Astrophysics (Stars) Flashcards
Describe the luminosity of a star.
The total amount of energy a star radiates per second (W).
Describe the intensity of a star.
The energy received by an observer from a star (Wm-2).
I = L / 4πr2
Define apparent magnitude.
The brightness of an object as it appears in the sky from Earth.
Define absolute magnitude.
The brightness of an object from a distance of 10 parsecs away.
m - M = 5 log10(d / 10)
What type of scale is used for brightness magnitudes?
Hipparchus scale, a reverse scaling system.
More positive values means a fainter star.
More negative values means a brighter star.
If a star is one magnitude brighter than another star, how many times brighter is it?
A difference in magnitude of one corresponds to a star that is 2.51 times brighter.
State two reasons why a star may appear brighter in the sky.
The star outputs more power at visible wavelengths.
The star is closer to the Earth.
Which equation relates the intensity and apparent magnitude of stars?
(I2 and m2 are typically the values for the brighter star)
I2 / I1 = 2.5m1 - m2
State the three most used units for astronomical distances.
Astronomical units (AU)
Light years (ly)
Parsecs (pc)
Define a light year and describe how it is calculated.
The distance travelled by light in a vacuum in one year.
Multiply the number of seconds in a year by the speed of light in vacuum
Define a parsec.
- The distance to a star when 1 AU subtends an angle of 1 arc second.
Describe the behaviour of a black body.
An object that absorbs all electromagnetic radiation of all wavelengths.
Can emit electromagnetic radiation of all wavelengths.
Stars can be approximated to behave as black bodies.
State the law that links a stars surface temperature and peak wavelength emission.
Wien’s Displacement Law: The shorter the peak wavelength, the higher the surface temperature.
λmaxT = 2.9 x 10-3 mK
State the law that links a stars luminosity with its surface temperature and surface area.
Stefan’s Law: Luminosity of a star is directly proportional to surface area, and proportional to the temperature raised to the power of four.
L = σAT4
Explain how line absorption spectra of stars are produced.
Cooler gas in a stars atmosphere will absorb photons of light of certain energies depending on the molecules present.
These photons will have specific wavelengths related to the energy of the photons.
When photons of these wavelengths are absorbed, they are absent from the continuous spectrum received from the star (and appear in the spectrum as dark lines).
Explain the importance of hydrogen Balmer lines.
Dark lines from wavelengths corresponding to electrons moving between hydrogen’s first excitation level (n = 2) and higher energy levels.
The intensity of these Balmer lines depends on the temperature of the star.
List the seven stellar classes.
O, B, A, F, G, K and M.
Spectral classification depends on the spectra of stars (strength of certain absorption lines), not distance or brightness magnitude.
Each class has a specific colour, temperature range and absorptions.
Describe an O class star.
Blue
Temperatures: 25,000 K – 50,000K
Strong helium (He) and helium-plus (He+) lines due to high temperatures.
Weak hydrogen (H) Balmer lines.
Describe a B class star.
Blue
Temperatures: 11,000 K – 25,000K
Strong helium (He) and hydrogen (H) lines.
Describe an A class star.
Blue-white
Temperatures: 7,500 K – 11,000K
Strongest hydrogen (H) Balmer lines, but some metal ion absorptions too.
Describe an F class star.
White
Temperatures: 6,000 K – 7,500K
Strong metal ion absorptions.
Describe a G class star.
Yellow-white
Temperatures: 5,000 K – 6,000K
Both metal ion and metal atom absorptions.
Describe a K class star.
Orange
Temperatures: 3,500 K – 5,000K
Spectral lines mostly from neutral metal atoms.
Describe an M class star.
Red
Temperatures: < 3500 K
Lines from neutral atoms, and molecular absorptions from compounds such as titanium oxide (TiO) since these stars are cool enough for molecules to form.
State the characteristics of stars that are plotted on an H-R diagram and the values of the axes.
A plot of absolute magnitude against absolute temperature (or spectral class.
