
Ten Trillion-Degree Quasar Astonishes Astronomers (2016) - virtualthings
https://www.iflscience.com/space/seven-trillion-degree-quasar-astonishes-astronomers/
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privong
Please note that this is not a temperature measurement in the sense of a
thermodynamic temperature. It's not made clear in the article, but the ten
trillion degrees is a "brightness temperature". This is a convention in radio
astronomy where you take a measured specific intensity (power / unit area /
unit solid angle / frequency) and compute the temperature a blackbody would
need to have to emit the same specific intensity. When the brightness
temperatures are this high, it indicates that the emission process is non-
thermal (in this case, synchrotron emission from relativistic electrons).

See equation 2.32 of:
[https://www.cv.nrao.edu/~sransom/web/Ch2.html](https://www.cv.nrao.edu/~sransom/web/Ch2.html)

~~~
adrianratnapala
Thank you for that, it seemed a bit implausible that they could measure such a
temperature using radio astronomy since the emission peak should be in the
gamma rays.

While I get it that we can't always be persnicketty about the definition of
(thermodynamic) temperature I find these alternate, phenomenological
temperatures often do more harm than good. It's not just that it results in
misleadingly juicy press releases.

In my experience, physicists themselves forget that they are talking about
what is little more than a fiction and start sticking their "temperature" into
equations where it doesn't belong.

~~~
privong
> While I get it that we can't always be persnicketty about the definition of
> (thermodynamic) temperature I find these alternate, phenomenological
> temperatures often do more harm than good. It's not just that it results in
> misleadingly juicy press releases.

My understanding is that there's a historical reason for this convention. Many
telescope observations are calibrated by comparing the sky measurements
against measurements of thermal loads with known temperatures. Since the
calibration sources are specified with a temperature, it's straightforward to
measure the source brightness in units of a pseudo-temperature. But I agree
that it's not intuitive for those familiar with it.

Another layer on this is fact that doppler broadening of emission lines from
molecules means they cover a finite width. Since typical velocities in
astronomy range from fractions of a km/s to hundreds of kilometers per second,
line widths are typically given in km/s. Since these line widths originate due
to doppler motions, a width in km/s is equivalent to a width in Hz (for a
given reference frequency for the spectral line).

The combination of these two things means people often report integrated flux
measurements in units of K * km / s! It seems bizarre at first, but this is
equivalent to specifying it as power / unit area (again, when there's a
reference frequency specified). But these units are often convenient in their
own way.

~~~
adrianratnapala
Well I am not an astronomer, but the story you tell feels very similar to the
how other parts of physics sometimes use weird kinds of temperatures for
similar reasons.

In my field we sometimes liked to quote frequencies in Kelvins. What's and
Planck-over-Boltzman between friends?

> But I agree that it's not intuitive for those familiar with it.

It's worse than that. The convention of using temperature usually arises from
some such instrument-calibration where the temperature more-or-less exists,
and is accurate. Then the same instrument gets used for things where
temperature doesn't exist at all and sometimes doesn't even give a fair
description of the energy scales. Yet people keep blithely reporting these
things as temperatures.

------
ilamont
This was scraped from another website. The original dates from 2016:

[https://www.iflscience.com/space/seven-trillion-degree-
quasa...](https://www.iflscience.com/space/seven-trillion-degree-quasar-
astonishes-astronomers/)

~~~
dang
Thanks. We changed the URL to that from [https://differentimpulse.com/ten-
trillion-degree-quasar-asto...](https://differentimpulse.com/ten-trillion-
degree-quasar-astonishes-astronomers/) and banned the latter.

~~~
pvg
Seems to be the entire purpose of that account, as well.

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zamalek
That's hotter than the Hagedorn temperature, there could be some very
interesting stuff going on.

~~~
privong
> That's hotter than the Hagedorn temperature, there could be some very
> interesting stuff going on.

Unfortunately it's not made clear in the article, but the temperature they
quote is not an actual temperature, but instead a way of describing the
specific intensity of the radiation. In this case the emission they're
measuring is non-thermal (synchrotron emission), so the "brightness
temperature" of the radiation field is not the same as the thermodynamic
temperature. I'd put in a top-level comment about it too:
[https://news.ycombinator.com/item?id=17898224](https://news.ycombinator.com/item?id=17898224)

~~~
andrewflnr
However, any matter in the way of the emissions would still reach something
like that temperature, right? There's got to be some dust out there that's
over the hagedorn temperature.

~~~
privong
Not likely, since the intervening material probably lets most of the radio
emission through. So the radiative coupling between the synchrotron emission
and that material is too weak to raise the temperature by much (if any).

> There's got to be some dust out there that's over the hagedorn temperature.

Interstellar dust grains are thought to be destroyed at temperatures of
1500-2000 K (depending on composition). And their absorption cross-section for
absorbing radio waves is fairly small.

