Digital Photography of Star Clusters

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Because nebulae and galaxies are extended diffuse objects, with the exception of the bright nebula M42, the great nebula in Orion, digital photography with small telescopes will not reveal more than the eye can see. Common fixed lens digital cameras do not have sufficient noise reduction for the long exposures needed. Although digital single lens reflex cameras are available with ISO speeds of 800 or more, resolution of 6 to 8 megapixels and much longer low noise exposure times, we consider them too expensive to be used within the context of this book's intentions.

Star clusters, however, consist of numerous point sources and a 30-second exposure with a fixed lens digital camera will reveal much more than the eye can see in the telescope. Figure 14.2, for example, is a 30-second exposure of the globular cluster M13 in Hercules obtained with an 80-mm f/6 refractor. Visually, in the telescope this cluster appears as a luminous, circular patch of light with no distinguishing features.

Stars clusters, in general, are of two types - globular and galactic. Globular clusters are spherical aggregates of 10,000 or more stars that lie in a halo surrounding the plane of the Milky Way. They contain the oldest stars in our Galaxy. Galactic clusters are loosely associated arrays of stars that formed in the plane of the Galaxy. Table 14.2 lists some star clusters that are good objects for small telescope digital photography.

For extended exposure photography of nebulae and clusters, a short focal length telescope (f/5) is preferred and accurate alignment of the mount with the celestial pole is essential. This will make 30-second exposures without elongating the star images. Use the same procedure described in Chapter 9 for variable star photography.

If the contrast is increased, downloaded photos will show fainter stars. For these objects, preserving the linearity of the camera response is not a concern.

Figure 14.2. A digital photo of the globular cluster M 13 taken with an 80-mm f/5 refractor.

Table 14.2. Star Clusters for Digital Photography

Designation

RA

Decl.

Magnitude

Description

M 13

16h42m

+36°26'

5.9

Globular Cluster

M 12

16h47m

-1°57'

6.6

Globular Cluster

M 10

16h57m

-4° 06'

6.6

Globular Cluster

M 92

17h 17m

+43-08'

6.5

Globular Cluster

M 15

21 h30m

+12° 10'

6.3

Globular Cluster

2802 Car

09h 12m

-64°52'

6.3

Globular Cluster

3201 Vel

10h18m

-46° 25'

6.7

Globular Cluster

m Centauri

13h27m

-47°29'

3.7

Globular Cluster

NGC 884, 869

02h20m

+57°08'

4.4

Double Cluster in Perseus

M 34

02h42m

+42°47'

5.2

Open Cluster

M 45

03h47m

+24°07

1.2

Pleiades

M 38

05h50m

+32°08'

6.4

Open Cluster

M 36

05h36m

+34°08'

6.0

Open Cluster

M 37

05h52m

+32°33

5.6

Open Cluster

M 44

08h40m

+20°00'

Beehive Cluster in Cancer

M 29

20h24m

+38°32'

6.6

Open Cluster

M 39

21h32m

+48° 26'

4.6

Open Cluster

M 52

23h24m

+61 °35'

6.9

Open Cluster

4755 Cru

12h54m

-60°20'

4.2

Jewel Box

M6

17h40m

-32° 13'

4.2

Butterfly Cluster

M7

17h54m

-34°49'

3.3

Open Cluster

Increasing contrast too much, however, will introduce a significant amount of electronic noise.

W. C. Bond and his son G. P. Bond obtained the first Daguerreotype image of a star in the 1850s. They needed a 20-minute exposure to record Vega with the Cambridge 380-mm refractor. Compare this with a 15-second exposure of a star cluster using your digital camera on a small telescope.

CHAPTER FIFTEEN

A Color-Magnitude Diagram for The Pleiades

As a source of radiant energy gets hotter, it gets brighter and its color shifts toward the blue end of the spectrum. Red stars have surface temperatures of about 4000 K; blue ones exceed 15,000 K. The quantitative measure of a star's color then is an indication of its temperature, is the color index. It is derived from measurements of the difference between the apparent magnitudes in two different regions of the spectrum.

A common method is to measure the magnitude of a star with a photometer in which filters are used to limit the measurement to different regions of the spectrum. When the measurement is limited to the blue region it is called the B magnitude. A filter that matches the response of the human eye to color is the V magnitude. The color index is then given by

The absolute magnitude is a measure of the intrinsic luminosity of a star. By comparing absolute magnitudes we are comparing them as if they were all at the same standard distance of 10 parsecs. Consequently, stars of lower absolute magnitude number are (for example, M = -2) intrinsically more energetic than stars of higher magnitude number (for example, M = +5). We should also expect to find the intrinsically brighter stars to have higher temperatures and bluer colors. A plot of absolute magnitude vs. color index for a population of stars is called a color-magnitude diagram. If a particular group of stars all belong to the same star cluster, absolute magnitudes need not be determined. Since they

Figure 15.2. A color-magnitude diagram for an old evolved star cluster.

A Color-Magnitude Diagram for The Pleiades 131

are all at approximately the same distance, their apparent magnitudes and color indexes can be compared. This can be done for some of the brighter relatively nearby star clusters by measuring prints from a digital camera attached to a small telescope.

Figure 15.1 is a color-magnitude diagram for a young star cluster, the stars of which occupy a narrow band called the main sequence. The curve running through the distribution defining the zero-age main sequence represents the point in the star's evolution at which nuclear fusion begins. In Figure 15.2 we see a much older cluster in which many of the main sequence stars have evolved along a horizontal giant branch; some to the red super giant stage.

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