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Why does the sun shine?
The source of the suns luminosity is the fusion of Hydrogen to Helium in it's core
What is luminosity?
It refers to the objects total energy output per second
How do you calculate luminosity?
The Stefan-Boltzmann law and the temperature and radius of the star
What is the Stefan-Boltzmann Law?
a statement that the total radiant heat energy emitted from a surface is proportional to the fourth power of its absolute temperature. F = σT4
What does G stand for in Astronomy?
Gravitational Constant
How do you calculate pressure of a gas?
Pressure = Constant × Temperature × Density
The energy produced by the fusion of H to He is given by Einstein's famous equation: E = mc2. What is m in this case?
The mass difference between 4 H atoms and 1 He atom.
Fusion only occurs in the core of the sun. Why is that?
The temperature and density are only high enough in the core
What are the Suns three interior layers?
Core, Radiative zone, Convective zone
What are the Suns two main exterior layers?
Photosphere, Stellar atmosphere
What is the corona of the sun?
the sun's outer atmosphere
What is the Chromosphere of the Sun?
The suns lower atmosphere
If the sun stopped generating energy in its core right now, when would we observe a change in the sun's brightness?
In about 100,000 years
What are sunspots?
areas of gas on the sun's surface that are cooler than the gases around them
What are prominences?
Reddish loops of gas that link different parts of sunspot regions
What are solar flares?
violent releases of energy and particles from small regions in the solar atmosphere
Sunspots appear dark because they are?
a bit cooler and thus dimmer than the rest of the photosphere
Which of the following does not change with the 11 year solar cycle?
the rate of fusion in the Sun's core
The Sun's surface, the "photosphere" is defined as the region where ...
radiation begins to travel freely outwards
Gravitational equilibrium ...
is what keeps the Sun stable and its rate of fusion nearly constant
The chemical composition of the Sun three billion years ago was different from what it is now in that it had
more hydrogen
Fusion in the Sun occurs ...
only in the core where the temperature and density are very high.
What is the ecliptic?
the path the Sun appears to trace around the celestial sphere each year
What does keplers third law help explain?
the retrograde motion of the outer planets
What are the three basic types of energy?
kinetic, potential, radiative
What is thermal energy?
kinetic energy associated with the random movement of atoms or molecules
Why do we want to classify stars?
In general, classify things (sorting them into related groups) in science helps us make sense of wide and complex subjects.
What is parralax
the apparent change in position of an object when you look at it from different places
What can we use a parallax to measure?
We can use parallax to measure distance
𝑑 = 1/𝑝
p = parallax angle in arcseconds
d = distance in parsecs
A star at a distance of 1 parsec has a parallax of 1 arcsecond
What are parsecs?
Parsecs are astronomers' preferred unit to measure distance
1 parsec (pc) is equivalent to:
- 206,265 AU
- 3.26 Light Years
- 3.086x1013 km
Inverse Square Law for light
If the distance is doubled, the brightness decreases by a factor of 4
B = L /4πd2
B = brightness
L = luminosity
d = distance
How does temperature affect spectral lines?
The higher the temperature of the gas, the wider the distribution of velocities in the gas. Since the spectral line is a combination of all of the emitted radiation, the higher the temperature of the gas, the broader the spectral line emitted from that gas.
What is an H-R diagram?
Scatter graph of stars measuring the star's absolute magnitude (brightness) against its temperature (color)
How to we measure the mass of stars?
we need to apply Newton's version of Kepler's third law
What does observing eclipsing binary tell us about stars?
It lets us measure the radii of stars 𝛿 =𝜋𝑅2B/𝜋𝑅𝐴
Where do stars form?
molecular clouds
What is a cloud collapse?
As the cloud size decreases, the cloud trades gravitational potential energy for kinetic energy. (Energy is conserved). Collisions excite molecules, and the molecules radiate away the energy. This lowers the gas temperature allowing the force of gravity to dominate over gas pressure causing further collapse
How is a protostar formed?
Stars start off as clouds of dust and gas, force due to gravity spirals them together to form a protostar. The temperature in the centre begins to rise at it becomes denser and more particles colllide eventually leading to nuclear fusion to form a helium nuclei and give out huge amounts of energy (star is born)
What are brown dwarfs?
Stellar figures, objects that formed though the contraction and fragmentation of an interstellar cloud, just as stars do, but fell short of the critical mass of about 0.08 solar mass needed to start hydrogen fusion in their cores.
