Download Ch. 25 Properties of Stars

Properties of
Stars
• Star Color – the color of
a star is a clue to its
temperature
a. The coolest stars
are red
b. The medium stars
are yellow
c. The hottest stars
are blue
Star Characteristics
• The color of the star can be a clue to its temperature.
– The hotter the star the shorter the wavelength the star emits.
Measuring Distance to Stars
• Parallax – using the
location of a star at two
different times to calculate
its distance from Earth
– The nearest stars have the
largest parallax (seem to
move the most) and the
farthest stars have the
smallest parallax (seem to
move the least)
The closer the
star the more it
“moves” in the
sky!
Measuring Distance to Stars
• Light-year – the distance light travels
in one year (9.5 trillion kilometers)
• We use this when determining astronomical
distances since miles and kilometers would
be way too small of a number
• The next closest star after our sun, Proxima
Centauri, is 4.3 light years away from our sun
Stellar Brightness
• Apparent magnitude – measure of how bright a
star appears from Earth
• This varies due to the star’s temperature, size, and distance
away from Earth
• Absolute magnitude – measure of how bright a
star actually is
• To get a stars absolute magnitude, astronomers have to
compare all stars at an equivalent distance
These three stars
can have the
same absolute
magnitude when
their sizes,
temperatures,
and distances
from Earth are
taken into
account
Comparison of Apparent Magnitude and Absolute Magnitude of Stars
Name
Distance (light
years)
Apparent
Magnitude*
Absolute
Magnitude*
Sun
N/A
-26.7
5.0
Alpha Centauri
4.27
0.0
4.4
Sirius
8.70
-1.4
1.5
Arcturus
36
-0.1
-0.3
Betelgeuse
520
0.8
-5.5
Deneb
1600
1.3
-6.9
The more negative, the brighter and the more positive, the dimmer
Astronomers estimate that there are 200-400 billion stars in our Milky Way Galaxy,
but we can only see about 2,500 visible to the naked eye on Earth
Life Cycle of a Star
1. Stars begin as large clouds of gas and dust called
nebula
2. The nebula then contracts and shrinks and its
temperature increases
– This process may take ~ 1 million years
3. Now the star is a protostar – still developing , large,
red object
– It can’t be classified as a star yet because it does not undergo
fusion
1. A star becomes a main-sequence star when fusion
begins
– Stars at this stage have a delicate balance between
gravity pushing inward and the gas pressure pushing
outward
• Stable, main-sequence stars have varying lifespans:
– Large, blue stars may only last a few million years
– Small, red stars may last hundreds of billions of years
• As a star begins the end of its lifecycle it will
become a red giant
– Red giants are red in color because they are cooler
– Red giants are HUGE in size compared to when they
were a main-sequence star
• When a small to medium sized star dies it becomes
a white dwarf
– White dwarf – a small, cool star near the end of its life
• When a large star dies it becomes a supernova
– Supernova – an exploding star
– When the largest stars explode, they can form black
holes
Hertzsprung-Russell Diagram
• H-R Diagram – shows the
relationship between the
absolute magnitude and
temperature of stars