Where to begin the adventure with variable stars?

The variable stars that are particularly suitable for observations for

beginners

Mateusz Bielski

Editors: A. Majczyna and M. Należyty Logo design: Armella Leung, www.armella.fr.to Translated to English by Magda Zarzycka.

The photometry of variable stars is ranked among the most difficult observations that can be done with

the use of a webcam built on the basis of a CCD. Learning how to gather and process data requires patience

and determination...

In reward for this you will get results that have not only didactic, but also

scientific value in the broadest sense of the word.

Therefore, teachers can create with their students a true scientific

laboratory at school!

Which stars should we choose for a start?

The process of developing your own technique is difficult and time-consuming. To be able to

observe weaker stars or those whose amplitudes are smaller, it is a good idea to practise with those

that are, so to say, ‘created’ for photometric observations with a webcam.

In the sky you will find precisely such stars, whose observations almost without exception return

results that impress laymen or beginners equally.

Below are three suggestions of stars whose amplitudes and magnitudes are so great that you can easily observe their minimum in the star light curve. The data come from observations conducted by the author.

• RZ Cas• TX UMa• U Cep (an example of a flat-bottomed curve)

RZ Cas

• Type: eclipsing binary

• Max magnitude: 6.4 mag

• Min magnitude: 7.8 mag

• Amplitude: 1.4 mag

• Period: 1.19525780 JD

RZ Cas – light curve

RZ Cas

-1

-0,5

0

0,5

1

1,5

2

2453411,30 2453411,35 2453411,40 2453411,45 2453411,50 2453411,55

Dni Juliańskie

C-V

[m

ag]

Julian Days

TX UMa

• Type: eclipsing binary

• Max magnitude: 7.06 mag

• Min magnitude: 8.8 mag

• Amplitude: 1.74 mag

• Period: 3.06329200 JD

TX UMa – light curve(incomplete, only one arm, but the variability is clearly visible)

Julian Days

U Cep

• Type: eclipsing binary

• Max magnitude: 6.74 mag

• Min magnitude: 9.81 mag

• Amplitude: 3.07 mag

• Period: 2.49309770 JD

U Cep – light curve

U Cep

0

0,5

1

1,5

2

2,5

2453267,45 2453267,5 2453267,55 2453267,6 2453267,65 2453267,7 2453267,75

HJD

V-C

[m

ag]

How to find these stars in the sky?

To be able to find the object of interest, it is necessary to know how to identify particular

constellations and stars.

This will require a few nights and some patience

but what you will get in reward is a great satisfaction.

A planisphere will be of great help while learning

how to navigate around the sky.

The easiest way to find it is on the internet (just search for ‘planisphere’ in any search engine) and buy it online (it costs

around $8).

This is what a planisphere looks like

Very simple instructions how to use it can be found at the back of the planisphere.

If you have a portable computer or have access to a stationary computer where you are, you can use electronic atlases of the sky.

Most of them are commercial programs but you can also find

freeware of this sort on the internet.

Cartes Du Ciel is the most popular atlas

It is available at:

www.stargazing.net/astropc

Electronic atlases are very helpful and allow users to print out accurate maps that help to identify variable stars and reference

stars that are later used in the process of photometry.

The navigation in the night sky

To navigate the sky quickly and effectively, it is necessary to learn first how to identify the main

constellations.

There are a few constellations in the sky whose main stars are very bright and whose shape is

very characteristic and therefore easy to recognise.

Ursa Major, also called Great Bear

Ursa Major, also called Great Bear

Mizar and Alcor

Cassiopeia

Cassiopeia

Andromeda

Andromeda

M31

Ursa Minor, also called Little Bear

Ursa Minor, also called Little Bear

Polaris

Lyra

Lyra

Vega

Cygnus, also called Northern Cross, Swan

Vega

Deneb

Lyra

Cygnus, also called Northern Cross, Swan

Cepheus

Cepheus

Polaris

Cassiopeia

Perseus

Perseus

Algol

HerculesDifficult to find

Hercules Difficult to find

Taurus, also called Bull

Taurus, also called Bull

Pleiades

Aldebaran

Orion

Orion

Aldebaran

Gemini, also called Twins

Gemini, also called Twins

Pollux

Castor

Bootes

Bootes

Arcturus

Aquila

Aquila

Altair

Auriga, also called Charioteer

Auriga, also called Charioteer

Capella

Leo, also called Lion

Leo, also called Lion

Regulus

Pegasus

Pegasus

Of course, there are far more constellations in the sky, but once we learn to find and recognise those most characteristic ones, finding the other

ones with the use of a planisphere should not cause any problems.

Constellations can be divided into those that are visible in the sky in winter, those that we can see

in summer, and those that are present all year round.

Winter constellations:

• Orion

• Taurus

• Monoceros

• Canis Major

• Cetus (?)

