonderful because it compensates the blurring due to the atmosphere

B. Wonderful: it compensates the thermal stresses due to the enormous heat load from the Sun

C. Nonsense!, it is a space telescope above the atmosphere

D. Nonsense!, the JWST is so large that is does not need any adaptive optics

What do the newer light-sensitive electronic CCD chips do better than the older photographic plates

coated with light-sensitive chemicals?

A. They have a greater sensitivity to light

B. They can detect both bright and dim objects in single exposure

C. The CCD images are easier to manipulate

Why must far-infrared telescopes be cooled to a low temperature?

A. To reduce interfering heat radiation emitted by the telescope

B. To protect the sensitive electronic ampliﬁers from overheating by sunlight

C. To improve their poor resolving power

D. To improve their poor magnifying power

If the temperature of star B is twice the temperature of star A, what can we say about the energy

emitted by the surface of star B compared to the energy emitted by star A?

A. Each square meter of B emits 2x as much energy per second as A

B. Each square meter of B emits 4x as much energy per second as A

C. Each square meter of B emits 8x as much energy per second as A

D. Each square meter of B emits 16x as much energy per second as A

Which subatomic particle(s) ha(s)(ve) no charge?

The amount of electromagnetic energy radiated from every square meter of the surface of a black body

A. proportional to temperature

B. inversely proportional to temperature

C. proportional to temperature to the fourth power

D. inversely proportional to temperature to the fourth power

The wavelength of maximum intensity that is emitted by a black body is:

A. proportional to temperature.

B. inversely proportional to the temperature

C. proportional to temperature to the fourth power

D. inversely proportional to temperature to the fourth power.

. Of the following, which color represents the lowest surface temperature for a star?

What conditions produce a bright (emission line) spectrum?

A. a hot solid, liquid, or high-density gas

C. light from a continuous spectrum source passing through a cooler low-density gas

Which of the following is true of an atomic nucleus?

A. It contains all of an atom&rsquos positive charge

B. It contains no electrons

C. It contains more than 99% of an atom&rsquos mass

What is the acceleration of gravity of Earth?

If your mass is 60 kg on Earth, what would be your mass on the Moon?

Suppose an object is moving in a straight line at 50 km/hr. According to Newton&rsquos ﬁrst law of motion, the object will

A. continue to move in the same way forever, no matter what happens

B. continue to move in the same way until it is acted upon by a force

C. eventually slow down and come to a stop

D. continue to move in a straight line forever if it is in space, but eventually come to a halt if it is on Earth

Gasoline is useful in cars because it has

A. gravitational potential energy

B. chemical potential energy

C. electrical potential energy

Which of the following statements correctly describes the law of conservation of energy?

A. An object always has the same amount of energy

B. Energy can change between many diﬀerent forms, such as potential, kinetic, and thermal

C. The fact that you can fuse hydrogen into helium to produce energy means that helium can be turned into hydrogen to produce energy

D. It is not really possible for an object to gain or lose potential energy, because energy cannot be destroyed

The wavelength of a wave is

B. the distance between two adjacent peaks of the wave

C. the distance between a peak of the wave and the next trough

D. the distance between where the wave is emitted and where it is absorbed

Which of the following statements about electrical charge is true?

A. Two negative charges will attract each other

B. Two positive charges will attract each other

C. A positive charge and a negative charge will repel each other

D. positive charge and a negative charge will attract each other

Which of the following statements about electrons is not true?

A. Electrons orbit the nucleus somewhat like planets orbiting the Sun

B. Within an atom, an electron can have only particular energies

C. An electron has a negative electrical charge

D. Electrons have a lot of mass compared to protons or neutrons

Observations of radio waves from astronomical objects suﬀer from poorer resolution than visible observations because

A. the signals are so weak in the radio region.

B. the wavelength of radio waves is much longer than the wavelengths of visible light.

C. radio telescopes are generally much smaller in diameter than optical telescopes.

D. it is very diﬃcult to detect radio waves.

You are in a space ship heading directly towards 3 stars. The stars are the same distance away from

you. One star is red, one star is yellow, and one stars is blue. Which star has a blueshifted spectrum?

Dubhe, in Ursa Major, is a spectroscopic binary star 2 stars each have more mass than the Sun, but

separated by 23 AU. They orbit every 44 yr. Why aren&rsquot these stars a visual binary?

A. One is always in front of the other

B. They are too far away to resolve

C. They are too faint to see

D. They both emit mostly non-visible light

Imagine that the Sun&rsquos core was somewhat cooler than it is today. What would that change about fusion

A. Fusion could not happen

B. Fusion reactions would be less frequent

C. Fusion reactions would happen at a higher rate

D. H would not fuse, but He would

Imagine you blow up a balloon and knot it. Then you take the balloon into the freezer department at

Costco. What will happen to the balloon?

