ASTRONOMY EXAM 3
How does a lightyear compare to a parsec?
1 Parsec = 3.26 lightyears
Approx what will be the main seq lifetime of a massive O star?
1 million years
What will be the total main sequence lifetime of the sun?
10 BILLION years
Today we say (by definition) that a difference of 5 magnitudes corresponds to what factor in brightness?
100 (one star is 100x brighter than the other)
At what temperature does the Triple Alpha Process in red giants start?
100 MILLION K
What is the radius of a black hole if its mass is the same as earths?
1cm
When we view a nearby star at two times 6 months apart, over what distance (or baseline) are we making our observations?
2 Astronomical Units (AU)
What factor in brightness corresponds to a difference of one magnitude?
2.512
The theoretical basis for the idea of black holes comes from Einstein's General Theory of Relativity. This is primarily a theory about gravity, and the four dimensional nature of our universe. What are these 4 dimensions?
3 Are space (Our world is 3D) and one is time.
Approx what will be the main seq lifetime of a LOW- mass O star?
300 BILLION years
In what form does most of the energy come out of a supernova explosion?
90% of the energy is carried off as neutrinos (10% is gas blown out into space)
After Helium fusion stops in a red giant's core, it may become a Planetary Nebula. What is a Planetary Nebula?
A Planetary Nebula is a star that no longer has enough gravitational pull to keep everything together. It has moved to the Second Red Giant Stage, making it a RED SUPERGIANT. Its outer layers of gas expand.
What is a lightyear?
A lightyear is the distance light travels in one year.
About how much mass do white dwarf stars have, and how big are they?
A low or medium mass star (with mass less than about 8 times the mass of our Sun) will become a white dwarf. A typical white dwarf is about as massive as the Sun, yet only slightly bigger than the Earth.
What is the system of Stellar Magnitudes?
A measurement of how bright a star woud appear at 32.6 lightyears from earth. brightest stars - first magnitude, slightly fainter stars were second magnitude, and the faintest. first magnitude (brightest) to sixth magnitude (dimmest).
What might be left after the explosion of the birth of a T2 Supernova star (after being a white dwarf)?
A neutron star
What is a Parsec?
A parsec is the distance at which a star has a parallax angle of one arcsecond.
What property of a star determines its position on the main sequence?
A star's mass is the property that determines its position on the Main Seq
What single factor determines a stars location on the main sequence?
A stars mass
What is a neutron star?
A very small star left after a supernova. It is extremely dense
What is absolute magnitude?
Absolute magnitude measures the TRUE brightness of a star; it removes the effect of distance
How does the main sequence lifetime depend on the mass and luminosity of the star?
Amount of fuel = mass/luminocity
Parallax angles are measured in fractions of an arcsecond. What is an arcsecond?
An arcsecond is an angular measurement equal to 1/3600 of a degree or 1/60 of an arcminute
Who gave us the spectral classification system we use today?
Annie J Cannon
At what distance would a stars apparent magnitude equal its absolute magnitude?
Apparent mag = absolute mag at 10 parsecs
What is apparent magnitude?
Apparent magnitude measures how bright a star appears in the sky.
How do astronomers define a star's spectral type?
Ast's use the various absorption lines in the spectrum of a star to define its spectral type.
In what 2 ways can a star transport energy outward?
Convection and Conduction Convection: cells of hot gas move out and up while cool cells move down Conduction: collation between electrons can move energy outwards.
Process: When helium fusion begins in the core of a star like the Sun, it begins as an explosion in the core called the "Helium Flash". This explosion does not blow the star apart. In fact, from the outside of the star we have no evidence of the explosion. The reason for believing that this explosion happens is that the rate of helium fusion is VERY sensative to temperature, so once the reaction starts, it should run out of control. After the Helium Flash, it goes left on the diagram on the Horizontal Branch.
Damn ok geez.
What are white dwarfs?
Dead stars; Snow White's pals
What is the relationship between the parallax angle in arcseconds and the distance in parsecs?
Distance in parsecs=1/parallax angle in arcseconds.
A white dwarf is supported by Electron Degeneracy Pressure. What is this pressure, and how is it different from gas pressure?
EDP is caused by electrons being squeezed together too tightly
The simplest type of Black Hole is called a Schwarzschild Black Hole. What is the outer boundary of this Black Hole called?
