Harvard Law Case

1. What are the Roche lobes of a close binary star system? What is the significance of the point

where the two lobes “touch” each other, and what can happen there?

The “Roche lobes” are the surfaces where the effects of the two stars balance. Anything at the “inner Lagrangian point,” L1, feels equal effects from both stars. Now the star of the show is L1 , the “inner Lagrangian point.” This is the place where the mass flows through from the Roche lobe of the donor star, to the star that receives the mass. Matter moving away from one star that reaches the “inner Lagrangian point” of gravity balance between the two stars can be captured by the other star. • If the companion is a normal star, such as Main Sequence star, it is large enough to intercept the mass stream and collect the transferred mass.

2. Why does the mass transferred from a star to a compact companion end up orbiting the

compact object in a so-called “accretion disk”? What kind of radiation does the disk emit?

Things get interesting when the companion is a compact object. The latter object’s mass is squeezed inside a small radius, leaving lots of space for the transferred mass to collect in a swirling accretion disk. The compact object may be a white dwarf, neutron star, or black hole. The accretion disk, which radiates optical light, can be the brightest thing in the system. Roughly regular brightness variations may be seen as the distorted companion star (with a hotter “disk-facing” side) orbits the center of mas and the “hot spot” cycles in and out of view.

3. What happens in a classical nova event? What causes a “thermonuclear” or Type Ia supernova?

What type of compact star is present in both cases, and what happens to it?

Classical novae are much brighter than disk instabilities. • They run nuclear fusion reactions up to the light alpha elements like Ne and Mg; they may also blast off some of the material from the white dwarf. • But they do not destroy the white dwarf, so after the system settles down, mass transfer can resume, and eventually another nova explosion can occur. -White dwarf or “thermonuclear” supernova: If so much mass accumulates on the white dwarf that its mass exceeds the Chandrasekhar limit of 1.4 M! , the WD will explode. This is called a “Type Ia” supernova, and is a very different phenomenon from a core-collapse supernova. White dwarf is present in both cases.

4. What discovery about the expansion of the universe was made using Type Ia supernovae?

Why were Type Ia supernovae useful in making this discovery?

Observations of Type Ia SNe indicate that the universe is actually expanding faster today than it was in the past. This is unexpected, since the influence of both luminous and dark matter should slow down the expansion. The Accelerating Universe The unknown agent that causing the acceleration has come to be called dark energy.

5. Describe the components of a so-called “X-ray binary” system. How can we tell whether such

a system contains a neutron star or a black hole?

The X-rays are produced by matter falling from one component, called the donor (usually a relatively normal star), to the other component, called the accretor, which is very compact: a neutron star or black hole. The infalling matter releases gravitational potential energy, up to several tenths of its rest mass, as X-rays. The lifetime and the mass-transfer rate in an X-ray binary depends on the evolutionary status of the donor star, the mass ratio between the stellar components, and their orbital separation. It’s a black hole if it’s not an ordinary star and its mass exceeds the neutron star limit (~2 M!)

6. Explain why the composition of the interstellar medium (ISM) is changing with time and, in

particular, the concentration of elements heavier than H and He keeps increasing.

Stars make new elements by fusion reactions • Dying stars expel gas and new elements, producing hot bubbles in the ISM (~106 K) • The hot gas cools, allowing atomic hydrogen clouds to form (~100-10,000 K) • Further cooling permits molecules to form, making molecular clouds (~30 K) • Gravity forms new stars (and planets) in cold molecular clouds

7. What are the effects of dust grains on starlight passing through interstellar clouds? Where did

the dust come from?

Mixed with the gas are multitudes of tiny solid particles called “astrophysical dust” or “dust grains.” These are heated by stars and motions, and radiate as thermal emitters, producing radiation mostly in the infrared range. At short wavelengths, they block light passing through. Dust clouds cut down its brightness by a factor of 1012

8. What kinds of electromagnetic radiation (what spectrum region) is emitted by interstellar dust