A. The balloon will expand

B. The balloon will shrink

C. The balloon will start leaking

Two stars have same Temperature (T), but 1 has 2x bigger radius (R) How do their Luminosities (L)

A. Bigger star has 4x bigger L

B. Bigger star has 2x bigger L

C. Bigger star has 2x smaller L

D. Bigger star has 8x bigger L

Two stars have same T, but one star has 4x bigger Luminosity. How do their Radii compare?

A. Brighter star has 16x smaller R

B. Brighter star has 4x smaller R

C. Brighter star has square root of 8x smaller R

D. Brighter star has 2x larger R

Two stars have the same apparent brightness (b), but one is further away than the other one. Which

has the larger luminosity?

A. They have the same luminosity

B. Can&rsquot tell with the provided info

C. The more distant one has higher luminosity

D. The more distant one has lower luminosity

The stars Antares and Mimosa have the same luminosity Antares is spectral type M and Mimosa is

spectral type B. Which star is larger in radius?

D. Insuﬃcient information to determine

A. a numerical scale that measures stellar brightness

B. a measure of the distance of a star

C. the location of a star in the HR diagram

D. a numerical scale that measures stellar faintness

The apparent magnitudes of the Sun, Proxima Centauri and Sirius are approximately -26, +11 and

-1.5, respectively. Rank these three objects from brightest to faintest (to the human eye).

A. a numerical scale that measures the intrinsic power of stars

B. a measure of the distance of a star

C. the apparent brightness of a star if it were at a distance of 10 pc

D. a measure of the temperature of a star

A Hertzsprung-Russel diagram is a plot of the following stellar properties:

B. color versus apparent brightness

D. temperature versus luminosity

The spectral sequence (OBAFGKM) sorts stars according to

On a Hertzsprung-Russell diagram, where would we ﬁnd stars that are cool and dim?

On a Hertzsprung-Russell diagram, where on the main sequence would we ﬁnd stars that have the

On a Hertzsprung-Russell diagram, where would we ﬁnd white dwarfs?

The mass-luminosity relation is valid for:

The stellar main-sequence is determined by:

A. hydrogen burning in the stellar core

B. helium burning in the stellar core

C. deuterium burning in a shell around the core

What would the HR diagram look like if we plotted brightness (b) on the y-axis instead of luminosity

A. About the same as if we plotted using L

B. A complete jumble, with no patterns

C. The main sequence would still be obvious, but the white dwarfs and giants/supergiants would be

D. The patterns would be reversed, like a mirror image

Considering supergiants (SG), white dwarfs (WD), main-sequence stars (MS) and giants (G), which

ordering in absolute luminosity is most correct? (from bright to faint)

What type of spectrum (in visible light) would you see from a reﬂection nebula?

A. Emission, because the dust is heated by starlight

B. Absorption, because the dust is reﬂecting starlight

C. Absorption, because the dust is scattered thinly in space

D. Continuous, because the dust is solid

Interstellar dust is made up of

D. silicon, carbon, oxygen & iron

A dark cloud is characterized by

A. the absence of stars in certain regions when taking optical images of the sky, while infrared images do show the stars

B. the absence of stars in certain regions when taking optical images of the sky

C. the absence of stars in certain regions when taking x-ray images of the sky

D. the absence of stars in certain regions when taking infrared images of the sky, while optical images do show the stars

When you see a reﬂection nebula, what else might you see nearby

Which of the following is the most common type of main-sequence star?

If the temperature of star B is twice the temperature of star A, what can we say about the energy emitted

by the surface of star B compared to the energy emitted by star A?

A. Each square meter of B emits 2x as much energy per second as A

B. Each square meter of B emits 4x as much energy per second as A

C. Each square meter of B emits 8x as much energy per second as A

D. Each square meter of B emits 16x as much energy per second as A

Which of the following is not known to be a component of the interstellar medium?

A giant molecular cloud typically needs an external triger to start collapings to start forming stars.

A. Galaxy-scale spiral shock waves

B. Colliding molecular clouds

During a star&rsquos formation, the protostar shrinks from gravity. How does its rotation change?

A. Its rotation will slow down.

B. Its rotation will stay the same

C. Its rotation will speed up

D. Its random, so we can&rsquot tell

Consider the formation of stars form the interstellar medium, which of the following phases do they

The temperature of a gas is a measure of the:

B. amount of heat that ﬂows out of the gas

C. total number of atoms in the gas

D. average motion of its atoms

If a star has weak Balmer lines in its spectrum, what are possible reasons?