Event Horizon
What is hydrostatic equilibrium
Every layer of a star must be in a pressure balance between gravity that wants to pull the gas inward and some internal pressure that pushes gas outward
What is the spectral type of the sun?
G2
As the core contracts there are now 2 sources of energy at the center of a star. What are they?
Gravitational energy and hydrogen fusion in the shell around the core.
Why do we think Helium Flash occurs, since we can't see it from the surface?
He fusion is VERY sensitive to temperature, so once the reactoin starts, it should run out of control.
What happens to a star when its core's helium runs out?
Helium Fusion stops in the core. At this point the core is now Carbon, and it contracts inward. The outer layers expand again, into the second Red Giant stage. A helium fusion shell is formed around the core, and a hydrogen fusion shell is formed just outside of that.
When does he Helium Flash occur in a red giant
Helium flash explosion occurs when He fusion begins in the core of a star like the sun.
Astronomers had no proof for the existence of neutron stars until the discovery of pulsars. What are pulsars?
Highly magnetic, rapidly-rotating neutron stars that give off pulses of radio emission at extremely regular intervals
What is along the Horizontal and Vertical axes of the HR diagram?
Horizontal: surface temp Vertical: absolute magnitude (brightness)
What was the original system of spectral classification based on?
Hydrogen absorption lines. The OG Spectral Classification was based on the strength of Hydrogen absorption lines.
Approx. how much mass do neutron stars have, and how big are they?
Infinite density; and they have a radius of 10-15 km. crazy
In what 3 ways does the surface of a star change during its main sequence lifetime?
It gets bigger, brighter, and cooler. Kinda like the difference between middle school you and high school senior you.
Which direction does a red giaint go on the diagram after the Helium flash?
It goes left on the horizontal branch
What is spacetime?
Its what Einstein called the four dimensions
How were pulsars initially discovered?
Jocelyn Bell Detected their radio emissions using an 81.5 megahertz radio telescope that was being used to track quasars
The amt of radiant energy from a SN explosion is huge, but what percent of the energy is this of the explosion?
Just 1%
Particles of matter, and photons of radiation, must also travel through the spacetime of the universe. How does this give rise to what we call gravity?
Light cant escape black hole gravity Gravity and electromag force are both carried by the same kind of quantum particles.
What is the lighthouse model of a pulsar?
Magnetic field surrounds it, beam of radio waves coming out of both poles; charged particles fall to magnetic poles.
What did Einstein say that matter does to spacetime around it?
Matter bends the spacetime around it. Spacetime can pushed and pulled, stretched and warped by matter.
What is the maximum mass for a white dwarf? What is the limit called?
Max mass for White dwarf: 1.4M O Called the Chandrasekhar limit
What was left over after the supernova of 1987 that proed theory of t2 supernovas was probably right?
Neutrinos!
What kind of pressure supports a neutron star?
Neutron degeneracy pressure
What stops the collapse of a core when a White dwarf is transforming into a Type 2 supernova?
Neutron degeneracy pressure
What does a supernova type 2's core turn into during its transformation from iron?
Neutrons
Why must a pulsar be a spinning neutron star, specifically?
Neutrons are the only things dense enough to be able to spin as fast as 30 times per second, that shit cray
When a star transitions to a Planetary Nebula (Red Supergiant) and its outer layers expand, is there an explosion involved?
No There is no explosion when a star becomes a planetary nebula
Will the sun every burn elements like helium?
No, the sun will never get that hot.
How do stars obtain their energy?
Nuclear fusion reactions
What is the sequence of spectral types, goiong from hottest stars to coolest stars?
O B A F G K M
Why do we say that a pulsar must be a spinning object? (Like the light in a lighthouse)
Only a rotating object at a constant speed could make such an accurate clock
How does the pressure inside a star change when going from surface to center?
Outer gravity that wants to pull the gas inward and some internal pressure that pushes gas outward
What was the first experimental test of Einstein's General Theory of Relativity?
Positions of stars near the sun were measured during a total solar eclipse
What is the typical range of periods for pulsars?
Pulses range from 30 per second to one every 4 seconds.
How was the system of stellar magnitudes originally set up?
Rankings 1-6 with the naked eye
What colors would stars be at 3,000 K; 6,000 K; 10,000 K; and 30,000 K?
Red; yellow; white; blue
How much larger is the Suns luminosity today, compared with when the sun was born 4.5 billion years ago?
