Chapter 16--Properties of Stars
Chapter 16--Properties of Stars
Chapter 16--Properties of Stars
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ment is not affected by the star’s distance. Instead, we determine<br />
surface temperature directly from the star’s color<br />
or spectrum. One note <strong>of</strong> caution: We can measure only a<br />
star’s surface temperature, not its interior temperature. (Interior<br />
temperatures are calculated with theoretical models<br />
[Section 15.3].) When astronomers speak <strong>of</strong> the “temperature”<br />
<strong>of</strong> a star, they usually mean the surface temperature<br />
unless they say otherwise.<br />
A star’s surface temperature determines the color <strong>of</strong><br />
light it emits [Section 6.4].A red star is cooler than a yellow<br />
star, which in turn is cooler than a blue star. The naked<br />
eye can distinguish colors only for the brightest stars, but<br />
colors become more evident when we view stars through<br />
binoculars or a telescope (Figure <strong>16</strong>.4).<br />
Astronomers can determine the “color” <strong>of</strong> a star more<br />
precisely by comparing its apparent brightness as viewed<br />
through two different filters [Section 7.3].For example, a cool<br />
star such as Betelgeuse, with a surface temperature <strong>of</strong> about<br />
3,400 K, emits more red light than blue light and therefore<br />
looks much brighter when viewed through a red filter than<br />
when viewed through a blue filter. In contrast, a hotter star<br />
such as Sirius, with a surface temperature <strong>of</strong> about 9,400 K,<br />
emits more blue light than red light and looks brighter<br />
through a blue filter than through a red filter.<br />
Spectral Type<br />
Figure <strong>16</strong>.4 This Hubble Space Telescope<br />
view through the heart <strong>of</strong> our Milky Way<br />
Galaxy reveals that stars emit light <strong>of</strong> many<br />
different colors.<br />
VIS<br />
The emission and absorption lines in a star’s spectrum<br />
provide an independent and more accurate way to measure<br />
its surface temperature. <strong>Stars</strong> displaying spectral lines <strong>of</strong><br />
highly ionized elements must be fairly hot, while stars displaying<br />
spectral lines <strong>of</strong> molecules must be relatively cool<br />
[Section 6.4].Astronomers classify stars according to surface<br />
temperature by assigning a spectral type determined from<br />
the spectral lines present in a star’s spectrum.<br />
The hottest stars, with the bluest colors, are called spectral<br />
type O, followed in order <strong>of</strong> declining surface temperature<br />
by spectral types B, A, F, G, K, and M. The time-honored<br />
mnemonic for remembering this sequence, OBAFGKM,<br />
is “Oh Be A Fine Girl/Guy, Kiss Me!” Table <strong>16</strong>.1 summarizes<br />
the characteristics <strong>of</strong> each spectral type.<br />
Each spectral type is subdivided into numbered subcategories<br />
(e.g., B0, B1,...,B9). The larger the number, the<br />
cooler the star. For example, the Sun is designated spectral<br />
type G2, which means it is slightly hotter than a G3 star but<br />
cooler than a G1 star.<br />
chapter <strong>16</strong> • <strong>Properties</strong> <strong>of</strong> <strong>Stars</strong> 527