Classification of Stars Flashcards
What is Luminosity
The energy per second that a star produces or power output of a star
What is the Intensity or Flux of a Star
- The power received from a star (its luminosity) per unit area
- The intensity of a star follows the inverse square law, meaning it is inversely proportional to the square of the distance from the star
Is brightness subjective or objective
Brightness is a subjective scale of measurement, meaning it varies depending on the observer
What is Apparent Magnitude
The brightness of star as seen from Earth
What is Absolute Magnitude
How bright a star would appear if it were placed 10 parsecs from the Earth
Describe the Magnitude Equation
m - M = 5 log (d / 10)
Apparent Magnitude - Absolute Magnitude = 5 log (Distance in Parsecs / 10)
What is the Hipparcos Scale
The scale classifies astronomical objects by their apparent magnitudes, with the brightest stars given an apparent magnitude of 1, and the faintest visible stars being given an apparent magnitude of 6
What is the Ratio on the Hipparcos Scale
- As the magnitude changes by 1, the intensity changes with a ratio of 2.51
- The intensity of a magnitude 1 star is 100 greater than a magnitude 6 star
What is Parallax
The apparent change of position of a nearer star in comparison to distant stars in the background, as a result of the orbit of the Earth around the Sun
What is the Parallax Angle
The angle subtended to the star by the line between the Sun and the Earth
What is an Astronomical Unit
The average distance between the centre of the Earth and the centre of the Sun - 1.5 x 1011 m
What is a Parsec
The distance at which the angle of parallax is 1 arcsecond or the distance at which 1 astronomical unit subtends an angle of 1 arcsecond
1 parsec = 2.06 x 105 AU = 3.08 x 1016 m = 3.26 ly
What is a Light Year
The distance travlled by light in a vacuum in one year
Describe the Equation to work out distance in parsecs
- tan θ = r / d
- d = r / θ
- d = 1 / θ
- d is measured in parsecs and θ is measured in arcseconds
What is Stefan’s Law
The power output of a black body radiator is directly proportional to its surface area and its absolute temperature4
What is Stefan’s Law Equation
P = σAT4
Power Output/ Luminosity = Stefan Constant x Surface Area x Absolute Temperature4
What is the Stefan Constant
5.67 x 10-8 W m-2 K-4
What is a Black Body Radiator
- A perfect emitter and absorber of all possible wavelengths of radiation
- Stars can be approximated as black bodies
Describe the Intensity and Inverse Square Law
Intensity is inversely proportional to the distance between the star and the observer
I = P / 4π d2
What assumption is made for the Intensity Inverse Square Law
It is assumed that light is emitted equally in all directions from a point, so will spread out (in the shape of a sphere)
What is Wien’s Displacement Law
The peak wavelength of emitted radiation is inversely proportional to the absolute temperature of the object
What is the Peak Wavelength
The wavelength of light released at maximum intensity
What is the Wien’s Displacement Law Equation
λmaxT = 2.9 x 10-3
Peak Wavelength x Absolute Temperature = 2.9 x 10-3
Describe the general shape of black-body curves
Explain the general shape of black-body curves
Wein’s law shows that the peak wavelength of a black body decreases as it gets hotter meaning the frequency increases so the energy of the wave increases
Describe the Full Spectral Classification Table
What are the Intensity of Hydrogen Balmer Lines dependent on
- The absorption lines are dependent on the temperature of the star
- This is because the energy of the particles which make up the star is dependent on its temperature
Explain why atoms need to be in an n = 2 state in order to be produce strong Hydrogen Balmer lines
- Hydrogen Balmer lines are caused by the excitment of hydrogen atoms from the n = 2 state to higher/lower energy levels
- If the temperature of a star is too high, the majority of hydrogen atoms will become excited to higher levels than n = 2 or electrons might even become ionised, so hydrogen balmer lines will not be present
- If the temperature of a star is too low, the hydrogen atoms are unlikely to become excited, or may not be present at all, so hydrogen balmer lines will not be present
How are the Spectral Classes related to the Hydrogen Balmer Lines
Describe the Hertzsprung-Russell (HR) diagram
Where would the sun be on the Hertzprung-Russell diagram
The sun is a main sequence star, its spectral class is G and its absolute magnitude is about 5
Describe the evolutionary path of a Main-Sequence Star on a HR diagram
Describe the stages of Stellar Evolution for a Small Star
- Protostar - clouds of gas and dust get clump together under gravity, eventually the protostar gets dense and hot enough so it begins to fuse elements
