10 experiments in astronomical spectroscopy, livre ebook

The field of investigation of astronomical spectroscopy is vast and can provide a wealth of information about the celestial objects observed (Planets, stars, nebulae, novae, supernovae, galaxies, quasars) through the light they emit. Indeed, this light can be broken down into an atomic spectrum, and based on its analysis and current knowledge of physics, it is possible to determine various characteristics of the star being studied.
This book presents methods and tools used for spectrum analysis and covers topics such as:
– the measurement of velocity (radial, orbital, rotational, redshift),
– the evolution over time in certain spectral lines,
– the explosion of a nova or supernova,
– the physical nature of an object (is it a planetary nebula? Is it a galaxy?).
Spectroscopy also allows for the reconstruction of an image by digitizing the surface of a planetary nebula at a given wavelength. It is also possible to estimate the temperature of a star and even evaluate its density.
For each chapter, the author indicates the type of equipment to be used (optics, camera, spectrograph) and explains step-by-step the methodology for producing the necessary data (acquisition, data processing, etc.) using ten concrete examples that the author has tested himself.
This book is intended for amateur astronomers who are curious to learn what can be gleaned from a spectrum so that they can then make their own observations and analyses sometimes rivaling those of professionals !

About the author xi

Acknowledgements xiii

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1 The goal of this book 1

1.2 Prerequisites in astronomy 2

1.3 Units, conventions and physical values in spectroscopy 3

1.3.1 Wavelengths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.3.2 Spectral resolution of a spectrograph . . . . . . . . . . . . . . . . . . . . . . . . 3

1.3.3 Wavelength ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.3.4 The ADU . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.3.5 1D and 2D files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.3.6 Binning area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.3.7 Sky background subtraction areas . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.3.8 Coordinates of a celestial object . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.3.9 Bias, darks, flats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.3.10 The F/D ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.4 The spectrographs 7

1.4.1 Building your own spectrograph . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.4.2 Buying a spectrograph . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.4.3 The spectrographs used in this book . . . . . . . . . . . . . . . . . . . . . . . . 10

1.4.4 The Star Analyser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.4.5 The Alpy 600 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.4.6 The LISA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1.4.7 The UVEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

1.4.8 The LHIRES III . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1.4.9 The eShel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

1.5 Spectrography software 17

1.5.1 Visual Spec . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

1.5.2 ISIS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

1.5.3 Demetra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

1.5.4 Spec INTI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2 The temperature of stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.1 The spectral classes 19

