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Studying Variable Stars
using Small Telescopes
Sudhanshu Barway
SAAO, Cape Town
What are Variable Star?
Stars showing variations in
their brightness as a function
of time.
Brightness variation: 0.001–
20 mag.
(in V band
∆λ=850 Å)
λeff=5500Å
Period: seconds to years
&
Over 30,000 variable stars
are known today, and roughly
equal number are suspected to
be variables!
Types of Variable Star
1. Intrinsic variables –
Stars which vary their light output, hence their
brightness, by some change within the star itself.
Provide a wealth of information about the internal
structure of stars, models of stellar evolution and
distance determination.
Further classified as:
Pulsating Variables – Cepheids, RR Lyrae, RV Tauri,
δ-Scuti
Eruptive Variables – Cataclysmic Stars, Supernovae,
Novae
Types of Variable Star
2. Extrinsic variables –
Light output varies either due to processes
external to the star itself or due to the
rotation of the star.
Further classified as:
Eclipsing Binaries – Algol, W Uma, β-Lyrae
Rotating Variables – RS CVn, BY Dra,
Light curve of Variable Stars
Light curve – brightness variation of star over time
Light curve of Pulsating Variables Cepheids
Variation in brightness is due to the periodic
expansion and contraction of the surface
layers of the stars i.e. the star actually
increases and decreases in size periodically.
Light curve of Eclipsing Binaries - Algol
A binary system of stars in which
the orbital plane lies edge-on to us
so that the component stars
periodically eclipse one another.
Light curve of Eclipsing Binaries –
more examples
Light curve of Rotating variables –
RS CVn binaries
Brightness variation
is due to the spots
(similar
to
sunspots)
on
the
surface of stars. As
a star with spots
rotates,
its
brightness changes.
Why should one study Variable Stars?
Systematic observations over a longer
period helps:
To determine the short-term and long-term
behavior of the stars.
To construct theoretical models.
Research on variables stars is important for
deriving stellar properties, e.g., mass,
radius, luminosity, temperature, internal and
external structure, chemical composition
and their evolution.
Studying Variable stars using
Small Telescopes
What is a small telescope?
Telescopes with aperture ≤1 m;
typically ~ 6 – 16 inch
16-inch Meade LX 200
Telescope
A typical Modern Small Telescope
Studying Variable stars using
Small Telescopes
State-of-the art Small Telescopes
8-inch
Celestron
CGE 800 Telescope
14-inch
Celestron
CGE 1400 Telescope
Studying Variable stars using
Small Telescopes
Detectors for Small Telescopes
A typical solid state photometer
(Model SSP 3A)
SSP 3A is a basic level instrument
for the measurement of stellar
magnitude.
Detector: P-N photodiode
Effective area: 1mm
For details:
http://www.optecinc.com/
Studying Variable stars using
Small Telescopes
Detectors for Small Telescopes
A small format CCD camera from
SBIG
(Model - ST7 XME imaging camera)
CCD - Kodak KAF-0402ME +TI TC-237
Pixel Array - 765 x 510 pixels
CCD Size - 6.9 x 4.6 mm
Pixel Size - 9 x 9 microns square
Exposure - 0.12 to 3600 seconds,
Computer Interface - USB 1.1
Computer Compatibility - Windows
98/2000/Me/XP and Mac OS
For details: http://www.sbig.com/
Studying Variable stars using Small
Telescopes
Advantages of having Small Telescopes –
1. Convenient access to a telescope.
2. For sufficiently bright stars, small telescopes achieve same
photometric accuracy as that of large telescopes.
3. With advanced increasing sophistications in optics and
electronics it is possible for smaller telescopes to do what
larger telescopes could do in the past.
e.g.. APT (Automated Photometric Telescope) and Robotic
telescopes.
Studying Variable stars using Small
Telescopes
Advantages of having Small Telescopes –
4. Photometry of variable stars using small telescopes is very
convenient and easy. Long term projects are specially good
for small telescopes.
5. The geographical location of an observing place can make it
critically important, independent of telescope aperture.
Studying Variable stars using
Small Telescopes
What can we do with small telescopes?
(a)
Teaching : observational projects for graduate
students
Excellent choice for conveying excitement of
observational Astronomy to the young minds!
(b) Research: suitable projects may be undertaken for
M Phil or Ph D students.
Publications in reputed astronomical journals!
Studying Variable stars using
Small Telescopes
Observational Projects for graduate students1.
2.
3.







Measurement of extinction coefficient.
Calibration of telescope-detector system.
Photometry* of well known variable stars of various types:
Delta-Cepheid variables - δ-Cepheid, ζ- Gem
β- Cephei type variable - ν Eri
δ- Scuti variable - V474 Mon
Eclipsing Binary - Algol-type: β- Per, λ- Tau
Eclipsing Binary - β-Lyrae type: β-Lyrae
Eclipsing binary - W UMa type: 44 Bootes
RS CVn binaries - V711 Tau, UX Ari
* Differential photometry technique is normally use for photometry of variable stars.
