Astronomy Temperature, Luminosity, & H-R Diagram

3.6 SPECTRAL CLASSES Absorption spectra are used to classify stars into nine principal types, called spectral classes. Hydrogen lines are much stronger in the spectra of some stars than in the Sun’s spectrum. Astronomers once mistakenly thought that these stars had more hydrogen than other stars. They classified stars by the strength of the hydrogen lines in their spectra, in alphabetical order, from the strongest (called Class A) to the weakest (called Class Q).

Spectral Class U.S. astronomer Annie J. Cannon (1863–1941), who examined and classified the spectra of 225,300 stars, modified this classification system to its still-used form: O B A F G K M L T. (Astronomy students remember this order by saying: “Oh Be A Fine Girl/Guy Kiss Me Love To.”) Oh Boy Alex Filippenko Gives Killer Midterms ( UC Berkeley Astronomy Prof-History Chan ) All visible stars are roughly uniform in composition, made mostly of hydrogen and helium. U.S. astronomer Cecilia Payne-Gaposhkin (1900–1979) showed that the differences in the dark line patterns of stars are due primarily to their different surface temperatures.

Spectral Class The sequence of spectral classes is a temperature sequence. The O stars are hottest, with the temperature continuously decreasing down to the coolest T stars. Each spectral class is arranged in 10 subclasses numbered 0 to 9, also in order of decreasing temperature. Today discoveries necessitate extra classes; L and T were added recently for “dwarfs” cooler than class M stars.

Spectral Class What property of the Star determines its spectral class ?

3.7 TEMPERATURE Today’s spectral classes of stars in order from highest to lowest temperatures, the approximate surface temperatures of these classes, and the main class characteristics are summarized below. What are the spectral classes of each of the stars shown above (A) __________ ; (B) __________

3.12 LUMINOSITY Astronomers distinguish a star’s apparent brightness—the way the star appears in the sky—from its luminosity—the actual amount of light a star shines into space each second. The star we know best is our Sun. The luminosity of other stars is often stated in terms of the Sun’s luminosity (L °.), which is 3.85 × watts (light bulb). The Sun’s luminosity is equivalent to 3850 billion trillion 100-watt light bulbs shining all together. The most luminous stars are over a million times as luminous as the Sun. The dimmest stars known are less than as luminous as the Sun. Deneb in Cygnus is about 60,000 times more luminous than the Sun. Explain why the Sun looks much brighter to us than Deneb does.

3.13 PROPAGATION OF LIGHT You cannot simply look at stars in the sky to determine which ones have the greatest luminosity. The farther away (d) a star of luminosity (L) is, the less bright (B) it appears. Light spreads out uniformly in all directions from a source so that the amount of starlight shining on a unit area falls. This relationship is called the inverse square law. The equation is: B = L/4πd 2. Our Sun is exceptionally bright because it is so close to us. If we moved it 100,000 times father away in space (100,000 x 93 million miles), how many times fainter would it look? Hint: 1/d 2

3.14 APPARENT MAGNITUDE

Apparent Magnitude How much brighter does the Sun appear than Sirius? Explain. 1. Find the apparent magnitude difference between |(Sun) – (Sirius)| = 2. Use above chart on the left. Absolute magnitude is like the wattage on a light-bulb. What is the Sun’s “wattage?” ________________. Venus ___________, Moon _________, 100-W bulb _______? Like light-bulb wattage

Apparent Magnitude

3.15 ABSOLUTE MAGNITUDE Absolute magnitude is a measure of luminosity, or how much light a star is actually radiating into space. (like light-bulb wattage) 1 LY ~ 6 trillion miles ~ 9.5 trillion km 1 parsec (pc) = 3.26 LY If you could line up all stars at the same distance from Earth, you could see how they differ in intrinsic, or “true,” brightness. Astronomers define a star’s absolute magnitude as the apparent magnitude the star would have if it were located at a standard distance of 10 parsecs from us. If a star is farther than 10 parsecs from us, its apparent magnitude is numerically bigger than its absolute magnitude If a star is closer than 10 parsecs, its apparent magnitude is numerically smaller than its absolute magnitude.

Consider the two bright stars Deneb and Vega. Use the table to fill in the chart below. Then tell (A) which looks brighter? __________ (B) Which is really more luminous? __________ (C) What factor did you use to answer A & B? ______________________________________________

3.16 Determining Distance m = apparent magnitude and M = absolute magnitude The difference between the apparent magnitude (m) and absolute magnitude (M) is called the distance modulus (m – M). In formula form: A star’s apparent magnitude can be measured directly. For a distant star whose absolute magnitude is known, the distance modulus can be used to calculate distance. Give the distance modulus of the stars that are (a) closer __________ ; (b) farther away __________ distance modulus (m – M)

3.16 Determining Distance another way The method of parallax is used in measuring the distances to nearby stars. The position of a star is carefully determined relative to other stars. Six months later, when Earth’s revolution has carried telescopes halfway around the Sun, the star’s position is measured again. Nearby stars appear to shift back and forth relative to more distant stars as Earth revolves around the Sun. The apparent change in a star’s position observed when the star is sighted from opposite sides of Earth’s orbit is called stellar parallax Stellar parallaxes are very small and are measured in seconds of arc (“), where 1 = 1/3600°. An aspirin tablet would appear to have a diameter of 1” if it were viewed from a distance of about 2 km (a mile) One parsec (pc), is the distance to an imaginary star whose parallax is 1 second of arc (1”). One parsec, 1 pc = 3.26 light-years. d (pc) = 1/parallax (“)

QUIZ: Four Nearby Stars. A. Which star is hottest? ___________________ B. coolest? _________________ C. brightest looking? ________________ D. faintest appearing? ________________ E. intrinsically (actually) most luminous? ________________ F. intrinsically least luminous? ___________________________ G. Most distant (show work) H. Closest (show work) Due at the end-of-the period for A-Day & B-Day Due on at end-of-the period on Tuesday for AB-45