Download Variables, Star Clusters, and Nebulae (Professor Powerpoint)

Cepheids and RR Lyrae
PULSATING VARIABLE STARS
Types of Intrinsic Variables:
Long-period variables
Cepheid variables
RR Lyrae variables
Flare stars
Cepheid variables are type F to type K supergiants yellow in color, pulsating stars, and relatively rare.
•Average Temperature = 4000 to 8000 Kelvin
•Average Luminosity = 300 to 40,000 Lsun
Some Information About Stars
On the Main Sequence
Cepheid & RR Lyrae Variables
Distances Using Cepheid Variables
These variable stars show intrinsic brightness variations.
d Cephei
RR Lyrae
Cepheid
From the `light curve', you can tell that it is a
Cepheid or RR Lyrae variable.
The period is simple to measure, as is the
apparent maximum brightness.
A complicating factor: There are two types of
Cepheids.
Classical
In addition to the
standard Type I
Cepheids, there
is also a Type II
Cepheids
To find the distance to a Cepheid Variable:
(1) Determine that the star is a Cepheid Variable
(2) Measure period of pulsation of the star.
(3) Magnitude (Mv) from the Period.
(4) Measure the apparent magnitude m
(5) Calculate the distance to the star using:
m – M = 5 log(d)-5
Why are Cepheid variable stars important?
The period of a cepheid variable is
directly linked to its average
brightness: the longer the period, the
brighter the star.
RR Lyrae Stars
Similar to Cepheids, only
smaller and fainter.
Period = 4 hours to 1 day
Variable stars are named by constellation and
in the order that they are identified.
The first variable star in a constellation is
called R (e.g. , R Orionus), the second S and
so on through Z. After that, the next set uses
double letters, beginning with RR, then RS ,
and on to RZ. Repeat to ZZ.
Then we next start with AA to QZ and after
that V followed by a number.An example is
V335 Tau. There are over 2000 known
variable stars in Sagittarius alone.
Nebulae
•Dust and Gas form three
different types of nebulae
•Emission (Bright) nebula (Red)
•Reflection nebula (Blue )
•Dark nebula
(Dark)
Reflection
&
Emission
Nebula
Emission Nebula are Red
HII Region and Recombination
Trifid Nebula
Hot stars illuminate a gas cloud and excites or
ionizes the gas. Electrons get kicked into higher
orbit, fall back to ground state, producing emission
lines.
Emission Nebulae
Dark Nebula
• Example: Snake Nebula
•Contain gas and dust that
block light
• Cool (10’s K)
•Larger than our Solar
System
•
•
•
•
•
Gas and Dust
Reflection Nebula
Absorption line spectra (stars)
Doesn’t generate own light
Scatters blue light from starlight passing through
Nebula appears blue (like sky)
Witch Head Nebula
Pleiades
A Panorama in Orion
Can you
ID the
different
types of
nebula
here?
Dark Nebula have lots of dense gas
and dust. More opaque than others
and blocks light.
Types of Nebulae
Dark Nebula
Emission Nebula
Reflection Nebulae
Reflection
Visible
UV
light
Nebula
(dust)
Blue
Hot Young Stars
Always in the spiral arms
Emission Nebula, gas is
ionized by UV from stars.
Electrons combine with
ions , drop to lower orbits
giving off visible light (red
in color)
When we see a star through interstellar clouds, it
appears redder than it actually is . This happens
because short wavelength , mainly blue starlight,
is scattered more by dust than longer wavelength
red light.
This interstellar reddening is different from
reddening due to the doppler shift.
Doppler shift causes all wavelengths to lengthen
equally, while interstellar reddening does not
change the wavelength of the star light, only their
intensity.
Star Clusters
Open Star Clusters (Galactic Clusters)
• Star clusters are roughly classified on how
“tight” they are.
•“Open” clusters are less compact, and generally
have a smaller numbers of stars (a few hundred).
Open clusters lie in the central plane of our galaxy.
where gas and dust are densest.
The stars within an open
cluster are, typically spaced
about 1 parsec apart. It is
estimated that there are 20,000
open clusters within our
galaxy.
Globular Star Clusters
 “Globular” clusters are more compact, and
generally have relatively large numbers of
stars (a few hundred thousand).
Old stars some with ages
of about 12 billion years.
No new star formation is
taking place.
Stars are tightly bound
and do not break apart
like galactic clusters.
Clusters
Two main types - Open Clusters and
Open Clusters
(Galactic Clusters)
Few hundreds stars
Globular Clusters
Globular Cluster
Few hundred thousands stars
30 pc wide
25 pc wide - densely packed!
Hot, young stars
Old, cool stars
Metal rich
Metal poor
Found in galactic disk Found away from galactic disk
Contain Type I
Cepheids
Contain RR Lyrae, Type II
Cepheids
Stellar Populations
Globular Clusters are nearly pure H & He with
little or no metals. They are old stars called
Population II
(two old, poor)
Open Clusters (Galactic Clusters), have metals
and they are hot and young. They are formed from
recycled material.
Population I
( one, hot young)
Young clusters in our Galaxy are called open
clusters due to their loose appearance.
During the exchange of energy between the
stars, some stars reach escape velocity
from the proto-cluster and become runaway
stars. The rest become gravitationally
bound, meaning they will exist as collection
orbiting each other forever.
Clusters are useful “laboratories'' for testing
our theories of star formation.
Stars in a cluster
•have the same age
•are at roughly the same distance from us.
•had the same initial chemical composition,
Cluster HR Diagrams:
The most massive stars at the top of the
main sequence evolve into red giants
soon because they die faster.
Therefore, the older the cluster, the
fewer stars to be found at the top of the
the main sequence, and an obvious
grouping of red giants will be seen at the
top right of the HR diagram.
This effect, of an evolving HR diagram
with age, becomes a powerful test of our
stellar evolution models.
The first diagram is of a cluster which is only
1 million years old. Why, the hottest O
star has already
been converted to
a red supergiant.
The cool K & M
stars have not yet
settled down onto
the main sequence,
and have not yet
ignited hydrogen
fusion in their cores.
The next diagram is of
a cluster which is 100
million years old. The
main sequence
lifetime of a 6 solar
mass star is 100
million years, so stars
with M = 6 Msun
(spectral type A) are
just turning off the
main sequence.
K,M stars getting closer to the Main Sequence
The final diagram is of a cluster which is 10 billion
years old. The main sequence lifetime of a 1 solar
mass star is 10 billion years, so stars with M = 1
Msun ( spectral type G) are just turning off the
B yr
main sequence
main
We can use the turning off the Main
Sequence to determine the age of clusters.
The youngest globular clusters are ~ 10 by old.
The oldest globular clusters are ~ 16 by old.
Globular clusters all formed when the
universe was young.
Open clusters formed more recently, out of
``recycled'' material containing heavy
elements.
Doppler Motion(vr)
(Radial Motion)
Actual Motion
Line of Sight
(vt).
Proper Motion
(Tangential Motion)
v
Radial Velocity
The radial velocity of a star is how fast it is moving directly towards
or away from us. (Doppler Effect)
Earth
Radial velocities are measured using the Doppler Shift of
the star's spectrum:
•Star moving towards Earth: Blueshift
•Star moving away from Earth: Redshift
•Star moving across our line of sight: No Shift
In all cases, the Radial Velocity is Independent of Distance.
Each of these velocities forms the legs of a right triangle
with the true space velocity (v) as the hypotenuse.
We can then use the Pythagorean Theorem to derive the
True Space Velocity (v):
v  v  (4.74  d )
2
2
Thank goodness, my brain is full