Some X-ray sources in nebulae are caused by hot materials in the millions of degrees that formed in supernovae. What's the cooling rate of this stuff? How long does it stay this hot?

If we would take our solar system for example, the time from the formation of the nebula (by a supernova?) until it was cool enough to form solid planet was only a few million years. That's very quick, for instance relative to the age of the Earth. How ephemeral are these X-ray sources? I would expect them to not last as long as IR sources.

Is there an order of magnitude for the lifetime of the emission? Does it take millions of years? Tens of millions? Tens of thousands of years?

A comment suggests it doesn't really cool down, but diffuse to the interstellar medium. How long does it take for it to diffuse until there isn't a clear X-ray source?

  • $\begingroup$ A gas only cools if it has an efficient cooling mechanism. Hot gas tends to be very tenuous, which makes it difficult for particles to meet and enchange energy. Cooling down to ~1e6 K happens most efficiently by Bremsstrahlung, but cooling below that is difficult. You might be interested in this discussion of cooling mechanisms. Supernova remnants don't really "cool down"; instead their hot gas dilutes and becomes part of the "hot ionized medium" (one of the phases of the interstellar medium). $\endgroup$
    – pela
    Jun 6, 2016 at 10:30
  • $\begingroup$ @pela difficult, but eventually the heat will radiate away. The link you provided is indeed interesting. However, it does not address cooling rates, which is the topic of my question. $\endgroup$
    – Gimelist
    Jun 6, 2016 at 12:12
  • $\begingroup$ The plots in the answer give the cooling rate in erg/cm3/s. For a given volume, you then have the cooling rate in energy per time. I think that if you do an order-of-magnitude calculation, you'll find that the cooling time scale is much smaller than the dilution time scale, in which case the heat won't radiate away because the gas is now a part of the hot ionized medium. $\endgroup$
    – pela
    Jun 6, 2016 at 12:45
  • $\begingroup$ Narrow down to the life-time of supernova remnants. There are a least 2 input sources of energy to heat the remnant: a central pulsar, collision between the outgoing gas of remnant and local interstellar remnant. The first is powered by the spin and magnetic field of the pulsar, the second is the kinetic energy imparted to the expanding nebula. $\endgroup$ Dec 7, 2016 at 5:35
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    $\begingroup$ The 2nd paragraph suggests that the gas cloud that the solar system condensed from was produced by a supernova. That's incorrect. It was mostly primordial hydrogen & helium created in the big bang, enriched with a tiny percentage of elements released by a huge number of stars spread over a large region of the galaxy, including asymptotic giant branch red giants, supernovae, colliding neutron stars, etc. The collapse of that cloud may have been triggered by shockwaves from a relatively nearby supernova, which may have also contributed a small amount of material. $\endgroup$
    – PM 2Ring
    Apr 12, 2019 at 3:06

1 Answer 1


This is not an answer, but it is too long to put in the comment.

There are formulas for cooling timescales of different radiative processes. Cannot recall right now about sources for reading.

One thing about nebulae. Some of them might have compact objects such as pulsars or black holes feeding energy from the centers. In those cases, you have to consider the energy input from the central sources and radiative cooling.

Another thing about X-ray emission processes. There are thermal X-ray emission (i.e., bremmstrahlung) and nonthermal ones (i.e., synchrotron and inverse Compton; IC). Each process dominates in different environment. Regarding to SN nebulae, nonthermal processes are the dominant ones, the electrons involving in the processes are relativistic and thermal process requires dense electron medium (for collision) to maintain the thermal equilibrium. IC normally dominates at early ages of nebulae due to the compactness of the system, while synchrotron self-absorption is also strong at these ages. At older ages, synchrotron dominates, while the IC can interact with the background photons (i.e., cosmic background radiation) and results in X/gamma-ray photons as well.

Last about IR emission. Since IR emission is mostly from thermal process while X-rays are from nonthermal processes, X-rays tend to last longer.

  • $\begingroup$ I'd added a bounty to this but so far no new info has been added. There's 17 hours plus 24 hours grace period to award it. Is there anything you can add to this to convert it from "This is not an answer... too long (for a) comment" to something more? $\endgroup$
    – uhoh
    Jan 28, 2021 at 6:55

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