Report your total points this week out of 4 points possible - one point per question.

1. Describe as accurately as you can what kind of stellar evolution process(es) is responsible for most of the iron in your blood (and anywhere else in the universe).

Iron in your blood or anywhere else comes mostly from type Ia supernovae, in which a white dwarf in a binary pair gets enough material transferred from its companion to send the dwarf over 1.4 SM, and it experiences fusion throughout the star to produce elements throughout the periodic table, including lots of iron (about half the 1.4 SM converts to mass 56 iron nuclei). Successive neutron absorption by the iron creates all the elements above iron, right up to (and beyond) uranium. The dwarf began this process as a high density compact object with a composition of carbon & oxygen. During the explosion, carbon & oxygen fusion builds the elements up to mass-56 nickel/iron. This type Ia supernova occurs with about the same frequency as the type II supernova in galaxies with ongoing star formation (such as spirals like ours). The type II supernova the mass-56 core collapses and becomes a neutron star (so that relatively little iron -56 gets recycled to the interstellar medium).

2. Suppose you take a picture of a certain star-field as a reference. Now, imagine a black hole comes between you and the center of the same star-field, and the black hole is relatively near to you. Describe how the appearance of the star-field might change. (Hint - one of the APOD images contains the clue).

This site shows what a star field would look like were a black hole to be relatively near you, with the star field well behind it - note that the picture at the APOD website is a theoretical one. In the language of Einstein’s general theory of relativity, space is severely warped in the vicinity of a black hole (but outside the event horizon), and light travels along those bent space lines. The overall effect is similar to light passing through a glass or plastic lens, but also different. Light rays from one star will appear to an observer to be coming from different places (i.e., more than one image per star), because distant starlight is "bent" regardless of which side of the black hole it went by, such that the light still enters your eye. I’m having trouble describing this, so refer to page 318 of your text, figure 14-10 - the so called gravitational lens effect. The example is different there, but the explanation for multiple images of one object is the same as described in the APOD figure caption. Astronomers have seen lots of gravitational lensing examples (at least one of which you will see in a future week), but (to my knowledge) none of them are due to a black hole all by itself.

3. Newton’s law of gravity, shown on page 66 in equation form, describes the force between the Sun and Earth that keeps the two bodies in orbit around each other. (Since the Sun is so much more massive than Earth, Earth does most of the moving and we say it orbits the Sun). If the Sun were instantly to become a black hole (with mass = 1 SM), how would Earth’s orbit change and why? Use the equation to justify your answer, and explain it well.

So long as the mass of the Sun does not change as it turns into the black hole, Earth would feel absolutely no effect and its orbit will remain the same. The gravitational force between Sun and Earth is proportional to the product of the mass of each object, and inversely proportional to the square of the distance between the CENTERS of the two objects. If none of those quantities changes, the force - and therefore the orbit -  remains the same.

4. The search for stellar black holes has so far turned up some fairly convincing candidates in binary systems. What observations could you make for an X-ray binary system to distinguish whether the unseen object is a black hole or a neutron star? (Note the term "stellar" black hole - in later chapters you will see that most large galaxies probably have a super-massive black hole in their core.)

The search generally starts with detection of a "point" (meaning unresolved) X-ray source whose intensity varies rapidly with time. The X-rays come from the inner part of an accretion disk, which in turn is due to a companion star losing mass to the black hole candidate. X-ray satellites above our atmosphere do find such objects. Then high resolution optical (visible light) telescopes look for a possible companion star - a visible object that is orbiting something that is NOT visible. If the optical telescopes can determine the period of the orbit, the size of the orbit, and (from spectroscopic information) an estimated mass of the visible star, then the mass of the invisible X-ray source can be estimated. If that mass is greater than about 3-5 SM, the invisible object is a black hole candidate. If the mass is less than about 3 SM, it is a likely neutron star (or even possibly a white dwarf whose brightness is too low to measure, but only if the mass is less than 1.4 SM). Remember that we can only "see" about 1/6 of all neutron stars as a pulsar, and most of the rest are invisible and not detectable, unless they are in a binary pair and emit X-rays. Other supporting spectral information in some cases is detection of gas falling from the visible star toward the invisible object (as the gas falls it heats and produces an emission line spectrum). Finally, the May 1999 issue of Scientific American provides evidence for stellar black holes in the observation of consistently less light emission per week mass from certain black hole candidates:  black holes have no surface for mass to smack into and produce intense, rapidly varying X-radiation upon impact; black hole-star binary systems are therefore much dimmer in X-ray emission than neutron star-star binary systems.