Variable and Binary Stars

Not so Unusual All stars are variable to some extent –Some over periods of days or weeks –Others over centuries or millennia –Often many modes of variability –The Sun is variable +/- 0.07% By a small majority, all stars are binary or trinary –53% of star systems are multiple –75% of O-type stars seem to have a companion –If Jupiter had been ~100 times more massive, the Sun would have a companion star

Variable When an Astrophysicist talks about variable stars, s/he usually means a dramatically variable star Some are variable even to the naked eye, once you know what you are looking for –Mira (  Ceti): T ~ 1 year AKA “Wonderful” in Latin –Algol (  Persei): T ~ 1 week AKA “The Demon” in Arabic –Suggests ancient peoples noticed these changes Intrinsic variability due to instability in the Hydrostatic Equilibrium Extrinsic variability due to shape, bright spots, rotation, neighbors

(some) Categories of Variables Cepheid RR Lyrae T Tauri Explosive variables (novae, discussed elsewhere) Eclipsing binaries –A combination of our two subjects! –Also ellipsoidal binaries

Cepheid The first Cepheid variable, Delta Cephei, is close enough for a parallax measurement Population I, giant yellow stars –F6-K2 –Young, metal rich –10 3,10 4 L sun T ~ days to months

Henrietta Leavitt and Cepheid Variables One of Edward Pickering’s “harem” “a straight line can readily be drawn... showing that there is a simple relation between the brightness of the variables and their periods...” “a straight line can readily be drawn... showing that there is a simple relation between the brightness of the variables and their periods...”

Rosetta Stone Since Delta Cephei is close enough for a parallax measurement, and since the magnitude variation a la Leavitt gives a good measure of intrinsic brightness, distance to other Cepheids can be obtained Used to find the distance to the Small Magellenic Cloud, changing ideas about the scale of the Universe –hmmmmmm

RR Lyrae Common in globular clusters –Spectral type F2-A2, yellow or white giant –Population II (as one would expect in a globular cluster) Periods of a day down to several hours Amplitude of magnitudes Also a reliable distance indicator etacar.umn.edu

T Tauri Very young low mass star: –> F, G, K, M –>10 Myr –> 3 M sun Still undergoing gravitational contraction Intermediate stage between a protostar and a main sequence star.

Binary Systems and Variability If two stars coalesce out of a globule near enough to each other but with sufficient relative velocity, they will orbit about their common center AKA Barycenter

  Proxima Centauri, currently the closest star to the Sun Alpha Centauri A Barnard’s star, not a binary but on its way. By 11,700 it’ll be the closest star Beta, not part of the system

Rho Ophiuchi Trinary System Antares M4 Binary stars

Mizar, an eclipsing binary

Spectral Red-Shift The shift in the spectrum can show the speeds in a binary pair also Also useful in finding extra-solar planets A new device called a laser comb produces a series of spectral lines at precisely spaced frequencies, allowing measuring star motions 60 times more precisely

Light Curve, one more time

Piper Reid, Dripping Springs High School, Austin TX Link Link to her paper

Contributions to a Nova This binary system has a giant and a dwarf star The dotted line is the Roche lobe If the giant expands during the course of its demise beyond the Roche lobe, the dwarf can steal material and go nova

Blue Stragglers: O-types merging gently

But That Is Just One Possible Fate of a Star