Transcript ppt

Cepheids
Lab 7
Cepheid Variables
• All stars, late in their lifetime, change from being
average stars for their mass (main sequence
stars) to becoming swollen red giants
• Most stars change from the swollen red giant
phase to pulsating variable stars before they
finally die, all reactions ceasing
• These are Cepheid Variables, which expand and
contract, glowing brightly and fading every so
often
How Does it Do it??
• Cepheid is a star of several solar masses
and ~104 solar luminosities whose
luminosity changes periodically
• As radiation streams out, some He+ in the
atmosphere of the star is ionized to He2+,
making the atmosphere opaque
Then……
• This decreased transparency of the stellar
material blocks the energy flux and heats
the gas, so the increased pressure pushes
the envelope out, which increases the
star's size and luminosity
• As the star expands, it cools, and He2+
gains an electron, converting back to He+
• The enhanced transparency causes the
atmosphere to shrink again
So… Cepheids are …
• pulsating stars
• have used up their main supply of
hydrogen fuel are unstable and pulsate
• have a relationship between the period of
a Cepheid and its luminosity
• Type I Cepheids are population I stars
whose brightness periods fluctuate from 1
to 100 days
Cepheids as Distance Markers
• This relationship allows us to work out how
much brighter than the Sun the star is
• From there we can calculate how much further
away the star must be than the Sun to make it
the brightness we see from Earth
• Delta Cephei shows has a period of about 5
days
• This is a reasonably bright star in the
constellation of Cepheus
• Cepheids are in other galaxies also, and used
similarly, to find distance
calculations
• The brightness at the distance of 1 lightyear > than the observed brightness
…..due to the fact that brightness drops
like the square of the distance
• From these numbers one can extract the
distance to the stars
• This method works up to 13 million lightyears when Earth-bound telescopes are
used
Example
• Population 1 Cepheid in the cluster with a period of 11
days and average apparent magnitude of 2.3
• From a graph we can see that the star's luminosity must
be 10,000 times that of the Sun. This corresponds to a
magnitude difference of 10
• Since the Sun's absolute magnitude is 4.8, the absolute
magnitude of the star must be
4.8 - 10 = -5.2
• We can now use a simple equation (Inverse Square
Law) connecting absolute and apparent magnitude with
distance to find that the distance of the star and also its
cluster is about 320 parsecs
Inverse Square Law
• The relationship between intensity of light
coming from a source and the distance of
the observer from that source
• It states that the intensity is inversely
proportional to the square of the distance
(i.e. if you are twice as far from a light, it
will appear four times as dim)
Definitions
• period
The length of time between successive peaks in the brightness of a
variable star
• apparent magnitude
A measure of the brightness of a star as seen by the observer.
Based on a system set up by the Greek astronomer Hipparchus
• absolute magnitude
A measure of the inherent brightness of a star independent of any
observer. Equivalent to the apparent magnitude of a star seen from
a distance of 10 parsecs.
• luminosity
Gives the total amount of radiation being given off by a star in 1
second
• parsec
The distance at which a star would have a parallax angle of 1".
(~3.26 light years)
Two Magnitude Systems
• One gives the brightness of a star as
observed from here on Earth. This is the
apparent magnitude.
• The other - absolute magnitude - gives a
measure of the total amount of light being
given out by the star in all directions
• Both systems are logarithmic with a
difference in magnitude of 5 being
equivalent to a factor of 100 in brightness
More Defs
• logarithmic
Applied to measures that increase by 1 whenever the
factor they depend on is multiplied by a certain number.
For example, if a value increased from 10 to 100 to
1000, then a logarithmic measure of that value could
increase, say, from 1 to 2 to 3.
• red giant
After a star has finished burning the hydrogen in its core,
it enters this stage before dying. The star swells up and
turns red.
• Doppler Effect
The apparent change in the wavelength of waves due to
the relative movement of the source of the waves
relative to the observer.
More Defs contd
• Hertzsprung-Russell diagram
A graph plotting absolute magnitude against spectral
type. Shows the correspondence between temperature
and luminosity
• Hubble's Law
Relationship between a galaxy's distance and its speed
of recession
v=H*d
where v is the speed, d is the distance, H is Hubble's
constant
• Hubble graph
A graph plotting a galaxy's recession speed against its
distance
• Stefan's Law
The relationship between luminosity, surface area and
surface temperature of a star