• Leo (?)

Summer constellations:

• Aquila• Serpens and Ophiuchus• Delphinus• Capricorn• Sagittarius• Pegasus• Lyra• Cygnus• Andromeda

Constellations visible all year round:

• Ursa Minor• Ursa Major• Cassiopeia• Draco• Cepheus• Camelopardalis• Perseus• Auriga• Lynx

When we know how to navigate across the night sky and we are equipped with printed maps

(e.g. from Cartes Du Ciel), we can start searching for interesting variable stars

in order to conduct photometric observations with a webcam.

In the case of the three stars presented before, the situation is simple because they belong to constellations that are visible all year round in the

northern hemisphere. They are located in relation to one another in the following way:

In the case of the three stars presented before, the situation is simple because they belong to constellations that are visible all year round in the

northern hemisphere. They are located in relation to one another in the following way:

PolarisUrsa Major

Cassiopeia

Cepheus

The location of this set may be different depending on the season and the time of observation but the stars

always remain in the same position in relation to one another.

(Ursa Major is opposite to Polaris in relation to Cepheus and Cassiopeia)

Steps to take:

• First, we should find the constellation with the variable star that is of interest for us (the second part of the name informs us about this, e.g. RZ Cas means that the star is in the constellation of Cassiopeia). No optical devices are necessary to do this.

• Using a map, we find the nearest neighbour of our variable star, that is visible with the naked eye.

Steps to take:

• Next, with the help of binoculars or a finderscope, we find our star of interest (the easiest and most efficient solution is to construct a set that will enable us to join a webcam and a finderscope, so that we can focus them on the same point in the sky).

• Lastly, we take pictures of the area of the sky and compare them with the map to make sure that our variable star and the star of reference are both in the field of view of the webcam.

From the suggested stars, the easiest is to find RZ Cas in the constellation of Cassiopeia.

Find Cassiopeia in the sky. The easiest way to do this, is to go from Polaris towards Andromeda in a straight line.

Polaris μ And

M31

When we find Cassiopeia...

Find the star ι (Iota) located at the extension of the arm that ends at the star Segin.

Segin

ι (Iota)

Next, already with the help of a telescope...

When the ι (Iota) star is in the centre of the view field of our telescope, we can easily find RZ Cas moving outwards Segin (see the arrow).

Segin

ι (Iota)

RZ Cas

It is best to have the variable star at the centre of the view field (the net of crosses in the telescope) and choose one of the four stars in the characteristic

zigzag as the star of reference. Another variable star, SU Cas, will be in your field of view as well. It is not suitable for a star of reference, however!

ι (Jota)

RZ Cas

SU Cas

It is somewhat more difficult to find TX UMa. It is in Ursa Major and we should begin with finding this constellation.

Finding the constellation does not pose any problem even to beginners. Following the straight line connecting two stars in Ursa Minor: Zeta Ursae

Minoris and Beta Ursae Minoris (see picture) you will find Mizar.

Mizar is the most popular star in Ursa Major thanks to its neighbour – Alcor, visible with the naked eye.

Mizar

Find Phecda in the constellation.

Moving from Megrez through Phecda find the star (see the arrow).

Phecda

Megrez

Moving still further from the bear from the χ star you will find the ψ and stars. In a dark sky they are visible with the naked eye but it is a good idea to use a finderscope. These two stars form a triangle with the star HIP 52469.

ψ

HIP 52469

TX UMa is situated a little below the line that connects the stars and HIP 52469, around 1/5 way from HIP 52469. It is not visible with the naked eye. One of the stars in the yellow circles can be used as a star of

reference.

HIP 52469

HIP 52831

HIP 52881

HIP 52702

HIP 53160

Finally U Cep

Let’s begin with finding the constellation of Cepheus, where the star is situated.

It will be easiest to begin with locating the star Alrai.

Moving in a straight line from Polaris towards Cassiopeia, and more precisely from the star Caph, we will find the bright star Alrai, belonging

to the constellation of Cepheus.

Polaris

Caph

Alrai

ι (Iota)

When we have found Alrai...

Moving along the straight line connecting Cephei and Alrai, further from Alrai towards Polaris, we will find HIP 760. It is not visible with the naked eye, so in

this step we will need a finderscope.

Alrai

HIP 760

Next, moving along the line that connects Alrai and

HIP 760 in the same direction as previously, we

will find U Cep.U Cep is situated more or less at the same distance

from HIP 760 that HIP 760 is from Alrai.

U Cep

HIP 760

Alrai

One of the stars in the yellow circles can be used as a star of reference.

HIP 5108

HIP 4966

TYC 4505-387-1

TYC 4505-558-1

HIP 3132

In this way we have found the stars and we can begin photometric observations. Their result will probably impress not only us, but also other students and our friends. Also the acquired skills of navigating

in the sky will be a source of satisfaction!