Notes

Leonid meteor in mid-November

Absolute Magnitude

m_sun= -26.8

M_sun= 4.85

Absolute magnitude, M, is the apparent magnitude of an object measured at a distance of 10 pc.

In Fig. 17.6 Star B is brighter, but more distant. An observer sees Star A and B appearing to have the same brightness

Spectral Classification

A star's spectrum uniquely identifies the type of star

Spectral lines depend on stellar temperature

Letters designate spectral class and strength of hydrogen lines

We now order spectral classes in order of temperature:

O, B, A, F, G, K, M

Oh Be A Fine Guy/Girl, Kiss Me

Absorption Lines in Spectral Classes

Hydrogen lines are most prominent at 10,000 K. O stars are too hot, most hydrogen is ionized. M stars are not hot enough to excite hydrogen's electrons.

Table 17.2

Spectroscopic Parallax

Distance Determination

Spectrum stellar class

Spectral class luminosity

Compare luminosity with apparent brightness

Luminosity distance

Has nothing to do with "parallax" itself

Color

Color of a star is determined by the ratio of the intensities at two different places on the spectrum curve

B-V are common filters to measure light intensity

Colors are indicative of temperature

Stellar Radii

The larger the radius, the more surface area, the larger the luminosity

L µ T⁴ R²

Hertzprung-Russell Diagram

H-R diagram of prominent stars

X axis: Temperature, spectral class

Y axis: Luminosity, absolute magnitude

H-R Diagram

Nearby stars on left, Bright stars on right

Group Discussion

What part of HR diagram has the:

Highest temperature

Highest luminosity

Lowest temperature

Lowest luminosity

Red giants

White dwarfs

Determination of Mass: Visual

Binary systems:

Visual

Spectroscopic

Eclipsing

Kepler's laws to get Mass

Visual binary on right

Fig. 17-19

Determination of Mass: Spectroscopic

Motion caused by orbit about center of mass in system produces Doppler shift that can be measured

Accurate within 25%

Fig. 17-20

Determination of Mass: Eclipsing

Light intensity changes due to blockage by one star in front of the other

Fig. 17-21

Mass

Mass increases upward on the H-R diagram

Stellar Lifetimes

Stellar radii and luminosity vs. Mass. Fig. 17.24

The more luminous or massive a star is, the shorter the lifetime

Extending the Distance Scale

Fig. 17-17