Absolute magnitudes from 15 to -10 - Temperatures from 50 000 K to 2 500 K (or OBAFGKM)
State the name and location of the three main groups of stars plotted on an H-R diagram.
Main sequence stars: long diagonal band from top-left to bottom right.
Red giants: Top-right
White dwarfs: Bottom-left
Describe the evolutionary path of a Sun-like star on an H-R diagram.
Starts off towards the centre of the long diagonal main sequence band.
Hydrogen fusion stops, the star becomes a red giant and moves to the top-right of the diagram.
Helium fusion stops, the star ejects it’s outer layers and becomes a white dwarf, moving left along the top of the H-R diagram and then down to the bottom-left of the diagram.
Describe how a star is initially formed and how it becomes a protostar.
Stars are born in a cloud of dust and gas leftover from supernovae.
These clouds contain denser clumps which slowly contract due to the force of gravity.
When these clumps are dense enough, they fragment and heat up to form protostars.
Describe how the protostar forms a stable main-sequence star.
Protostars eventually contract and heat up enough for hydrogen fusion to take place in the core.
An outwards force is generated from nuclear fusion (radiation pressure) which balances the inwards force of gravity.
The star reaches a stable equilibrium and remains like for as long as hydrogen is being fused (typically billions of years).
What is core hydrogen burning?
The stage of a star where the inwards force of gravity is balanced by the outwards radiation pressure produced by the nuclear fusion of hydrogen in the core of a star.
What is shell hydrogen burning?
When all the hydrogen in the core has fused into helium, the radiation pressure is lost and the helium core begins to contract.
As the core contracts, it heats up and becomes hot enough to cause fusion of the hydrogen surrounding the core.
What is core helium burning?
As the helium core continues to contract and heat up, it becomes hot enough to cause helium to fuse into carbon and oxygen
What is shell helium burning?
When the helium in the core runs out, the carbon-oxygen core begins to contract and heat up as well.
As the core heats up, it becomes hot enough to cause fusion of the helium layer surrounding the core.
Describe a white dwarf and how they are formed.
In low mass stars, the carbon-oxygen core doesn’t get hot enough to fuse further.
Once the core has shrunk to approximately the size of the Sun, electron degeneracy pressure prevents the core from collapsing.
The star ejects it’s outer layers, leaving behind a very hot and dense core (white dwarf).
Describe how a red supergiant is formed.
For high-mass stars, the force of gravity is great enough for fusion beyond the carbon-oxygen core.
Layers of elements are built up around the core (like an onion) forming a red supergiant.
For the most massive stars this continues all the way up to iron (beyond which fusion is energetically unfavourable).
Describe a supernova
For stars with core greater than 1.4 times the mass of the Sun, electron degeneracy pressure is insufficient to stop the core from contracting.
The core contracts and the outer layers of the star fall in and rebound off the core, causing huge shockwaves causing the star to explode.
State the defining features of a Type 1a supernova.
A Type 1a supernova can be plotted on a light curve – a plot of absolute magnitude over time.
These supernova always have a sharp initial park at -19.4, followed by a gradually decreasing curve (they are used as standard candles).
Describe a neutron star and how they form.
If a star’s core is between 1.4 and 3 solar masses, the electrons in the core material become compressed and form neutrons and neutrinos.
After supernova occurs, a dense star made mostly of neutrons is left behind.
State the key features of a neutron star.
Incredibly dense
Very small (approximately 20 km diameter)
Can rotate very quickly (some also emit two beams of radio waves, called pulsars).
Describe a black hole and how they form.
If a star’s core is greater than 3 solar masses, the force due to gravity is too great for neutrons to withstand.
The core collapses to an infinitely dense point – this is a black hole.
State the key feature of a black hole.
A gravitational field that is so strong that the escape velocity is greater than the speed of light – nothing can escape, not even light.
Describe what is meant by an event horizon and how this can be calculated.
The boundary of the region around the black hole at which the escape velocity is equal to the speed of light.
The radius of the event horizon is called the Schwarzschild radius and is calculated using:
Rs = 2GM / c2
