Astrophysics (Stellar Characteristics & Evolution) Flashcards
How do we know the composition of stars?
Stellar Spectra
the missing wavelengths of a star’s absorption spectra (the black lines) correspond to the absorption spectrum of the elements in the star
How do we know the temperature of stars?
Larger atoms have more kinetic energy and are excited first (before lighter atoms). at high enough temperatures, the electrons are bounced off and the atoms are ionized and have no electrons to produce the absorption lines.
low-temperature stars:
have absorption spectra showing heavier elements (like calcium & magnesium)
high-temperature stars:
fewer absorption lines from heavy elements. mainly only of lighter elements (like hydrogen)
What is the Hertzsprung–Russell (HR) diagram?
The Hertzsprung-Russell (HR) diagram shows the relationship between luminosity and temperature
- Luminosity - y axis. Measured in Solar units (based off luminosity of our sun)
- Temperature - x axis. Measured in Kelvin (NOTE: horixontal axis is DECREASING in temp. from left to right)
What are the “Main Sequence” Stars in an HR diagram?
Line which is like a negative straight line crossing the middle of the graph (from top left to bottom right)
- many stars are on this “line”
- normal, stable stars that only differ in mass.
- fuse hydrogen to helium
What are the “Supergiants” Stars?
Top area of HR graph (on the right side of “Main sequence” line)
*Cool, bright stars
What are the “White Dwarf” Stars?
- Small, white, relatively hot, dim
- is cooling down b/c Fusion is not happening anymore *Once runs out of light fuel, will turn into a Brown dwarf
- one of the densest forms of matter, (second only to neutron stars and black holes)
- Has very intense gravity
Bottom left region of HR graph
What are “Neutron” Stars?
- post-supernova from large mass stars
- Gravity pressurized it and now it’s only made of neutrons.
- Ultra-dense
- Rotating types are called “pulsars”.
What are “Cepheid Variables”?
slightly unstable stars.
- Regular variation in luminosity.
- b/c of fluctuating size, their outer layer contracts (dimmer) and expands (brighter) periodically.
- the period of their variations is correlated to their average luminosity, which is extremely bright and is what we call a “standard Candle”.
- Scientists can calculate distances of far galaxies with them because their value is so consistent
What is the Chandrasekhar limit?
The largest mass a white dwarf can have (or for a star to become a white dwarf
1.4 solar masses.
What is the Oppenheimer-Volkoff limit?
the largest mass a neutron star can have (more than this will be a black hole b/c neutron degeneracy can only overcome so much gravity)
around 2-3 solar masses.
The uncertainty in this limit is because we’re not too sure on the equation for the state of matter in a neutron star
Explain Stellar Evolution
It begins with a NEBULA.
under the influence of gravity, it begins to condense and eventually forms a PROTOSTAR. In this stage, the process of nucleosynthesis begins.
When a star runs out of helium, the core collapses, and, under the additional gravitational pressure, the helium in the core will start to undergo fusion, causing the outer layers of the star to expand and cool, and the star becomes a RED GIANT.
The core continues to react and elements such as carbon, neon, oxygen, silicon and iron are produced.
(This is where the elements that compose our world are created. Without stars the universe would be composed solely of hydrogen)
When the star completely runs out of fuel; usually when the core becomes iron, the red giant star collapses. The next stage of the star is determined by the mass of that star and the Chandrasekhar limit.
If a star is LESS than the Chandrasekhar limit, the energy from the gravitational collapse is not sufficient to produce the neutrons of a neutron star so the collapse is halted by electron degeneracy to form a WHITE DWARF along with a PLANETARY NEBULA. The white dwarf star continues to cool and eventually becomes invisible
If a star is MORE than the Chandrasekhar limit, it will become a SUPER RED GIANT. In this case, when the star dies, it becomes a SUPERNOVA. if the star’s mass is LESS than the Oppenheimer-Volkoff limit it will go on to become a NEUTRON STAR
if it is MORE than the Oppenheimer-Volkoff limit, no known mechanism is enough to prevent further collapse, so it will become a BLACK HOLE
Define Nucleosynthesis
The fusion of light nuclei being smashed together and fused to produce a heavier nucleus, and gamma rays. The star will react its core of hydrogen into helium, this process called a proton-proton cycle, for all of its main-sequence lifetime.
What are Black Holes?
A singularity in space-time
post-supernova from large mass stars.
*Nothing to stop their gravitational collapse.
*Their escape velocity is faster than light
Outline Stellar Evolution on an HR diagram
Small Stars:
Follow the Main Sequence line until a low point where it diverges to the right up to being a RED GIANT, going up and around back to near the top of the Main Sequence line, and continue the curve downwards to be a WHITE DWARF
Large Stars:
Begin near the top of the Main Sequence line, go horizontally to the right to the SUPER RED GIANT zone, and zig-zag upwards towards the top-right corner up to the SUPERNOVA corner.
What is Neutron and Electron Degeneracy?
IDK man