Observational Projects for Graduate
Students
Light curve of Cepheid variables •
Periods : 1-70 days
•
<Te> - 4000-5000 K
•
<L> - 300 – 40,000 LΘ
•
Amplitude – 0.1 - 2 mag
•
Massive stars of spectral type F
or G-K
Object: δ Cephei
•
Giant star
•
Period : 5.4 days
•
∆V = 0.89
•
Steep rise but slow decline
Observational Projects for graduate students-
Observational Projects for
Graduate Students
Variable Star
Comparison
Star
Check Star
δCep
εCep
ζ-Cep
RA (J2000.0)
22h 29m 24.21s
22h 15m 15.23s
22h 11m 04.43s
Dec (J2000.0)
+58° 26' 35.6"
+57° 4' 15.1"
+58°13' 42.2"
Sp. Type
F5 Iab
F0 IV
K1.5 Ib
B-V
+0.757
+0.261
+1.590
4.13
4.18
3.34
Name
V
Amplitude V
1.0
Period
5.37 days
Epoch
2451621.151
Variability
Type
δCep
Observational Projects for
Graduate Students
Cepheid variables exhibit a relationship between
their period and their intrinsic luminosity known as
“Period-Luminosity relation”.
Period-Luminosity
relationship can be used to
determine the distance to
a Cepheid.
This relationship also used
to
determine
the
extragalactic
distance
scale
by
observing
Cepheids in other galaxies.
Studying Variable stars using
Small Telescopes
Research projects in observational astronomy –
1.
Variable stars/suspected variable stars
Photometry of rotating variables – RS CVn binaries,
Chromospherically Active Stars or spotted stars.
2.
Nova, Supernova search and follow up observations
3.
Meteor and Asteroid observation
4.
Lunar and Planetary Occultation
5.
Comet observation
Research projects in
observational astronomy
Photometry of rotating variables –
•
Rotating variables are normally late type stars
with enhanced solar like magnetic activities
such as dark spots, highly energetic flares,
facular
networks,
chromospheric
plages,
emission from transition region and corona.
•
Starspots can blanket up to 50% of a
hemisphere of the active star, whereas
sunspots cover only about 0.2% of the solar
hemisphere during the sunspot maximum.
Research projects in
observational astronomy
Photometry of rotating variables –
•
Presence of Strong Ca II H & K and Hα
emission lines.
The rotating variables are also known as
Chromospherically Active Stars include single and
binary
stars.
Different
groups
of
chromospherically active stars are - RS CVn
binaries, BY Dra, W Uma, T Tauri, FK Com
These stars needs continuous monitoring Short term variation: Light curves vary over couple of rotation periods
Long term variation: Light curve varies over long periods like sun spot
cycle
Research projects in
observational astronomy
Photometry of rotating variables –
•
•
•
•
Short-term and long-term activity cycles in
Chromospherically Active Stars.
To investigate the relationship among various
activity indicator with dynamo parameters.
Comparative study of various starspot models
and explore the role of various least square
optimization technique in starspot models.
Search for new Chromospherically Active
Stars.
Research projects in
observational astronomy
Photometry of rotating variables –
Short-term variation
Chromospherically Active Stars
display appreciable changes in
amplitude, shape, phase of
minimum light as well as mean
light level within a short period
of time.
Spot modeling analysis of
photometric data reveals the
variation in spot parameter
within very short period of time.
Light curve of UX Ari
Research projects in
observational astronomy
Photometry of rotating variables –
Long-term variations
In addition to the short-term variation due to rotational
modulation of stellar brightness in rotating variables, it is
believed that there should also exist a systematic long-term
variations.
These long-term variations or Stellar Activity Cycles are very
similar to the solar 11-year sunspot cycle or 22-year
magnetic cycle.
The study of stellar activity is one of important areas where
knowledge of existence of activity cycle can provide a better
understanding of the vital force responsible for a variety of
atmospheric magnetic activities occurring in the Solar as well
as other stellar systems.
Research projects in
observational astronomy
Photometry of rotating variables –
Photometry of suspected variablesPadmakar et al. identified new RS
CVn type rotating variable star HD
61396 using 16” telescope and SSP3A photometer later verified by
Barway et al. 2004.
Barway et al. identified 4 new
rotating variables using IUCAA’s 16”
Meade telescope & SSP- 3A
photometer.
Light curve of suspected variable
star HD 61396
Research projects in observational
astronomy
Research publications using small telescopes
•
Barway, S., Pandey, S.K., Parihar, P.S. and Chaware, L., Long-term
spot activity variation in Chromospherically Active Stars, Poster
presented in IAU 2006.
•
Barway, S., Pandey, S. K., Parihar, P. S., BVR photometry of a newly
identified RS CVn binary star HD 613963, 2004, NewA, 10, 109
•
Barway, S., Pandey, S. K., HD 52452: New BVRI Photometry, 2004,
IBVS 5553, 1
•
Padmakar, Singh, K. P., Drake, S. A., Pandey, S. K., Optical, X-ray
and radio observations of HD 61396: a probable new RS CVn-type
binary, 2000, MNRAS 314, 733
•
Padmakar, Barway, S., Pandey, S. K., Study of sky condition at Raipur
and calibration of the photometer-telescope system, 2000, BASI, 28,
437
•
Padmakar, Pandey, S. K., New BVR photometry of six prominent RS
Canum Venaticorum binaries, 1999, A&AS, 138, 203
ReferencesBooks:
1. Light Curves of Variable Stars
C. Sterken and C. Jaschek
(C.U.P.)
2. The Study of Variable Stars using Small
Telescope
Ed. John R. Percy (C.U.P.)
3. Variable stars
M. Petit (John Wiley &Sons)
4.
CCD observing
telescope
manual
from
Sky
and
ReferencesBooks:
5.
Hands on
telescope
Astrophysics
from
Sky
and
6. Astronomical Photometry Henden, A.A. and Kaitchuck, R.H.
7. Photoelectric Photometry of Variable Stars Hall, D.S. and Genet, R.M.
ReferencesWebsites:
http://www.aavso.org
http://www.skyandtelescope.com
http://www.rssd.esa.int/Hipparcos
http://www.iucaa.ernet.in/html/facilities_meadetel
escope.htm