Transcript: Consider a binary system where a giant or a supergiant star is in orbit around a white dwarf and where a mass transfer between the lobes pushes the white dwarf over the Chandrasekhar limit. At this point the white dwarf collapses catastrophically to form either a neutron star or a black hole blasting off a lot of material in a titanic explosion called a supernova. This particular type of supernova that occurs in binary systems is a Type II supernova, and because of the regulated...

Transcript: Consider a binary pair where the more massive star has expanded to fill its Roche lobe, and the smaller star is a white dwarf of nearly 1.4 solar masses. As the massive star dumps hydrogen onto the white dwarf the gas is compressed, rapidly heated, and it ignites nuclear reactions that blow off a shell of gas. The resulting emission is called a nova. This process is cyclic because gas can then accumulate again, and on a time scale ranging from a hundred to ten thousand years a...

Transcript: How does mass transfer occur in a close binary pair? The Roche surface or lobes define the region of space where gas is bound to one or both stars. If either star becomes a giant the outer layers swell to fill the lobe, and the teardrop shape becomes the actual surface of the star. In the first stage of binary evolution neither star fills its Roche lobes. In the second stage the more massive star becomes a giant first and fills its Roche lobe. Mass then flows at the point...

Transcript: Most binary stars orbit slowly with a large distance between the two stars. However, a small fraction of binary systems are in close, tight, rapid orbits. In this situation mass transfer can occur from one star to the other. The system of two co-orbiting stars is surrounded by two teardrop regions of space called Roche lobes. The cross-section of these imaginary regions of space is like the figure eight. At the point of contact between the two stars mass can flow in one...

Transcript: It’s a great challenge to determine the true incidence of binary and multiple star systems. The nearest stars are well observed, but it’s a very small sample. As we go to more distant stars a faint companion might not be detectable. Spectroscopic binaries are biased towards small separations because those orbits have the fastest velocity and the largest Doppler shifts. On the other hand, visual binaries are biased towards wide separations or low speeds because those are the...

Transcript: Binaries provide an essential way to measure a star’s most fundamental property: its mass. Kepler’s Law relates the period of an orbit to the semi-major axis in a situation like the solar system where the Sun dominates the motions. Newton generalized Kepler’s Law to apply to any two objects orbiting each other. In Newton’s formulation the sum of the two masses equals the semi-major axis cubed divided by the period squared. Thus if we can measure both the period of the orbit...

Transcript: An eclipsing binary is a pair of stars generally unresolved whose orbit is seen nearly edgewise. The stars alternatively eclipse each other which requires a very favorable orientation so only a small fraction of all possible systems are seen this way. The timing of the eclipse gives the period of the orbit, and the depth of the eclipses gives information both on the size of stars and the opacity of their atmospheres. For example, if a binary pair are in a circular orbit of a...

Transcript: An astrometric binary occurs when a bright star is moving around an unseen companion. In this case the orbit is detected and measured based on the astrometric motion of a single star. This requires extremely careful positional measurements, usually with an accuracy of a thousandth of an arcsecond or less, and careful observations over several months. Astrometric binaries are an important category of binary stars because in many cases binary stars combine an object of...

Transcript: In a binary star the brighter or more massive star is usually called A, and the fainter or less massive star is called B. Castor A and Castor B is an example. The most common type of binary is a visual binary. Visual binaries can be resolved by a telescope. The separate images can be tracked over a period of months or years with images that show the orbit directly. The space motion is derived from a combination of the motion on the plane of the sky, the transverse velocity,...

Transcript: A spectroscopic binary occurs when the two stars are too close to be resolved by a telescope or when one member of the pair is too faint to be detected. The binary is observed in terms of a periodic Doppler shift of narrow spectral features, absorption lines in the atmosphere of the stars, which reveal both the motion and the orbit. Sometimes the motion of only one of the stars is visible in this way. Several thousand spectroscopic binaries are known.

Transcript: In 1767 amateur astronomer John Michell, who also speculated about the existence of black holes, noticed that a lot of stars in the night sky had apparent close companions. He compared the distribution of angular separations of bright stars in the sky to a random distribution and found that on average there were more stars with small angular separations than you would expect by chance. He concluded that some stars were physically associated, that they were attached by gravity in...

Transcript: Sometimes stars appear to be close to each other on the plane of the sky, but how do we know if these stars are physically close to each other in three dimensional space? For an individual pair of stars, without additional information, we don’t know, but for a distribution of stars we can estimate it based on their spatial distribution in two dimensions. A cluster distribution looks different from a random distribution because the average distance between any two objects is less...

Transcript: Stars are not distributed randomly in space. More than this, stars often have companions. Four of the six nearest stars to the Sun have companions, and two-thirds of all stars in the Milky Way are in systems of two or more stars. Two stars is called a binary system, and more than two stars is called a multiple star system. Beyond that we find stars in groups. The Pleiades is one clear example, the seven sisters, or clusters, and some of these are visible to the naked eye in...

Transcript: Astronomers are often interested in the clustering of objects in space whether they be stars, or galaxies, or quasars. To understand the nature of clustering it’s better to start with the situation of a random distribution, and it’s sometimes easiest to think of two dimensions rather than three. So imagine objects spaced on the sky randomly. There is a mean distance between objects in a random distribution. The mean distance corresponds to a distance where half the stars have...