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Allen Downey
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02_coords.html
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@@ -416,7 +416,8 @@ Created TAP+ (v1.2.1) - Connection:
<div class="section" id="getting-gd-1-data">
<h2>Getting GD-1 Data<a class="headerlink" href="#getting-gd-1-data" title="Permalink to this headline"></a></h2>
<p>From the Price-Whelan and Bonaca paper, we will try to reproduce Figure 1, which includes this representation of stars likely to belong to GD-1:</p>
<img src="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-4.png"><p>Along the axis of right ascension (<span class="math notranslate nohighlight">\(\phi_1\)</span>) the figure extends from -100 to 20 degrees.</p>
<img alt="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-4.png" src="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-4.png" />
<p>Along the axis of right ascension (<span class="math notranslate nohighlight">\(\phi_1\)</span>) the figure extends from -100 to 20 degrees.</p>
<p>Along the axis of declination (<span class="math notranslate nohighlight">\(\phi_2\)</span>) the figure extends from about -8 to 4 degrees.</p>
<p>Ideally, we would select all stars from this rectangle, but there are more than 10 million of them, so</p>
<ul class="simple">

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03_motion.html
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@@ -904,7 +904,7 @@ The coordinates in <code class="docutils literal notranslate"><span class="pre">
<div class="section" id="plot-proper-motion">
<h2>Plot proper motion<a class="headerlink" href="#plot-proper-motion" title="Permalink to this headline"></a></h2>
<p>Now we are ready to replicate one of the panels in Figure 1 of the Price-Whelan and Bonaca paper, the one that shows the components of proper motion as a scatter plot:</p>
<img width="300" src="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-1.png">
<a class="reference internal image-reference" href="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-1.png"><img alt="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-1.png" src="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-1.png" style="width: 300px;" /></a>
<p>In this figure, the shaded area is a high-density region of stars with the proper motion we expect for stars in GD-1.</p>
<ul class="simple">
<li><p>Due to the nature of tidal streams, we expect the proper motion for most stars to be along the axis of the stream; that is, we expect motion in the direction of <code class="docutils literal notranslate"><span class="pre">phi2</span></code> to be near 0.</p></li>
@@ -915,7 +915,7 @@ The coordinates in <code class="docutils literal notranslate"><span class="pre">
<div class="section" id="selecting-the-centerline">
<h2>Selecting the centerline<a class="headerlink" href="#selecting-the-centerline" title="Permalink to this headline"></a></h2>
<p>As we can see in the following figure, many stars in GD-1 are less than 1 degree of declination from the line <code class="docutils literal notranslate"><span class="pre">phi2=0</span></code>.</p>
<img src="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-4.png">
<img alt="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-4.png" src="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-4.png" />
<p>If we select stars near this line, they are more likely to be in GD-1.</p>
<p>We’ll start by selecting the <code class="docutils literal notranslate"><span class="pre">phi2</span></code> column from the <code class="docutils literal notranslate"><span class="pre">DataFrame</span></code>:</p>
<div class="cell docutils container">
@@ -1077,7 +1077,7 @@ Name: phi2, dtype: bool
<h2>Filtering based on proper motion<a class="headerlink" href="#filtering-based-on-proper-motion" title="Permalink to this headline"></a></h2>
<p>The next step is to select stars in the “overdense” region of proper motion, which are candidates to be in GD-1.</p>
<p>In the original paper, Price-Whelan and Bonaca used a polygon to cover this region, as shown in this figure.</p>
<img width="300" src="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-1.png">
<a class="reference internal image-reference" href="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-1.png"><img alt="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-1.png" src="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-1.png" style="width: 300px;" /></a>
<p>We’ll use a simple rectangle for now, but in a later lesson we’ll see how to select a polygonal region as well.</p>
<p>Here are bounds on proper motion we chose by eye,</p>
<div class="cell docutils container">

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04_select.html
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@@ -902,8 +902,8 @@ Results: None
</div>
<p>We’re starting to see GD-1 more clearly.</p>
<p>We can compare this figure with one of these panels in Figure 1 from the original paper:</p>
<img height="150" src="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-2.png">
<img height="150" src="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-4.png">
<a class="reference internal image-reference" href="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-2.png"><img alt="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-2.png" src="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-2.png" style="height: 150px;" /></a>
<a class="reference internal image-reference" href="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-4.png"><img alt="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-4.png" src="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-4.png" style="height: 150px;" /></a>
<p>The top panel shows stars selected based on proper motion only, so it is comparable to our figure (although notice that it covers a wider region).</p>
<p>In the next lesson, we will use photometry data from Pan-STARRS to do a second round of filtering, and see if we can replicate the bottom panel.</p>
<p>We’ll also learn how to add annotations like the ones in the figure from the paper, and customize the style of the figure to present the results clearly and compellingly.</p>