- Main Sequence Star - the inward force of gravity and the outward force due to fusion are in equilibrium – the star is stable, hydrogen nuclei are fused into helium
- Red Giant - once the hydrogen runs out, the temperature of the core increases and begins fusing helium nuclei into heavier elements (e.g Carbon and Oxygen) The outer layers of the star expand and cool
- White Dwarf - when a red giant has used up all its fuel, fusion stops and the core contracts as gravity is now greater than the outward force. The outer layers are thrown off. The core becomes very dense
- Black Dwarf - a white dwarf will eventually cool to a black dwarf
Describe the stages of Stellar Evolution for a Large Star
- Protostar - clouds of gas and dust get clump together under gravity, eventually the protostar gets dense and hot enough so it begins to fuse elements
- Main Sequence Star - the inward force of gravity and the outward force due to fusion are in equilibrium – the star is stable, hydrogen nuclei are fused into helium
- Red Supergiant - when the core runs out of hydrogen, the temperature of the core increases fusing helium nuclei into heavier elements (e.g Carbon, Oxygen and Iron)
- Supernova - when all fuel runs out, fusion stops and the core collapses inwards very suddenly. This causes gamma ray bursts and the outer layers of the star fall inwards and rebound off of the core, launching them out into space in a shockwave
- Neutron Star - when the core of a large star collapses, gravity is so strong that it forces protons and electrons together to form neutrons, this is incredibly dense
- Black Hole - when the core of a giant star collapses, the neutrons are unable to withstand gravity forcing them together., the gravitational pull of a black hole is so strong that not even light can escape
What are Pulsars
Spinning neutron stars that emit beams of radiation from the magnetic poles as they spin
Describe the Composition and Density of Neutron Stars
- Gravity is so strong that it forces protons and electrons together to form neutrons
- A neutron star is incredibly dense – about 1017 kg m- 3 (the density of nuclear matter)
What is the Escape Velocity for a Black Hole
- The gravitational pull of a black hole is so strong that not even light can escape
- Escape velocity > speed of light
What is the Event Horizon of a Black Hole
The point at which the escape velocity becomes greater than the speed of light
What is the Schwarzchild Radius
The radius of the event horizon
Rs = 2GM / c2
Schwarzchild Radius = 2 x Gravitational Constant x Mass of Black Hole / Speed of Light2
Compare the energy output of a supernova with total energy output of the Sun
Supernovae may release around 1044 J of energy, which is the same amount of energy as the sun outputs in its 10 billion year lifetime
What is a Binary System
A system where two stars orbit a common centre of mass
What are the Two Types of Supernovas
- Type I - when a star accumulates matter from its companion star in a binary system and explodes after reaching a critical mass
- Type II - the death of a high-mass star after it runs out of fuel
What is a Standard Candle
It is an object of which has a known absolute magnitude
Explain how you would use a Type 1a Supernovae as a Standard Candles
- All types of supernovae occur at the same critical mass, meaning they all have a very similar peak absolute magnitude and produce very consistent light curves
- This allowing astronomers to use them as standard candles to calculate distances to far-off galaxies
What is the Critical Mass of a Supernova
When the white dwarf star reaches a critical mass, fusion begins and becomes unstoppable as the mass continues to increase, eventually causing the white dwarf to explode in a supernova
Describe the Light Curve of Typical Type 1a Supernova
What is the Centre of Every Galaxy
- Scientists believe that there are supermassive black holes at the centre of every galaxy
- This is because stars and gas near the centre of galaxies appear to be orbiting very quickly
- They concluded that there must be a supermassive object at the centre with a very strong gravitational field attracting them
What is Dark Energy
- Dark energy is thought to be the reason behind the universe accelerating
- It is described as having an overall repulsive effect throughout the whole universe. Since gravity follows the inverse square law, it decreases with distance
- Dark energy however remains constant all throughout the universe, meaning it has a greater effect than gravity and is therefore causing expansion speed to increase
What is Dark Energy Controversial
Dark energy is controversial because there is evidence for its existence but no one knows what it is or what is causing it