2.2 Selecting targets 24

2.3 Spectra production 26

2.4 Highlighting the results 28

2.5 Estimating the temperature of a star 31

2.6 Bibliography 35

3 Redshift of galaxies and quasars . . . . . . . . . . . . . . . . . . . . . 37

3.1 A brief history 37

3.2 Choice of targets 40

3.3 Making a galaxy spectrum 43

3.3.1 Non-point target . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

3.3.2 Point target . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

3.4 The calculation methods 46

3.5 Example with NGC 7469 49

3.6 Identifying the spectral lines of the galaxy NGC 7469 50

3.7 Correcting the heliocentric velocity spectrum 51

3.8 Measuring spectral line positions 54

3.9 Major redshifts: z >1 the quasar of "Andromeda parachute" 56

3.10 The case of special galaxies 59

3.11 Find the intruder 61

3.12 Redshift measurement frame of reference 62

3.13 Bibliography 63

4 Spectro-imaging of nebulae . . . . . . . . . . . . . . . . . . . . . . . . . 65

4.1 Principles of spectro-imaging 66

4.2 Target selection 67

4.3 Practical case on M57 71

4.4 Spectral sequence processing 73

4.5 Image reconstruction 76

4.6 M57 at various wavelengths 81

4.7 Making science with planetary nebulae 82

4.8 Reddening correction applied to a spectrum 84

4.9 The Balmer decrement 86

4.10 Balmer decrement represented on an image 89

4.11 Calculating the electronic temperature 92

4.12 The electronic temperature map of a nebula 96

4.13 Calculating electron density 98

4.14 The density map of a nebula 100

4.15 Bibliography 100

5 Tracking Be stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

5.1 Observe the Be stars 105

5.1.1 Medium and high resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

5.1.2 Echelle-type spectrographs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

5.1.3 Low resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

5.2 Selecting targets for observation: the BeSS database 108

5.3 Submit your first spectrum to BeSS database 109

5.4 The various profiles of Be stars 112

5.5 Detecting an outburst 114

5.6 Long-term measurement and monitoring 115

5.7 Measuring the V and R peaks of a spectral line 124

5.8 Measurement of equivalent width (EW) 125

5.9 Bibliography 130

6 Spectra of novae and supernovae . . . . . . . . . . . . . . . . . . . 133

6.1 Nova or supernova? 133

6.2 Where to find information? 134

6.3 Novae 135

6.4 Observing the spectrum of a nova 138

6.4.1 Low-resolution spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

6.4.2 Medium/high resolution spectra . . . . . . . . . . . . . . . . . . . . . . . . . . 140

6.5 The P Cyg profile 141

6.6 Converting a wavelength graph into a velocity graph 142

6.6.1 Using ISIS software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

6.6.2 With a Python script . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

6.7 A practical case study with the nova V339 Del 145

6.8 Supernovae 150

6.8.1 Type Ia supernovae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151

6.8.2 Other types of supernovae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152

6.9 Supernova spectrum acquisition 154

6.10 Example with SN 2021 pit 155

6.11 Calculating the velocity of the supernova ejecta 158

6.12 Bibliography 161

7 Confirming a planetary nebula . . . . . . . . . . . . . . . . . . . . . . 163

7.1 The story of the LDu 1 planetary nebula 164

7.2 How to confirm a planetary nebula? 171

7.3 Some examples of the spectra obtained 172

7.3.1 Pre 24 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173

7.3.2 Ra 69 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175

7.3.3 Hu 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177

7.3.4 DeGaPe 32 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178

7.3.5 Pa 30, a very strange object... . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179

7.4 Bibliography 183

8 Measuring rotational velocities . . . . . . . . . . . . . . . . . . . . . . 187

8.0.1 Influence of the rotation axis inclination . . . . . . . . . . . . . . . . . . . . 188

8.1 The rotation of the planets 189

8.2 Practical case study with Jupiter 190

8.3 The rotation of Saturn and its rings 195

8.4 Full width at half maximum of a line (FWHM) 198

8.5 Measurement of rotational velocities based on its FWHM 202

8.6 The rotation of a Be star’s disk 203

8.7 The rotation of matter near a black hole in a galaxy 208

8.8 The rotation of the arms of the M77 galaxy 212

8.8.1 Conversion of distance in pixels / distance in parsecs . . . . . . . . . 217

8.8.2 A clue to dark matter? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220

8.9 Bibliography 221

9 Time series in spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . 225

9.1 Dynamic 2D graphics 226

9.2 BW Vul, a short-period pulsating star 229

9.2.1 Processing a series with ISIS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231

9.3 Study RR Lyr 234

9.4 Study the flickering of spectral lines 237

9.5 Bibliography 242

10 Spectroscopic binaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245

10.1 Principle of measuring spectroscopic binaries 245

10.2 A bit of history 247

10.3 Observe a spectroscopic binary star 249

10.4 Practical case study based on Mizar A 251

10.5 Order the spectra according to phase 252

10.6 The spectral evolution of Mizar A 254

10.7 Measuring the period of a spectroscopic binary: the periodogram 256

10.8 The eccentricity of an orbit 261

10.9 HIP 45080, a binary star with low eccentricity 263

10.10 HIP 26241, a highly eccentric binary star 264

10.11 3D printing of the period of a spectroscopic binary 266

10.12 Bibliography 270

11 Measuring filters, slits and more... . . . . . . . . . . . . . . . . . . . . 273

11.1 WARNING: Precautions to be taken! 274

11.2 Building your own filter test bench 275

11.3 The Lab’Ex with an Alpy 600 or a Star’Ex? 276

11.4 The light module 278

11.5 Measuring the bandwidth of a filter 279

11.6 Practical case with 3 narrowband SHO filters 281

11.7 Measuring a light source 284

11.8 The spectral signature of a liquid 286

11.9 Measuring the width of a slit 288

11.10 Bibliography 291