Download Binary Stars/Star Clusters

Binary Stars
Two stars gravitationally bound after formation (~ 55% stars in MW)
Each star orbits the center of mass (COM) (“balance point”)
Stars of equal mass: COM equidistant from each star
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Binary Stars
Stars of unequal mass: COM closer to more massive star
Massive star ⇒ small orbit; Low mass star ⇒ larger orbit
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Only direct method of measuring star masses!!!
Methods of Observation
Binary Stars are classified according to how they are observed
• two stars close in sky may not be close in space
Mizar
Alcor
Big Dipper
Optical Double: two stars that appear close from our perspective
but are not gravitationally bound
Need to look for binary signatures.
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Methods of Observation
Visual Binary System
• Stars directly observed to orbit each other over an
extended time
Simulated orbit of Sirius A & B
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Visual Binaries
Center on one star & watch the other move around it
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Orbital properties will provide masses
Methods of Observation
If a binary system is too far away, then we’ll see the system as one
source of light.
Need to interpret light in order to determine if a binary star
is present.
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Doppler Effect
A perceived change in wavelength (frequency) of a wave that is
produced by a source moving toward or away from an observer
λ
Doppler Equation:
λ
• As source moves toward an observer, the waves are compressed
• As source moves away from observer, the waves are stretched
The faster the source, the more compression/stretching
Sound: frequency (pitch) of a siren gets higher as ambulance
moves toward an observer (vice versa)
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Doppler Effect
Light – wavelength of light shrinks as star moves toward Earth
• speed of a star is << c, λ changes very little (same color)
• use a star’s spectrum as a guide
Hα
Lab Spectrum
Star moving away
from Earth
Spectrum is redshifted
Star moving toward
from Earth
Spectrum is blueshifted
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Methods of Observation
Spectroscopic Binary
• Single point source produces two different spectra
• Each spectrum will exhibit a periodic redshift and blueshift
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Methods of Observation
Eclipsing Binary
As each star passes in front of the other, the total brightness changes.
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Methods of Observation
The properties of a binary system can be determined by its light curve
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Effects on Stellar Evolution
As one star matures more quickly and becomes a (Super) Giant…
… matter can transfer to its companion to form an accretion disk
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Novae
• Matter accretes onto white dwarf
• H fusion will ensue
• Explosion of surface material
Nova Cygni 1992
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Supernovae (Type I)
If material accretes too quickly then fusion reactions can blow up
the white dwarf entirely
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Light curve indicates type of Supernova
Black Holes
Accretion disk heats around a black hole to produce X-rays
Cygnus X-1
Steps for finding a black hole:
(1) find a point source of X-rays
(2) look for a star orbiting an unseen companion
(3) determine mass of the unseen companion
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If M > 3 MSuns, then the object is a black hole
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Star Clusters
Large groups of gravitationally bound stars
Open
Globular
Diameter
75 L.Y.
30 – 100 L.Y.
# of stars
50 – 1000
104 - 106
Location
Disk
Halo
Young, metal-rich
Old, metal-poor
Types of stars
Shape
Example
Asymmetric
Spherical
Pleiades
Omega Cen
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Pleiades
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Omega Centauri
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Variable stars used to obtain distance
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Star Clusters
Assumption: all stars of a given cluster formed from same Nebula
∴ All stars of a single cluster are (1) the same distance from Earth &
(2) the same age
Create an HR Diagram using all stars
of a single cluster
HR Diagram will change in appearance
over time because the stars evolve
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Determining Ages
Mass of a star determines its spectral type, formation time, &
Main Sequence lifetime
Spectral Type Formation Time
(years)
O&B
100,000
A
1 million
F
7.5 million
G
50 million
K&M
300 million
MS Lifespan
(years)
5 million
125 million
1 billion
10 billion
50 billion
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Determining Ages
All stars of a cluster begin to contract at the same time
After 3 Million years:
- All high mass stars (O, B, A)
are in MS stage
- Lower mass stars not formed
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Determining Ages
After 30 Million years
- Moderate mass stars (F & G) have
entered MS stage of evolution
- Lowest mass stars not formed
- Highest mass stars have evolved to
later stages
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Determining Ages
Star found on the Main Sequence Turnoff will give the cluster’s age
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Determining Distances
Spectroscopic Parallax
- Determine the Luminosity
of MS Turnoff
- Measure the Brightness of
MS Turnoff
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