Classification of Stars Flashcards
What is apparent magnitude?
The perceived brightness of a star as seen from Earth.
What was the Hipparcos scale originally like?
A scale that classifies stars by there apparent magnitude.
mag 1 - Brightest stars
Mag 6 - Very faint
What is the Hipparcos scale like now?
Mag 1 star is 100 times brighter than mag 6 star.
Brightness from a m1 to m2 star decreases by a factor of 100^1/5 (2.51)
How do you compare the brightness of 2 objects?
I₁/I₂ = 2.51^(m1 - m2)
I₁/I₂ = ratio of intensities of object s
If the apparent magnitude is negative, is it bright or faint?
The more negative the apparent magnitude, the brighter the object appears.
(Sun has an apparent magnitude of -26, Pluto has one of 15).
At what order of magnitude do stars become invisible to the naked eye?
After magnitude 6.
What is luminosity defined as?
The total power output of radiation/power emitted by a star.
What are the units for luminosity?
Watts (W)
What is the brightness of a star defined as?
The intensity of radiation received on Earth from a star.
What are the units for brightness?
Wm⁻²
How does the inverse square law tie into intensity and radiation?
Apparent brightness / intensity observed at Earth = Luminosity / 4π(distance between star and Earth)²
I = L/4πd²
When the light is twice as far away, it has spread over four times the area, hence the intensity is four times smaller.
What is an astronomical unit (AU)?
The mean distance from the centre of the Earth to the centre of the Sun.
(1.50x10¹¹m)
What is a light year?
The distance travelled by light in one year.
= 9.5x10¹⁵m
What is a parsec?
The distance at which the radius of the Earth’s orbit (1AU) around the Sun subtend sat an angle of 1 arcsecond.
3.08 × 10¹⁶m
What is an arcminute and arcsecond?
1 degree = 60arcminutes
1 arcminute = 60 arcseconds
1 arcsecond = 1/3600 degree
What is absolute magnitude of a star?
The apparent magnitude it would have if it were observed 10 parsecs away from Earth.
What is the relationship between apparent magnitude and absolute magnitude? (in equation form)
m - M = 5log(d/10)
M = absolute magnitude
m = apparent magnitude
d = distance of the star from Earth (in parsecs)
What is a black body?
A surface that absorbs all radiant energy falling on it.
What is an ideal black body radiator?
One that absorbs and emits all wavelengths.
(Theoretical but stars are the closest thing to it)
What is Wien’s displacement law?
The black body radiation curve for different temperatures peaks at a wavelength that is inversely proportional to the temperature.
What is the formula that describes Wien’s law?
λmax ∝ 1/T
λmax = the maximum wavelength emitted by an object at the peak intensity (m)
T = surface temperature of an object (K)
What can Wien’s law also be written as and what does it show? (the constant)
λmaxT = 2.9 × 10⁻³mK
- The higher the temp of a body, the shorter the wavelength at the peak intensity and the greater the intensity of the radiation at each wavelength.
What is an assumption we make about stars? (in terms of their radiation)
They are a black body
What is Stefan’s Law? (Written)
The total energy emitted by a black body per unit area per second is proportional to the fourth power of the absolute temperature of the body.
What is Stefan’s law? (Equation)
P = σAT⁴
P = total power emitted across all wavelengths (W)
σ = the Stefan-Boltzmann constant (formula sheet)
A = surface area of the body (m)
T = absolute temperature of the body (K)
What is the formula for when Stefan’s law is used to find luminosity?
L = 4πr²σT⁴
L = luminosity of star (W)
r = radius of the star (m)
σ = Stefan-B constant
T = surface temperature of the star (K)
What are the 3 types of light spectra?
- Continuous emission spectra
- Emission line spectra
- Absorption line spectra
When is continuous line spectra created?
When photons of all wavelengths are emitted.
What do continuous line spectra look like?
A broad range of colours (depending on a star’s temperature)
What are continuous line spectra produced by?
hot, dense sources, such as the cores of stars.
When is emission line spectra created?
When photons are emitted by excited electrons in a hot gas.
What do emission line spectras look like?
Discrete (the opposite of continuous, something that is separate, distinct and individual) wavelengths represented by coloured lines on a black background.
What produces emission line spectra?
Hot, low-pressure gases, such as a nebula surrounding a star.
When is absorption line spectra created?
When photons are absorbed by electrons in a cool gas.
What does the absorption spectrum look like?
Discrete wavelengths represented by dark lines on a continuous spectrum.
What is the absorption spectrum produced by?
Light passing through cool, low-pressure gases, such as the photosphere of a star. (photosphere = outer shell from which light is radiated)
What is the Balmer series and what does it show?
Hydrogen gas in the atmosphere around the star absorbs photons of light from the star. These are then emitted leaving the hot surface and absorption spectrum for hydrogen. In the balmer series the energy transitions converge at energy level n=2. So when a photon is excited to a higher energy level is always releases a photon with energy between its current energy level and n=2 energy level (in visible spectrum).
What are the 7 spectral classes in order?
O B A F G K M
Out of the spectral classes which has the strongest hydrogen absorption lines?
A
(then B)
What is the intrinsic colour of O?
Blue
What is the intrinsic colour of B?
Blue
What is the intrinsic colour of A?
Blue-White
What is the intrinsic colour of F?
White
What is the intrinsic colour of G?
Yelllow-white
What is the intrinsic colour of K?
Orange
What is the intrinsic colour of M?
Red
What is the temperature of O?
