Download Integrative Studies 410 Our Place in the Universe

Variable Stars & The Milky Way
Variable Stars
• Eclipsing binaries (stars do not change
physically, only their relative position changes)
• Nova (two stars “collaborating” to produce
“star eruption”)
• Cepheids (stars do change physically)
• RR Lyrae Stars (stars do change physically)
• Mira Stars (stars do change physically)
Eclipsing Binaries (Rare!)
• The orbital plane of the pair almost edge-on to our
line of sight
• We observe periodic changes in the starlight as one
member of the binary passes in front of the other
Cepheids
• Named after δ Cephei
• Period-Luminosity Relations
• Two types of Cepheids:
– Type I: higher luminosity, metal-rich, Pop. 1
– Type II: lower lum., metal-poor, Population 2
• Used as “standard candles”
• “yard-sticks” for distance measurement
• Cepheids in Andromeda Galaxies established
the “extragalacticity” of this “nebula”
Cepheids
• Henrietta Leavitt (1908) discovers the
period-luminosity relationship for
Cepheid variables
• Period thus tells us luminosity, which
then tells us the distance
• Since Cepheids are
brighter than RR Lyrae,
they can be used to
measure out to further
distances
Properties of Cepheids
• Period of pulsation: a few days
• Luminosity: 200-20000 suns
• Radius: 10-100 solar radii
Properties of RR Lyrae Stars
• Period of pulsation: less than a day
• Luminosity: 100 suns
• Radius: 5 solar radii
• Extends the cosmic
distance ladder out
as far as we can see
Cepheids – about 50
million ly
• In 1920 Hubble used
this technique to
measure the distance
to Andromeda
(about 2 million ly)
• Works best for
periodic variables
Distance Measurements
with variable stars
Cepheids and RR Lyrae: Yard-Sticks
• Normal stars undergoing a
phase of instability
• Cepheids are more massive
and brighter than RR Lyrae
• Note: all RR Lyrae have
the same luminosity
• Apparent brightness thus
tells us the distance to
them!
– Recall: B  L/d2
The Milky Way
• Appears as a milky band of
light across the sky
• A small telescope reveals
that it is composed of many
stars (Galileo again!)
• Our knowledge of the Milky
Way comes from a
combination of observation
and comparison to other
galaxies
How do we know?
Obviously a bogus picture of our milky way!
• Question: How can
we say anything
about our Milky
Way, if we cannot
see it from outside?
Enter: the Genius
• William Herschel (XVIII century)
• Simple model:
– Assumed all stars have the same
absolute brightness
– Counts stars as a function of
apparent magnitude
– Brighter stars closer to us; fainter
stars further away
– Cut off in brightness corresponds to
a cut off at a certain distance.
• Conclusion: there are no stars
beyond a certain distance
Herschel’s Findings
• Stars thinned out very fast at right angles to Milky Way
• In the plane of the Milky Way the thinning was slower
and depended upon the direction in which he looked
• Flaws:
– Observations made only in visible spectrum
– Did not take into account absorption by interstellar gas and
dust
Discovering other Island Universes
• Data: Lots of nebulous
spots known in the night
sky
• Questions: What are they?
All the same? Different
things?
• Need more observations!
Build bigger telescopes
(The Leviathan of Parsonstown shown, 1845
Biggest telescope of the World until 1917)
The first nebula discovered to have
spiral structure: M51
Enter: next genius
• Harlow Shapley used variable
stars, e.g. RR Lyrae stars, to
map the distribution of
globular clusters in the galaxy
• Found a spherical distribution
about 30 kpc (30,000 pc)
across
– This is the true size of the
galaxy
• Sun is (naturally!) not at the
center – it’s about 26,000 ly
out
Standing on the shoulders of Giants
• Shapley used methods developed by others
to measure the distance to globulars
• Cepheid variables show luminosity-period
correlations discovered by Henrietta Leavitt
• Shapley single-handedly increase the size of
the universe tenfold!
Structure of a Spiral Galaxy
• Three main parts
of a galaxy:
– Bulge (center of
galaxy)
– Disk (rotating
around center)
– Halo (orbiting
around bulge
with randomly
inclined orbits)
Properties of Bulge, Disk and Halo
Disk
Highly flattened
young and old stars
has Gas and dust
Star formation
White colored,
blue spiral arms
Halo
spherical
Bulge
football-shaped
only old stars
young and old stars
none
lots in center
none since 10 billion yrs
reddish
in inner regions
yellow-white