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Flashcards in Stars & EA 3 Deck (28):

What 2 types of properties do stars have and how are they connected?

Stars have physical and observable properties. We must use the observable properties to find the physical properties.


List some physical properties.

Absolute magnitude


List some observable properties.

Spectral type


Describe 3 characteristics of electromagnetic radiation.

All EMR travels at the speed of light (c = 3x10^8 m/s)
It has wave-like properties; described by wavelength (lambda) or frequency (f), e.g interference.
It has particle-like properties; described in photons as 'packets' of energy, e.e photoelectric effect


Write an equation to describe wave-like properties of EMR.

c = f * (lambda)


Write an equation to describe particle-like properties of EMR.

E = hf or E =hc/lambda
h -> Planck's constant


Describe qualitatively the Doppler effect.

The wavelength that we measure is dependent on the relative motion between the source and the detector (us).
If the source doesn't move, two observers on either side see the same wavelength. But if the source is moving to the right, the observer on the right sees a blue shift as the source is moving towards them and the observer on the left sees a red shift as the source is moving away from them.


Describe mathematically blue and red shift.

The source emits wavelength (lambda0)
The observer measures wavelength (lambda)

If (lambda) < (lambda0) = blue-shifted
If (lambda) > (lambda0) = red-shifted


Describe quantitatively the Doppler effect.

(Delta)(lambda) / (lambda0) = v/c for v << c

c : speed of light
(Delta)(lambda) : differences in wavelength (lambda)-(lambda0)
v : radial velocity


Define radial velocity

The relative speed along the line of sight of source and observer.


Why do stars appear to move on the sky and what is actually happening?

Stars move due to:
-Companion stars
-Motion around galactic centres

In reality there is:
-Radial velocity which is what we observe
-Proper motion which is what movement actually occurs


What are binary star systems?

This is 2 stars in a mutual gravitational interaction orbiting their common centre of mass.
Higher multiples are also possible e.g. triples, quadruples


Why are binary star systems useful?

Their orbits are determined by gravity or the masses of the stars.
The eclipses are determined by orbits and the radii of the stars.


Describe visual binaries.

You can see 2 stars and you can see them move in space together.


Describe close binaries.

These stars are too close for you to see separately.


Describe eclipsing binaries.

These star regularly show eclipses of each other.


Describe spectroscopic binaries.

These are 2 stars with Doppler shifts in opposite directions that vary as they orbit their common centre of mass.


What is the equation for the orbital period?
How do you describe the 2 periods?

P = 2 (pi) r / v
P1 = P2


Define the centre of mass equation in terms of a binary star system.

m1 r1 = m2 r2


Describe the radii and radial velocities of a binary star system

r1 / v1 = r2 / v2


Combine the centre of mass equation and the equation describing the radii and radial velocities in terms of a binary star system. What does this equation mean?

v1 / v2 = r1 / r2 = m1 / m2

This means the more massive star is orbiting at a shorter distance from the centre of mass and with a lower velocity.


Write Newton's law of gravity and an equation to describe the force required to keep the star in orbit in terms of a binary star system.

F = G m1 m2 / (r1 + r2)^2
F = m1 v1 ^2 / r1


Using Newton's law of gravity, an equation to describe the force required to keep the star in orbit, v1 = 2(pi)r1/P and R = r1 + r2, write the full verison of Kepler's 3rd law and the simplified version (due to solar system units)
HINT use the tutorial!

4(pi)^2 * R3 /G = (m1 + m2)P^2

R^3 = (m1 + m2)P^2


What is Kepler's 3rd law used for?

To measure the sum of the masses.


What information is neglected when calculating Kepler's 3rd law?

Elliptical orbits, inclination of orbits against the line of sight or the sky.


What information does a spectroscopic binary star system have that can be used in Kepler's 3rd Law?

The radial velocities v1 and v2 can be measured, together with the period. This immediately gives r1 and r2 and thus R, as well as the ratio m1 and m2.


What depends on the properties of stars in eclipsing binary star systems?

Shape, depth and length of eclipse.


How can the sizes of stars in an eclipsing/spectroscopic binary star system be determined?

Create a lightcurve during the eclipse to measure 4 t-value (t1, t2, t3, t4). These can be input into 2 different equations to determine diameters of the 2 stars.

(t4 - t1)/ P = (D1 + D2) / 2(pi)R
(t3 - t2)/ P = (D1 - D2) / 2(pi)R