Each star is born with a specific mass. This mass is the main factor in determining the star s brightness, temperature, expected lifetime, type of

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1 Each star is born with a specific mass. This mass is the main factor in determining the star s brightness, temperature, expected lifetime, type of death, and spectra. Stars are classified according to their spectra.

2 Properties of Stars Mass The single most important property that determines other properties of the star. Luminosity The total amount of energy (light) that a star emits into space. Temperature surface temperature, closely related to the luminosity and color of the star. Spectral type closely related to the surface temperature Size together with temperature determine the luminosity

3 What can we measure directly? The Easy Ones: Apparent brightness: a well-calibrated detector. Temperature: spectroscopy Spectral type: spectroscopy The Hard ones: Distance: stellar parallax, but the stars are so farrrrr away Size: The stars are so far away. Their small angular size makes it really difficult to be measured directly. Mass: Newton s version of Kepler s Third Law Need to find the right targets

4 The Apparent Brightness Apparent brightness The brightness of the a star as it appears to our eyes (or detectors). It depends on both the luminosity AND distance between the star and the Earth. The apparent brightness of a star is related to its luminosity and distance by the formula: apparent brightness luminosity 4π (distance) 2 The total energy in this cone is fixed At a larger distance from the star, the same amount of energy is spread into a larger area. Thus, the apparent brightness of a star is lower if we are

5 Astronomers realized that large numbers of stars exhibit a small number of distinct patterns in their spectral features.

6 Spectral lines are caused by different elements present in a star. The overall spectrum is strongly related to the surface temperature of the star. Stars are then categorized into classes distinguished by the strength of the spectral lines and their shape.

7 The 7 classes of stars are: O B A F G K M From O M: Stars decrease in temperature Stars generally become less massive (smaller)

8 O B A F G K M Oh Be A Fine Guy/Girl, Kiss Me! One Bug Ate Five Green Killer Moths These are two of many mnemonic devices students and astronomers use to remember the proper order of stellar classification; from the hottest blue stars (O-type) to the coldest red stars (M-type).

9 For analysis purposes, stars are placed on the H-R diagram according to their luminosity and their temperature (Spectral Class).

10 The Hertsprung-Russell or H-R diagram reveals that about 90% of all stars lie along a smooth diagonal curve called the main sequence with hot, luminous stars in the upper left and cool, dim stars in the lower right. Not all stars fall on the main sequence. Stars below the main sequence are called white dwarfs and those above it are called giants.

11 Since hotter stars are bluer, and cooler stars are redder, a White Dwarf is hotter than a Red Giant.

12 HR Diagram in Luminosity vs Temperature

13 Hertzsprung- Russell Diagram Sizes scale 1 R sun 10 R sun 100 R sun 1000 R sun Notice that Temperature scale decreases from left to right. The scale of luminosity is in power of 10 (log scale). Mass increases from lower right to upper left Size increases from lower left to upper right.

14 Classification of Stars in H-R Diagram 1. The Main Sequence stars healthy stars, fusing hydrogen in the core. High-mass, high-luminosity, high-temperature, and shortlived stars on the upper-lefthand corner Low-mass, low-luminosity, low-temperature, and longlived stars on the lower-righthand corner 2. The Supergiants, 3. The Giants, Supergiants and giants are dying stars, fusing helium and heavier elements. 4. The White Dwarfs. dead stars, exposed core of dead main-sequence stars. Sizes scale 1 R sun 10 R sun 100 R sun 1000 R sun

15

16 Color: blue Temperature: 28,000-50,000 K Size: very large and massive Spectra: ionized helium, ultraviolet continuum Examples: 10 Lacertra, Zeta Puppis, and Iota Orionis A The rock stars of the universe-they live fast, die young and leave a spectacularlooking corpse!

17 Color: blue Temperature: 10,000-28,000 K Size: large and massive Spectra: neutral helium lines,some hydrogen Examples: Beta Centauri, Rigal, Spica B stars are relatively rare, comprising only 0.1% of main sequence stars.

18 Color: white Temperature: 75,000-10,000 K Size: moderate sized, very luminous Spectra: strong hydrogen lines, ionized metals Examples: Alpha Canis Majoris (Sirius) and Alpha Lyrae (Vega) A stars are amongst the most common naked eye stars.

19 Color: white-yellow Temperature: 6,000-75,000 K Size: 1.2 to 1.6 times bigger than the Sun Spectra: weak hydrogen lines, strong Calcium and other ionized metals Examples: Canopus, Procyon Often used as targets for extrasolar planet searches and SETI programs.

20 Color: yellow Temperature: 5,000-6,000 K Size: 0.8 to 1.1 times the mass of the Sun Spectra: weak hydrogen lines, neutral and ionized metals Examples: Alpha Centauri A, Capella The best known example of a G star is our SUN!

21 Color: orange Temperature: 3,500-5,000 K Size: smaller and cooler than the Sun Spectra: faint hydrogen lines, strong neutral metallic lines Examples: Alpha Boötis (Arcturus) and Alpha Tauri (Aldebaran) Also used as targets for extrasolar planet searches like project Ozma in 1960.

22 Color: red Temperature: 2,500-3,500 K Size: range from (main sequencesupergiants) times the mass of the Sun Spectra: strong metallic lines and wide titanium oxide bands Examples: Antares and Betelgeuse Most common class by number of stars, since 90% of all stars are red dwarfs.

23 Other Spectral Types W: Up to 70,000 K - Wolf-Rayet stars L: 1,500-2,000 K - Stars with masses insufficient to run the regular hydrogen fusion process (brown dwarfs).also contain lithium which is rapidly destroyed in hotter stars. T: 1,000 K - Cooler brown dwarfs with methane in the spectrum. C: Carbon stars. R: Formerly a class on its own representing the carbon star equivalent of Class K stars N: Formerly a class on its own representing the carbon star equivalent of Class M stars S: Similar to Class M stars, but with zirconium oxide replacing the regular titanium oxide. D: White dwarfs

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