LDS 1 - Stars & Stellar Structure Flashcards
The surface temperature of the sun is approximately
6,000 K
On the main sequence stars are powered by nuclear reactions termed ______ burning
hydrogen
The brightest star in the night sky is
Sirius
Which type of binary star can also be described as a photometric binary?
Eclipsing
What units are normally used to describe proper motion?
A. Arc seconds
B. Light years
C. Kilometres per hour
D. Seconds
A. Arc seconds
Which star has the greatest observed proper motion?
A. Proxima Centauri
B. Barnard’s Star
C. Betelgeuse
D. Epsilon Eridani
B. Barnard’s Star
What is the name given to the process by which elements are formed in stars?
Stellar nucleosynthesis
Which element is the most abundant in the universe?
Hydrogen
Which element is the second most abundant in our universe?
Helium
Which of the following correctly lists the spectral stellar classes in order of descending temperature?
A. OBAGFKM
B. OBAFGKM
C. OBAKMGF
D. OBAMFGK
B. OBAFGKM
blue –> red
large –> small size
high –> low temp
large –> small mass
The sun’s visible surface is called the
photosphere
Before it reaches us light from the sun has been travelling for about
8 minutes
A first magnitude star is _________ than a second magnitude star.
brighter
The absolute magnitude of a star is the magnitude it would have if it was at a distance of
A. 1 AU
B. 1 parsec
C. 10 parsecs
D. 100 parsecs
C. 10 parsecs
Which star has the greatest observed proper motion?
A. Proxima Centauri
B. Barnard’s Star
C. Betelgeuse
D. Epsilon Eridani
B. Barnard’s Star
The amount of solar energy reaching the surface of the Earth’s atmosphere is known as the solar
constant. What is this?
1400 Wm-2 (watt per square metre )
Which process allows for the building up of elements heavier than iron?
Neutron capture
Star A has magnitude 4.5. Star B has magnitude 2.5. How much brighter is star B than star A?
about 6.25 times brighter
A star has a parallax of 0.125”. How far away is it?
8 parsecs
Star A and star B are identical in all respects except star B is 10 times further away from us. How would
their apparent magnitudes mA and mB compare?
A. they would have the same apparent magnitude, mA = mB
B. A would have the smaller magnitude with mB - mA = 5
C. B would have the smaller magnitude with mA – mB = 5
D. A would have the smaller magnitude with mB - mA = 100
B. A would have the smaller magnitude with mB - mA = 5
The phrase “proper motion” means
a star’s motion across the sky due to its physical movement through space
A star whose absolute magnitude is 2.0 is at a distance of 10 parsecs. What would its apparent magnitude
be?
2.0
To the eye, a star of spectral type M would most likely appear
A. white
B. blue
C. red
D. green
C. red
A star is a Main Sequence star while energy is generated in its core by which of the following?
A. alpha capture reactions
B. fusion of helium to carbon
C. fusion of hydrogen to helium
D. gravitational contraction
C. fusion of hydrogen to helium
How do we know that a spectroscopic binary star is in fact a binary star?
from Doppler shifts of spectrum lines
A Main Sequence star of spectral type M would have
A. an outer convection zone
B. a convective core
C. an outer convection zone AND a convective core
D. none of the above
A. an outer convection zone
s-process nuclear reactions build up heavier nuclei by
absorption of neutrons until the nucleus becomes unstable
Write a couple of sentences in each case to explain how we know the following:
(i) the distance to another star (3)
(ii) the surface temperature of another star (3)
(iii) the luminosity of another star (3)
(i) Earth’s annual motion around the Sun means that stars appear to trace out a parallactic circle in the course of the year (1). Since we know the Astronomical Unit (1), if we measure the angular size of the parallactic circle we can deduce the distance to the star, as an exercise in triangulation (1), with conclusion: If the parallax P of the star is measured in arc seconds, the distance in parsecs is 1/P.
