Stars - Basic Properties/H- R Diagram

1. Solar Composition
•The Sun consists of hydrogen, about 73.4 percent
by mass, and helium, 25 percent, as well as a
small amount of other elements.
The Sun
•This composition is
very similar to that of
the gas giant planets.
•The Sun’s
composition
represents that of the
galaxy as a whole.

2. Groups of Stars
•Constellations are the 88 groups of stars
named after animals, mythological characters, or
everyday objects.
Measuring the Stars
–Circumpolar constellations can be seen all year long as
they appear to move around the north or south pole.
–Summer, fall, winter, and spring constellations can be
seen only at certain times of the year because of
Earth’s changing position in its orbit around the Sun.

3. Groups of Stars
Star Clusters
Measuring the Stars
–A group of stars that are gravitationally bound to each
other is called a cluster.
•In an open cluster, the stars are not densely packed.
•In a globular cluster, stars are densely packed into
a spherical shape.

4. The Pleiades Star Cluster

6. M55 is a globular
cluster in the
constellation
Sagittarius. Quite
large on the sky
(about two thirds
of the Moon's
diameter), but
with a very
loose, almost
non-globular,
appearance, M55
is about 18000
light-years away
and about 100
light-years
across.

7. Groups of Stars
Binaries
Measuring the Stars
–A binary star is two
stars that are
gravitationally bound
together and that orbit
a common center of
mass.
–More than half of
the stars in the
sky are either
binary stars or
members of
multiple-star
systems.

8. Binary Stars
The top image shows a blob
without adaptive-optics
correction. The bottom one
shows the same image after
an adaptive-optics correction
that resolves the top blob into
a double star.

9. Binary Stars
vary in shape
and position.

10. Stellar Position and Distances
•Astronomers use several units of measure for long
distances.
a)A light-year (ly) is the
distance that light
travels in
one year, equal to
9.461 × 10
12
km.
b)A parsec (pc) is equal
to 3.26 ly, or 3.086 ×
10
13
km.
c)(Write out) – An
Astronomical Unit (AU)
is equal to 93 million
miles, the distance
between the sun and
Earth. Pluto is 39.5 AU.

11. Earth’s diameter: 7,926 Miles

15. Excerpted from a commencement
address delivered May 11, 1996. Dr.
Sagan's book Pale Blue Dot
expands on these ideas.
Image from Voyager 1, 1990 3.7
billion miles away (a little pass
Pluto)
Carl Sagan died Dec. 20, 1996. He
was an astronomer, astrochemist,
and pioneered SETI (Search for ET
intelligence).
Reflections on a mote of dust...
Carl Sagen’s “Reflections on a
mote of dust”
Earth

16. Stellar Position and Distances
•To estimate the distance of stars from Earth,
astronomers make use of the fact that nearby
stars shift in position as observed from Earth.
Measuring the Stars
•Parallax is the apparent shift in position of an
object caused by the motion of the observer.
•As Earth moves from one side of its orbit to the
opposite side, a nearby star appears to be shifting
back and forth.

17. Stellar Position and Distances
•The distance to a star, up to 500 pc using the
latest technology, can be estimated from its
parallax shift.
Measuring the Stars

18. Basic Properties of Stars
•The basic properties of stars include diameter,
mass, brightness, energy output (power), surface
temperature, and composition.
Measuring the Stars
•The diameters of stars range from as little as
0.1 times the Sun’s diameter to hundreds of
times larger.
•The masses of stars vary from a little less than
0.01 to 20 or more times the Sun’s mass.

19. Basic Properties of Stars
Magnitude
Measuring the Stars
–One of the most basic observable properties of a star is
how bright it appears.
–The ancient Greeks established a classification system
based on the brightnesses of stars.
–The brightest stars were given a ranking of +1, the next
brightest +2, and so on.

20. Basic Properties of Stars
Apparent Magnitude
Measuring the Stars
a) Apparent magnitude is based on the ancient Greek
system of classification which rates how bright a star
appears to be.
–In this system, a difference of 5 magnitudes
corresponds to a factor of 100 in brightness.
–Negative numbers are assigned for objects
brighter than magnitude +1.

21. Basic Properties of Stars
Absolute Magnitude
Measuring the Stars
–Apparent magnitude does not actually indicate how
bright a star is, because it does not take distance
into account.
b) Absolute magnitude is the brightness an object would
have if it were placed at a distance of 10 pc.

22. Basic Properties of Stars
Luminosity
Measuring the Stars
–Luminosity is the energy output from the surface of a
star per second.
–The brightness we observe for a star depends on both
its luminosity and its distance.
–Luminosity is measured in units of energy emitted per
second, or watts.
–The Sun’s luminosity is about 3.85 × 10
26
W.

23. Stars: Basic Properties
–Mass determines a star’s temperature, color, and brightness.
Mass also determines the age/life span of a star.
–Stars on the Main Sequence (stable stars) are classified into
“spectral” classes: O, B, A, F, G, K, M.
–“M” is the coolest, least massive, and dimmest.
–“G” is average – the sun is in this class.
–“O” class is the fewest in number, “M” has the most stars.
–“O” is the hottest, most massive, and brightest (write out).

24. Spectral
Class
Effective
Temperature (K)
Color L/L
Sun
Main
Sequence
Lifespan
O 28,000 - 50,000 Blue
90,000 -
800,000
1 - 10 Myr
B 10,000 - 28,000
Blue-
white
95 -
52,000
11 - 400 Myr
A 7,500 - 10,000 White 8 -55
400 Myr - 3
Gyr (Billion
Years)
F 6,000 - 7,500
White-
yellow
2.0 - 6.53 - 7 Gyr
G 4,900 - 6,000 Yellow0.66 - 1.57 - 15 Gyr
K 3,500 - 4,900 Orange
0.10 -
0.42
17 Gyr
M 2,000 - 3,500 Red
0.001 -
0.08
56 Gyr
*M
a
s
s
60
18
3.1
1.7
1.1
0.8
0.3
*Mass is
compared
to our
sun..
“Oh,
Be
a
Fine
Girl,
Kiss
Me.
Brightness
Highest
Lowest
Brightest
Dimmest
Shorter life
Longer life

25. Spectra of Stars
H-R Diagrams
Measuring the Stars
–A Hertzsprung-Russell
diagram, or H-R diagram,
demonstrates the
relationship between mass,
luminosity, temperature,
and the diameter of stars.
–An H-R diagram plots the
absolute magnitude on the
vertical axis and
temperature or spectral
type on the horizontal axis.
Red Giants

26. Spectra of Stars
H-R Diagrams
Measuring the Stars
–The main sequence, which
runs diagonally from the
upper-left corner to the
lower-right corner of an
H-R diagram, represents
about 90 percent of stars.
–Red giants are large, cool,
luminous stars plotted at the
upper-right corner.
–White dwarfs are small, dim,
hot stars plotted in the lower-
left corner.

28. •In the Hertzsprung-Russell (HR) diagram, the star is no
longer on the main sequence. If of modest mass, it is a red giant.
If very massive, it is a yellow or red supergiant.
•The Sun, whose current age is 4.5 billion years, will become a
red giant in about 5.5 billion years.



Main Sequence Star

Red Giant