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05_join.html
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@@ -309,7 +309,7 @@
<p>This is the fifth in a series of notebooks related to astronomy data.</p>
<p>As a continuing example, we will replicate part of the analysis in a recent paper, “<a class="reference external" href="https://arxiv.org/abs/1805.00425">Off the beaten path: Gaia reveals GD-1 stars outside of the main stream</a>” by Adrian M. Price-Whelan and Ana Bonaca.</p>
<p>Picking up where we left off, the next step in the analysis is to select candidate stars based on photometry. The following figure from the paper is a color-magnitude diagram for the stars selected based on proper motion:</p>
<img width="300" src="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-3.png">
<a class="reference internal image-reference" href="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-3.png"><img alt="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-3.png" src="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-3.png" style="width: 300px;" /></a>
<p>In red is a theoretical isochrone, showing where we expect the stars in GD-1 to fall based on the metallicity and age of their original globular cluster.</p>
<p>By selecting stars in the shaded area, we can further distinguish the main sequence of GD-1 from younger background stars.</p>
<div class="section" id="outline">

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06_photo.html
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@@ -325,7 +325,7 @@
<p>As a continuing example, we will replicate part of the analysis in a recent paper, “<a class="reference external" href="https://arxiv.org/abs/1805.00425">Off the beaten path: Gaia reveals GD-1 stars outside of the main stream</a>” by Adrian M. Price-Whelan and Ana Bonaca.</p>
<p>In the previous lesson we downloaded photometry data from Pan-STARRS, which is available from the same server we’ve been using to get Gaia data.</p>
<p>The next step in the analysis is to select candidate stars based on the photometry data. The following figure from the paper is a color-magnitude diagram for the stars selected based on proper motion:</p>
<img width="300" src="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-3.png">
<a class="reference internal image-reference" href="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-3.png"><img alt="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-3.png" src="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-3.png" style="width: 300px;" /></a>
<p>In red is a theoretical isochrone, showing where we expect the stars in GD-1 to fall based on the metallicity and age of their original globular cluster.</p>
<p>By selecting stars in the shaded area, we can further distinguish the main sequence of GD-1 from younger background stars.</p>
<div class="section" id="outline">
@@ -393,7 +393,7 @@
<div class="section" id="plotting-photometry-data">
<h2>Plotting photometry data<a class="headerlink" href="#plotting-photometry-data" title="Permalink to this headline"></a></h2>
<p>Now that we have photometry data from Pan-STARRS, we can replicate the <a class="reference external" href="https://en.wikipedia.org/wiki/Galaxy_color%E2%80%93magnitude_diagram">color-magnitude diagram</a> from the original paper:</p>
<img width="300" src="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-3.png">
<a class="reference internal image-reference" href="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-3.png"><img alt="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-3.png" src="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-3.png" style="width: 300px;" /></a>
<p>The y-axis shows the apparent magnitude of each source with the <a class="reference external" href="https://en.wikipedia.org/wiki/Photometric_system">g filter</a>.</p>
<p>The x-axis shows the difference in apparent magnitude between the g and i filters, which indicates color.</p>
<p>Stars with lower values of (g-i) are brighter in g-band than in i-band, compared to other stars, which means they are bluer.</p>

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@@ -384,7 +384,8 @@
</ol>
<p>Not necessarily in that order.</p>
<p>Let’s start by reviewing Figure 1 from the original paper. We’ve seen the individual panels, but now let’s look at the whole thing, along with the caption:</p>
<img width="500" src="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-5.png"><p><strong>Exercise:</strong> Think about the following questions:</p>
<a class="reference internal image-reference" href="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-5.png"><img alt="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-5.png" src="https://github.com/datacarpentry/astronomy-python/raw/gh-pages/fig/gd1-5.png" style="width: 500px;" /></a>
<p><strong>Exercise:</strong> Think about the following questions:</p>
<ol class="simple">
<li><p>What is the primary scientific result of this work?</p></li>
<li><p>What story is this figure telling?</p></li>