14: Astrophysics - Classification of Stars Flashcards
What is luminosity?
Rate of light energy released/power output of a star
What is the intensity of a star?
The power received from a star (its luminosity) per unit area
Unit: Wm^-2
How is the distance from a star connected to the intensity of a star?
Intensity of a star is inversely proportional to the square of the distance from the star
What is apparent magnitude?
(m)
How bright an object appears in the sky
How is apparent magnitude measured?
Hipparcos scale
- Brightest stars have apparent magnitude 1
- Faintest visible stars have apparent magnitude 6
- Intensity of magnitude 1 star is 100 greater than a magnitude 6 star
How does the change in apparent magnitude affect the intensity?
Hipparcos scale is logarithmic
- As the magnitude changes by 1, the intensity changes with a ratio of 2.51
- A magnitude 5 star is 2.51 times brighter than a magnitude 6 star
What is the absolute magnitude of an object?
(M)
What the apparent magnitude would be if it were placed 10 parsecs away from the Earth
What is the relationship between apparent magnitude and absolute magnitude?
m - M = 5log(d/10)
where d is the distance is parsecs
What is parallax?
The apparent change of position of a nearer star in comparison to distant stars as a result of the orbit of the Earth around the Sun
Measured by angle of parallax (θ)
- Greater θ, closer to Earth
What is an astronomical unit?
The average distance between the centre of the Earth and the centre of the sun
1 AU = 1.5 x10 ^11 m
What is a parsec?
The distance at which the angle of parallax is 1 arcsecond
or
The distance at which 1 AU subtends an angle of 1 arcsecond
1 pc = 2.06 x10^5 AU = 3.08 x10^16 m = 3.26ly
What is a light year?
The distance that an EM wave travels in a year in a vacuum
1 ly = 9.46x10^15m
What is the equation to work out the distance between the Earth and the Sun?
d = r/θ
where r is the radius of the Earth’s orbit around the sun
and θ is small and in radians
What is a black body radiator?
A perfect emitter and absorber of all possible wavelengths of radiation
- Stars can be approximated as black bodies
What is Stefan’s law?
The power output (luminosity) of a black body radiator is directly proportional to its surface area and its absolute temperature^4
What is the equation to show Stefan’s law?
P = σAT^4
where T is absolute temperature and σ is the Stefan constant 5.67x10^-8
What can Stefan’s law be used for?
Comparing the power output, temperature and size of stars
What is Wien’s displacement law?
The peak wavelength (λmax) of emitted radiation is inversely proportional to the absolute temperature (T) of the object
What is the peak wavelength (λmax)?
The wavelength of light released at maximum intensity
What is λmaxT?
λmaxT = constant = 2.9x10^-3
What happens to the λmax of a black body when it gets hotter?
- Decreases
- meaning the frequency increases so the energy of the wave increases
How do you work out the intensity of a star?
I = P/4πd^2
where P is the power output of the star and d is the distance from the star
What are hydrogen Balmer lines?
Absorption lines that are found in the spectra of O, B and A type stars
How are hydrogen Balmer lines formed?
- the excitation of hydrogen atoms from the n=2 state to higher/lower energy levels
Why would hydrogen Balmer lines not be formed when the temperature of a star is too high?
The majority of the hydrogen atoms will become excited to higher levels than n=2, or electrons might even become ionised
Why would hydrogen Balmer lines not be formed when the temperature of a star is too low?
The hydrogen atoms are unlikely to become excited, or may not be present at all
The intensity of hydrogen Balmer lines is dependent on…
Temperature
What is the order of the spectral classes from highest temperature to lowest temperature?
O,B,A,F,G,K,M
What are the properties of O class stars?
Colour: Blue
Temperature range: 25000-50000
Prominent absorption lines: He+, He, H
Prominence of hydrogen Balmer lines: Weak
What are the properties of B class stars?
Colour: Blue
Temperature range: 11000-25000
Prominent absorption lines: He, H
Prominence of hydrogen Balmer lines: Slightly stronger than O
What are the properties of A class stars?
Colour: Blue/white
Temperature range: 7500-11000
Prominent absorption lines: H, ionised metals
Prominence of hydrogen Balmer lines: Strongest
What are the properties of F class stars?
Colour: White
Temperature range: 6000-7500
Prominent absorption lines: Ionised metals
Prominence of hydrogen Balmer lines: Weak
What are the properties of G class stars?
Colour: Yellow/white
Temperature range: 5000-6000
Prominent absorption lines: Ionised and neutral metals
Prominence of hydrogen Balmer lines: None
What are the properties of K class stars?
Colour: Orange
Temperature range: 3500-5000
Prominent absorption lines: Neutral metals
Prominence of hydrogen Balmer lines: None
What are the properties of M class stars?
Colour: Red
Temperature range: <3500
Prominent absorption lines: Neutral atoms, Titanium Oxide
Prominence of hydrogen Balmer lines: None
What is the spectral class of the sun?
G
What is the absolute magnitude of the sun?
4.83
What is the evolutionary path of a main sequence star?
- once the main sequence star uses up all the hydrogen in its core, it will move up and to the right on the HR diagram as it becomes a red giant, which is brighter and cooler than a main sequence star
- Once the red giant uses up all the helium in its core, it will eject its outer layers and will move down and to the left on the HR diagram as it becomes a white dwarf, which is hotter and dimmer than a main sequence star
What are the stages of stellar evolution?
- Protostar
- Main sequence
- Red giant (for a star < 3 solar masses)
- White dwarf (for a star < 1.4 solar masses)
- Red supergiant (for a star > 3 solar masses)
- Supernova (for a star > 1.4 solar masses)
- Neutron star (for a star between 1.4 and 3 solar masses)
- Black Hole (for a star > 3 solar masses)
How is a protostar formed?
- Clouds of gas and dust (nebulae) have fragments of varying masses that clump together under gravity
- Irregular clumps rotate and conservation of angular momentum spins them inwards to form a denser centre - a protostar
Main sequence
- 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
- The greater the mass of the star, the shorter its main sequence period because it uses its fuel more quickly
Red Giant
- for a star < 3 solar masses
- Once the hydrogen runs out, the temperature of the core increases and begins fusing helium nuclei into heavier elements
- The outer layers of the star expand and cool
White dwarf
- for a star < 1.4 solar masses
- when a red giant has used up all of its fuel, fusion stops and the core contracts as gravity is now greater than the outward force
- the outer layers are thrown off, forming a planetary nebula around the remaining core
- The core becomes very dense
- eventually cools to become a black dwarf
Red Supergiant
- for a star > 3 solar masses
- when a high mass star runs out of hydrogen nuclei, the same process for a red giant occurs, but on a larger scale
- the collapse of red supergiants in a supernova causes gamma ray bursts
- red supergiants can fuse elements up to iron
Supernova
- for a star > 1.4 solar masses
- when all fuel runs out, fusion stops and the core collapses inwards very suddenly and becomes rigid
- the outer layers of the star fall inwards and rebound off the core, launching them out into space in a shockwave
- as the shockwave passes through surrounding material, elements heavier than iron are fused and flung out into space
- the remaining core depends on the mass of the star
- rapidly increasing absolute magnitude
- release a lot of energy
Neutron star
- for a star between 1.4 and 3 solar masses
- when the core of a large star collapses, gravity is so strong that it forces protons and electrons together to form neutrons
- incredibly dense
- pulsars are spinning neutron stars that emit beams of radiation from the magnetic poles as they spin
Black hole
- for a star > 3 solar masses
-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
- The event horizon of a black hole is 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, and can be calculated using the formula:
Rs = 2GM/c^2
What is a binary system
Where two stars orbit a common mass
What is a Type I supernovae
When a star accumulates matter from its companion star in a binary system and explodes after reaching a critical mass
What is a Type II supernovae
The death of a high-mass star after it runs out of fuel
What is Type 1a supernova
A Type I supernova with a white dwarf
- When the companion stars in the binary system runs out of hydrogen, it expands, allowing the white dwarf to begin accumulating some of its mass
- 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
Features common across all types of supernovae
- all occur at the same critical ass
- all have similar peak absolute magnitude (-19.3)
- produce very consistent light curves
- astronomers use them as standard candles to calculate distances to far-off galaxies (they can be seen up to 1000Mpc away)
Supermassive black holes
- scientists believe they are at the centre of every galaxy
- because stars and gas near the centre of galaxies appear to be orbiting very quickly
- very strong gravitiational field attracting them
How can supermassive black holes form
- collapse of massive gas clouds while the galaxy was forming
- a normal black hole that accumulated huge amounts of matter over millions of years
- several normal black holes merging together
How does Hubble’s law show that the universe is expanding
- if the expansion of the universe was slowing down, more distant objects would be observed to be receding more quickly, since expansion was faster in the past
- they would also appear brighter than predicted because they would be closer
- type 1a supernovae were dimmer than they were expected to be, meaning they are more distant than Hubble’s law predicted
- this suggests that the expansion of the universe is accelerating and is older than Hubble’s law estimates
what do scientists believe to be the reason why the universe is accelerating
dark energy
Dark energy
- has an overall repulsive effect throughout the whole universe
- since gravity follows the inverse square law, it decreases with distance, but dark energy remains constant
- it has a greater effect than gravity, causing expansion speed to increase
why is dark energy controversial
there is evidence for its existence but no one knows what it is or what is causing it
