In a number of posts I will be studying the composition of the Milky Way, with the goal of better understanding the sources of the light from objects in the galaxy observed here on the surface of this planet we call Earth. I will focus on radiation that I can capture in my telescope –the visible spectrum.
Figure 1: Messier 13, the Hercules Globular Cluster, a collection of several hundred thousand stars in the Milky Way galaxy.
Stars
When we think of a galaxy we immediately think of the billions of stars that dominate its mass and brightness. The Milky Way is believed to have between 100 and 400 billion stars. The total mass of the stars is estimated to be approximately 50 billion times that of our sun, meaning that our sun is more massive than the average star.
Looking out from the Earth we see stars of differing brightness and colour, in all directions.
The brightest is Sirius, the dog star. The brightest stars often appear bright because they are nearby, not because they are intrinsically bright. The obvious example of this is our Sun, which is not a particularly bright star but is wildly brighter than any other star when viewed from the surface of our planet.
Of the twenty brightest stars (not including the Sun), ten are within 100 light years of us, sixteen are within 500 light years and just four are more distant than 500 light years. The familiar Sirius, Vega, Arcturus, Altair and Aldebaran are all within 100 light years. Betelgeuse and Rigel are a bit farther away and are intrinsically bigger and brighter stars.
The superstar of this group of stars is Deneb, 19’th brightest, easily found high in our northern skies in the constellation Cygnus. Deneb is a blue-white supergiant, 2600 light years away. Deneb is believed to be 200,000 times more luminous than our Sun and is large enough that it would engulf the Earth if we were in orbit around it.
The diameter of the Milky Way disk is approximately 100,000 light years. Anything within 500 light years is in our galactic neighborhood. Our night sky is dominated by stars quite close to us… plus Deneb. Even Deneb is relatively close when we look at a map of our Galaxy.
The proximity of these stars is also why they appear to be equally distributed across the sky. If the brightest visible stars were more distant we’d start to see more of them along the disk of the Milky Way (galactic latitude of 0deg) and fewer away from the Milky Way’s disk (galactic latitude 90/-90deg). If we were to look only at very dim stars their distribution in the sky would align with the structure of the Milky Way - and when we see the Milky Way from a very dark site that's exactly what is happening.
While the dominant factor is proximity to us, star types come in a wide variety, with the main factor being the mass. More massive stars burn more brightly, evolve differently and have a different end of life. A second factor is the chemical composition of the star. Stars are born from clouds of gas and dust and their composition will reflect the materials present at their birth and this in turn affects how they evolve with time.
Figure 2: Star Temperature versus Luminosity with colour illustration
Figure 2 shows the relationship between temperature and luminosity for the general star population. The more massive the star the greater the luminosity. Stars will also change their luminosity and temperature as they evolve. As shown here there are bluish stars and red giants and others more yellowish (like our Sun). Deneb has a surface temperature of approximately 8500K, so it would appear roughly in the centre at the top, a blue supergiant. Betelgeuse is a red giant, with a surface temperature of approximately 3500K - the upper right of Figure 2.
The vast majority of stars will have surface temperatures less than 10,000K. Emission nebulae are illuminated by different processes, one of which is radiation from bright stars. To energize Hydrogen and thereby create the common Hydrogen-Alpha (red) emission the star must be able to give off substantial amounts of ultraviolet radiation at wavelengths shorter than 91.2nm. To do this the star must be hotter than approximately 25,000K! If the star is much cooler than this it cannot energize the Hydrogen gas effectively and reflection of the star light from the nearby gas will dominate over emission from these gases.
We mentioned that stars appear to the naked eye (or through a small telescope) as if they were equally distributed over the sky, which is an artifact resulting from their proximity to us. Consider also that many of what we see as single stars are in fact multiple star systems in which the stars are too close for us to separate them visually. Most of the brightest and most massive stars are parts of multiple star systems (the star is bound gravitationally to one or more companions). The North Star, Polaris, appears as a single star but careful observation reveals a companion. Most stars, however, are smaller and dimmer, and of these smaller stars only 25% are multiple star systems. Overall, it seems that multiple star systems are very common, but most star systems have one star, like our solar system.
Stars also appear in clusters, generally of two types. Globular clusters are giant collections of very old stars that are tightly bound to each other, and which are distributed all around the galactic centre – they are not concentrated in the galactic disk. Open clusters are collections of stars that are much less tightly bound. Within a given open cluster the stars will generally be of a similar age. Open clusters themselves have a wide range of ages and we generally find them within the galactic plane.
Astrophotographers will rarely take images of fields of stars, instead the targets will be star clusters. Globular clusters are spectacular because of the sheer size, while open clusters may have a broader range of star types and colours.
To sum it up:
The bright stars that we see are generally very nearby. This causes them to appear in all directions of the sky, which should not be if we look at the shape of the Milky Way galaxy.
Stars have varying brightness and colour, associated with their mass and to a lesser extent their chemical composition.
Stars are often found in multiple star systems or in clusters.
Deneb is pretty hot, but Betelgeuse is cooler.
The image of Messier 13 in Figure 1 was imaged at Lac Teeples on June 16, 2021. The telescope was the RASA8 and the camera was the ZWO ASI 6200MC.
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