4. Size of a Star
anywhere from 20km to 1 trillion km in diameter
Main Group by Star
1. Neutron Star
• **Can be the distance you travel to school
2. White Dwarf Star
• **Can be the size of Asia
3. Medium-sized Star
• **Our own Sun
5. 4. Giant Star
• *The diameter 10 to 100X larger than our
sun
5. Super-giant Star
• *The diameter 1000X larger than sun
Size of a Star
6. Color: star can be
Blue, White, Orange
or Yellow, red
Temperature:
Blue – hottest (35000⁰C)
Yellow – our Sun
(5500⁰C)
Red - coolest(3000⁰C)
Color and Temperature
7. Characteristics of stars
Color: stars can be red, blue, white, orange or yellow
Composition: contain different elements determined using
spectra
• Temperature: Blue is the hottest (35000 C)
Yellow (our sun) (5500 C)
Red is the coolest (3000 C)
8. Composition
Astronomers use a spectroscope to determine
what stars are made of by their spectral lines
• Hydrogen (70% of the total mass of a star )
• Helium (28% of the total mass of a star)
• Trace Elements- O, Ne, C, N and other
elements (2% )
10. Magnitude of Star
1. Apparent magnitude
is the brightness of a star when viewed from
Earth
The lower the number on the scale, the brighter
the star appears to us.
factors :
how big it is
how hot it is
how far away it is.
11. Analyzing the Apparent Magnitude of
Stars
Relating Magnitudes to Brightness Ratio
Magnitude Difference Ratio of Brightness
1 2.512:1
2 2.5122 = 6.31:1
3 2.5123 = 15.85:1
4 2.5124 = 39.8:1
5 2.5125 = 100:1
.
.
.
10 2.51210 = 104:1
12. EXAMPLE:
Suppose stars 1 & 2 have magnitudes of
m1=6 m2= 1: the ratio of their
brightness is:
SOLUTION:
=2.512m1-m2
=2.5126-1
=100.02
13. Analyzing the Apparent Magnitude
of Stars
• Most powerful
telescopes can
detect stars with
apparent
magnitude of +29
• Faintest star seen
by eye has
apparent
magnitude of +6
• This is called a
6th magnitude star
14. 2. Absolute Magnitude
the actual brightness of a star, assuming all stars
were set at a standard distance from Earth.
true brightness
If sun was 32.6 light-years away, it would be
5th magnitude
•So absolute magnitude of sun = +5
•Most stars are between -5 and +15
Magnitude of Star
15. Analyzing the Absolute Magnitude of
Stars
Relating Absolute Magnitude to Luminosity
Absolute Magnitude Approximate Luminosity
in Solar Units
-5 10,000
0 100
5 1
10 0.01
16. EXAMPLE:
If Sirius A has Absolute Magnitude of 1.5,
how many times that sirius is brightre than
the sun?
SOLUTION:
5-1.5=3.5
2.5123.5 = 25
17. Comparison of the Nearest Stars in terms of
Apparent and Absolute Magnitude
Name Distance (ly) Apparent
Visual
Magnitude
AbsoluteVisual
Magnitude
Sun -26.8 4.83
ProximaCentauri 4.23 11.09 15.5
Alpha CentauriA 4.35 0.01 4.4
B 4.35 1.34 5.7
Barnard’s Star 5.98 9.55 13.2
Wolf359 7.80 14.45 16.6
20. Analyzing Starlight
The surface temperature
of a star is indicated by
its color.
Blue stars shine with the
hottest temperatures and
Red stars with the
coolest.
21. Stellar Classification
• The luminosity class is added in Roman numerals
after the temperature spectral class.
– Indicates the size of the star.
Stellar Luminosity Classes
Class Description
I Supergiants
II Bright Giants
III Giants
IV Subgiants
V Main-sequenceStars
VI Subdwarfs
VII WhiteDwarfs
22. Getting Familiar to the Spectral Sequence
Spectral type- a way of classifying a star by the
lines that appears in its spectrum; it is related
to surface temperature.
Basic spectral types are designated by letters :
OBAFGKM with O for the hottest and M
for the coolest.
subdivided with numbers from 0-9
26. Hertzsprung-Russsell Diagram
developed by Einar Hertzsprung and Henry
Russell in the early 20th century.
graph that exhibits the intrinsic stellar
properties such as the sizes, colors and
temperatures
Plots the individual stars as points , with stellar
luminosity on the vertical axis and surface
temperature con the horizontal axis
29. Stellar Motion
Two kinds of motion are associated with stars
• 1. APPARENT MOTION
• Earth’s rotation causes the illusion of stars moving around a
central star, Polaris, commonly known as the North Star.
• Earth’s revolution around the Sun causes stars to be visible
during different seasons.
• The apparent change of position of a star on the celestial sphere is
called the proper motion of the star.
• Angular Motion on the Celestial Sphere
• -Proper motion is usually denoted by the Greek symbol "mu", and
is a velocity that is usually quoted in units of seconds of arc per
year.
31. 2. ACTUAL MOTION
– First, they move slightly across the sky (only see the
closest ones).
– Second, they may revolve around another star
(binary system).
– Third, they may either move away from or toward
our solar system.
• The apparent shift in the wavelength of light emitted by
a light source moving away from or toward an observer
is called the Doppler Effect.
– Also used for sound waves on Earth.
Stellar Motion
34. Why is it important to learn about the stars?
1. studying stars is important because it helps
tell us how we got all the other elements that
make up things around us (and in us!).
2. To learn from other stars may help us
understand our own Sun, which is also a star.
The Sun only seems different to us because it
is so much closer to us than other stars.
35. 3. we can also learn something about how they
are born and die. This helps us understand
how our own solar system was formed.
4. Stars contain a large fraction of all the visible
mass in galaxies. As a result, their combined
gravitational forces affect the 'dynamics' of
galaxies, i.e. the ways in which galaxies move
and evolve in shape.
36. The gravitational pull of one particular star, our
Sun, is especially important since it is the Sun's
gravitational attraction that keeps the Earth in
orbit.
-Scott Sandford
-Astrophysics Branch
-NASA/Ames Research Center
