Unit: The
Universe
Topic: Properties of Stars
(U5L2 Exp.2)
1. Describe 5 properties of stars that
astronomers are able to determine by
studying light spectra.
2. Describe the relationship shown on a H-R
diagram.
Draw or type 2 things you already know about today’s topic:
How do scientists study distant
stars?
● Scientists study the light using
telescopes and spectroscopes.
○ They study the observed
spectrum.
■ Spectrum: a representation of an
object’s light spread into component
frequencies
Properties of Stars
1. Composition
2. Distance
3. Brightness
4. Temperature & Color
5. Size & Mass
1. Composition
● Recall that each
element has its own
unique spectrum, much
like a fingerprint.
● By studying the light
spectra of a star and
matching the spectral
lines to the known
elements, scientists can
identify which elements
are in the star.
The absorption spectrum of an unknown element is
pictured as well as the emission spectra for helium,
carbon, and hydrogen.
Determine which element the absorption spectrum
represents by comparing it with the emission
spectra.
2. Measuring Distance to Stars
● Parallax is the slight shifting of
the apparent position of a star
due to the motion of Earth.
○ Closer stars have the largest
parallax angles.
Parallax
Original
Photo
Photo taken 6 months
later
Photograph nearby star against the background distant
stars. Photograph 6 months later. Compare--position of
nearby star, seems to have shifted with respect to
background stars.
Light-Year
● Distances to stars are so large
that light-years are the unit of
measurement.
● A light-year is the distance light
travels in one year.
○ about 9.5 trillion kilometers
(9.5 x 1012 km)
● Proxima Centauri, the next
closest star, is about 4.3
light-years away from the sun.
Which unit is used to measure
distances to stars?
3. Stellar Brightness/Luminosity
● Apparent magnitude is the
brightness of a star when
viewed from Earth.
○ Brighter stars have smaller
magnitudes
● 3 factors control the apparent
brightness of a star as seen
from Earth:
○ how big it is
○ how hot it is
○ how far away it is
● Absolute magnitude is the
apparent brightness of a star if
it were viewed from a distance of
3.26 light-years.
○ Our sun has an apparent
magnitude of -26.7 and an
absolute magnitude of 4.8.
■ Stars with absolute magnitudes less
than 4.8 are brighter than our sun but
look dimmer because they are farther
away.
Why does the sun look like the
brightest star in our sky?
4. Temperature & Color
● Color is generally a good indicator of
a star’s surface temperature.
● Recall when studying the EMS, lower
frequencies are associated with
lower energy.
○ Red light has the lowest frequency
of visible light.
■ A red star is cooler (emitting less
energy) than a blue star of the
same size
Each circle represents a different
star and each color a different
temperature. Order the stars from
(1) hottest to (5) coolest.
5. Size & Mass
● In general, larger stars have more
atoms than smaller stars and are
more massive.
● Starts with larger diameters typically
have higher mass, are brighter,
hotter, and have shorter life cycles.
● Size can be estimated using:
○ temperature
■ color
○ amount of energy emitted
■ brightness
■ distance
● Outliers:
○ Giants & supergiants: very large &
bright but not very dense
○ White dwarf stars: small, very hot &
dense
Order the given stars in order of
mass, from (1) highest to (5) lowest.
The masses are expressed in terms
of solar mass M (M = mass of our
sun).
•A
Hertzsprung–
Russell
diagram
shows the
relationship
between the
absolute
magnitude
and
temperature
of stars.
• 85-90% of stars
are
main-sequence
stars
• H-R diagram is a
model that
predicts the
characteristics of
stars at different
life-cycle stages.
Draw or type 2 things you learned in today’s lesson: