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Why is Mars cold?

Firstly, Mars has a mean distance from the Sun of 1.524 AU, so by the inverse square law the energy it gets from the Sun is about 40% of what the Earth gets. But the main reason that Mars is so cold ...
PM 2Ring's user avatar
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48 votes
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Could liquid water have existed in open space 15 million years after the Big Bang?

Let's interpret your question to be about whether the conditions would permit blobs of water to remain liquid, whether or not water existed yet. And the answer is No, because the pressure was by then ...
Mark Foskey's user avatar
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38 votes
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Why didn't the Big Bang produce heavier elements?

I think that your thought process is flawed in that you assume that by drastically increasing the temperature you are guaranteed to get heavy elements. As odd as this may sound, this isn't the case (...
zephyr's user avatar
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26 votes

Why is Mars cold?

I'm just going to expand and deepen on what the other answers already said. In the following I contrast the atmospheric transmission ($T$) and absorption ($A$, which is $A=1-T$) of Mars and Earth. ...
AtmosphericPrisonEscape's user avatar
24 votes
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How cold is interstellar space?

You can stick a thermometer in space, and if it is a super-high-tech one, it might show you the temperature of the gas. But since the interstellar medium (ISM) is so dilute, a normal thermometer will ...
pela's user avatar
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21 votes

Could liquid water have existed in open space 15 million years after the Big Bang?

As others have mentioned in the comments, there wouldn't have been any oxygen to form water. Soon after the Big Bang, the protons were hot or dense enough to fuse up to helium and some lithium but ...
Warrick's user avatar
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16 votes

Why is Mars cold?

Mars does have a greenhouse effect, only somewhat weaker than Earth's. Mars' atmosphere is very dilute, with a with a surface pressure only 0.6% of Earth's. So even if 95% of it is CO2, that's not a ...
pela's user avatar
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16 votes

Is the Sun hotter today, in terms of absolute temperature (i.e., NOT total luminosity), than it was in the distant past?

From the Wikipedia article on the sun, it is about 150 K: The Sun is gradually becoming hotter in its core, hotter at the surface, larger in radius, and more luminous during its time on the main ...
planetmaker's user avatar
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12 votes
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Why does lithium fuse at lower temperatures than hydrogen?

The slowest reaction rate in the pp chain determines how quickly hydrogen can "burn" in the core of a sun-like star. That rate-determining step is actually the fusion of two protons to form ...
ProfRob's user avatar
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12 votes
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Why can't neutron stars ignite and explode?

In a white dwarf, the dense matter is not in its lowest energy configuration. Energy can still be extracted from the white dwarf material by fusion, provided it can be ignited. What exothermic ...
ProfRob's user avatar
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11 votes
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Is there a formula for calculating surface temperature of a rocky world based on atmosphere and solar input?

Guillot et al. 2010 give a comparatively simple solution (their Eqn. 27) for a double-gray purely radiative atmosphere. It reproduces the equilibrium temperature in the optically thin limit. Robinson &...
AtmosphericPrisonEscape's user avatar
10 votes

Why is the Sun so bright, but you can feel it far away?

The answer to your first question has to do with luminosity. It's a measure of power, the energy given off by an object in a certain amount of time, which you can think of as brightness. The more ...
HDE 226868's user avatar
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10 votes
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What is the temperature 55 km beneath the surface of Mars?

If we look at Mars' possible geothermal gradient (see Earth's) which is about 25 °C per km. Using the low estimate of Mars's gradient to be 1/4 that of Earth's Source, that's a bit over 6° C per km. ...
userLTK's user avatar
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10 votes
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Why are there no ISM clouds with temperatures between 100 and 6,000K?

Let $n$, $T$, and $x_i$ be the number density of hydrogen, the temperature of the gas, and $n_i/n$, where $n_i$ is the number density of the $i$th component of the interstellar medium. We can then ...
HDE 226868's user avatar
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9 votes

How cold is interstellar space?

The title of the question asks about interstellar space, but the body asks about the interstellar medium. These are two very different questions. The temperature of the interstellar medium varies ...
David Hammen's user avatar
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9 votes
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What spectral type of star has an absolute magnitude of exactly 0?

There isn't a one-to-one relationship between spectral type and absolute magnitude. Instead, there is a mean relationship with a fair bit of scatter around it. The reason is that the luminosity of a ...
ProfRob's user avatar
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9 votes
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What is the RGB curve for blackbodies?

Ok, here's my take on calculating the color of a blackbody, or any spectrum in fact: Disclaimer: I'm not a color theorist, and there may be more accurate methods. But the result, shown in the bottom, ...
pela's user avatar
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9 votes
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How did the temperature of the solar system evolve?

There is no "nuclear flash". The approach to a stable, hydrogen-burning star is rather smooth and in fact the Sun was more luminous during its first few million years as a protostar than it ...
ProfRob's user avatar
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8 votes
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Hottest Possible Hydrogen-Fusing Stars

An answer to your question is contained within What is the largest hydrogen-burning star? The hottest observed main sequence stars are of type O3V, with photospheric temperatures of about 50,000 K. ...
ProfRob's user avatar
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8 votes
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Why is our Sun hotter than UY Scuti?

UY Scuti is a red supergiant star. When stars start to run out of hydrogen fuel, their cores start to collapse, causing the core of the star to heat up, and heavier elements start to be used as fuel....
James K's user avatar
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8 votes
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What's the temperature of Pluto's core?

TL;DR: 1000 K (according to differentiated model of Pluto) According to the density value of Pluto, astronomers proposed three types of structural models: Undifferentiated or "cold" model: ...
Nilay Ghosh's user avatar
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8 votes
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What is the coolest white dwarf known?

The one from 2014 is still the record holder I believe - in the sense that it is reasonably convincing that the unseen companion of the pulsar PSR 2227-0137 is consistent with being a white dwarf with ...
ProfRob's user avatar
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8 votes
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At high temperatures, do planets glow like blackbodies?

At high temperatures, do planets glow like blackbodies? Yes, and at low temperatures too!1 1As @DavidHammen points out, since there's likely going to be a star nearby the planet, it will also be ...
uhoh's user avatar
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7 votes

How to Distinguish between Temperature and Doppler effect using Black-body radiation?

Without any other information, you cannot distinguish between the two effects. $$ T = T_0 (1 + z) $$ A blackbody spectrum of temperature $T$ is identical to a blackbody spectrum of temperature $...
BowlOfRed's user avatar
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7 votes

What spectral type of star has an absolute magnitude of exactly 0?

A star with magnitude 0 would be 85 times brighter than the sun (since Magnitude=-2.5 log(Luminiosity)) Referring to the H-R diagram on Wikipedia shows that there is quite a range of spectral types ...
James K's user avatar
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7 votes
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Exactly how long does it take for the exposed core of a star to cool from its starting temperature (several billion K) to ~50,000 K?

The answer is of order 1 million years to cool from a standard end of He burning temperature of just over $10^8$ K to the top end of the white dwarf temperature range you give in your question. The ...
ProfRob's user avatar
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7 votes
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If the sun would be two times as big and with the same power output would temperature on Earth change?

Second question first: If the emitted power is the same does it matter if the emitting body is a large sphere of gas or a solid sphere? Not really. For thermal radiation discussed below, as long as ...
uhoh's user avatar
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6 votes
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Are gas giants supported by thermal pressure?

I am not sure what you mean by "thermal" pressure. Jupiter is supported by pressure, just like all objects that are in (approximate) hydrostatic equilibrium. That pressure is provided by your ...
ProfRob's user avatar
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