1. Explain the basis of the stellar classification scheme that involves
the “spectral type” (O,B,A,F,G,K,M).
Astronomers classify stars according to surface temperature by assigning a
spectral type determined from the spectral lines present in a star’s
The hottest stars with the bluest colors are called spectrial type O, followed
in the orderof declining suface temperature by spectral types B,A,F,G,K,M
“Oh, be a fine guy kiss me!”
we measure a star’s surface temperature from its color or spectrum, and
we classify spectra according to the sequence of spectral types, OBAFGKM,
which runs from hottest to coolest. Cool, red starts of spectral shape M are
much more common than hot, blue stars of Spectral
How do the colors of these stars vary?
Some stars fail to achieve a proper balance between the amount of fusion
energy welling up from their cores and the amount of radiative enegery
emanating from their surfaces.
The surfaces of these stars therefore
pulsate in and out, periodically rising and falling in luminosity.
Stars come in different colors because they emit thermal radiation.
Recall that an ideal thermal radiation spectrum depends only
on the surface temerpature of the object that emits it.
What is really being measured by the spectral type?
A spectral type is determined from the spectral lines present in a star’s
The hotest stars with the bluest colors are called spectral type
O, and M being the coolest.
Cool, red stars are much more common than hot, blue stars.
2. What is the difference between “apparent” and absolute”
magnitude when describing the brightness of a star?
Apparent magnitudes is how bright stars “appear” in the sky. Apparent
magnitudes are directly related to apparent brightness, except the scale
A larger apparent magnitude means a dimmer apparent
Example: A star of a magnitude 4 is dimmer in the sky than a star
magnitude of 1.