9.2 Classification of stars Flashcards
What is apparent magnitude?
How bright a star appears from Earth.
What is absolute magnitude?
How bright a star would appear if it were placed 10 parsecs frm Earth.
What is the equation relating apparent and absolute magnitudes?
What do the letters represent in this equation?
How is the Hipparcos scale defined?
- Dimmest visible stars are magnitude 6
- Brightest visible stars are magnitude 1
What is a difference of 1 on a magnitude scale equal to as an intensity ratio?
2.51
Define the parsec.
The distance to an object that subtends an angle of one arcsecond to the line that runs from the centre of the Earth to the centre of the Sun.
What is the equation for parsecs?
Angle is in arcseconds.
Define the light year.
The distance it takes light to travel in 1 year in a vacuum.
State Stefan’s law.
What do the letters represent in this equation?
State Wien’s displacment law.
What do the letters represent in this equation?
What is a black body?
A black body has equal rates of absorption and emission of EM radiation.
Draw 3 black body radiation curves for 3 black bodies of different temperatures.
What do we assume stars to be?
Black bodies.
What are the colours of the different spectral classes?
- O: Blue
- B: Blue
- A: Blue/White
- F: White
- G: Yellow/White
- K: Orange
- M: Red
Rank the spectral classes in order of their temperature ranges in Kelvin.
- O: 25,000-50,000
- B: 11,000-25,000
- A: 7500-11,000
- F: 6000-7500
- G: 5000-6000
- K: 3500-5000
- M: <3500
What are the prominant abosroption lines for the different spectral classes?
- O: He+, He, H
- B: He, H
- A: H (strongest), ionised metals
- F: Ionised metals
- G: Ionised and neutral metals
- K: Neutral metals
- M: Neutral atoms, TiO
What is the prominance of the Balmer lines for the different spectral classes?
- O: Weak
- B: Slightly stronger
- A: Strongest
- F: Weak
- G, K, M: Very weak/none
What is the explanation for the prominance of the Balmer lines in O and B type stars?
Star’s atmosphere too hot, hydrogen is likely to be ionised.
What is the explanation for the prominance of the Balmer lines in A type stars?
High abundence of hydrogen in n=2 state.
What is the explanation for the prominance of the Balmer lines in F type stars?
Too cool, hydrogen unlikely to be excited.
What is the explanation for the prominance of the Balmer lines in G, K, and M type stars?
Too little atomic hydrogen, far too cool to be excited.
What are the hydrogen Balmer lines?
Absorption lines that are produced as a result of the excitation of electrons from the n=2 state of hydrogen atoms.
Draw a Hertzsprung-Russel diagram
Where is the Sun on a HR diagram?
At Absolute magnitude +5,
In G spectral class or about 5500K for temperature.
Sketch the evolution path of a star similar to the Sun on a HR diagram.
(From formation to white dwarf)
What is a supernova?
A supernova is the explosion of a star, which causes it to very suddenly and rapidly increase in absolute magnitude.
What happens to the star during a supernova?
When a star of mass greater than 1.4 solar masses uses up all of its nuclear fuel and gravity collapses the core.
Outer layers fall inward and rebound off the core, causing heavy elements to be fused and distributed into space.
What is a type 1 supernova?
The consequence of a star in a binary system accumulating matter from its companion star. When the star reaches critical mass, it explodes.
What is a type 1a supernova?
A consequence of a white dwarf star exploding. They can be used as a standard candle since they always reach the same maximum absolute magnitude (-19.3).
What is a type 2 supernova?
The consequence of a high mass star dying when it runs out of fuel.
How many times greater is the energy output of a supernova compared to that of the Sun?
About 1015 times greater.
How are Gamm Ray Bursts caused?
From the collapse of supergiant stars to form neutron stars or black holes.
What is a white dwarf?
The result of a star of mass <1.4 solar masses using up its nuclear fuel
* Core contracts, outer layers blown off
* Very dense (108-109 kg)
What is a neutron star?
The result of a star of mass 1.4-3 solar masses using up its nuclear fuel
* force electrons and protons togther to form neutrons during core collapse
* Incredibly dense, same as nuclear matter (1017 kg)
What is a pulsar?
A rapidly spinning neutron star that emits beams of radiation from magnatic poles.
What is a black hole?
The result of a star of mass >3 solar masses using up its nuclear fuel
* All matter is condensed to a singular point during core collapse.
* Escape velocity greater than speed of light
What is a standard candle?
An astronomical object of known intrinsic brightness.
Why can type 1a supernovae be used as standard candles?
They have the same peak absolute magnitude (-19.3), occur at the same mass, and last for the same amount of time.
What does the light curve of a type 1a supernova look like?
Why can’t type 2 supernovae be used as standard candles?
Type 2 peak absolute magnitudes have a range of values since the supernovae don’t occur for a specific mass.
What controversy did the use of type 1a supernova as standard candles lead to?
The accelerating expansion of the Universe and the proposal of dark energy.
How would we find the Schwarzschild radius?
(Consider energy equations)
What do the letters represent in this equation?
Where do we find supermassive black holes?
In the centres of galaxies.
