Transcript Answer to question 1 - Northwestern University

What are Cepheid Variables
• Cepheids are unstable (on human time scales) stars
with cycles of 1-50 days. And the longer the period
the intrinsically more luminous they are =>
QuickTime™ and a
decompressor
are needed to see this picture.
Fun animation on how standard
candle works
QuickTime™ and a
decompressor
are needed to see this picture.
Basic Concept:
Measure the period
know the intrinsic brightness
infer the distance using:
F = L/4pd2; where
F = the apparent brightness
L = the intrinsic brightness
luminosity
d= distance
What is Cepheid Variable
• A Cepheid is a star that has a core which
generates so much pressure, the envelope
expands and “over shoots the point where the
internal heat (and light) pressure will hold up
the envelope.
•The result is that the envelope then comes
falling down,
• Over shoots inward
• Then starts over
• Simplistically, the envelope acts like a
“brick wall” (in physics terms it’s optically
thick and radiates as black body), so ...
•The more expansion, the larger the surface
area and the higher [or larger] the intrinsic
brightness. But more expansion takes longer
=>
• Longer periods go with higher brightness.
Time scale OK for Human endevor
But be careful, life is complex:
Complications:
Two types of Cepheids: type I (also called
“classical Cepheids”) and type II; Type I’s
are about 4 times brighter for the same
period and are found in young star groups
called open clusters.
Type II’s are also called WW Virginis stars are
found in old star clusters called Globular clusters
La vie n’est pas facile, cont.
DUST can make the stars appear dimmer than
otherwise.
Edwin Hubble over corrected for dust
Was looking at II’s when thought looking at I’s
And used objects too close
He ended up deriving H0 = 500 km/sec-Mpc!
Hubble’s Original Diagram
Notice
top
distance,
=>
amazing
he saw
anything
at all =>
From Ned Wright
It’s better to be lucky than good.
or maybe
• Generate an overlap with Cepheids and go out.
• Method is to cross check/overlap as far as we can to
calibrate other methods.
Requirements for methods:
• Bright enough to measure far beyond 15 Mpc
(distance to Virgo z = about 0.003,
v = about 1000 km/sec)
• Numerous or frequent enough to get 30-100 data
points ( to define a “Hubble relation” => v = H0D)
BTW, before we go too far
Pun intended
How far out can we go and
still use the simple Hubble
relation v = H0D
Answer is z < 0.1
And, how close is too close?
Certainly z must be > about 0.003 (the
distance to Virgo where we still feel a
“tug.” (More on the “Great Attractor”
later). And ideal is to start with z = 0.02
Also for the record 1+z = lob/lem; works
even for blueshift, just doesn’t correspond
to R0/R then.
1 v / c
 1 v / c for v/c << 1
1+z =
1- v / c
Or what is commonly used is:
z = v/c or v = zc
v > 0 means recession; v = < 0 means
coming towards us. We usually don’t quote a
z value for approaching objects, just a
velocity.
Some “classic” distance measures
Must cross calibrate all of these with Cepheids
(1) Tully-Fisher/Jackson-Faber Relation...
(2) Brighter standard candles
A. Supernovae
B. Red giants, globular clusters,
brightest galaxy in a cluster
If Supernovae are so good, why bother?
Supernovae are not frequent => not frequent enough if
we want to measure the distance to some objects
directly.
Indirect = Hubble relationship (or a fancier form for z
> 0.1) and measure a redshift.
Indirect doesn’t work if it’s too close = closer than
about 20Mpc (z= about 0.003)
When the objects become faint and the necessary large
(10 meter) telescope time is not available
What is Tully-Fisher?
Relationship between the absolute (visible or
infra-red) brightness of a spiral galaxy versus
the width of the 21 cm line.
21 cm line is intrinsically narrow which helps a
lot
Physics of line width: relational quantities and the
Heisenberg Uncertainty Principle:
Time and Energy => Line is made from a very very
slow transition ( about 107 years*!) and
Dt x DE > h => High Dt => Very low DE =>
line is well defined or line width is small
h is called Planck’s constant
*http://tesla.phys.unm.edu/phy537/8/node3.html
Did you know?
Famous colleague of
Planck’s was Max Born who
had a famous grand daughter
Olivia Newton-John; cf
http://www.onlyolivia.com/onj.html
Also a famous X-ray Joan Baez
optics design called
http://imagine.gsfc.nasa.gov/
docs/science/how_l2/
Kirkpatrick … was
and
named after who has a xtelescopes_systems.html
http://baez.woz.org/jbchron.html
famous daughter
What is Tully-Fisher, cont.
A spiral galaxy is one that has a spiral shape, and
these enough atomic hydrogen in them to allow a
detection of the 21 cm line.
This image of NGC 628
(M-74) was obtained by
the 8.1-meter Gemini
North Telescope on
Mauna Kea Hawaii using
the newly
commissioned Gemini
Multi-Object
Spectrograph. To make
the color image, three
images were combined
to make this red, green
and blue composite.
18 kpc
Need to look edge-on, or no motion relative to us
Need spiral galaxy to get regular circular motion
(more on that later) And to have enough atomic
hydrogen to be detectable
The 21 cm line is very special, cont.
Also, 21 line is in the radio = 1420 MHz (Mega
Herz) on your radio dial (FM is about 100)
Also 21 cm can be observed during day and
through clouds. => Dutch astronomers got
good at this.
From J. Schombert