Astro A2 - Stars Flashcards
What is apparent magnitude scale
The brightness of a star as seen from earth
What is luminosity
Total power radiated by a star
What is intensity
Power per unit area at the observerq
What is the apparent magnitude value for the faintest stars we can see
6
How much brighter is a fourth magnitude star compared to a fifth magnitude star
2.51 times
What is our Sun’s apparent magnitude
-26
What is the astronomical unit
The mean distance from the Sun to the Earth
How to define a parsec
The distance from which 1AU subtends an angle of 1 arc second
What is the absolute magnitude
The apparent magnitude a star would have at a distance of 10 pc from an observer
What is a black body
A body that absorbs all the em radiation that falls on it
What does the graph of light intensity look for 3 stars of different temps
Hotter stars have a peak at a shorter wavelength. Hotter stars have a higher intensity of each wavelength of light than colder stars
What is the assumption for the use of the inverse-square law
No light is absorbed or scattered between the source and the observer. Source is treated as a point
What are the spectral classes in descending temperature
OBAFGKM
Describe an O-class star
Blue, 25000-50000K, absorption at He+, He, H
Describe a B-class star
Blue, 11000-25000K, absorption at He, H
Describe an A-class star
Blue-white, 7500-11000K, absorption at H, ionised metals
Describe an F-class star
White, 6000-7500K, absorption at ionised metals
Describe a G-class star
Yellow-white, 5000-6000K, absorption at ionised and neutral metals
Describe a K-class star
Orange, 3500-5000K, absorption at neutral metals
Describe an M-class star
Red, <3500K, absorption at neutral atoms, TiO
What are Balmer lines
Spectrum produced when transitions from the energy level n=2 are made
Where are balmer lines strongest
A-class star as there is a high abundance of hydrogen in n=2 state
Where are balmer lines weak
O (too hot so hydrogen would be ionised), F,G,K,M (too cool to be excited)
What are the axis on the Hertzsprung-Russell diagram
Spectral class on x (or temperature from 2500-50000k) and absolute magnitude on y (-10 to 15)
Describe the evolution of the Sun
The Sun is currently a G-class star, a main sequence star. As hydrogen is used up, it will fuse hydrogen in the outer layers and expand, to become a giant. It will collapse, increase temp and will initiate helium fusion. Helium shell fusion will then occur and the Sun will expand again. The outer material will then be pushed away to form a planetary nebula leaving a white dwarf. After fusion is complete, it becomes a black dwarf
What are supernovae
Objects which exhibit a rapid and enormous increase in absolute magnitude
What are type 1a supernovae
They reach the same peak value of absolute magnitude
What are standard candles
Absolute magnitude is known
What causes a type 1a supernova
A white dwarf star attracts mass from its binary system and fuses again of carbon. When it reaches a critical mass, the white dwarf star explodes
What is a neutron star
A core with a mass twice the mass of the Sun
Describe the motion of neutron stars
Due to conservation of angular momentum as
the core collapses, they tend to be spinning very rapidly
What are pulsars
Radio sources produced by strong magnetic fields and spinning of a neutron star
What are black holes
More massive cores (more than about 3 solar masses) can continue to collapse
and form black holes
What is a key feature of a black hole
Its escape velocity is greater than the speed of light
What is the event horizon
The boundary at which the escape velocity is equal to the speed of light
What is the Schwarzschild radius
Radius of the event horizon
How is a quasar produced
From supermassive black holes at the centre of young active galaxies
How can type 1a supernovae be used to measure distance
Type 1a supernovae has the same peak absolute magnitude (-19.3), so apparent magnitude can be measured
What is a significant feature of quasars
Bright radio sources; brightest objects in the universe
