CLASSIFICATION OF STARS Flashcards
Why are giants brighter than the sun?
Giants are cooler than the sun but bigger than the sun hence brighter.
What is on the y/x-axis of the Hertzsprung-russell diagram?
y-axis: absolute magnitude
x-axis:temperature/k or spectral class
what is apparent magnitude?
The perceived brightness of the star seen from earth.
what is absolute magnitude?
The absolute magnitude of a star is the apparent magnitude it would have at a distance of 10 pc from an observer.
what is Wien’s displacement law?
the wavelength of a star’s emission at peak intensity is inversely proportional to its absolute temperature.
what is 1AU?
1 AU(astronomical unit) is the average distance between earth and the sun.
what is 1 pc?
1 parsec is the distance to a star with parallax angle of 1 arc second.
what is 1 ly?
1 ly is the distance light can travel in a vacuum in 1 year.
what is parallax?
parallax is the apparent shift of a nearby object (star) relative to its (fixed) background when observed from different positions.
State Stefan’s law
the power output of a star is directly proportional to its surface area and its (absolute temperature)^4.
what is a black body?
A black body absorbs all the EM radiation incident
3 Key things: black body radiation curve
- the hottest objects has a peak at the shortest wavelength
- Hotter stars will produce more of their light at the blue/violet end of
the spectrum and will appear white or blue-white
-Cooler stars look red as they produce more of their light at longer wavelengths
Assumptions of inverse-square law
that no light is absorbed or scattered between the source and the observer and that the source can be treated as a point.
explain how absorption spectra takes place.
- When the light created within a star passes through its ‘atmosphere’, absorption of particular wavelengths takes place.
-This produces gaps in the spectrum of the light from the star, resulting in an absorption spectrum. - Electrons in the atoms and molecules of the star’s atmosphere are absorbing the light, and therefore ‘jumping’ to higher energy levels. The difference in these energy levels are discrete (have particular values) and therefore the frequencies of the absorbed light are discrete.
What is the relationship between temperature and spectra?
- At low temperatures, there may not be enough energy to excite atoms/break molecular bonds resulting in TiO and neutral atoms in the M class spectra.
- At high temperatures, atoms have too much energy to form molecules and ionisation can take place seen in class F and G.
Where are Balmer lines the most prominent?
The prominence of Balmer lines are strongest at A as there is a high abundance of hydrogen in the n=2 state.
what do the wavelengths absorbed by the corona depend on?
1) elements/gases present
2) temperature of the photosphere- because hotter stars can excite electrons to higher energy levels so more drops are possible.
what is the abs mag and the temperature of the sun approx?
Abs mag:+5
Temp: 5800K
What are the characteristics of a 1a supernovae?
- no strong H lines present.
- strong silicon lines present
- standard candle- all have the same peak abs mag.
When does a 1a supernova occur?
A white dwarf in a binary system attracts matter from a companion star. When the white dwarf reaches the critical mass (1.4 times the Mass of sun), it explodes.
What are the characteristics of 1b supernova?
- No strong H lines present
- Strong He lines present
How does a 1b supernova occur?
It occurs when a supergiant without hydrogen in its outer layers collapses
What are the characteristics of a 1c supernova?
- No strong H lines present
- No strong He line present
What are the characteristics of a Type II supernova?
- H and He lines are present?
When does Type II supernova occur?
A supergiant with both hydrogen and helium in its outer layers collapses.
What spectral class is the sun?
Currently a G class star hence on the main sequence.
What will happen to the sun in the future? HINT: fusion
- As it uses hydrogen fuel in its core, it will start to fuse hydrogen in its outer layers and expand.
- It will then leave the main sequence and become a giant star.
- When it expands the outer surface of the sun will cool.
- As it uses up the fuel, it will collapse and the increase in temperature and pressure in the core will initiate helium fusion.
- When this fuel is used up then helium fusion can take place and the sun will expand again.
- Outer layer of sun will be pushed away to form planetary nebula leaving small core of white dwarf and when fusion stops then it will cool into a black dwarf.
How are neutron stars formed?
Neutron stars are objects which form after a supernova has ejected the outer layers of a star into space. A core with a mass of about twice the mass of the sun will become a neutron star.
- reverse beta decay.
What is the definition of a neutron star?
An extremely dense collapsed star made up of neutrons.
What is a spinning neutron star?
A pulsar which emits beams of radio waves at their magnetic poles that sweep like a lighthouse.
What is the event horizon?
The boundary at which the escape velocity is equal to the speed of light.
What is the Schwarzschild radius, Rs?
The distance from the centre of the black hole to the event horizon.
What is the centre of the black hole known as?
The singularity.
Why are pulsars easy to identify compared to slow neutron stars?
- They emit radiation periodically which makes them easier to detect
What are gamma ray bursts?
A short, extremely high energy burst of gamma radiation emitted by a collapsing supergiant star.
