The Gould Belt.
1.0 Star Spectral Types
1.1 OB associations
0.0 Preface
I've always noticed a bright chain of huge blue stars in the southern sky while out taking photographs, but never knew anything about them beyond guessing there must be some connection.
I started looking around wikipedia articles trying to figure out what I was looking at, and eventually stumbled onto the Gould Belt, a feautre of the southern sky first reported by John Herschel and Benjamin Gould in the 19th century.
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The Gould Belt is one of the most visually beautiful parts of the night sky, and something I felt like writing a short article about. It is also one of my favorite regions to photograph, with an overwhelming number of bright stars strung along the sky.
image 1: the southern milky way with the Sco-Cen OB stars above it
1.0 Star Spectral types
Before jumping into what the Gould Belt is, you first have to understand the basics of why some stars shine brighter than others, and specifically what O and B type stars are. There are countless ways to identify differences in stars, from their size to age or composition etc, but perhaps the most simple is how they look. More specifically what color they are, aka their spectral type.
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Stars are put into the categories O, B, A, F, G, K, M* - and are assigned a class based on their color (the color is caused by the temperature of a star's photosphere). On this scale, O stars are the hottest and bluest stars, and M stars are the coolest and reddest (nice and intuitively as usual). Therefore, O and B class stars are the hottest of them all, accounting for less than 1% of main sequence (hydrogen fusing) stars in the sky.
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While there are some other examples of non main sequence O and B class stars on the Hertzsprung-Russell diagram, the main sequence stars in this class are huge. Their enormous size, and therefore pressures within the star's core, release incredible amounts of fusion energy that result in these ultra bright and blue stars. And as a result we can see these O and B class stars many hundreds of light years away.
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* It's worth noting there are many sub categories of star spectral type.
image 2: the Sco-Cen OB stars (circled) in relation to the milky way
image 3: the Orion OB1 stars are another nearby OB association
1.1 OB associations
An OB association is essentially a group of O and B class stars, clustered together in the night sky. They're not as gravitationally bound as stars in an open cluster, but they do have a connection. These groups of large bright stars (and many other smaller stars too) all form out of the same cloud of interstellar gas and dust - what we call a nebula. As the nebula starts to run out of gas and disappear from the sky, the freshly formed stars within will start to drift away, giving us the loose associations we see in the sky.
However, large stars fuse through their fuel very quickly, and may only live for a few million years as a result (particularly O class stars). This means that O and B class stars within an association will not be around for long, and as such OB associations have to be relatively young.
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The Sco-Cen OB association, is the closest such association to earth, located a little over 400 light years away. Because of this, it is the largest and brightest one in the night sky, and can be seen as a beautiful chain of giant blue stars littered across the southern sky.
Some of the brightest stars in the sky are contained within this association, including Antares and Hadar (Beta Centauri), and stretches from the head of Scorpius, to well past Crux (Southern Cross).
image 4: self portrait under the string of OB stars