Activity 3.1.2 – Cepheid Variable Star Observation
Beta Doradus – A Cepheid Variable Star
Background
Among the most important of all stars are the Cepheid variables, named after the prototype, Delta
Cephei. Beta Doradus, (the second brightest in the constellation) is a fourth magnitude (averaging ~3.76)
pulsating variable star measured to be approximately 1000 light years from the Earth. The modern
southern constellation of ‘Dorado’ is the Spanish name for "dolphin fish" although in the 17th and
18th century, the constellation was also known as ‘Xiphias’, the swordfish however it is still referred
to as the ‘Goldfish’on some star maps in use today. It was one of the constellations observed by the
Dutch navigators Pieter Dirkszoon Keyser and Frederick de Houtman and listed by the Dutch
astronomer Petrus Plancius in the 16th century. The Dutch navigators observed the dolphinfish
chasing flying fish, which is why Dorado was placed in the sky next to the constellation Volans, which
represents the flying fish.
At its peak brightness, Beta Doradus actually reaches into third magnitude, 3.41, and then falls to 4.08,
oscillating between the two over a slightly variable 9.842...day cycle. Cepheid stars are evolved
supergiants, and this one is no exception, changing its spectral class during its cycle and temperature as
well (the average is about 6000 Kelvin). From the distance of 1040 light years, the star shines with a mean
luminosity equivalent to 3000 of our Suns, which implies a radius of approximately 50 times our sun and
a mass 6.5 greater. Direct measure of angular diameter through interferometry, however, suggests a
larger star, closer to 65 times bigger than our Sun.
Cepheid stars pulsate, alternately growing larger then smaller, hotter then cooler (which may explain part
of the difference), as a deep layer within them traps and releases heat. In an unstable state, they expand,
shoot past equilibrium, fall back under the force of gravity, then over-compress and expand again. The
effect involves only the outer layers -- the core has no idea of what is going on. Though we know they
long ago gave up hydrogen fusion in their cores, they can be in any of a variety of conditions.
Cepheids follow a strict "period-luminosity" relation. The more luminous they are -- in total power output
-- the longer the pulsation period (which makes sense, since brighter stars are the larger, and they just
take longer to go through the cycle). The pulsation period of any random Cepheid thus gives its absolute
brightness, while comparison with apparent brightness gives the distance. Find a Cepheid in another
galaxy, and from measurements of its period you can calculate its absolute luminosity from which you
can determine the galaxy's distance. They are the key to finding the structure of the Universe and the
nature of its expansion.
Activity 3.1.2 – Cepheid Variable Star Observation
Visual Magnitude
Period of Beta Doradus
4.2
4.1
4
3.9
3.8
3.7
3.6
3.5
0
2
4
6
8
10
Number of Days
Drawn from 430 observations taken from 1937 to 1941 by Cousins, A. W. & Williams E. G.
Observational activity - Beta Doradus
This first investigation is designed to provide you with insights into the world of observational
astronomy. It is intended to challenge you with an example of an authentic observational task
where you can see the obvious benefits of joint collaboration. If the weather permits, it will allow
you to become familiar with the skills of navigating the night sky using star charts and introduce you
to the practical difficulties of collecting and recording reliable scientific data for analysis.
This task encourages you to recreate (in a small way) the extensive naked eye observational work
undertaken by Andrew Cousins from the Astronomical Society of South Africa from 1937 to 1941.
During this time he recorded over 420 observations of Beta Doradus as part of a much larger study
of variable stars (during which he completed more than 1400 observations). His work has been
instrumental in accurately determining the period of varying brightness of this Cepheid variable star.
As discussed in the course, measuring the periodic changes in the light intensity of Cepheid stars
provides us with evidence of their absolute luminosity. Knowing how bright the star shines in its
region of space and how bright it appears to us from ours allows us to accurately determine how far
away it is. This provides us with a very powerful method of measuring the vast distances to galaxies
that contain Cepheid variable stars if we can measure their periodic changes in brightness.
What to do
Down load the two star maps. The first A4 map provides you with a general view of the night sky as
it will appear in the southern hemisphere in February, 2013. This will encourage you to become
Activity 3.1.2 – Cepheid Variable Star Observation
familiar with the location and names of several of the brightest stars from which you can begin to
navigate around. Located on this map is the position (rectangle) of a more detailed A3 map giving
the location of the fainter star Beta Doradus. This detailed map also contains a number of reference
stars (contained in red circles) who's brightness (magnitude) is provided for you. These are similar in
rage to Beta Doradus during its cycle but importantly these stars remain constant. You will use these
starts to estimate the magnitude of Beta Doradus on different nights as it changes in brightness by
comparing it with the comparison stars.
Hints on how to estimate the magnitude
1) Most importantly, before attempting this task avoid watching TV or viewing a bright
computer screen for at least 30 mins immediately before you venture out doors.
2) You will need to allow at least 10-15mins for your eyes to adjust to the dark so you have
good night vision. Avoid using bright white torches, particularly LED torches while
waiting for your eyes to adjust. Try using this time to see how many earth satellites you
can count passing over head in 10mins. You will begin to see more and more as your
eyes become more receptive to the low light conditions.
3) Start by looking at the reference stars and identifying the ones that clearly appear
brighter and fainter than Beta Doradus. As your eyes become more sensitive estimate
the magnitude of the Cepheid using the reference stars which appear to have a similar
brightness. Use the reference stars to estimate the maximum and minimum brightness
which you observe beta Doradus to be somewhere in between.
4) Try not to stare at any one star for too long as you will begin to think it is brighter than it
really is. Instead look back and forth from the reference star to the target star about
every half second. Your eyes will also naturally assess reddish stars to be brighter than
equivalent stars of other colours. This is called the Purkinje Effect.
5) Record the date and time of your observation and the conditions of the sky at the time.
E.g. Patches of thin cloud, or a bright quarter moon nearby. Upload your estimate of the
magnitude of Beta Doradus and the date of the observation on the Google doc shared
with you for this purpose. Also make a backup of your recordings.
For more details about how to improve the accuracy and reliability of your estimates see the advice
provided by the American Association of Variable Star Observers, (AAVSO).
References:
Cousins, A.W, (1962) Visual Light Curves of Eta Carina and Beta Doradus, Monthly notes of the
Astron. Soc. Southern Africa, Vol. 21, p. 77
Kaler, Jim, (2012), Blog Page, Prof. Emeritus of Astronomy, University of Illinois.
TopAstronomer.com (2012)
Activity 3.1.2 – Cepheid Variable Star Observation
Activity 3.1.2 – Cepheid Variable Star Observation