Stars Flashcards
(23 cards)
What is the main driver of a star’s evolution
Its mass
Explain the importance of knowing a stars mass
We can estimate how long it will shine and what its ultimate fate will be
Explain binary stars
- Make up about half of all stars
- Two stars that orbit each other, bound together by gravity
- Masses of binary stars can be calculated from measurements of their orbits
- Physical (or angular) separation between two stars in a binary system is so small, we never see them as two separate stars when viewing with the naked eye
What are double stars
- Any pair of stars that appear to be close to each other in the sky
- But not all of these form a true binary, that is, not all of them are physically associated
Explain visual binary
A binary star system in which both of the stars can be seen with a telescope
Explain spectroscopic binaries
- A second class of binary stars
- Two stars making up the binary can’t be seen separately using a telescope (i.e., they’re not visual binaries)
- However, evidence of the binary nature of the system is seen in its spectrum: double spectral lines that shift back and forth with time
Explain centre of gravity
In a binary system, both stars actually orbit a point between them
What is a radial velocity curve
A plot showing how the velocities of the stars change with time
Explain Newton’s reformulation of Kepler’s
third law
If two objects are in mutual revolution, then the period (P) with which they go around each
other is related to the semi-major axis (D) of the orbit of one with respect to the other:
D^3 = (M1 + M2) P^2
( D is in astronomical units, P is in years, and the sum of the masses is in units of solar
masses )
How can we determine the total
mass of the system
If we can measure the size of the orbit and orbital period
What can the radial velocity curve be used for
To determine the sizes and periods of the orbit
- The x-axis separation of two crests gives the orbital period (P) of the system.
- Given that the y-axis has units of distance/time and the x-axis units of time, the
separation (D) of the two stars can be calculated. - Kepler’s law then yields the sum of the two masses (M1+M2).
Explain mass-luminosity relation relationship
- More massive stars are generally also the
more luminous
L directly prop to M^3.9
- Luminosity (expressed in units of the
Sun’s luminosity) varies as the fourth power
of the mass (in units of the Sun’s mass)
Explain Dwarf Stars
- 7% of the true stars (spectral types O–M) in our local neighbourhood are white dwarfs
- White dwarfs are extremely hot, yet not very luminous (small surface area)
- Small radius (~ 1.5% of Sun) but relatively high mass (~ 0.5 Msun) —> very high density
- A ton of white dwarf material would fit into a match box
- White dwarfs are dying stars, reaching the end of their productive lives
Explain Giant stars
- Few extreme stars that are a million times more luminous than the Sun, with masses that exceed 100 times the Sun’s mass
- Are superluminous stars, which are at the upper left of the H–R diagram, are exceedingly hot, very blue stars of spectral type O
- Cqool supergiants in the upper corner of the H–R diagram are as much as 10,000 times as
luminous as the Sun- In addition, these stars have diameters very much larger than that of the Sun
Explain HR Diagrams
- As a star consumes its nuclear fuel, its source of energy changes, as do its chemical composition and interior structure
- These changes cause the star to alter its luminosity and surface temperature so that it no longer lies on the main sequence on our diagram
- Because stars spend much less time in these later stages of their lives, we see fewer stars in
those regions of the H–R diagram.
How can we measure star sizes using indirect methods
1- Stars occulted by the Moon
2- Eclipsing binary system
3 - Using the Stefan-Boltzmann law
Explain Stars occulted by the Moon
- Observe the dimming of light that occurs when the Moon passes in front of a star.
- Knowing the speed at which the Moon moves across the sky, and by measuring very
carefully the amount of time it takes for all of the light from the star to be blocked by the
Moon, and knowing the distance to the star … allows for an accurate determination of its
size. - This method works only for bright stars near the orbital path of the Moon (i.e., within the
zodiac)
Explain eclipsing binary system
- Can be used to accurately determine sizes of stars
- Is when a binary star system is oriented in such a way that allows for each star to pass in front of the other during every revolution
- When one star blocks the light of the other, preventing it from reaching Earth, the luminosity of the system decreases, and astronomers say that an eclipse has occurred
Explain Using the Stefan-Boltzmann law
Gives relationship between amount of energy emitted per unit time at the surface of the star (F) and the star’s surface temperature (T):
F= To^4
- If a star has radius R, then its surface area is A = 4PiR^2
- uminosity of the star (units: J/s) is then given by L = F x 4PiR^2 = oT^4 x 4PiR^2
Explain diameters of sclipsing binaries
- Assume we have an eclipsing binary
system consisting of a small, hot star (very
luminous) and a larger, cooler star (less
luminous) - When the smaller star just starts to pass
behind the larger star (a point we call first
contact), the brightness begins to drop - The eclipse becomes total (the smaller star
is completely hidden) at the point called
second contact - At the end of the total eclipse (third
contact), the smaller star begins to emerge - When the smaller star has reached last
contact, the eclipse is completely over
Explain the distances during eclipsing binaries
- During the time interval from the first to third contacts, the smaller star has moved a distance equal to the diameter of the larger star.
Explain doppler shift and eclipsing boundaries
- If the spectral lines of both stars are visible in the spectrum of the binary, then the speed of the smaller star with respect to the larger one can be measured from the Doppler shift.
Explain how to determine the diameter of ecplising binaries
Knowing the speed with which the smaller star is moving and how long it took to cover some distance can tell us the span of that distance—in this case, the diameters of the stars.
- The speed multiplied by the time interval from the first to second contact gives the diameter of the smaller star.
- We multiply the speed by the time between the first and third contacts to get the diameter of the larger star.