Since its birth, The suns luminosity has increased by 30%
What is right at the center of the Black Hole?
Singularity -- all the mass is here
What types of stars are on the Main Sequence?
Stars in the prime of life that get energy from fusion of Hydrogen to Helium
What are red giants?
Stars in the process of dying
The star is moving into the red giant region of the HR diagram. How have its surface temperature and luminosity changed?
Temperature- It has gotten hotter Luminocity- it has gotten dimmer
What is the Hertzsprung-Russel (HR) Diagram?
The HR diagram is a graph of intrinsic brightness of a star, vs the surface temperature
What heats the gas in a planetary nebula and what causes it to glow?
The Outer layers deep inside the sun make it hot; Ultraviolet Radiation makes it glow. UV is probably the answer to both
We may find the distance to nearby stars using what method?
The Stellar Parallax
Fusion reactions combine light atoms into heavier ones. How does this relate to Ein's Theory of Relativity?
The ToR equation is Mass to energy, which is exactly what happens in nuclear fusion
The core is continuing to contract, becoming hotter and denser, in preparation for the next stage of nuclear fusion. The fusion of helium occurs through a reaction called the "triple alpha process". What is this reaction, and what element results from this reaction?
The Triple Alpha Process formula: 3Helium -> 1 Carbon + Energy The reaction is about when Helium fusion starts in the core of a red giant, when its temp reaches 100 million K. Here, the nuclear fusion turns it into carbon and has excess energy
How does Electron Degeneracy Pressure depend on density or temperature?
The amt of this pressure depends ONLY on DENSITY NOT temperature.
How does the color of a star change with temperature?
The color gets cooler as the temperature gets hotter; Red Yellow Blue White
How is the core of a star changing during the entire main sequence lifetime?
The core gradually changes from hydrogen to helium
Once a core is all Iron, it is finna become a Type 2 supernova. What happens to the core at this point?
The core of Iron collapses and becomes a core of neutrons -- the process takes just 1/1000 seconds
What is a white dwarf star?
The dead embers of a star. they are born hot and spend forever cooling down.
How does energy from the contracting star's center now compare to the energy produced when the star was on the main sequence?
The energy from the star's center now produces MORE ENERGY than when the star was on the main sequence
Nuclear burning proceeds past carbon into heavier elements. This burning continues until Iron is reached. Why CANT nuclear burning of IRON occur?
The fusion of iron into something heavier doesn't give energy out of the reaction.
What is the "Main Sequence"?
The main sequence runs through the HR diagram (the squiggly line)
What is the Chandrasekhar limit?
The max mass 1.4 M O of a WHITE DWARF star.
How can you understand what happens to the core in terms of hydrostatic equilibrium?
The nuclear reactions have stopped and there is no more pressure pushing the core out, so it begins to shrink inwards
What is an Event Horizon?
The outer boundary of a Schwarzschild black hole.
What happens to outer layers of a star after there is no more hydrostatic equilibrium and it begins to die?
The outer layers expand
When the nuclear reactions in the core end, the core begins to contract. Why?
There is no more hydrostatic equilibrium
What do all Main Seq Stars have in common with how they obtain energy?
They all obtain energy from the nuclear fusion from Hydrogen to Helium
In which spectral class of stars are the Hydrogen absorption lines the strongest?
Type A
From the position of its birth on the main sequence, how will a star appear to move on the HR diagram during its Main Seq lifetime?
Up, and to the right.
About how much mass do white dwarf stars have?
WD's are low/medium mass stars, less than 8x that of our sun
Why can't white dwarfs be more massive than the Chandrasekhar limit?
When the core of iron reaches the Chandrasekhar Limit of 1.4, it collapses. White dwarf stars get smaller as they get more massive. When a massive star finally has to stop its fusion at iron, it tries to fuse Iron into something even heavier, and doesn't give energy out of the reaction.
At what point will the main seq lifeitime be over
When there is no more H in the core and fusion has stopped
Are there any nuclear reactions going on inside White Dwarf stars?
Yeah, Iron is being fused into something heavier
What is ZAMS?
Zero Age Main Sequence
What is the radius of the Black Hole if its mass is one solar-mass?
just 3km
What are the apparent and absolute mags for the sun?
m= -26.8; M= +4.8
How big are white dwarfs?
only slightly bigger than earth.