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Variable Stars
Katrien Kolenberg
Ulaanbaatar
May 2007
Variable Stars
Stars…
massive, luminous ball of plasma
Astronomers can determine the mass,
age, chemical composition and many
other properties of a star by observing its
spectrum, luminosity and motion through
space.
The total mass determines its evolution.
Stars…
massive, luminous ball of plasma
Astronomers can determine the mass,
age, chemical composition and many
other properties of a star by observing its
spectrum, luminosity and motion through
space.
The total mass determines its evolution.
Stars…
Stars…
Stars…
The HR diagram
What is a variable star?
The physical reason for the brightness changes determines the
classification of the stars into the different classes and
subclasses.
What is a variable star?
pulsars
Variable Star Naming
•
Catalogues: e.g., the General Catalogue of Variable Stars
•
Naming:
first double capital letters according to the following scheme:
R, S, …, Z,
RR, RS, …, RZ,
SS, ST, …, SZ,
…,
AA, AB, …, AZ,
BB, BC, …, BZ,
…,
QQ, …, QZ
NB: no J, no BA, CA, CB, etc. …
334 possibilities per constellation
then further with V335, V336, …
V2412: highest number in the GCVS
What is a variable star?
Different Types of Variability
• intrinsic variables:
variation is due to physical
changes in the star or stellar
system
• pulsating and
eruptive variables
• extrinsic variables:
variability is due to the eclipse
of one star by another or the
effects of stellar rotation
• Eclipsing binaries
and rotating stars
Intrinsic Variables: Pulsating Stars
Oscillations are excited in almost
all types of stars and in many
stages of stellar evolution
Periodic expansion and
contraction of surface layers
• Radial: star keeps
spherical symmetry
• Nonradial: deviations from
a sphere
Intrinsic Variables: Pulsating Stars
•
Some lightcurves for different
types of pulsating stars
Cepheids:
P = 1-70 days
A = 0.1-2.0 mag
RV Tauri stars:
P = 30-100 days
A = up to 3.0 mag
Intrinsic Variables: Pulsating Stars
Long Period Variables:
also LPVs or Miras
P = 80-1000 days
A = 2.5-7.0 mag
Semiregular Variables:
P = 30-1000 days
A = 1.0-2.0 mag
Also irregular variables occur!
Intrinsic Variables: Cataclysmic Variables
•
Cataclysmic variables (or eruptive variables) are stars that have occasional
violent outbursts caused by thermonuclear processes either in their surface
layers or deep within their interiors.
What is a variable star?
pulsars
Intrinsic Variables: Cataclysmic Variables
•
•
Double stars with mass transfer
Main sequence star and compact star
Supernovae
Stellar explosions, look like a new star!
Maxima at Mv=-20
Can be brighter than the whole
galaxy!
Important for the chemical
evolution of the galaxy
Supernovae
Supernovae are divided into two basic physical types:
Type Ia. These result in some binary star systems in which a carbon-oxygen
white dwarf is accreting matter from a companion.
Supernovae
Supernovae are divided into two basic physical types:
Type II. These supernovae occur at the end of a massive star's lifetime,
when its nuclear fuel is exhausted and it is no longer supported by the
release of nuclear energy. If the star's iron core is massive enough then it will
collapse and become a supernova. (core-collapse supernovae)
Supernovae Type II
Supernovae
Use of Supernovae
Use of Supernovae
Historic Supernovae
Remnants of first ever recorded supernova? (7 Dec 185 A.D.!!)
Historic Supernovae
SN 1054  supernova which caused the Crab Nebula, seen by
Chinese and Arab astronomers and maybe native Americans
SN 1572  Tycho Brahe‘s Supernova in Cassiopeia
Historic Supernovae
SN 1604  Kepler‘s Supernova; last one in our Milky Way
Historic Supernovae
SN 1885  S And, in the Andromeda Nebula
Historic Supernovae
SN 1987a  in the LMC, first supernova, for which the star was
known before
Historic Supernovae
SN 1987a: discovered 24.2.1987 by Ian Shelton & Albert Jones
Maximum brightness V = 2.9
Progenitor star Sanduleak -69o 202
Intrinsic Variables: Eruptive Variables
•
Supernovae: P = none
A = +20 mag
A catastrophic stellar explosion!
Intrinsic Variables: Eruptive Variables
•
Supernovae: P = none
A = +20 mag
Pulsars
Emit regular radio pulses („pulsating radio source“).
Duration: a few microseconds
Periods: 0.25 – 2 seconds, also Millisecond pulsars
Magnetic, fast rotating
neutron stars,
Radiation along
magnetic axis
Pulsars
Original star had between 8 and 12 solar masses.
Intrinsic Variables: Eruptive Variables
• Supernovae: P = none
A = +20 mag
A catastrophic stellar explosion!
• Novae (5-15 mag)
• Recurrent novae
• Dwarf novae (3-5 mag)
S Doradus Stars
Eta Car
1843: V = -0.8
Ca. 120 Msun
Next 100000 years:
Supernova (Mv=-6)
Eta Car
1843: V = -0.8
Ca. 120 Msun
Next 100000 years:
Supernova (Mv=-6)
S Doradus Stars
Binaries
About half of the stars are in
binaries
Optical and visual binaries
What is the difference?
Optical Binary
Visual binary
Extrinsic Variables: Eclipsing Binary Stars
Extrinsic Variables: Eclipsing Binary Stars
•
•
•
Binary systems with an orbital plane lying near the line of sight to the
observer.
Components periodically eclipse one another → brightness decreases.
Eclipse period = orbital period of the system.
Eclipsing Binaries
Fainter component
eclipses the brighter one
Light curve
Spherical components: no
variation outside of the
eclipse
Total eclipse: lght curve
shape depends on the
sizes of the components,
flat minimum
Partial eclipse: sharp
minimum
Main eclipse
Orbit
Variable Stars: a bit of history …
•
β Persei = Algol = The Devil‘s Eye
1667, Montanari (Bologna):
‚Algol is a variable‘
1782, Goodricke: ‚Algol is a
periodic variable‘ (variation of 1.5 mag
in 70 hours)
Algol is an eclipsing variable, see later!
Extrinsic Variables: Eclipsing Binary Stars
Extrinsic Variables: Eclipsing Binary Stars
Extrinsic Variables: Eclipsing Binary Stars
3 types
Algol-Type (EA)
β Lyr-Type (EB)
W UMa-Systems (EW)
Extrinsic Variables: Rotating Stars
Extrinsic Variables: Rotating Stars with Spots