# Are the SI units used in astrophysics?

Just a curious question, do astrophysicist use the SI units, for example in this equation,

$r = 5pc$, will this be converted to meters? And what does this $\nu$ stand for?

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$\nu$ is proabably the frequency of light. –  jkej Jan 15 '13 at 18:18
i thought of that too , and i asked a lot of my friends and all of them said "probably". So i need a sure answer :) –  Abdelrahman Esmat Jan 15 '13 at 18:24
Ok, it's definitely frequency. Happy? :) –  jkej Jan 15 '13 at 18:27

Yeah, convert it to meters. Other units are used for quick-and-dirty conceptual stuff, but when it comes to the math, you need to keep your units consistent.

$\nu$ is frequency. Hence the exchange:

"$\nu$ is frequency!" answered Betty.

Alan blinked twice, and began looking for an exit to this conversation.

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And is that nu 0 in integral lower limit? –  Abdelrahman Esmat Jan 15 '13 at 18:38
Yeah, although we're at the limit of my understanding of astro. We're definitely dealing with frequency, but what does the equation signify? I see luminous flux per unit area times whatever $a_\nu$ is, divided by the energy of each photon, and integrated by frequency, but this is the first time I've seen this particular equation. Clearly you want to look at it from 5 parsecs away (probably from earth, and it's like 15 light years away). –  Will Cross Jan 15 '13 at 19:17
@thatnerd, this equation concerns the number-flux of photons with a frequency above $\nu_0$. $L/(4 \pi r^2)$ is the flux--as you say--and dividing by energy ($h \nu$) converts that to flux of number of photons. I'm not sure what $a_\nu$ is, but maybe an absorption or transmission coefficient (i.e. calculating the number of photons absorbed, transmitted etc.). Apparently the result is about 1 photon per few years, per square-cm. –  zhermes Jan 15 '13 at 19:57
Oops, it's a period, not a frequency. And I have to defer to zhermes on the Gaussian units. They're common enough in physics, but I didn't question whether or not they were here, and I'm not an astro guy (I'm a gravity guy), so I don't fully know the conventions. –  Will Cross Jan 17 '13 at 13:48

Astronomy does not use SI (mks) units1. CGS (Centimeter-Gram-Second)-Gaussian2 units are the standard in astro fields. I've yet to hear a good explanation for why cgs is used, but gaussian units can definitely be convenient.

In your above equation, $\nu$ (pronounced: 'nu') is the standard symbol for frequency, and has units of inverse time (1/s). The standard astronomical procedure would be to use distance ($r$) in cm, planck's constant ($h$) in erg-seconds, and the luminosity in erg/s.

1: Some experimentalists, who are often associated more closely with 'physics' than astronomy or astrophysics, sometimes do use SI (MKS).

2: Gaussian is a sub-unit system of CGS involved in electricity-and-magnetism. In this system, you can write the electromagnetic force simply as $F = \frac{q_1 q_2}{r^2}$, without the usual numerical prefactor that you see in SI $\left( \frac{1}{4\pi \epsilon_0} \right)$.

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so basically the units i use depend on the units of the plank constant, is that what you're saying? –  Abdelrahman Esmat Jan 16 '13 at 5:33
In the end, the only thing that matters is unit consistency --- so yes, as long as you use consistent units, you will get a consistent result. You're question also asked what the astronomical standards were, so I tried to answer that as-well. –  zhermes Jan 16 '13 at 17:26