1. There are three main ways to determine the distance to stars: comparing absolute and apparent magnitude, using parallax to calculate distance, and applying the inverse square law relating luminosity and brightness to distance. 2. Key properties of stars can be determined by their spectra, luminosity, temperature, and location on the Hertzsprung-Russell diagram showing the main sequence of star development. 3. Binary star systems provide information about stellar characteristics and evolution through their gravitational interactions, and some binaries involve mass transfer or lead to supernovae.

1. Other Suns
Distant Stars

2. What is a Star?
• Stars begin as cold molecular hydrogen
that its own gravity causes to collapse.
• As it collapses, it fragments and these
fragments form stars because of
nuclear fusion at the core.
• Gravity pushes inward, the energy from
the core pushes outward. This balances
the star.

3. Finding distance to stars
• 1. Absolute Magnitude: A scale which
uses a specific distance to compare the
brightness of stars. (10 parsecs, or 32.6
Light years, almost our sun distance.)
Means the brightness a star would have
at that distance. (Similar to Luminosity)
• With the scale THE LOWER THE
NUMBER THE BRIGHTER THE STAR.

4. How Powerful are the Stars?
• Luminosity: Amount of energy a star gives off
in a given amount of time.
• Analogy:
Think of headlights: Car headlights are basically
the same. But the closer they are the brighter
they are. So the same headlights can show
different brightness depending on the distance
they are away.
So Stars can have the same brightness but we
see them differently because of distance.

5. Here is how it works
• 1. Calculate how bright a star would
appear at standard distance of 32.6 lt-
yrs. So all stars are compared at that
distance. This is called absolute
magnitude.
• For a star at this distance is has an
absolute magnitude and apparent
magnitude that are the same.

6. Explanation step
• 2. Consider a star that is farther away
from us than the standard distance. To
get absolute magnitude you bring it
closer. That means its magnitude at
standard distance is brighter than
apparent magnitude (brightness at
actual distance).

7. 1 Continued: Apparent Magnitude
Apparent Magnitude is the brightness of
a star at its actual distance.
Apparent Magnitude is calculated using a
telescope.
Compare Absolute Magnitude to Apparent
Magnitude gives the distance to the star.

8. 2. Trigonometric Parallax
parallax = “p” is defined as half of the angular
shift produced over a 6 month baseline (2 au)
which is the diameter of the earth’s orbit.

9. Parallax explained.
• Example: The distance of a star whose
parallax is 1 is 1 parsec. 1 pc is 3.26 lt-
yrs.
• It turns out then that the star’s
distance in parasecs is the inverse of its
parallax in seconds of arc:
• d = 1/p

10. Examples
• A star whose measured parallax is .5 arc
seconds has a distance of 1/.5 or about
6.5 lt-yrs.
• The closest star Proxima Centauri has a
parallax of .77 arcseconds so how many
light years away is it?
• 1/.77 = 1.3 pc 1.3 pc = ? lt yrs?
• 4.2 lt-yrs

11. 3. b = L/(4pd2)
• A star’s brightness (b) depends on its distance because of how
the light spreads out uniformly in all directions.
The larger the distance
the greater surface area
of the sphere that
the light spreads out
over, so the smaller the
amount of
light that will pass
through a section of the
sphere.

12. Inverse Square Law
• The energy we receive from a star
(luminosity) decreases with
• the square of
• the distance
• the star is
• away.

13. Summary
• 3 ways to find distance to stars.
• 1. use telescope, find apparent
magnitude, calculate absolute magnitude
and compare them.
• 2. use relationship between parallax
and distance d = 1/p
• 3. use luminosity and brightness
• b = L/(4pd2) where d is distance to
star.

14. Proper Motion
• Proper Motion: Stars motion in sky
relative to OTHER stars, not Earth.
• Units are typically arc/century.
• Easier to measure stars motion toward
or away from us rather than side to
side.

15. Doppler Effect
• Reminder of Doppler Effect:
• change in
• the spectrum
• because
• the source
• is moving.

16. Red Shift/Blue Shift
• Blue Shift means the star is moving toward
us because the wavelengths get shorter.
• Red Shift means the star is moving away
from us because the wavelengths get
longer.
• Song??
http://www.astrocappella.com/doppler.sht
ml

17. eye eye

18. How Fast do stars go?
change in wavelength = speed of emitter
rest wavelength speed of light
speed of emitter is speed of source
either toward or away from us
rest wavelength is emitted by a source of
light that is not moving relative to the
observer.
speed of light is 3E8 m/s.

19. Classification of Stars
• Main Sequence: On a
• Temperature -
Luminosity
diagram stars
at the main
sequence are
burning
hydrogen into
helium.

20. OBAFGKM
Spectral type in order of hottest to coolest.
More recently: Cooler objects like brown
dwarfs and failed stars.

21. Absolute Magnitude vs. Spectral
Type
• Same graph results plotting Absolute
Magnitude (equivalent to luminosity) vs.
Spectral type (derived because of
temperature)

22. Binaries
Binary star systems have enabled
astronomers to deduce characteristics
and make up of the stars as they
influence one another.
Some orbit black holes: One such
system consists of a blue supergiant
star and a suspected black hole which
has 14.8 times the mass of the Sun.
Some binaries are invisible: The second
star might be a brown dwarf or neutron
star, only making itself known through
its gravitational influence on its partner.
They cause Supernovae: Binaries transfer mass between them.
When one star takes on so much mass it can no longer support
itself and it explodes.
4 out of 5 stars are multiples.

23. Variable Stars
• Stars that change brightness because
of an eclipsing spectroscopic binary.
• A very famous one is Algol which can be
seen in the North American sky.
• http://csep10.phys.utk.edu/astr162/lec
t/binaries/algol.html

24. Some Star Stories
• http://www.astrocappella.com/backgrou
nd/wolf_background.shtml
• http://www.astrocappella.com/activities
/land_safely.html
• Binaries: http://www.space.com/22509-
binary-stars.html