CLASSNOTES 11

The H-R diagram is used extensively in the coming class. I provide the following simple exercise so that you may become more familiar with this diagram.

I challenge you to plot up two samples of stars on the blank H-R diagrams at the end of the classnotes.

Sample One: the stars Within 12 Light Years of the Earth

 

Sample Two: the 20 Brightest Stars

The blank diagrams show luminosity (in solar units, L#) versus the color B-V. Luminosity is given on a log scale. B-V is a numerical measure (in magnitudes) of the color: crudely the ratio of blue to red light. Large B-V denotes a red star. Small B-V denotes a blue star.

On the diagram labelled 'Nearest Stars' I've placed 61 Cyg. On the diagram labelled 'Brightest Stars,' I've placed Regulus.

These two examples should help you place the other stars. It is not necessary to write the names of the stars besides the points.

After completing the two diagrams, please think about the following questions:

1. Describe in your own words the key differences between the HR diagrams of the two samples.

 

2. Main sequence stars of spectral types K and M are the commonest stars in the Galaxy. Why are there none in the sample of brightest stars?

 

3. Red giants such as Arcturus and Betelgeuse are very rare - in large part because they are in phase of their lives which is short-lived. These rare stars are well represented in Sample 2 but there are none in Sample 1. Why do red giants turn up in Sample 2 but not in Sample 1?

Star Designation	Constellation Designation	Distance (ly)	Spectral Type	B-V Color	Luminosity L
Sirius	Cma	8.6	A1 V	+0.00	29
Canopus	Car	120	F0 II	+0.15	2200
Arcturus	Boo	35	K1 III	+1.28	370
Rigel Kent	Cen	4.3	G2 V	+0.68	1.5
Vega	Lyr	25	A0 V	+0.00	63
Capella	Aur	40	G5 III	+0.80	140
Rigel	Ori	850?	B8 I	-0.03	120,000
Procyon	CMi	11.4	F5 IV-V	+0.42	6.6
Achemar	Eri	125	B3 V	+0.16	6,000
Beteguese	Ori	650?	M1 I	+1.85	120,000
Hadar	Cen	360?	B1 III	-0.23	61,000
Altair	Agl	16.1	A7 V	+0.22	10.6
Aldebaran	Tau	60	K5 III	+1.54	300
Acrux	Cru	400?	B1 IV	-0.26	71,000
Antares	Sco	300?	M2 I	+1.83	16,000
Spica	Vir	260?	B1 III	-0.23	23,000
Pollux	Gem	35	K0III	+1.00	50
Fomalhaut	PsA	22	A3 V	+0.09	15
Deneb	Cyg	1600?	A2 I	+0.09	90,000
Mimosa	Cru	500?	B0 III	-0.23	82,000
Regulus	Leo	70	B7 V	-0.11	310
Adhara	CMa	650	B2II	-0.21	72,000
Castor	Gem	50	A1 V	+0.03	55
Shaula	Sco	330?	B2 IV	-0.22	13,000
Gacrux	Cru	230?	M4 III	+1.59	8,700
Bellatrix	Ori	300?	B2 III	-0.22	17,000
El Nath	Tau	120	B7 III	-0.13	240
Miaplacidus	Car	160	A2 IV	+0.00	550
Alnilam	Ori	55	B7 IV	-0.19	550,000
Al Nair	Cru	55	B7 IV	-0.13	150

The excitement of doing science is not conveyed by textbooks such as ours. I came across the following piece in an essay ("Blood, Birds, and the Old Road") written by Sir Denys Wilkinson whose lectures on nuclear physics I attended long, long ago.

Sir Denys suffered extensively at an early time in his career from radiation sickness. His doctor ordered him to take a complete rest. He took up bird watching. I hope he conveys the excitement of experiments -- when they work!

He was fascinated by how birds released, far from home, find their way back. Here is his account of one experiment:

The Manx Shearwater (Puffinus puffinus), a strictly pelagic bird never seen inland unless tempest-tossed, comes to land for only a few weeks per year to breed in a burrow close to the sea. We brought these birds, deprived of sensory clues, from Skokholm, their breeding island off the Welsh coast, and released them singly from the top of the Cambridge University library tower, for them totally unknown territory, and watched until each was out of sight before releasing the next. The angular distribution of their disappearances formed a tight fan pointing toward their island home 230 miles away. I have had many thrills in my life in physics, when something has worked or when I have thought that I have understood something, but never that eerie feeling as when I watched those shearwaters disappear.

His next paragraph opens with the question 'How is it done?' We don't know!!