Download 29.2 Measuring the Stars - Mr. Tobin`s Earth Science Class

 Students
will be able to determine how
distances between stars are measured.
 Students will be able to distinguish
between brightness and luminosity.
 Students will be able to identify
properties used to classify stars.
 Star: A
self-luminous celestial body
consisting of a mass of gas held together
by its own gravity.
 Page
837
 Main
Idea: Stellar classification is
based on measurement of light
spectra, temperature, and
composition.
 Page
837

Constellations: Groups of
stars.
• There are 88 of them.
• Circumpolar Constellations:
Visible in Northern
Hemisphere all year.
• Summer, Fall, Winter, Spring:
Only visible during certain
seasons.
• Constellations are used by
people to know when to
prepare for planting, harvest,
and ritual celebrations.
Circumpolar Star
Trails
Seasonal Constellations
Hercules: Visible in NH
only during summer
Orion: Visible in NH only
during winter
 Page
838


Star Clusters: Groups of stars that are
gravitationally bound to each other.
2 kinds:
1.
2.
Open Cluster: Not densely packed.
Globular Cluster: Densely packed into
spherical shape.
Open Star Cluster
Globular Star Cluster
 Page
838

Binary Stars: Two stars that are
gravitationally bound together and
that orbit a common center mass.
•
•
Hard to tell two stars apart.
One star often is much brighter than the
other.
 Constellation
in a cup
 Page
840
 Stellar
Position and Distances:
 2 Units used to measure long
distances:
1. Light Year (ly): The distance that light
12
travels in 1 year = 9.461 x 10 km.
2. Parsec (pc): 3.26 ly.
Distances in Light Years
 Page
840
 Parallax: Apparent
shift in position of
an object caused by the motion of
the observer.
• Used to calculate distance.
• Earth’s orbit around the Sun means that
an observer’s position in space is always
changing.
• Distance to a star can be estimated
using the star’s parallax (angle of
change.)
• Closer the star, the larger the shift.
• Estimates are accurate up to 500 pc
away.
 Page
841
4
1.
2.
3.
4.
Basic Properties of Stars:
Mass
Diameter
Luminosity
Temperature – Estimated by
finding spectral type of star.
Controls nuclear reaction rate and
governs luminosity and magnitude.
H-R Diagram
 Page
842
 Magnitude:
• Apparent Magnitude: How bright a star
appears.
 Brightest stars = +1
 Next brightest = +2
 A difference of 5 magnitudes = 100 x more
bright.
 Ex) +1 is 100 x more bright than +6.
 Page
842
 Magnitude:
• Absolute Magnitude: Accounts for
distance (apparent does not.)
 How bright a star would appear if it were 10 pc
away.
 Page
842
 Luminosity: Energy
output from
surface of star per second – a star’s
power.
• Requires that one know a star’s apparent
magnitude and distance away.
• Measure in watts – energy emitted per
second.
26
 Sun’s luminosity = 3.85 x 10 w.
26
 Page
483
 Classification
of Stars:
 Word of the Day:
• Spectral Line: An isolated bright or dark
line in a spectrum produced by
emission or absorption of light of a
single wave length.
 Page
483
 Spectral
Lines: Provide information
about a star’s temperature and
composition.
 Page
483
 Temperature: Corresponds
to a star’s
spectral lines.
 Types: O
Hot 50,000 K

B

A

F

G

K

M
Cool 2,000 K
 Each type is subdivided into more
specific divisions by numbers 0 – 9.
Spectral
Lines of
Stars
Hotter
Stars
Cooler
Stars
 Page
 Composition: All
485
compositions.
stars have similar
• Hot Stars: Less lines in spectra (fewer
kinds of gasses.)
• Cool Stars: More lines in spectra (more
kinds of gasses.)
 Typical
Composition of Stars:
• 73% hydrogen (H)
• 25% helium (He)
• 2% other elements.
Composition of Stars
“Heavier Elements” = Anything heavier than Helium
 Page
 Hertzprung
485
Diagram):
– Russell Diagram (H-R
• Shows the lifetimes of stars.
• A graph on which absolute magnitude is
plotted on vertical axis and temperature
or spectral type is plotted on horizontal
axis.
 Each
class of star has a specific
mass, luminosity, magnitude,
temperature and diameter.
 Page
485
 H-R
Diagram: Plots evolution of stars
from one class to another.
 Main Sequence: Area of the H-R
Diagram in which most stars are
located.
• Runs diagonally from upper left corner
(Hot Bright Stars) to lower right corner
(Cool Dim Stars.)
 Page
486
 Main
Sequence: Contains 90% of
stars including Sun (which is at the
center because it has an average
temperature and luminosity.)
• Stars here fuse hydrogen.
• As hydrogen runs out stars fuse helium
and begin to evolve off of the main
sequence.
 Page
486
 Mass
determines a star’s lifetime.
• More Mass = More Pressure = Shorter
Lifetime
• Higher mass stars burn hydrogen faster.
• Lower Mass = Less Pressure = Longer
Lifetime
• Lower mass stars burn hydrogen slowly.
 Page
486
 Red
Giants and White Dwarfs:
 Red Giants: Top right of H-R diagram
= Cool, Bright Stars.
• Because these stars are cool, but still
bright, they must have large surface
areas.
• Mass is more than 100 x greater than the
Sun.
 Page
486
 Red
Giants and White Dwarfs:
 White Dwarfs: Lower left of H-R
diagram = Hot, Dim Stars.
• Appear dim, though they are very hot,
must have small surface areas.
• Size of Earth, but has a mass same as the
Sun.