(possibly with a diagram)
(ii) Stars are approximately black bodies (1). The distribution of black body radiation over wavelength depends only on the temperature of a black body so some observed quantity reflecting this distribution yields an estimate of surface temperature (1). A measurement of the wavelength of peak intensity, for instance, yields an estimate of
temperature through Wien’s Displacement Law, lmaxT = const (1). Alternatively a Colour Index, the difference between the magnitudes of the star measured with two standard filters in place (e.g. B-V) may be converted e.g. via standard tables to give an estimate of temperature. (alternative 1)
(iii) The apparent magnitude of a star tells us the rate at which we receive radiation from it, in Watts per square metre (1). If we know its distance d, from a measurement of parallax (1), then we can scale up from the radiation passing through one square metre at Earth, to the total that has passed through all of a sphere of surface area 4πd2 (1).
Equivalently, we appeal to the inverse square law and the radiation measured from a body of known power at known distance, to deduce the luminosity of the star at a different, known distance.
Which two quantities are used as the axes of the Hertzsprung-Russell Diagram?
temperature on the x-axis, luminosity on the y-axis
Briefly describe the three stages of the proton-proton chain that underpins nuclear fusion in solar
mass stars. (3)
p + p ➝ d + ν + e^+ (1)
d + p ➝ 3^He + γ (1)
3^He + 3^He ➝ 4^He + p + p (1)
Above 1.3M⊙ the proton-proton chain is no longer the dominant process by which stars sustain
nuclear fusion. What is? (1)
The CNO (Carbon-Nitrogen-Oxygen) cycle
What is the common outcome of both of these two processes (e proton-proton chain/nuclear fusion) in terms of the element generated? (1)
Four protons are fused together to create one Helium nucleus (alpha particle)
Briefly describe how nuclear fusion gives rise to the release of energy, and by what equation is
this release of energy quantified? (2)
The mass of four protons is different to that of a Helium nucleus (nuclear binding energy) (1) and the
difference is calculated by E=mc2 (1)
At core temperatures greater than 100 MK, “Helium burning” can begin. By what two processes
are heavier elements formed in this regime? (2)
The triple alpha process (1) and Helium capture (1)
What consequence do these two processes have on the elemental abundances in the universe? (1)
There is a relatively higher abundance of even atomic numbered elements than odd ones due to the availability of Helium atoms (Z=2)
Briefly explain the concept of a blackbody. (2)
A blackbody is a hypothetical object that absorbs all the radiation that falls on it. But it must also reemit this energy otherwise it would heat up indefinitely, and therefore has a finite temperature.
i.e.
it is a perfect absorber (1)
and a perfect emitter (1)
What two changes happen to a blackbody curve as a star’s temperature increases? (2)
The blackbody curve increases in intensity (1) and its peak shifts to shorter wavelengths/higher
frequencies (1).
The colour index of a star is an indicator of its surface temperature. Describe how to measure a star’s colour index from its blackbody spectrum. (2)
Measure the solar flux/irradiance at two separate wavelengths (traditionally B – blue - and V - visible) (1)
and calculate the difference between the two (1).
How does the luminosity of a star scale with its blackbody temperature, and how can one use this relationship to infer the radius of a star? (2)
Luminosity scales with temperature to the fourth power (L∝T4). (1)
Specifically, L=AσT4, where σ is
Stefan’s constant, and A = 4πr2
Therefore r = √(L/4π σT^4). (1)
Briefly describe what would happen to the luminosity of a star if its temperature were to increase
slightly? (1)
A fairly slight increase in a star’s temperature can lead to as much as a factor of 2 increase in luminosity.
(1)
Why do the temperatures of some stars differ from their blackbody approximation? (1)
Some stars cannot be well approximated by a blackbody due to the presence of strong absorption lines in
their spectra. These indicate the presence of heavier elements (metals), typically found in older stars. (1)
What physical parameter will primarily determine how a star evolves?
Its mass
The temperature at the centre of a solar-like star is of the order of:
millions of degrees K
The solar chromosphere presents strong Hydrogen spectral lines while the solar corona does not. Why?
The temperature of the corona is too large to give rise to hydrogen spectral lines.
The Hertzsprung-Russell diagram is a plot showing
the luminosities of stars as a function of their surface temperatures.
