Chapter 25.1 – Properties of Stars
I.
Characteristics of Stars – This chapter will begin with examining some properties of stars, such
as color, temperature, and mass.
A.
Star Color and Temperature
1.
B.
Color is due to a star’s temperature.
i.
Blue Stars = very hot!!! Surface temperature >30,000 K (emit energy in
form of short-wavelength light and therefore appear blue)
ii.
Red Stars = much cooler (emit energy in form of longer-wavelength
light and therefore appear red)
iii.
Yellow Stars = medium temperature…Surface temperature between
5,000 and 6,000 K (like our sun)
Binary Stars and Stellar Mass
1.
Binary stars – pairs of stars that orbit each other due to their gravitational pull
(more than 50% of the stars in the universe may occur in pairs or multiples)
i.
Binary stars are used to determine the star property most difficult to
calculate – its mass.
ii.
Orbit each other around a common point called the center of mass.
a.
b.
c.
II.
Stars with equal mass: mass lies exactly halfway between them
1 star > other: common center closer to the more massive one
***If the sizes of their orbits are known, the star’s mass can be
determined!!!
Measuring Distances to Stars
A.
Parallax – the most basic way to measure star distance; slight shifting in the apparent
position of a nearby star due to the orbital motion of Earth.
1.
First astronomers photograph nearby star against the background of distant
stars.
2.
Six months later, when Earth has moved halfway around its orbit, a second
photograph is taken.
3.
When the photographs are compared the position of the nearby star appears to
have shifted with respect to the background stars.
B.
III.
Light-Year – distance light travels in one year—about 9.5 trillion km
Stellar Brightness – measure of a star’s brightness is its magnitude
A.
Apparent Magnitude – a star’s brightness as it appears from Earth
1.
2.
Three Factors control the apparent magnitude as seen from Earth:
i.
How big it is (size)
ii.
How hot it is (temperature)
iii.
How far away it is (distance)
Numbers are used by astronomers to rank apparent magnitude.
i.
The larger the number, the dimmer the star
a.
B.
a first-magnitude star is about 100 times bright than a sixthmagnitude star
Absolute Magnitude – how bright a star actually is
1.
Two stars of the same absolute magnitude usually do not have the same
apparent magnitude because one may be much farther from us than the other
ex. Two people of the same height standing different distances from the person
viewing them
i.
Astronomers determine what magnitude the stars would have if they
were at a standard distance of about 32.6 light-years
IV.
Hertzsprung-Russell Diagram – a graph used to study stars; shows the relationship between the
absolute magnitude and temperature of stars (teaches about the sizes, colors, and temperatures
of stars)
A.
Types of Stars
1.
main sequence stars – 90% of stars are main-sequence stars that fall along a
band that runs from the upper-left corner to the lower-right corner of the
diagram (hottest=brightest::coolest=dimmest)
i. Appear in decreasing order, from hotter more massive blue stars to cooler,
less massive red stars.
2.
red giants – above and to the right of the main sequence stars
i. Objects with = surface temperatures radiate the same amount of energy per
unit area. Therefore any difference in the brightness of two stars having the
same surface temperature is due to their relative size.
B.
3.
supergiants – largest of all the stars
4.
white dwarfs – stars in the lower-central part of the H-R diagram (very small in
size)
5.
variable stars – stars may fluctuate in brightness
Interstellar Matter
1.
Nebula – clouds of gas and dust that stars and planet form from
i.
if a nebula is close to a very hot star, it will glow and is called a bright
nebula
a.
emission nebula – consists largely of hydrogen; absorb
ultraviolet radiation emitted by a nearby hot star and emit the
energy as visible light
b.
reflection nebula – merely reflect the light of nearby stars
c.
dark nebula – not close enough to a bright star to be lit up