Astronomy201 - Midterm
What are the contents of the universe (present-day)
68.3% dark energy
26.8% dark matter
4.9% ordinary matter
What is special relativity?
It explains the behaviour of things that move very fast.
- it only applies to specific situations where the different frames or reference aren't accelerating (someone riding a train and someone standing on a platform)
What is time dilation?
- the faster you move through space, the slower you move through time
What is length contraction?
Says that objects can appear shorter in the direction they're moving if they're travelling close to the speed of light.
Postulates of Special Relativity...
1. the laws of physics are all reference frames that are moving at constant speeds
2. the speed of light (in a vacuum) is the same for all observers, regardless of relative motion of source or observer
What is a light year?
The distance light travels in one year
1 light year = 95 trillion km
Newton's First Law of Gravity...
An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
Newton's Second Law...
The change in speed (acceleration), a, is equal to the applied force, F, divided by the mass of the object, m.
a= f/m
Newton's Third Law...
For every applied force, there is an equal and opposite reaction force (you push on ball, ball pushes on you)
What are gravitational fields?
The invisible areas around objects where the force of gravity pulls other objects towards them. (think of them like invisible magnets that attract things with mass)
What are fields?
1. convey the information about the charge and position of each particle
2. they are "quantized", meaning that for each field there is an associated particle
What are the carriers of electromagnetic force?
Photons (particles of light)
What is the equivalence principle?
Gravity and acceleration are indistinguishable from each other
What is general relativity?
The result of the way mass and energy curve the fabric of space and time.
What is an escape speed?
The speed required to break free from an object's gravity (from Earth, the escape speed is about 11.2 km/s)
What is a black hole?
A region in space where matter is compacted so densely that you would have to move faster than the speed of light to escape its gravity.
What is an event horizon?
The outer boundary of a black hole (this is the distance from the center at which the escape speed = the speed of light)
What is the singularity of a black hole?
The center of a black hole, where the density goes essentially to infinity
What is a Schwarzschild Radius?
The distance from the singularity to the event horizon. (depends only on the mass of the black hole
What are the 4 types of black holes?
1. Microscopic black holes
2. Stellar Mass black holes
3. Intermediate Mass black holes
4. Supermassive black holes
What is a microscopic black hole?
Are hypothetical objects (we don;t know if they really exist)
What is a stellar mass black hole?
Have masses that range from 5 to tens of solar masses.
- they form when very massive stars die explosively in supernovae
- there are probably tens of millions of these in our own galaxy
What is an intermediate mass black hole?
Have masses that range from around 100 to 100,000 solar masses
- these may form from the collision of smaller objects or have originated in the early universe
- evidence that they exist comes from gravitational waves
What is a supermassive black hole?
Have masses of millions or billions of solar masses
- reside at the hearts of most large galaxies
- they may have formed by direct collapse, or as stellar mass black holes that grew
What is a tidal force?
a gravitational force that stretches or compresses an object due to the difference in gravitational pull from another nearby object.
How do we know that black holes exist?
- X-ray binary systems
- Direct imaging using the Event Horizon Telescope
- Gravitational wave measurements of black hole mergers
- The motion of the stars around Sagittarius A*
What is a binary star (or binary system)?
A pair of stars that orbit one another
What is an accretion disk?
Often one star in a binary will gravitationally "steal" matter from the other, forming an accretion disk. Material in the accretion disk collides with itself and can reach extremely high temperatures
What is a spectograph?
An instrument used to measure the intensity of light as a function of its wavelength.
- it separates into what appears to be a perfectly continuous rainbow of colours
Spectrum
- Increasing energy and frequency = bluer (has the shortest wavelength)
- Increasing wavelength = redder (has the longest wavelength)
What is the term blackbody?
To describe an idealized object that absorbs all wavelengths of light that strike it.
What are spectral lines?
Tiny gaps in the Sun's spectrum, they tell us what the Sun is made of...
What determines the colour of a star?
- depends on their wavelength
What colour does a cool star emit?
- a cool star emits mainly red light (so you perceive it as reddish)
What colour does a hot star emit?
- a hot star emits some light of every colour, but significantly more blue than the others (perceive it as blue)
What is nuclear fusion?
The fusion of four protons at a time into a helium nucleus
1. the fusion of four protons
2. results in a helium nucleus, 2 anti-electrons, 2 neutrinos, and light
3. the results have a smaller mass than the 4 protons
What is the proton-proton chain
- the main energy source of the Sun
- It's a sequence of nuclear reactions by which hydrogen nuclei (protons) are combined to form helium nuclei, releasing a tremendous amount of energy in the process.
What is a neutrino?
They interact only via gravity and the weak nuclear force. They do not know the electromagnetic or strong nuclear forces- by far the strongest forces
- neutrinos are "ghostly" --> they pass through ordinary matter without difficulty
What is luminosity?
Tells us how bright a light source is intrinsically
What is apparent brightness?
Tells us how bright a light source appears to a given observer
--> it changes with the position of the observer relative to the source
--> the fraction of the star's light the Earth receives determines the apparent brightness of the star to us
What is a parallax?
Refers to the apparent shift in the position of a nearby star relative to more distant background stars as seen from different vantage points in Earth's orbit around the Sun.
Stellar spectral sequence surface temperatures
OBAFGKM (hot to cold)
What is the main sequence?
Stars that cluster along a diagonal sequence from red and low luminosity to blue and high luminosity.
- stars on the main sequence are still fusing hydrogen in their cores (we say they are "alive")
- stars that are not on the main sequence, cease to fuse hydrogen in their core
Characteristics of main sequence stars...
- M-class main sequence stars are called red dwarfs
- G-class main sequence stars are called yellow dwarfs
- all M stars are small and red
- all O stars are large and blue
*as we move up the main sequence, stars get larger in mass but while their lifetimes decrease*
Main Sequence stars... are they dying or are they dead?
- most of the stars above the main sequence (subgiants, giants, supergiants, etc.) are in the process of "dying"
- those below the main sequence, primarily the white dwarfs, are dead
What are luminosity classes?
Terms such as dwarf, subgiant, giant, supergiant, etc.
- categorizes stars based on their brightness and size relative to other stars.
How do we know the age of a star cluster on the main sequence?
- if the star cluster is spread along the main sequence, we know that it is young
- the age of the star is roughly equal to the lifespan of the shortest-lived star still on the main sequence. We call this the main-sequence turnoff age.
What is the Sun a big ball of...?
Hydrogen
Star Spectrum...what is it?
Contains information about its temperature, chemical composition, and intrinsic luminosity.