A. The star is much cooler than 10,000 K.

B. The star is much warmer than 10,000 K.

C. The star contains no hydrogen

D. either the star is much cooler than 10,000 K or the star is much warmer than 10,000 K

The luminosity (total energy emitted per second) of a star is an excellent measure of:

B. the mass lost from the Sun due to magnetic reconnection (per second)

C. the temperature of the star

D. the amount of hydrogen converted into helium (per second)

Which is closest to the temperature of the core of the Sun?

How does the Sun generate energy today?

D. gravitational contraction

The light radiated from the Sun&rsquos surface reaches Earth in about 8 minutes, but the energy of that light

was released by fusion in the solar core about

C. about a hundred years ago

D. about one hundred thousand years ago

Since all stars begin their lives with the same basic composition, what characteristic most determines

## Ask a NASA astronomer! Is there proof that the Earth is round?

Dr. Michelle Thaller is an astronomer who studies binary stars and the life cycles of stars. She is Assistant Director of Science Communication at NASA. She went to college at Harvard University, completed a post-doctoral research fellowship at the California Institute of Technology (Caltech) in Pasadena, Calif. then started working for the Jet Propulsion Laboratory's (JPL) Spitzer Space Telescope. After a hugely successful mission, she moved on to NASA's Goddard Space Flight Center (GSFC), in the Washington D.C. area. In her off-hours often puts on about 30lbs of Elizabethan garb and performs intricate Renaissance dances. For more information, visit NASA.

Michelle Thaller: So, Oscar, you asked the question, &ldquoWhat are some of the easiest ways that you can prove that the Earth is round?&rdquo Because apparently, this is something that we&rsquore debating&mdashI have no idea why.

That&rsquos a hard thing for me to even start talking about because there are so many proofs that the Earth is round, it&rsquos difficult to know where to start. And it&rsquos not okay to think that the Earth is flat. This is not a viable argument.

I have friends who have been on the International Space Station, they have orbited the Earth once every 90 minutes I've had personal experience with people who have been up in space and can see with their own eyes that the Earth is round. And of course, we&lsquove taken all of these amazing pictures from space they&rsquore so beautiful, all those pictures of the Earth.

So I don&rsquot really know what&rsquos going on right now with this 'Earth is flat' thing, but I will tell you that this is one of the things I really enjoyed teaching my own astronomy class about because there are proofs all around you. It is not difficult to know that the Earth is round. In fact, people have known of this for way more than 2,000 years. The ancient Greeks actually had a number of really elegant, wonderful proofs that the earth was a sphere.

So let&rsquos start from the simple to the slightly more complicated. One of the things you can see yourself, with a pair of binoculars, is if you actually go out to a lake and there are boats on that lake, the farther away a boat is the more the bottom of the boat will disappear, and you&rsquoll basically just see the mast of the boat. And as a boat goes farther and farther away the last thing you will see is the very top of the mast of that boat, and that&rsquos because the boat is actually going over the horizon that&rsquos curved&mdashand that means that as it goes farther and farther away you see less and less of the bottom of it, and more of the top of that. You can see that with binoculars by an ocean, by a lake, it&rsquos really easy. That wouldn&rsquot happen if the Earth were flat&mdashyou would simply see the boat getting smaller and smaller and smaller as it went farther away, but you&rsquod be able to see the whole thing with the same proportions.

Now, another way that you can tell that we&rsquore on a sphere is to think about how there&rsquos something called the tropics on the Earth, and the tropics are places near the equator of the earth were sometimes the sun is overhead in the sky. This was actually something that the Greeks used, not only to prove that the Earth was round about 2000 years ago, but they actually measured the circumference of the Earth, accurate to within just a couple percent. 2,000 years ago we&rsquove known that the Earth was round.

There was a really brilliant Greek scientist called Eratosthenes, and Eratosthenes noticed that there was a town called Syene, and on a certain date the sun would actually shine straight down to the bottom of a well. That meant the sun was directly overhead you could look down a well and see the sun shining back at you.

And on the very same date, farther away in the city of Alexandria, that didn&rsquot happen. The sun was not directly overhead, it was a slight angle, and all that Eratosthenes did was he measured the difference in the angle of the sun. It was straight overhead in Syene in Alexandria it was a little bit less than overhead, and he rationed that that change in angle from one city to another was probably indicative of us being on a curved surface, and you could make all kinds of measurements even between those two cities and see that the angles were different&mdashthe sun was at a different place in the sky. Using this, he actually measured the circumference of the Earth, and he got it right 2,000 years ago.

So another really simple proof is that on any given date, at different cities and different places around the world, the sun is at different angles in the sky. That wouldn&rsquot happen if the Earth wasn&rsquot round.

