Comments on Homework 5

Part A

A1.	b.
A2.	d. The supernova of AD1054 has left a remnant (the Crab Nebula) with a pulsar ( neutron star) at its center.
A3.	White dwarf.
A4.	b. The smaller the star the closer the surface is to the rest of the star. Thus, the gravitational force scales as l/d2, or in words, increases strongly.
A5.	b.
A6.	Low mass main sequence stars are cool. Cool stars emit most of their light at red and infrared wavelengths and none (strictly very little) at ultraviolet wavelengths. Therefore, you need an IR telescope.
A7.	Main sequence star red giant planetary nebula white dwarf black dwarf.
A8.	Main sequence star red supergiant SN Type II neutron star OR black hole. A cepheid variable should be added here but we discussed these stars after HW5 was handed in. The Cepheid variable stage occurs between main sequence and red supergiant. Actually, the story is more complicated!
A9.	This is the maximum mass possible for a white dwarf. Below this mass, the WD is supported against collapse by the degenerate electrons. Above this mass, the electrons cannot provide the pressure required to withstand collapse.
A10.	Very young cluster = O, B main sequence star. Very old cluster = low mass red giants.
A11.	This illustrates the method of spectroscopic parallax (main sequence fitting). Pick your favorite spectral type. Say G. Look at the diagram for , the G main sequence star has a brightness of 1. The same star in has a brightness of 1/10,000. Therefore, is more distant. Recalling that l/d2 for stars of the same luminosity, we see that must be = 100 times more distant than a. That is is at 100 x 200 = 20,000 parsecs.
A12.	b.
A13.	dust!
A14.	globular clusters.
A15.	The Sun orbits the Galaxy once every 240 million years (Seeds, p.313). Then, in 5 billion years it has done so
A16.	Our Galaxy, Kiloparsec, light year (1pc=3.26 ly), Betelgeuse (earth's orbit around the Sun), White dwarf (earth's size), a 50M Black hole, neutron star, kilometer, centimeter, H atom, nucleus.
A17.	The key here is note that the masses of the stars are pretty similar but their radii vary greatly. Density is mass volume, and volume is proportional to radius3. For the stars, the density increases: Betelgeuse (R~Earth's orbit) Sirius A (main sequence star, R~R) Sirius B (White dwarf, R~Earth's radius) Crab Pulsar (neutron star, R~10 km). And the book says the average density of Betelgeuse is less than that of air (at sea level).

Part B

B1.	a.	Seeds.
	b.	Low mass stars spend 'forever' on the main sequence. Recall, but the Galaxy (indeed, the Universe) is much younger than this (say, 15 billion years). Therefore, all 0.4M and smaller stars are happily burning H He on the main sequence and will be so for billions of years to come.
	c.	WD's are of low luminosity. Even though common, the distances to the nearest WD's are sufficient to depress their brightness below the limit for naked eye detection.
B2.	a.	Seeds.
	b.	Galaxies suffer a SN explosion at a rate of roughly one every 50 years. If you observe ONE galaxy, you may have to wait about 50 years to see a SN - of course, you could be lucky and see one your first night. If you observe a sample of 100 galaxies you might expect to see one or two SN in a year of observing.
	c.	Dust!
B3.	a-b.	Seeds.
	c.	As a real supergiant, Betelgeuse is burning He or a "more advanced" fuel. Table 13-3 (also Classnotes 14) shows that the duration of these burning stages decreases dramatically after He - burning. Betelgeuse has been around for thousands of years so presumably it was and is burning helium, which it can do for a few hundred thousand years. If it ceased to burn He more than a 1000 years ago, it would have gone bang by now.
B4.	a.	Seeds.
	b.	Radio signals from a pulsar are beamed like a lighthouse beam. As the star spins, we will detect pulses if the beam shines in our direction. Many pulsars will direct their beams in other directions.
	c.	This question from Seeds can not, however, be answered adequately from the text! The key is that neutrons are composed of three quarks. At high densities, neutrons break up into their quarks and degenerate neutron pressure declines. At this time, we do not know enough about how quarks are bound into neutrons nor about the stability of a star made of quarks.
B5.	a.-b.	Seeds.
	b.	The 'light' that reveals a BH is emitted from an accretion OUTSIDE - just outside - the Schwarzchild radius of the BH. Add description of the accretion process - include a drawing - and explain where the energy comes from to make the accretion disk so very hot. To distinguish BH from NS one must prove that the mass of the accreting object is unambiguously greater than 3M, the maximum mass of a NS. This needs a binary system of such a kind that you can derive a reliable mass for the companion to the visible quasi-normal star.
B6.	a.	Seeds.
	b.	O-Type stars were formed recently from gas polluted by 'metals' made in and ejected by earlier generations of stars. Those O stars formed at earlier times from less polluted clouds have long since evolved and died as Type II SN. M-type main sequence stars (see B1.b. above) live forever. Old - that is metal-poor - M-stars are still present as main sequence stars. Young - that is metal-rich M-stars are present too.
	c.	Seeds.
B7.		This question will be addressed in Comments on HW6.