Published on March 9, 2014
The Properties of Stars Stellar Spectra & The H. R. Diagram
Stellar Spectra There is a lot of information hiding in individual stellar spectra besides color! We can also learn much about the chemical composition of stars by investigating the spectral lines that show up in the spectra.
“spectral types” Annie Jump Cannon Born 1863, Dover Delaware She developed the widely adopted classification scheme in 1910. Strength of Hydrogen Balmer (to n=2) Lines along with the appearance of other chemical line features This is a surface temperature scale based on spectral lines features.
The Sun’s Complete Spectra!
O BAFGK M Early Types Late Types Hot Cool Bluish Reddish
Hertzsprung-Russell Diagram: Ejmar Hertzsprung (1873-1967) – Copenhagen – Began his career as a Chemical Engineer. While working and independently at the same time… Henry Norris Russell (1877-1957) – Princeton – Student then professor. A graph that separates the effects of temperature and surface area on stellar luminosities. The HR Diagram is much like the same thing as producing a graph of people’s height vs. weight.
What does the luminosity of a star depend on? Temperature (proportional to T4) Size (proportional to R2) Full blown formula? L=4πR2σT4
Bright Supergiants Absolute Magnitude Luminosity Red Giants Main Sequence White Dwarfs Dim O B Early Types Hot A F M G K Late Types Cool
Main Sequence Extends from the hot, bright, bluish stars in the upper left to cool, dim, reddish stars in the lower right. Size (Radius) of the stars: R~R (or slightly bigger/smaller) ~90% of the stars in space.
Red Giants Cool, luminous stars. They are very luminous because of their large size. Size R~100R but they are only about 0.9% of the stars by number.
Supergiants Exceptionally luminous extra large sized stars! (humongous!) Size R~1000R but they are only about 0.1% of the stars by number.
Where is the missing 9%? White Dwarfs: HOT but faint stars. The surface temperatures of these stars are very hot, but since they are so small they are not very luminous. Size R~(1/100)R ~R⊕ but they are only about 0.9% of the stars by number.
Stellar Information: Some Example Stars Name Spectral Type Temp (K) Mass Kind? 13 M M.S. Bluegiant 1 M Main Sequence 14 M Red Supergiant Spica B1 ~20,000 K Sun G2 ~5,800 K Betelguese Sirius B M2 A0 ~ 3,000 K ~18,500 K 1 M White Dwarf
Caution! Do not confuse the size of an object with the mass of an object. Just because an object is large in dimension does not necessarily mean it is also large in mass. For example, you can have a forty foot tall by three foot across marshmallow that looks “large,” but that does not mass as much as that of a “small” football sized hunk of lead.
Mass Ranges of Main Sequence Stars Most Massive Stars ~ 55-100(?) times more massive than the Sun (very rare) Least Massive Stars ~ 0.1-0.08 times the mass of the Sun. (very common) This pattern stretches from the higher mass O,Bs to lower mass K,Ms.
Mass-Luminosity Relationship The Main Sequence also exhibits a “MassLuminosity Relationship.” Simply, a star’s location on the main sequence depends on its mass. The more luminosity a star has the larger its mass. The lower the star’s mass the smaller its luminosity. But this is true only on the main sequence.
Bright Absolute Magnitude Luminosity Dim M K G F A B O
Mass-Luminosity Relationship Main Sequence Stars ONLY!! L = M 3.5
Examples Suppose the mass of a star is 4 M then L = (4)3.5 = (4) × (4) × (4) × (4)0.5 = 128 times more luminous than Sun. L = 128 L Suppose the luminosity of a star is 5 L then 5 = (M)3.5 M=(5)-3.5 = 1.6 times more mass than Sun. M = 1.6 M
Density ranges of Stars Sun ~ 1.0 grams/cm3 (The same density of water!) Giants ~ 0.01 grams/cm3 0.1 grams/cm3 Supergiants ~ 0.000001 grams/cm3 (That is less than the density of air!) White Dwarfs ~ 10,000,000 grams/cm3 (1 sugar-cubed size of white dwarf material would weigh nearly 20 tons!)
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