What is the core accretion
Planets mostly form from the "bottom up" as small pieces in the protostellar disk stick together and form larger objects
What is a protostellar disk?
is a rotating circumstellar disc of dense gas and dust surrounding a young newly formed star
What is an exoplanet?
A planet outside of our Solar System.
What is the radial velocity method?
It measures the doppler shift of the star as it orbits around the star-planet center-of-mass
What is an exoplanet transit?
happens when an exoplanet's orbit causes it to pass in front of its star from our perspective on Earth
𝐹𝑖𝑛 =𝐹𝑜𝑢𝑡 What is 𝐹𝑖𝑛?
The brightness of a star
How do you calculate the temperature of exoplanets?
by figuring out the amount of energy the planet is getting from its star. The planet must be hot enough to emit this same amount of energy back into space
The length of time a star spends on the main sequence depends on?
how long it can continue to fuse H into He in its core
Main Sequence Lifetime = Amount of fuel/Rate at which fuel is used
= Mass/Luminosity
What kind of stars have shorter lifetimes?
More massive stars have shorter lifetimes
More mass means there's more H available to undergo fusion
But more massive stars are much more luminous and burn H at a higher rate.
When core H-burning ends ..
The star becomes a Red Giant
The star moves up and to the right on the H-R diagram (more luminous, cooler)
What prevents the helium core of a Red Giant from collapsing?
by electron degeneracy pressure
Electrons are packed as tightly as allowed by quantum mechanics.
The gas that makes up a planetary nebula is hot and diffuse, what kind of spectrum does it produce?
the light produced by a planetary nebula should be an emission spectrum, with spikes of emission at specific wavelengths corresponding to the elements in the gas.
In a red giant phase, stars are so large and luminous that?
they start to drive off their outer layers
The outer layers of the star can form a planetary nebula
What is a white dwarf star?
After ejection of the outer layers, all that's left is a tiny, hot, dense electron degenerate carbon core
Why does a low mass star end its life as a slowly cooling white dwarf —a ball of ultra-dense carbon? Why doesn't carbon fuse to something else?
The stars that white dwarfs form from aren't massive enough to produce the high core temperatures needed for Carbon fusion
Why does the CNO cycle require higher temperatures than the P-P chain?
The steps of the CNO cycle involve protons fusing with carbon and heavier nuclei. Because a carbon nucleus has a charge of 6, it strongly repels an approaching proton. Therefore, the CNO cycle requires a much higher temperature than the proton-proton chain, to overcome the stronger Coulomb barrier.
When a star becomes a red giant ...
the core temperature increases and the surface temperature decreases
The heaviest element that can be synthesized in a star's core is
Iron
Why do stars go supernova after they build up and iron core?
The fusion of iron into heavier elements does not release energy
a white dwarf does not collapse because
electrons pressing up against each other oppose gravity
You observe two stars in a binary and find that one is a white dwarf and the other is a red giant. Which star was originally the more massive?
The white dwarf
What is a neutron star?
A ball of neutrons left over from a massive star supernova and supported by neutron degeneracy pressure
What is a supernova?
a star that suddenly increases greatly in brightness because of a catastrophic explosion that ejects most of its mass.
Which of the following statements about pulsars is correct?
Only a few neutron stars are pulsars
What are pulsars?
rotating neutron stars
What are black holes?
Stellar black holes are made when the center of a very big star falls in upon itself, or collapses. When this happens, it causes a supernova. A supernova is an exploding star that blasts part of the star into space. Scientists think supermassive black holes were made at the same time as the galaxy they are in.
What is an event horizon?
In the case of a black hole, there's a point where your orbital speed would have to be equal to the speed of light to avoid falling in. Rs = 2 GM/c2
What would happen if the sun suddenly became a 1 Msun black hole?
Life on Earth would perish, but Earth's orbit would be unchanged
What would happen if you fell into a black hole?
Your infall would appear slow to an outside observer due to the warping of spacetime - in fact you would never appear to cross the event horizon
But for you, time would seem run at its normal pace
Any signal you sent would be sifted to redder and redder wavelengths
If light can't escape, how do we know black holes exist?
Look for its effect on its environment!
Black holes in binary systems can accrete matter from their companion. The gas forms a hot disk around the black hole that emits X-rays
We can monitor the orbit of the companion star to determine the mass of the black hole
Merging black holes create
gravitational waves — ripples in the fabric of spacetime that can be detected on Earth using a giant system of lasers and mirrors
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