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Variable Stars & Distance
The
“Standard Candle”
Cepheid Variables
• Stars that exhibit pulsation periods on the order of a few days to
months, are 4–20 times more massive than the Sun, and up to
100,000 times more luminous.
• Cepheids are supergiants of spectral class F6 – K2 and their radii
change by several million km (30%) in the process.
• There exists a well-defined relationship between a Cepheid
variable's luminosity and pulsation period, securing Cepheids as
viable standard candles for establishing the Extragalactic Distance
Scale.
• Over 700 classical Cepheids are known in the Milky Way Galaxy,
and several thousand extragalactic Cepheids have been discovered.
• The Hubble Space Telescope has identified classical Cepheids in
NGC 4603, which is 100 million light years distant.
• http://www.phys.ufl.edu/~buchler/ceph/anim.html
Discovery of Cepheids
• On September 10, 1784 Edward Pigott detected the
variability of Eta Aquilae, the first known representative
of the class of Cepheid variables.
• The namesake for classical Cepheids is the star Delta
Cephei, discovered to be variable by John Goodricke a
few months later.
• The period-luminosity relation of Cepheids was
discovered in 1908 by Henrietta Swan Leavitt in an
investigation of thousands of variable stars in the
Magellanic Clouds.[9] She published it in 1912 with
further evidence.
RR Lyrae variables
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RR Lyrae variables are periodic variable stars, commonly found in globular clusters,
and often used as standard candles to measure galactic distances.
This type of variable is named after the prototype, the variable star RR Lyrae in the
constellation Lyra.
RR Lyraes are pulsating horizontal branch stars of spectral class A (and rarely F),
with a mass of around half the Sun's. They are thought to have previously shed mass
and consequently, they were once stars with similar or slightly less mass than the
Sun, around 0.8 solar masses.
RR Lyrae stars pulse in a manner similar to Cepheid variables, so the mechanism for
the pulsation is thought to be similar, but the nature and histories of these stars is
thought to be rather different. (The average absolute magnitude of an RR Lyrae is
0.75, only 40 or 50 times brighter than our Sun.[citation needed]) Their period is shorter,
typically less than one day, sometimes ranging down to seven hours.
The relationship between pulsation period and absolute magnitude of RR Lyraes
makes them good standard candles for relatively near objects, especially within the
Milky Way. They are extensively used in globular cluster studies, and also used to
study chemical properties of older stars.
The RR Lyrae stars are divided
[1]
into 3 main types ,
following a classification of S.I. Bailey based on the shape of the stars' brighness curve:
• RRab — the majority, with steep rise in
brightness (about 91%)
• RRc — having shorter periods, more
sinusoidal variation (about 9%).
• RRd — rare double-mode pulsators.
RR Lyrae vs Cepheid Variables
In contrast to Cepheids, RR Lyraes are old, relatively low mass, metal-poor
"Population II" stars.
RRs are much more common than Cepheids, but also much less luminous.
RR Lyrae stars were formerly called "cluster variables" because of their strong (but
not exclusive) association with globular clusters; conversely, about 90% of all
variables known in globular clusters are RR Lyraes.
RR Lyrae stars are found at all galactic latitudes, as opposed to classical Cepheid
variables, which are strongly associated with the galactic plane.
Several times as many RR Lyraes are known as all Cepheids combined; in the
1980s, about 1900 were known in globular clusters. Some estimates have about
85000 in the Milky Way[1].
He+ is ionized to He++ within the partial-ionization layer near the surface
of the star whenever T > 40,000K.
The outer layers
and reheat to
- - contracts
- 40,000K, and the ionization cycle begins
again.
This sort of stuff happens all the time within
stars – including the Sun – wherever the
internal temperature crosses the ionization
threshold of important elements.
Along the instability strip, however, the
period for He++ ionization cycling is
approximately equal to the overall acoustic
oscillation period within the star, and so
these fluctuations become greatly amplified
and readily detectable.
When this happens, the opacity of the layer goes up due to the
increased number of free electrons.
The greater opacity drives up the temperature and pressure gradients
across the partial-ionization layer,
which physically expands outward and cools.
As the layer cools, He++ recombines with free electrons to form He+,
driving the opacity back down and
decreasing the temperature and pressure gradient.
The outer layers contracts and reheat to 40,000K,
and the ionization cycle begins again.