<p>Brγ is a line in the emission spectrum of Hydrogen, or single-electron (and therefore) hydrogen-like <a href="https://en.wikipedia.org/wiki/Rydberg_atom" rel="nofollow noreferrer">Rydberg</a> atoms in general. In particular, it is the emission due to an electron moving from the 7th to the 4th energy level.</p>

<p>Br is short for <a href="https://en.wikipedia.org/wiki/Hydrogen_spectral_series#Brackett_series_(n%E2%80%B2_=_4)" rel="nofollow noreferrer">Brackett</a>, after <a href="https://en.wikipedia.org/wiki/Frederick_Sumner_Brackett" rel="nofollow noreferrer">Frederik Brackett</a>, and gamma because </p>

<ul>
<li>$\alpha: \ \ 5 \rightarrow 4,$ </li>
<li>$\beta:  \ \ 6 \rightarrow 4,$ </li>
<li>$\gamma: \ \ 7 \rightarrow 4,$ </li>
</ul>

<p>...and so on.</p>

<p>Brγ has a wavelength of 2166nm, as you would get from the <a href="https://en.wikipedia.org/wiki/Rydberg_formula" rel="nofollow noreferrer">Rydberg formula</a>, which is well in the infra-red part of the spectrum. It is probably chosen as it has low absorption by the dust towards the centre of the galaxy.</p>

<p>Screenshot from <a href="https://en.wikipedia.org/wiki/Hydrogen_spectral_series#Brackett_series_(n%E2%80%B2_=_4)" rel="nofollow noreferrer">here</a>.</p>

<p><a href="https://i.sstatic.net/vDj5l.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/vDj5l.png" alt="enter image description here"></a></p>

CC BY-SA 3.0
2018-02-24T23:32:18.260Z
Thanks! I've added some embellishments, feel free to roll back.
uhoh
CC BY-SA 3.0
2018-02-24T07:40:40.253Z
2018-02-24T23:36:28.960Z
2018-02-24T23:36:28.960Z
James K
<p>The Phys.org article <a href="https://phys.org/news/2018-02-astronomers-s0-star-ready-big.html" rel="nofollow noreferrer">Astronomers discover S0-2 star is single and ready for big Einstein test</a> describes the results of a careful analysis of radial velocity measurements of the star S0-2, which orbits the supermassive black hole at the center of our (Milky Way) galaxy with a period of only 16 years. The paper itself (Devin S. Chu et al, 2018) is available for open access at <a href="https://arxiv.org/abs/1709.04890" rel="nofollow noreferrer">ArXiv</a>, and also <a href="http://iopscience.iop.org/article/10.3847/1538-4357/aaa3eb/meta" rel="nofollow noreferrer">ApJ</a>.</p>

<p>The only mention of the word wavelength is in Section 2.2:</p>

<blockquote>
  <p>The radial velocity of S0-2 is measured from both data sets by fitting a Gaussian to the Brγ absorption line.</p>
</blockquote>

<p><em>What exactly is</em> a Brγ ("Br" + gamma) absorption line?</p>

<hr>

<p><strong>below:</strong> "The orbit of S0-2 (light blue) located near the Milky Way's supermassive black hole will be used to test Einstein's Theory of General Relativity and generate potentially new gravitational models. Credit: W. M. Keck Observatory (Click for full size)</p>

<p><a href="https://i.sstatic.net/9Iv1I.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/9Iv1Im.jpg" alt="stars orbiting supermassive black hole center of Milky Way"></a></p>

<spectroscopy>
What spectral feature was used to confirm that S0-2 is "single and available"? What is Brγ?
uhoh
# What spectral feature was used to confirm that S0-2 is "single and available"? What is Brγ?

(Asked by: username_0 on 2018-02-24)

The Phys.org article [Astronomers discover S0-2 star is single and ready for big Einstein test](https://phys.org/news/2018-02-astronomers-s0-star-ready-big.html) describes the results of a careful analysis of radial velocity measurements of the star S0-2, which orbits the supermassive black hole at the center of our (Milky Way) galaxy with a period of only 16 years. The paper itself (Devin S. Chu et al, 2018) is available for open access at [ArXiv](https://arxiv.org/abs/1709.04890), and also [ApJ](http://iopscience.iop.org/article/10.3847/1538-4357/aaa3eb/meta).

The only mention of the word wavelength is in Section 2.2:

> The radial velocity of S0-2 is measured from both data sets by fitting a Gaussian to the Brγ absorption line.

*What exactly is* a Brγ ("Br" + gamma) absorption line?

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**below:** "The orbit of S0-2 (light blue) located near the Milky Way's supermassive black hole will be used to test Einstein's Theory of General Relativity and generate potentially new gravitational models. Credit: W. M. Keck Observatory (Click for full size)

[![stars orbiting supermassive black hole center of Milky Way](https://i.sstatic.net/9Iv1Im.jpg)](https://i.sstatic.net/9Iv1I.jpg)

**Tags:** spectroscopy

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## 1 answers

### Answer by username_1 on 2018-02-24. Score: 4 | Accepted answer

Brγ is a line in the emission spectrum of Hydrogen, or single-electron (and therefore) hydrogen-like [Rydberg](https://en.wikipedia.org/wiki/Rydberg_atom) atoms in general. In particular, it is the emission due to an electron moving from the 7th to the 4th energy level.

Br is short for [Brackett](https://en.wikipedia.org/wiki/Hydrogen_spectral_series#Brackett_series_(n%E2%80%B2_=_4)), after [Frederik Brackett](https://en.wikipedia.org/wiki/Frederick_Sumner_Brackett), and gamma because

* $\alpha: \ \ 5 \rightarrow 4,$
* $\beta: \ \ 6 \rightarrow 4,$
* $\gamma: \ \ 7 \rightarrow 4,$

...and so on.

Brγ has a wavelength of 2166nm, as you would get from the [Rydberg formula](https://en.wikipedia.org/wiki/Rydberg_formula), which is well in the infra-red part of the spectrum. It is probably chosen as it has low absorption by the dust towards the centre of the galaxy.

Screenshot from [here](https://en.wikipedia.org/wiki/Hydrogen_spectral_series#Brackett_series_(n%E2%80%B2_=_4)).

[![enter image description here](https://i.sstatic.net/vDj5l.png)](https://i.sstatic.net/vDj5l.png)

> **Comments:**
> **username_0**: Thanks! I've added some embellishments, feel free to roll back.  

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