A star’s luminosity is proportional to the product between
the square of its radius and the fourth power of its surface temperature.
Compared to any blue star, a red star is
cooler
Compared to a G2 star, an A3 star is
hotter
On the magnitude scale, the naked-eye limit corresponds to a magnitude of
6
The transverse velocity of a star can be measured if the star is
close to us and moving at a significant speed in the plane of the sky.
Among the following elements, which one is most abundant in the Universe?
16O
Which spectral class of stars shows strong levels of ionised helium in their spectra?
A. M
B. F
C. O
D. A
C. O
Which of the following lists the elements Carbon, Helium, Hydrogen and Oxygen in descending order of
abundance in our universe?
Hydrogen, Helium, Carbon, Oxygen
As the temperature of a blackbody falls, what happens to its peak emission frequency?
As temperature falls, the peak emission frequency falls.
Barnard’s star exhibits the largest observed proper motion for a star. Roughly how fast is this motion?
90 km s-1
What is the largest baseline that Earth-based telescopes can have for stellar parallax observations?
2 AU
The alpha process of element building in stars ends at which element?
A. Hydrogen
B. Iron
C. Technetium
D. Plutonium
B. Iron
What name is given to the chain of reactions by which carbon nuclei are formed from helium nuclei?
Triple-alpha process
What is the approximate temperature at the centre of our Sun?
15 million K
How does the apparent magnitude of a star vary with its luminosity?
The apparent magnitude of a star decreases when its luminosity increases.
A Hertzsprung-Russell diagram is a plot of stars’
A. spectral types as a function of their distances.
B. distances as a function of their mass.
C. mass as a function of their absolute magnitudes.
D. absolute magnitudes as a function of their spectral types.
D. absolute magnitudes as a function of their spectral types.
Primordial elements were created in
the Big Bang.
In the modern definition of the magnitude scale, a change of 5 in the magnitude of an object corresponds to a change in apparent brightness of exactly a factor of
100
The Sun is the brightest object in the sky and has an apparent magnitude of
-26.7
Spectroscopic parallax is a technique allowing astronomers to
measure distances to relatively far-away stars
The apparent brightness of a star is
proportional to its luminosity and inversely proportional to the square of its distance.
A star’s luminosity class is determined by
the width of its spectral lines
The lifetime of a main-sequence star that is twice as massive as the Sun is
10 times shorter than our Sun’s lifetime
A spectroscopic binary is a pair of stars that
are so close to each other that they cannot be seen separately with a
telescope
Stars in the Milky Way
orbit around the Galactic centre.
The annual movement of the stars across the sky as seen from Earth and corrected for parallax is known as
proper motion.
The region of the Sun where most of its visible radiation is released is called
the photosphere.
Stars produce energy during most of their lifetime by
fusing light nuclei
Neutron capture is a process in which
. free neutrons are captured by nuclei.
As the temperature of a blackbody increases, the peak of intensity occurs
at a higher frequency.
When comparing colours of main-sequence stars, those stars which appear red in colour are
cooler than those which appear blue.
The apparent brightness of a star is
proportional to its luminosity and inversely proportional to the square of its distance from
the observer.
In a spectroscopic binary system, some information about stellar masses can be obtained
by studying
a periodically variable Doppler shift in the observed spectrum.
The lifetime of a massive star
A. is shorter than that of a less massive star.
B. is longer than that of a less massive star.
C. is the same as that of a less massive star.
D. cannot be compared to that of a less massive star.
A. is shorter than that of a less massive star.
Define what the Hertzsprung-Russell diagram is. Explain how it is used in the spectroscopic
parallax technique, and name the law that is then applied to work out distances.
[4]
The HR diagram is a plot of the luminosity of a group of stars as a function of their
temperature (or some equivalent definition). [2 marks]
Suppose we observe a star and determine its apparent magnitude. If we have some
additional information – e.g. star lies on main sequence and has a given spectral type – then
the H-R diagram can tell us its luminosity. [1 mark]
Once we know the apparent magnitude and the luminosity, the distance can be worked out by
applying the inverse-square law. [1 mark]
