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Project Cepheus, 20 May - 11 August 2015

Introduction

As of May 2015, I've just begun Project Cepheus. I've been observing a very small part of Cepheus for years... Now I aim to concentrate on the constellation as a whole and examine its more interesting objects, including nebulae, variable stars and clusters, in as much detail as possible.

Variable Stars & Other Objects, 19 May 2015

There are three very different variable stars in Cepheus:

  1. T Cephei, a classic long period red giant variable. The star has a period of about 390 days and varies in brightness from magnitude 5.3 to magnitude 11. It is a relatively cool star.
  2. VV Cephei, an eclipsing binary with a very long period of 20.3 years. The system comprises two massive stars, one hot and blue, the other relatively cool and red. The red star has similarities to ε Aurigae in that it is very large with an extended atmosphere. Primary eclipses occur when the red star obscures the blue one; they last for months and the next one (at the time of writing) starts in late 2016. I plan to continue observations from May 2015 through to the eclipse to look for changes.
  3. S Cephei, an extremely red star. It appears red visually and its spectrum really shows why it is so colourful: it emits very little blue light compared to a typical A-type star. It has a period of 480 days.

On 19 May 2015, I captured images of the first two variables above, and of galactic cluster NGC 188 and the passing Comet Lovejoy (C/2014 Q2).

Figure 1 is my first observation of T Cephei. It shows the field containing the star and the nearby Iris nebula (NGC 7023). (Gyulbudaghian's Nebula is also nearby, but outside the field of view). Figure 2 shows the spectrum of T Cephei (and of the star SAO 19214). T Cephei is just past maximum so will fade over the next few months. I plan to follow the spectrum of the star and look for any changes as it fades. I took figure 2 with the 12° flint objective prism borrowed from the BAA mounted on my old 11 cm F/5 Newtonian (focal length 55 cm). As such, it has only modest resolution. However, it still shows the dark bands associated with titanium oxide in the atmosphere of the star. When I attach the prism to the main telescope next month the resolution will increase by a factor of four and the CCD used with it will give an improved spectral range.

20150519_T_Cep_fld_MJH.jpg Fig. 1.

20150519_T_Cep_spec_MJH.jpg Fig. 2.

Figures 3 and 4 are my first observation of VV Cephei, and used the same set-up as above. Currently the two stars of the system are separated so that the observed spectrum is the superposition of the individual spectra. During eclipse, only the red star is visible so the spectrum should change significantly... this should be fascinating! The next eclipse is due towards the end of 2016 so this is a good time to make baseline observations.

20150519_VV_Cep_fld_MJH.jpg Fig. 3.

20150519_VV_Cep_spec_MJH.jpg Fig. 4.

Comet C/2014 Q2 (Lovejoy) is well past its best but in late May 2015 passed close to star cluster NGC 188 in Cepheus. NGC 188 is a very old galactic cluster so is faint and sparse but nonetheless shows up nicely in the wide field views below, taken on 19 May. Figure 5 is a single 30 second shot. Figure 6 is a combination of 15 one-minute exposures, presented as a negative; it shows galaxies to magnitude 15, and demonstrates the lack of an ion tail associated with the comet.

20150519_NGC188+C2014Q2_MJH.jpg Fig. 5.

20150519_NGC188+C2014Q2_MJH.jpg Fig. 6.

Equipment Upgrade, 03 June 2015

In 2014, I had my new 30 cm F/3.8 mirror finished by Es Reid in Cambridge and in mid-2015 I completed construction of a new prime-focus assembly to convert it to F/6.8. This is achieved with an Astro Systeme Austria Barlow corrector which increases the focal length of the mirror by x1.8. Figure 7 below shows the top end of the telescope with the new corrector/DSLR set-up.

On the night of 03 June 2015, I took the opportunity to test the latest equipment upgrade. Although it doesn't get astronomically dark at this time of year, it is still possible to do useful work on bright stars. So I aimed the new astrograph plus objective prism at a few bright stars to see what it was capable of. Vega was the first target. I took an ordinary image first; the 30 second exposure shows stars to magnitude 13 (figure 8). Next I mounted the prism just above the DSLR body and aimed the telescope at Vega and four other bright stars. Figure 9 shows the results with the stars in order of descending temperature and type: γ Cas is of spectral type B0 with emission lines of hydrogen, Vega A0, Deneb A2, γ Cyg F8 and, finally Arcturus K2.

DSLR+Barlow.jpg Fig. 7.

20150603_Vega_MJH.jpg Fig. 8.

20150603_spectra_MJH.jpg Fig. 9.

μ Cephei and χ Cygni Compared, 22 July 2015

After a summer break I thought it would be fun to compare the spectra of μ Cephei, the famous Garnet star, and χ Cygni, currently rising to maximum brightness (figures 10 and 11 respectively). Both are very red stars, but their spectra show significant differences. Through the eyepiece of the 100 mm F/5 Newtonian finder scope, μ Cephei appeared to be the redder of the two, and the spectra in fact confirm this (figure 12). There is more blue light in the spectrum of χ Cygni. The other significant difference is the hydrogen emission lines in the spectrum of χ Cygni which are missing in that of the other star.

I must get back to T Cephei and VV Cephei with this new, improved, higher resolution equipment!

20150722_mu_cep_MJH.jpg Fig. 10.

20150722_chi_Cyg_MJH.jpg Fig. 11.

20150722_mu_chi_comp_MJH.jpg Fig. 12.

χ Cygni spectrum, 25 July 2015

I had a look at the DSLR spectrum of χ Cygni (above) with Visual Spec software (http://www.astrosurf.com/). This enables the non-linear dispersion of the objective prism to be corrected and the spectrum to be shown on a linear scale. Figure 13 shows data extracted from the DSLR image in the "raw" non-linear form (A) and transformed to a linear scale (B), in Ångströms, with hydrogen lines labelled. It shows how much a prism stretches the blue end of the spectrum.

More work is needed to positively identify the broad bands but they are thought to be from zirconium oxide in the cool atmosphere of the star.

20150722_chi_Cyg_spec_comp_MJH.jpg Fig. 13.

S, T and VV Cephei Spectra, 07-08 August 2015

Figures 14, 15 and 16 are new spectra of the three variable stars in Cepheus. Still using the DSLR camera but now with the 30 cm telescope at F/6.8 to give higher spectral resolution. I've had a look at T and VV Cephei before (above) but this is the first observation of S Cephei. The star is approaching maximum brightness whereas T Cephei is fading and becoming too faint to capture a spectra with a DSLR at F/6.8. In the spectrum of VV Cephei it is easier to see the hydrogen emission lines in this higher resolution view. (Spectra below are shown at the same scale but just ¼ of their original size.)

20150808_S_Cep_MJH.jpg Fig. 14.

Red_stars_comp.jpg Fig. 15.

20150807_VV_Cep_spec_MJH.jpg Fig. 16.


Mike Harlow