94 votes
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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 ...
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88 votes
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What would happen if an ice cube is left in space?

It depends on where in outer space you are. If you simply stick it in orbit around the Earth, it'll sublimate: the mean surface temperature of something at Earth's distance from the Sun is about 220K,...
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47 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 ...
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  • 2,525
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 (...
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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. ...
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23 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 ...
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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 ...
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20 votes
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What is the difference between gas and dust in astronomy?

Yes, metals and other elements and molecules can exist in gaseous form under the right conditions of temperature and pressure. A "gas" is simply one of the fundamental states of matter, as in solid, ...
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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 ...
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13 votes

What would happen if an ice cube is left in space?

It would sublimate. The frozen mass of water would decrease in size as the water converts from a solid to a gas (without becoming a liquid) and drifts away.
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10 votes

Why is the Boomerang Nebula colder than the CMB?

The Boomerang Nebula (or Bow Tie Nebula) is a cloud of gas being expelled from a dying low-mass star, at $164~\mathrm{km}~\mathrm{s}^{-1}$ (cf. Raghvendra Sahai and Lars-Åke Nyman: The Boomerang ...
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10 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 ...
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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 ...
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9 votes
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Is there a limit to how hot a star can be?

Yes, there is a limit. If the radiation pressure gradient exceeds the local density multiplied by the local gravity, then no equilibrium is possible. Radiation pressure depends on the fourth power of ...
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9 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. ...
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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 ...
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9 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 ...
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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 ...
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  • 115k
8 votes

Is a supernova's core temperature absolute zero just before collapse?

No, absolutely not. The core of a core-collapse supernova is one of the hottest places in the present-day universe. The temperature as the star runs out of nuclear fuel in its core is around 6-10 ...
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8 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 deuterium ...
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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. ...
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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....
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8 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, ...
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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: ...
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  • 3,236
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 ...
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  • 115k
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 ...
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  • 31.5k
7 votes

What is the difference between gas and dust in astronomy?

In astronomy, there is no formal definition of the threshold between gas and dust. Gas can be monoatomic, diatomic, or molecular (or made of photons, in principle). Molecules can be very large, and in ...
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  • 32.3k
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 $...
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