Visual Observations Visual Observations of Delta Cephei: of Delta Cephei:

Time to Update the Time to Update the Finder ChartFinder Chart

David TurnerSaint Mary’s University

Cephei has a long history as a variable star, its variability being discovered by John Goodricke in October 1784. His friend Edward

Piggot had discovered the variability of the Cepheid Aquilae a month earlier in September 1784.

Polaris — the brightest of 40

classical Cepheids visible without

optical aids.

Cephei

Aquilae

The ready accessibility of δ Cephei for amateur or student observations has been pointed out in the literature from time to time, e.g. van de Kamp, Sky & Telescope, 12, 208, 1953.

Recent article on δ Cephei on the AAVSO website.

AAVSO finder chart with ζ Cep and ε Cep as reference stars.

A more detailed comparison chart for δ Cephei from the AAVSO. Problem: ζ is

clearly brighter than ι to the eye, and closer to β in brightness.

Specifically: 10 years of regular observation by two AAVSO observers (AAP, SSW) relative to

the photoelectric light curve. Visual observations do not match the amplitude or extrema of δ Cep.

Alternate method (Turner JRASC, 93, 228, 1999).

Alternate comparison chart for δ Cephei tied to photoelectric V-magnitudes from the Bright Star Catalogue. At faint limits the eye can distinguish

differences of 0m.1 or better (Turner 2000).

See Turner (JAAVSO, 28, 116, 2000).

Cephei 3.35

3.52

3.77

4.29

4.19

Cassiopeia

Cepheus

Lacerta

Delta Cephei comparison

chart

An accurate ephemeris for a Cepheid is essential for proper phasing of the observations. For δ Cephei:

HJDmax = 2442756.49 + 5d.36627 E

First observations of δ Cephei tied to the alternate comparison chart.

Phased photoelectric observations of δ Cephei from the literature.

A comparison of eye V magnitudes with the photoelectric V light curve.

Note the 0m.15 offset.

A similar comparison with eye estimates by the author for η Aquilae.

There is no offset here.

Ditto for ζ Geminorum.

A combination of observations from 1999-2000 with every five data points averaged in phase.

The results are comparable to photoelectric data.

Additional observations of δ Cephei from 2009.

... and from 2010.

A study of the evolutionary period decrease in δ Cephei from 230 years of observation.

John Goodricke

David Turner

The problem lies in the adopted reference magnitudes for the two main comparison stars: ζ Cep and ε Cep. van de Kamp (1953) lists values of 3.60 and 4.36 for the stars from Potsdam Durchmusterung photovisual pv magnitudes. Their photoelectric V magnitudes are 3.35 and 4.19, respectively, while AAVSO chart values are 3.6 and 4.2. Since all good eye estimates of δ Cephei are made under normal light conditions, the use of AAVSO charts artificially depresses the observed light amplitude and skews light maximum from its true brightness.

Star V BV SpT Vtrans Vchart

ζ Cep 3.35 1.57 K1.5 Ib 3.60 3.6ε Cep 4.19 0.28 F0 IV 4.21 4.2ι Cep 3.52 1.05 K0 III 3.68 3.5α Cep 2.44 0.22 A7 V 2.45 2.4υ Cep 4.29 0.52 A2 Ia 4.35 ...α Lac 3.77 0.01 A1 V 3.74 ...

The study by Richard Stanton (JAAVSO, 10, 1, 1981) relating pe V magnitudes to visual magnitudes on AAVSO charts. He derived the relationship: mvis = V + 0.182 (BV) 0.15

Ron Zissel’s (1998) study of the visual magnitude system adopted the same formula… for observations with the dark-adapted eye.

The wavelength dependence of the photoelectric V filter relative to that for the dark-adapted eye.

But the eye’s spectral response for normal lighting conditions is much closer to the

wavelength sensitivity of the Johnson V filter.

Since most observations of Cephei are done in normal lighting conditions, use

of a comparison chart with star magnitudes adjusted for dark adaption is inappropriate. Observations near the visual limit are also more accurate than

those made of bright stars using telescopes, so the ideal observations of δ Cephei are made by eye or binoculars before the eye becomes dark adapted.