27-07-2012, 02:18 PM
A STUDY OF VARIABLE STARS
A STUDY OF VARIABLE STARS.pptx (Size: 414.15 KB / Downloads: 37)
Introduction
Stars that changes their brightness.
Time scales of seconds, hours, days, years or centuries.
Understanding variable star behavior sheds light on other astronomical phenomena
Two types – Extrinsic variable & intrinsic Variable – based on the mode of varying brightness.
Discovery
Omicron Ceti - 1st variable star identified by Johannes Holwarda in 1638.
This discovery, combined with supernovae observed in 1572 and 1604, proved that the starry sky was not eternally invariable.
Algol, 2nd variable star by Geminiano Montanari in 1669; - eclipsing variable
John Goodricke gave the correct explanation of its variability in 1784.
Chi Cygni was identified in 1686 by G. Kirch,
Then R Hydrae in 1704 by G. D. Maraldi.
By 1786 ten variable stars were known.
John Goodricke himself discovered Delta Cepheid and Beta Lyrae.
Since 1850 the number of known variable stars has increased rapidly.
1890 – use of photography
Today 46,000 variable stars in our own galaxy, as well as 10,000 in other galaxies, and over 10,000 'suspected' variables.
Variable star nomenclature
In a given constellation, the first variable stars discovered were designated with letters R through Z, e.g. R Andromedae.
developed by Friedrich W. Argelander, who gave the first previously unnamed variable in a constellation the letter R.
Letters RR through RZ, SS through SZ, up to ZZ are used for the next discoveries, e.g. RR Lyrae.
Later discoveries used letters AA through AZ, BB through BZ, and up to QQ through QZ (with J omitted).
if 334 combinations are exhausted, variables are numbered in order of discovery, starting with the prefixed V335 onwards
Rotating Variables
Brightness changes due to rotation.
Non-spherical stars
Ellipsoidal variables
Stellar spots
FK Comae Berenices variables
BY Draconis variable stars
Variations due to Magnetic fields
Alpha-2 Canum Venaticorum variables
SX Arietis variables
Optically variable pulsars
Eclipsing Binaries
Binary stars.
One component eclipsed by other.
Primary eclipse – large drop in brightness.
Hotter star is eclipsed.
Secondary eclipse – hot star pass in front of cooler star.
Eg. Algol variables,beta lyrae
Pulsating Variable stars
Periodic expansion & contraction of surface layers.
Distinguished by period of pulsation & shape of light curve.
Light curve provide info about the interior processes.
Relationship b/w period & luminosity.
Mainly 5 types – Cepheid ,RV Tauri, Mira type , Beta canis , Red semiregular & irregular.
Red semi regular & Irregular variables
star in which variation in brightness in certain aspects is related to the long period variables and in certain others to the semi-regulars.
stars which are related to long period variables have a range of brightness variation over a quarter of a magnitude.
These stars are of high luminosity
Mira Type Variables
It has a period of 331 days and varies its brightness by almost 6 magnitudes in the visible waveband during a cycle.
A red giant, its radius varies by 20 percent, peaking at 330 times that of our Sun.
effective temperature ranges from 1,900 K to 2,600 K