Then there are some other proofs that are a little more obscure, but they&rsquore actually really lovely. One is to observe what happens during a lunar eclipse. Now, a lunar eclipse happens when the Earth casts a shadow on the moon. The moon actually goes dark, in fact, if you&rsquove seen one you can actually see the Earth&rsquos shadow go across the moon, and when the moon is entirely in the Earth&rsquos shadow the moon looks kind of dark and even kind of red-colored it&rsquos really, really beautiful.

What&rsquos happening, in that case, is that the sun is on one side of the Earth&mdashthe Earth is in the middle&mdashand the Earth is casting a shadow on the moon, and as the shadow moves across the moon you&rsquoll notice that the shadow is curved, it&rsquos round.

And so something like the sun that&rsquos bigger than the Earth and is able to cast a shadow of the Earth on the moon can actually show you the shape of the Earth. &ldquoAh-ha!&rdquo you might say, &ldquobut could the Earth to be a disk? Could it be flat but it&rsquos actually still shaped like a disk, not like a sphere?&rdquo

There was a Greek scientist called Aristarchus and what he noticed was that you can get a lunar eclipse at many different angles where the sun is sometimes the shadow goes straight across the moon, sometimes it just kind of glances the moon&mdashjust a little bit is in shadow just on the top or on the bottom. From every different vantage point, every different angle the sun is casting a shadow, you always get a perfectly curved shadow. The only shape that can cast a shadow that&rsquos curved from any direction you put the light is a sphere.

So people have known that the Earth is spherical for thousands of years. It&rsquos not okay to say that the Earth is flat. This is some sort of strange denial, I don&rsquot know where it comes from, and it&rsquos something where I keep getting this question. We really need to put this question to bed because we&rsquove known the Earth is a sphere for a long time.

There&rsquos even some well-meaning people who say, &ldquoI don&rsquot really believe the Earth is flat, but I&rsquom not really sure what to think about it.&rdquo And they&rsquove asked me some interesting questions, like they&rsquove heard that space is a very hot, that when you go up above the atmosphere the temperature of space is millions of degrees, which is true. The problem is there&rsquos basically no air at all. So the gas right around the Earth is actually millions of degrees hot. That&rsquos actually true, but there&rsquos almost none of it, there&rsquos almost nothing. Like one single proton whizzes by you at a temperature of a million degrees, it&rsquos not the same as temperature in the air, it&rsquos not the same thing at all. So that's one that I get sometimes.

And the other one is&mdashI actually said this to somebody, and I couldn&rsquot believe they had never thought of it&mdashthat with binoculars you can see planets, you can see Saturn and Jupiter, you can see Mars with a telescope, the sun and the moon, everything else you see in the solar system is a sphere. So we&rsquore the one thing that is different? And that actually made somebody who was more interested in actually hearing information, that actually got them to think. They were like, &ldquoYou&rsquore right&hellip everything else we take a picture of is a sphere!&rdquo

Hey flat Earthers, it's time to put your theory to bed once and for all! A curious stargazer by the name of Oscar has submitted a question to Big Think's 'Ask an astronomer' series with NASA's Michelle Thaller. Oscar wants to know: "What would be the easiest proof that the Earth isn’t flat, that I could come back with whenever I get challenged on this issue?" Thaller sets the record straight. "There are so many proofs that the Earth is round, it’s difficult to know where to start. And it’s not okay to think that the Earth is flat this is not a viable argument," she says. The ancient Greeks figured out we were living on a sphere over 2,000 years ago, and there are things you can do to prove that the Earth is indeed round—just go to a body of water and look at ships or boats on the horizon with binoculars. Thaller explains three observable proofs that instantly debunk flat-Earth theory with irrefutable evidence of the Earth's round, curvaceous, gloriously spherical shape. You can follow Michelle Thaller on Twitter at @mlthaller.

## Curious Kids: Why don’t the planets closest to the Sun melt or burn up?

Chris Tinney is an employee of UNSW Sydney, a Fellow of the Astronomical Society of Australia, a member of the Australian Academy of Science's National Committee for Astronomy, and a Non-Executive Director of Astronomy Australia Limited.

### Partners

UNSW provides funding as a member of The Conversation AU.

The Conversation UK receives funding from these organisations

This is an article from Curious Kids, a series for children. The Conversation is asking kids to send in questions they’d like an expert to answer. All questions are welcome – serious, weird or wacky!

Can you tell me why the planets closest to the sun don’t melt or burn up, please? – Sophie, aged 6, Brisbane.

Hi Sophie. That’s a good question.

The planets closer to the Sun than the Earth are indeed hotter than the Earth is. But that still doesn’t make them hot enough to melt the rocks that they are made from!

Mercury is the small, rocky planet nearest the Sun. The side that faces the Sun has a temperature of around 430℃. Remembering that 100℃ is the temperature at which water boils, that make 430℃ very hot indeed. In fact, it’s hot enough to melt some types of metal, like lead.