Stars! A star is a big ball of gas, with fusion going on at its center, held together by gravity! Massive Star Sun-like Star Low-mass Star There are variations between stars, but by and large they’re really pretty simple things.

What is the most important thing about a star? MASS! The mass of a normal star almost completely determines itsLUMINOSITY TEMPERATURE The mass of a normal star almost completely determines its LUMINOSITY and TEMPERATURE!  Note: “normal” star means a star that’s fusing Hydrogen into Helium in its center (we say “hydrogen burning”). HOW and WHY is that so?

The mass of a star determines the pressure in its core: The core supports the weight of the whole star! The more mass the star has, the higher the central pressure! The core pressure determines the rate of fusion… RATE OF FUSION … which in turn determines the star’s MASS PRESSURE & TEMPERATURE luminosity!

Usually, what we know is how bright the star looks to us here on Earth… We call this its Apparent Magnitude The Magnitude Scale  Magnitudes are a way of assigning a number to a star so we know how bright it is Rigel and Betelgeuse, stars in Orion with apparent magnitudes 0.9 and 0.3

The historical magnitude scale…  Greeks ordered the stars in the sky from brightest to faintest…  …so brighter stars have smaller magnitudes.  Greeks ordered the stars in the sky from brightest to faintest…  …so brighter stars have smaller magnitudes. MagnitudeDescription 1stThe 20 brightest stars 2ndstars less bright than the 20 brightest 3rdand so on... 4thgetting dimmer each time 5thand more in each group, until 6ththe dimmest stars (depending on your eyesight)

Later, astronomers accepted and quantified this system.  Modern measurements showed it was actually a logarithmic scale (99.626) Every one magnitude corresponds to a factor of 2.51 change in brightness because (2.51) 5 = 2.51 x 2.51 x 2.51 x 2.51 x 2.51 = magnitudes change is 100 change in brightness

Brighter = Smaller magnitudes Fainter = Bigger magnitudes  Magnitudes can even be negative for really bright stuff! Object Apparent Magnitude The Sun Full Moon Venus (at brightest) -4.4 Sirius (brightest star) -1.5 Faintest naked eye stars 6 to 7 Faintest star visible from Earth telescopes ~25

log scale, no kidding? b – apparent brightness m – apparent magnitude

The last thing to introduce is the question of mass That question can be translating into question: What are binary stars ????????

star A large ball of gas that creates and emits its own radiation. Binary Stars >60% of Stars are in Binary Systems two balls – not necessarily gas, not necessarily emitting radiation

Contains two (or sometimes more) stars which orbit around their common center of mass. Importance - only when a star is in a binary system that we have the possibility of deriving its true mass. The period – watching the system for many years. The distance between the two stars - if we know the distance to the system and their separation in the sky. → the masses can be derived. The more unequal the masses are, the more it shifts toward the more massive star. The masses of many single stars can then be determined by extrapolations made from the observation of binaries.

Visual Binaries The ideal case: Both stars can be seen directly, and their separation and relative motion can be followed directly. Visual Binary Stars

the most common case: Spectroscopic Binaries Usually, binary separation d can not be measured directly because the stars are too close to each other. information from: Doppler Shifts for Binary Stars

Idealized binary star system: two stars have equal masses and are in circular orbits and each star has a single spectral line at the same frequency when the stars are at rest. The approaching star produces blue shifted lines; the receding star produces red shifted lines in the spectrum. Doppler shift → Measurement of radial velocities → Estimate of separation d → Estimate of masses Spectroscopic Binary Star have patience

There is the rare case when the system is turned so that we see it directly edge-on. This is called an eclipsing binary system. In the case of an eclipsing binary, we see each star pass directly in front of the other one. In these cases, the masses can be directly determined for the stars. Algol known colloquially as the Demon Star, is a bright star in the constellation Perseus. It is one of the best known eclipsing binaries, the first such star to be discovered.constellationPerseuseclipsing binaries Eclipsing Binary Star Eclipsing Binaries

Peculiar “double-dip” light curve

NASA X-ray Binaries A special class of binary stars is the X-ray binaries, so-called because they emit X-rays. X-ray binaries are made up of a normal star and a collapsed star (a white dwarf, neutron star, or black hole). These pairs of stars produce X-rays if the stars are close enough together that material is pulled off the normal star by the gravity of the dense, collapsed star. The X-rays come from the area around the collapsed star where the material that is falling toward it is heated to very high temperatures (over a million degrees!).binary starsX-rayX-rayswhite dwarfneutron starblack holegravity

An eclipsing binary, with an indication of the variation in intensity. An animation of an eclipsing binary system undergoing mass transfer.

What is the defining characteristic of an eclipsing binary system? That at some point in its orbit one star eclipses the other along our line of sight. How many eclipses occur during a complete orbital cycle? (Two for this system) Add that most systems have two eclipses but not all systems. When does the large dip in the light curve occur? (When the hot blue star is eclipsed.)