Stars Flashcards
What is one light year?
The distance light travels through space in one year
What is parallax?
Where the position of nearby stars appears to shift against the background of more distant stars as the earth moves around its orbit
What is one astronomical unit?
The mean distance between the centre of the sun and the centre of the earth
What is the parallax angle?
The angle subtended to the star by the line between the sun and the earth. It is also half the angler shift of the star’s line of sight over 6 months. Generally measured in arc seconds.
What is one arc second?
1 degree over 3600.
What is one parsec?
The distance to a star which subtends an angle of one arc second to the line from the centre of the earth to the centre of the sun.
What does a smaller parallax angle to a star mean?
The star is further away.
Describe the Hipparcos scale
A logarithmic scale to measure the magnitude of brightness of stars. A increase of 5 is a division of 100. Smaller number represent bigger magnitudes.
What is apparent magnitude?
A measure of brightness of a star that depends on the light intensity received from the star on earth.
What is the absolute magnitude of a star?
The star’s apparent magnitude if it was 10pc from earth
What is d measured in for the magnitude equation?
Parsecs
What is a blackbody?
A perfect absorber of radiation at all wavelengths and therefore emits a continuous spectrum of wavelengths. Eg. a star
Describe the blackbody radiation curve
y-axis is power radiated at each wavelength. x-axis is wavelength in micrometers. Visible range is about from 0.3 to 0.7 micrometers. Line curves up from near origin to peak wavelength then curves back down not as steep and levels off. Higher temperature blackbodies have a higher peak and a shorter peak wavelength.
Why can we assume a star is a blackbody?
Because any radiation incident on it would be absorbed and none would be reflected or transmitted by the star. Also, the spectrum of thermal radiation emitted by the star is a continuous spectrum with an intensity distribution that matches the shape of a blackbody radiation curve.
What is Wien’s displacement law?
The wavelength at peak intensity is inversely proportional to the absolute temperature of the photosphere of a star.
What is the photosphere of a star?
The light-emitting outer layer of a star. Sometimes called the surface of a star.
What is Stefan’s law
The total energy per second emitted by a blackbody (luminosity or power output) is proportional to its surface area, and its absolute temperature to the power 4.
Order of class of stars from hottest to coolest
OBAFGKM
Recite table of class of stars
See grid on specification
Why do spectra from stars contain absorption lines?
There is an atmosphere of hot gases above the photosphere. Atoms, ions and molecules of these gases absorb light photons of certain wavelengths emitted from the photosphere. The light that passes through these gases is therefore deficient in these wavelengths so its spectrum contains absorption lines.
What are Hydrogen Balmer lines?
The absorption lines on an emission spectrum from a star that correspond to excitation of hydrogen atoms from the n=2 energy levels to higher energy levels.
Why are Hydrogen Balmer lines only found in the spectra of O, B and A stars?
Other star’s are not hot enough for excitation of hydrogen atoms, due to collisions, to the n=2 state.
Why don’t hydrogen atoms in the n=1 state absorb visible photons?
Because visible photons don’t have enough energy to causes excitation from n=1.
Describe the Hertzsprung-Russel diagram
y-axis is absolute magnitude ranging from +15 up to -10. x-axis is temperature or spectral class ranging from 50k to 2.5k or OBAFGKM. Sun(G,+5). Main sequence band class B to M magnitude -10 to +15 and is straight, then levels, then straight with same gradient as start. Red giants(G-M,0to-5). White dwarfs(B-F,+15to+10).
Describe the evolution of a star
Protostar, main sequence star, red giant.
If low mass then white dwarf, invisible.
If high mass then supergiant, supernova, neutron star, black hole if mass of neutron star would be 3x mass of sun.
How does a star form?
Dust and gas clouds in space contract under their own gravitational attraction becoming denser and denser to form a protostar. In the collapse GPE is converted into thermal energy as the atoms and molecules gain kinetic energy so the interior of the collapsing matter becomes hotter. If sufficient matter accretes to form the protostar, the temp at the core gets hot enough to do nuclear fusion. Otherwise it gradually cools. Energy from fusion of H increases core temp so more fusion occurs so outer layers heat and photosphere forms. Star.
Main sequence stage
Newly formed star reaches internal equilibrium as the inward gravitational attraction is balanced by the outward radiation pressure. The star becomes stable at a constant luminosity. Star remains like this for most of its lifetime emitting light as a result of ‘hydrogen burning’ in its core.
Red giant stage
When most H is converted to He, the core starts to collapse on itself and outer layers of star expand and cool. The temp of the He core increases as it collapses and causes surrounding H to form a ‘hydrogen burning’ shell which heats the core further. When core temp reaches 10^8K He can fuse. Luminosity increases.
White dwarf stage.
When star stops doing fusion, it cools and core contracts causing outer layers to be thrown off as shells of hot gas and dust to form a planetary nebula around the star. The star is now little more than its core and is white hot due to release of gravitational energy. The contraction of core stops as electrons can’t be forced closer. Star is stable and is white dwarf which gradually cools as it radiates its thermal energy into space and eventually becomes invisible.
Death of high mass star stage
Iron nuclei are most stable so no more fusion occurs. If the core mass is greater than 1.4 solar masses, the electrons in iron core can’t prevent collapse as they are forced to react with protons to form neutrons. This sudden collapse of core makes it more and more dense until the neutrons can’t get closer and it becomes rigid. The collapsing matter surrounding the core hits it and rebounds as a shock wave which propels the surrounding matter outwards into space in an explosion.
Typical supernova characteristics
A sudden and very large increases in the luminosity of a star. Absolute magnitude of -15 to -20. Increase in luminosity occurs within about 24h. Luminosity gradually decreases over a number of years.
Type 1a supernova
Reach peak luminosity about 10^9 times the sun. Strong absorption lines due to silicon. The absolute magnitude is known (-18 to -20) so they can be used to find distances between earth and its galaxy. Therefore they are standard candles.
How did type 1a supernovae lead to controversy?
The distance from earth calculated was more than expected so the universe must be expanding at an accelerating rate which would require the input of energy. This lead to the theory of dark energy.
Describe neutron stars
It is the core of a supernova after all the surrounding matter has been thrown off into space. Incredibly dense (similar to a nucleus). Contains only neutrons. Very small compared to sun.
What are pulsars?
Pulsating radio stars. About 2 solar masses. Rapidly rotating neutron stars that produce beams of radio waves. How we discovered neutron stars.
Black hole stage
If the core of a supernova is greater than 3 solar masses, the neutrons are unable to withstand the immense forces that are pushing them together. The core collapses in on itself and becomes so dense not even light can escape from it. The object is then a black hole.
Describe a black hole
So dense not even light can escape from it because the escape velocity is greater than c. It can’t emit any photons and it absorbs any photons incident on it.
What is the event horizon of a black hole and what is its radius called?
A sphere surrounding a black hole from which nothing can ever emerge. Schwarzschild radius.
Supermassive black holes
Almost unimaginable mass thought to be at the centre of many galaxies. At the centre of a galaxy, star’s are much closer together than at the edges. A supermassive black hole at the centre could pull in millions and millions of stars and therefore gain enormous quantities of matter.
Gamma ray bursts
Release vast amounts of energy in the form of jets of gamma radiation. They shoot out of the poles of supergiant stars when they collapse or fall into a black hole.
