Activity 9 – Roller Coaster Stars
 Context
- Once you are familiar with some of the most impressive
binocular targets, it is time to explore variable stars
– stars that grow brighter and fainter over time.
Three of the easiest variable stars to observe are featured
in this activity. The three are all of a type known as
Cepheid variable stars.
Specific Learning Outcomes
- You will locate and observe a Cepheid variable star,
recording your observations as a light curve showing the
variations in brightness over several weeks.
Planning and Preparation
- Observing the light curve of a variable star usually
requires a series of observations spread over several
weeks.
- Northern Hemisphere observers are recommended to
observe delta Cephei. Southern Hemisphere observers are
recommended to observe l
Carinae. Both of these
stars are circumpolar at most latitudes so they can be
observed all year round. However, in the equatorial zone
you might find that both of these stars are too low in the
sky. In that case choose eta Aquilae, which will be best
observed in the months of June to September. (Use your
Star
Wheel to find out more about when and where these stars
will appear at your latitude.)
- Next print the finder chart and comparison chart for
the appropriate star (download here).
- These stars are visible to the naked eye under dark
conditions, but binoculars are essential if there is some
light pollution at your observing site. For most city
observing sites you will need binoculars.
What You Need
Science Background Knowledge
Vocabulary checkpoint
-
Magnitude: A measure of the brightness of
a star. The Ancient Greek astronomers divided the stars
into six magnitudes. First-magnitude stars were the
brightest, and sixth-magnitude stars were the faintest.
This system has been kept in modern times, although
magnitudes are now measured more accurately, and it has
been extended to cover stars fainter than the eye can see
unaided. This table shows how the modern numbers match up
with the ancient magnitudes:
|
Ancient Greek magnitude
|
Modern magnitude
|
|
1st
|
Numbers lower
than 1.5
|
|
2nd
|
1.5 to
2.5
|
|
3rd
|
2.5 to
3.5
|
|
4th
|
3.5 to
4.5
|
|
5th
|
4.5 to
5.5
|
|
6th
|
5.5 to
6.5
|
|
Too faint to
see unaided
|
Numbers
higher than 6.5
|
- The three Cepheid variable stars that feature in this
activity are all 4th magnitude stars, never
brighter than 3.3 or dimmer than 4.4.
- Period: When something repeats in a
regular pattern, its period is the amount of time
for one complete cycle. For a variable star this is the
time between it reaching a peak of brightness, and reaching
its next peak of brightness.
- Light curve: A light curve is a graph
showing the showing the variations in brightness of a
variable star over time. For a Cepheid variable star the
light curve usually covers several days or weeks. There is
an example below.
Variable Stars
- Variable stars are stars that vary in brightness. Some
of them vary in an irregular way, due to flares, fades, or
cataclysmic outbursts. Others vary in a regular way. The
regular variable stars include:
- stars that vary because one part of the surface is
brighter than the rest and they are rotating,
- stars that vary because there are two stars orbiting
each other and the fainter star regularly passes in front
of the brighter star causing it to dim, and
- stars that vary because the star itself is pulsating
(pulsating variable stars).
Cepheids
- Cepheids are a particular type of pulsating variable
star. They are named Cepheids after the star
delta Cephei which is a prime example of the
type.
- Cepheids have a special part to play in astronomy. The
period of a Cepheid can be used to determine how much light
it is giving out, and this information can be used to work
out how far away it is. Cepheids are used to find out how
far away distant galaxies are.
- Cepheids are stars at a particular stage in their life
cycles. Typically a star is stable for most of its lifetime
while it gets energy from turning hydrogen into helium. The
energy rushing out from the centre of the star balances the
pressure of gravity pushing in, and the star settles into a
stable state. This is called the main sequence
stage. In the case of our sun we believe that it has been
in its main sequence stage for 5 billion years, and it will
last another 5 billion years before the hydrogen is all
gone.
- When the hydrogen is gone from the core, the star uses
up the hydrogen from around the core and it swells up to
become a red giant. When the last of the hydrogen has gone
the force of gravity overwhelms it, and the star collapses.
But not for long. The increased pressure brought about by
this collapse is enough for the helium to start turning
into carbon, and this marks the beginning of a new stage in
the star’s life; helium burning.
- A star does not settle into a steady state so easily in
the helium burning stage. Large stars, between three and
nine times as massive as our sun, usually go through
lengthy periods during which the star expands and contracts
on a regular basis; something like a cross between a jelly
wobbling and a ball bouncing, but much bigger and slower
and much more regular. During these parts of its helium
burning stage, the star is known as a Cepheid variable
star.
- Cepheids typically change brightness by between half
and two magnitudes, and have periods of between 1 and 50
days. The three Cepheids that feature in this activity vary
in brightness by about one magnitude. They are among the
brightest Cepheids in the night sky.
The five brightest Cepheids
|
|
Star
|
Constellation
|
Magnitude when
brightest
|
Magnitude when dimmest
|
Change in magnitude
|
Period
|
|
Polaris
|
Ursa Minor
|
1.8
|
2.1
|
0.3
|
4.0 days
|
|
l Carinae
|
Carina
|
3.3
|
4.2
|
0.9
|
35.5 days
|
|
Beta Doradus
|
Dorado *
|
3.5
|
4.1
|
0.6
|
9.8 days
|
|
eta Aquilae
|
Aquila
|
3.5
|
4.4
|
0.9
|
7.2 days
|
|
delta Cephei
|
Cepheus
|
3.5
|
4.4
|
0.9
|
5.4 days
|
source:
General
Catalogue of Variable Stars
* Dorado is between Hydrus and Carina.
-
Here is a typical Cepheid light curve:
|
