The Stars
Vincent Van Gogh's Starry Night on the Rhone (Arles, September 1888) is a masterpiece of the "en plein air" style and reveals that Van Gogh was a consummate observer of the night sky. In this chapter you will learn the basic ideas of naming stars and assigning brightness measures to the stars. You will also be able to estimate within a few minutes when Van Gogh looked across the Rhone on a warm September evening more than a century ago!
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| Figure 2.1 Vincent Van Gogh's Starry Night on the Rhone (Arles, September 1888). Image in public domain. |
Constellations and Asterisms
Some arrangements of stars form such striking patterns that they are recognized around the world by different cultures as logical groupings (although they may well have different names and meanings). The astronomical term constellation derives from such groupings. For example, Ursa Major or the Great Bear is an example of a constellation. You might consider Figure 2.1 as showing this but in fact Van Gogh's painting only shows the part of the constellation that North American's call "The Big Dipper" or the British call "The Plough" or "The Butcher's Cleaver". Such a grouping is called an asterism. The Big Dipper (asterism) is part of the much larger Ursa Major (constellation).
There are 88 constellations as officially defined by the International Astronomical Union ion 1928 as well as distinct regions of the sky that accompany these constellations. Figure 2.2 shows this.
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| Figure 2.2 Ursa Major and its boundary as defined by the International Astronomical Union in 1928. |
Example 2.1 Which of the following is a constellation and which is an asterism? a) Orion, b) The Northern Cross, c) Leo - the Lion.
Solution: Both a) and c) are named in the official list of 88 constellations. The Northern Cross is part of Cygnus the Swan and hence is considered an asterism.
Star Names
Astronomy is the oldest science with a history pre-dating the Mesopotamians. As such, it has inherited much from previous cultures and epochs. Nowhere is this clearer than in the way in which we name stars. For example consider Table 2.1 in which some of the possible names for Vega - the brightest star in the summer evening sky.
| Vega | modern common name (IAU approved!) |
| alpha-Lyrae (a-Lyrae) | Bayer designation |
| 3 Lyrae | Flamsteed designation |
| Wega Al Nasr al Waki | arabic name |
| HD172167 | Henry Draper Catalogue |
| HR7001 | Harvard Revised Catalog |
| HIP7001 | Hipparchus Catalogue |
| SAO67174 | Smithsonian Astrophysical Observatory Catalogue |
| BD+38 3238 | Bonner Durchmusterung Catalogue |
| Table 2.1 A few of the different names that could be used for Vega | |
The last five names in Table 2.1 refer to astronomical catalogues which reflects the role that technology has played in changing how we name stars. The first two names are ones most commonly used for bright stars. For Vega the Bayer designation a-Lyrae indicates that Vega is the brightest star in the constellation Lyrae.
Example 2.2 Betelgeuse is the second brightest star in Orion. What is the Bayer designation for this star?
Solution: Since it is 2nd in brightest star in Orion it could also be referred to as b-Orionis (the Latin name of Orion).
Magnitude scales
The "unit of brightness" in astronomy is the magnitude. The idea of ranking the brightness of stars into magnitude groupings goes all the way back to the Greek astronomer Hipparchus who divided the stars into six classes from brightest (magnitude 1 stars) to the faintest (magnitude 6) visible with his eyes. This means that, like golf scores, the magnitude scale goes backwards. The modern scale is a variant of his and is defined the following way: For every 1 magnitude difference in brightness there is a 2.512 times factor in brightness or intensity. Thus, if star A is 5 magnitude brighter than star B then we would conclude that star A is (2.512)5 = 100 times brighter than star B. An other way of thinking about this is that we would receive 100 times as much energy from star A than from star B. Figure 2.2 provides you with an interactive graph that will enable you to convert between magnitude units and brightness units.
| Figure 2.3 A magnitude-brightness interactive graph. You can adjust the magnitude difference range by typing a new number and then pressing enter. If you move your mouse over the graph a dot will appear and two numbers appear in the upper right corner. The first is the magnitude difference, the second is the corresponding brightness factor. You can zoom in to increase the scale by left-mouse click and drag on the graph. Double click restores the original scale. |
Example 2.3 The star Vega has a magnitude of 0.55 while the star Sheliak (b-Lyrae) has a magnitude of 3.50. Which is the brightest and by what intensity or brightness factor? What does this mean?
Solution: Magnitudes go "backwards!" so a small magnitude is brighter than a large magnitude. So, Vega is the brighter of the two (You could also deduce this from the names!). The difference in magnitude is 2.95 so, using the "2.5 brightness factor for each magnitude difference" rule you can conclude that Vega is approximately (2.5 X 2.5 X 2.5) = 15.6 times brighter than Sheliak. If you use the applet in Figure 2.3 you get a more precise value of 15.1 times. This means that we receive 15.1 times as much energy from Vega as we do from Sheliak.
Light Pollution!
A major challenge to astronomy (both amateur and professional) today is light pollution. Few people living in urban areas have ever seen the Milkyway! Un-controlled lighting is the culprit. Figure 2.3 shows this for the province of Alberta and the King's University Observatories.
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| Fig 2.4a: Light pollution map for the central corridor in Alberta | Fig 2.4b: Light pollution map showing laction of The King's University Observatories. |
Astronomers have quantified the darkness of observing sites using the "Bortle Scale. Figure 2.5 shows the scale:
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| Figure 2.5: The Bortle scale and simulation of how the sky will appear from different sites. |
Figure 2.6 shows a photo of the Milkyway taken from a dark rural site (Bortle Class 3)
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| Fig 2.6 Milkyway as seen from a Bortle 3 site. Click on image for a bigger view. |
Practice
- The magnitude scale is "backwards" - like golf scores. Brightest stars having lower magnitudes, fainter stars larger magnitude measures.
| Sirius | Arcturus | Vega | Altair | Deneb | Mizar | Albiereo | |
| -1.46 | -0.04 | 0.03 | 0.77 | 1.25 | 2.3 | 3.4 | |
- Which is the brightest star in the table given here?
- How much brighter is Altair than Mizar?
- How much more energy do we receive from Sirius than from Albiereo?
- The faintest star that one can see under a very dark sky is 6.5 magnitudes. How much brighter is Sirius than this?
- What is the Bayer designation for Sirius? (Hint - it is in the constellation Canis Major)
- Do a web search to find three alternate names for alpha-Ursa Minoris (a UMi)
- True or false - Polaris or the "north-star" is important because it is the brightest star in the sky? Explain your answer.
- Use the Bortle Scale to estimate the Bortle Classification for The King's Univeristy Observatories.
To understand how astronomers name stars and assign brightness measures to the stars
Constellations are regions of the sky officially defined by the International Astronomical Union in 1928
Asterisms are smaller, "informal" groupings of stars. The big dipper is an example of an asterism.