25000- 50000K
What is the temperature of B?
11000-25000K
What is the temperature of A?
7500-11000K
What is the temperature of F?
6000-7500K
What is the temperature of G?
5000-6000K
What is the temperature of K?
3500-5000K
What is the temperature of M?
<3500
What are the prominent absorption lines of O?
He+, He, H
What are the prominent absorption lines of B?
He, H
What are the prominent absorption lines of A?
H (strongest), ionized metals
What are the prominent absorption lines of F?
ionized metals
What are the prominent absorption lines of G?
ionized and neutral metals
What are the prominent absorption lines of K?
neutral metals
What are the prominent absorption lines of M?
neutral atoms
What is the relationship between the temperature of a star and its absorption spectra related to?
The effect of the energy of the state of the atoms or molecules present in the atmospheres of the star.
What happens to absorption spectra of low temp stars?
- Not enough energy to excited atoms or break molecular bonds = results in TiO and neutral atoms
What happens to absorption spectra of high temp stars?
- Atoms have too much energy to form molecules = ionisation
What happens to the absorption spectra of the hottest stars?
- Hydrogen and helium are found to be in higher abundance in the atmospheres of the hottest stars = spectral lines start to dominate
Where do electron transitions occur in the Balmer series?
Electron transitions either to or from the second energy level.
Why do the Balmer lines become stronger from O to A and then week after A to M?
O and B - star’s atmosphere too hot, hydrogen ionised
A - high abundance of hydrogen in n=2 state
F - too cool, hydrogen not excited
G,K,M - too little atomic hydrogen, too cool for excitation
What are the conditions for nuclear fusion?
Both nuclei must have sufficiently high kinetic energy to overcome the electrostatic repulsion between protons.
- Very high temperature
- Very high pressure and density
How does fusion work (on an atomic scale)?
4 hydrogen nuclei (protons) are fused into one helium nucleus, producing two gamma-ray photos, two neutrinos and two positrons
- massive amounts of energy are released
- the momentum of the gamma-ray photons results in an outward acting pressure (radiation pressure)
What is the life cycle of a star (up to equilibrium?
- Once the core temperature of a star reaches millions of Kelvin, fusion occurs
- Protostar’s grav field attracts more gas + dust, increasing temperature and pressure of the core
- With more frequent collisions, the kinetic energy of particles increases - more fusion
- An outward radiation pressure is produced which balances the inward pull of gravity
- star reaches a stable state where inward = outward forces
- As temp of star inc, volume decreases due to gravitational collapse, the gas pressure increases
- gas + radiation pressure act outwards and balance gravitational force inwards
What is considered to be a low mass star?
<8Msun
What is the lifecycle of a low mass and high mass star?
Not writing all this out - you should know this from gcse
What are supernova defined as?
An object which exhibits a rapid and enormous increase in absolute magnitude.
What is a type II supernova?
A supergiant star collapses and then explodes.
What is a type 1a supernova?
A white dwarf accrues matter and explodes.
What is a gamma ray burst (GRB)?
A short, extremely high energy burst of gamma radiation emitted by a collapsing supergiant star. (before forming neutron stars or black holes)
What is a standard candle?
An astronomical object of known brightness that can be used to calculate galactic distances.
What is an example of a common standard candle?
Type 1a supernovae.
Why can type 1a supernovae be used as standard candles?
They reach the same peak value of absolute magnitude each time.
(and they’re extremely bright)
What is the Chandraeskhar limit?
The maximum mass of a stable white dwarf star: 1.4 times the mass of the Sun.
What is the light curve for a type 1a supernova?
- Peak magnitude of -19
- Always has same peak magnitude and shape
(search it up and remember it)
What is a neutron star defined as?
An extremely dense collapsed star made up of neutrons.
How are all the neutrons produced in a neutron star?
The immense gravitational forces acting on the core crush the electrons and protons until they combine into neutrons via reverse beta decay
p + e- = n + v
What is further collapse in neutron stars prevented by?
Neutron degeneracy pressure - An outward pressure caused by densely-packed neutrons pushing against each other in the collapsing core of a star.
What are pulsars?
Rapidly rotating neutron stars (600 times per second).
They emit bursts of highly directional electropmagnetic radiation.
What makes pulsars easier to detect that neutron star?
The emit radiation periodically
What is a singularity?
A theoretical point at which matter is compressed to an infinitely small point and the laws of physics, as they are currently understood, break down
What does the mass of a supernova core have to be to create a black hole?
> 3Msun
What is escape velocity?
The minimum velocity required for an object to escape a gravitational field with no further energy input.
What is the escape velocity of a black hole greater than?
The speed of light - why black holes cannot be seen directly - photons cannot escape beyond the event horizon.
What is the event horizon?
The boundary at which light and matter cannot escape the gravitational pull of the black hole.
What is the equation for the Schwarzchild radius of a Black Hole?
Rs = 2GM/c²
Rs = the radius
G = gravitational constant
M = mass of black hole
c = speed of light
Where are supermassive black holes always located?
In the centre of every galaxy.
What are the axis of the Hertzsprung-Russell Diagram?
Luminosity and absolute magnitude on the y axis, temperature and spectral class on the x axis. (like a box)
What is the scale range of absolute magnitude in a HR diagram?
-10 to +15
What is the scale range of temperature in a HR diagram?
2500 to 50000K
Do stars with higher luminosity’s have greater or shorter life spans?
Shorter, as they burn up more nuclear fuel per second.
