Space Flashcards
What is the Astronomical Unit?
The astronomical unit is the mean distance between the Earth and the Sun.
1AU = 1.5X10¹¹m
What is the unit of a light year a measure of?
Distance
What is a light year?
The distance that light travels in a year
1ly = 9.46 x10¹⁵ m
What can parallax be used for?
To calculate distances in space, using observations and trigonometry
What is a parsec?
A parsec is an astronomical unit of distance. A star is one parsec (pc) away from the Earth if the parallax angle is one arcseond
What is a Hertzsprung-Russell diagram?
A H-R diagram is a plot of a star’s stellar luminosity against its surface temperature
What category of star is represented by the main diagonal on a H-R diagram?
Main Sequence
What category of stars lie below the main sequence diagonal on a H-R diagram?
White Dwarfs
What categories of stars lie above the main sequence diagonal on a H-R diagram?
Red giants and red supergiants
State the equation linking the instensity, luminosity and distance of a star
I = L / 4πd²
What is the doppler effect?
The apparent change of wavelength as a result of relative movement between a receiver and source
What happens to the observed wavelength of a wave when a source is moving away from the observer?
The observed wavelength increases
What happens to the observed wavelength of a wave when a source is moving towards the observer?
The observed wavelength will decrease
What happens to the observed frequency of a wave when a source is moving towards the observer?
The observed frequency will increase
What is red-shift?
The apparent shift of the wavelength of light emitted by a star towards the red end of the light spectrum, as a result of it receding from the observer
What equations are used to calculate red-shift?
z = Δλ/ λ ≈ Δf/ f ≈ v/ c
Describe how the relative speed value inputted into the equation (red shift equation) differs depending on whether the source and receiver are nearing or receding.
For receding, the value is positive.
For nearing, the value is negative
What is the difference between a positive and a negative z value?
Positive means reshift is happening, the objects are receding from one abother. A negative value represents blue shift, objects are getting closer
What are the two main conclusions that can be drawn from observations of red shift from distant galaxies?
- All galaxies show red shift and so all galaxies are moving away
- The more distant a galaxy is, the greater its red shift and so the faster it is receding
What does the red-shift of distant galaxies provide evidence for?
Red-shift of distant galaxies suggests that they are moving away from us. This supports the concept that the univers is expanding, which supports the Big Bang theory
What does CMBR stand for?
Cosmic Microwave Background Radiation
How does the existence of CMBR support the Big Bang theory?
In the big bang, lots of high energy radiation was produced. As the universe has expanded over time, the wavelength of this radiation has been ‘stretched’ so that it is now in the microwave region. Other theories of the universe can’t account for the existence of CMBR
What is cosmic microwave background radiation?
Radiation in the microwave of the Em spectrum that is present in very small quantities all around us
What is the Big Bang theory?
A theory for the formation of the universe. it proposes that the universe began from a very small, hot and dense region, and expanded from this point.
What two quantities does Hubble’s law relate?
The distance of a galaxy to the speed at which it is moving away
Explain Hubble’s law in words
The speed at which a galaxy is moving away from us is directly proportional to the distance it is away
State the defining equation for Hubble’s law.
v = H0 d
H0 is Hubble’s constant
Describe what was observed when observations of galaxies’ brightnesses were compared to the predicitions made by Hubble’s Law.
- The actual and predicted brightnesses of distant galaxies don’t always agree with each other
- Distant galaxies aren’t as bright as predicted by Hubble’s Law, suggesting that they are more distant than predicted
State the conclusion that can be made from Hubble’s Law and observations
The rate of expansion of the universe is accelerating, since galaxies are further away than predicted.
give one possible explanation for the acceleration of the universe’s expansion.
The existence of dark energy. This would explain where the energy required for the expansion rate to constantly accelerate comes from.
Why is the existence of dark energy still only a hypothesis?
Dark matter and energy can’t be easily observed
What has led scientists to believe in the existence of dark matter?
When measuring the rotational speed of the stars in our galaxy, the mass suggested is way too small to keep the galaxy spinning, so they must contain a lot of unseen matter that does not emit light
Why is it difficult to know the exact value of Hubble’s constant?
It is very difficult to get an accurate measurement for a galaxy’s velocity and its exact distance from Earth. There is a lot of variation in vlaues of H0 depending on various methods
What is the Stefan-Boltzmann equation ?
L = 4πr²σT⁴ (for a sphere)
L = σAT ⁴
L is luminosity, σ is Stefan constant, T is temperature K, A is surface area m^2
Why do hotter stars more blue whereas cooler stars redder?
Hot stars show very little hydrogen and most of the hydrogen is in the form of plasma so cannot absorb or emit light. As temp cools, hydrogen can hold onto an electron, so cooler stars are able to absorb more light, relating to the redder colour of the star
What is the differene between brightness and luminosity?
Luminosity is the true brightness of a star wgich is their power output whereas brightness is the object’s apparent birghtness meaured from Earth
What is Wien’s law?
λmaxT = 2.898 × 10⁻³ m K
peak wavelength x Temp (K) = 2.898x10⁻³
Describe the relationship between peak wavelength and temperature
As the temperature increases, the overall radiated energy increases and the peak wavelength decreases
What happens if the density of matter in the universe is less than the critical density?
The universe will expand forever, there isn’t enough gravitational pull from the material to stop/reverse the expansion. known as the ‘Big Chill’ or ‘big Freeze’ as it slwoly cools as it expands until it is unable to sustain life
What does the inverse sqaure law tell us?
It tells us that the energy emitted by a star will spread out in all directions over the surface of an increasing sphere
What are standard candles?
Objects of known luminosity
What is an arcsecond?
An angle equal to 1/3600 th of a degree
What is the critical density
The avergae density of matter required for the universe to just halt its expansion
Why can we only use parallax to measure distances less than 100 lightyears?
Beyond 100 light years it becomes very hard to measure as the parallax angle is very small and so the measurement error becomes too big
What is the absorption spectrum?
A spectrum of EM radiation, transmitted through a substance showing dark lines when light is absorbed at particular wavelengths
For objects too far to be measured using parallax what can you do?
You can use ‘standard candles’ to estimate how far away it is from Earth by how bright it appears
What is a perfect black body?
A theoretical object which absorbs ALL the radiation tha falls on it, doesn’t reflect or transmit any radiation
What are the 3 possible outcomes for the universe?
Open universe
Flat universe
Closed universe
How do you get from degress to kelvin?
add 273.15
Describe a ‘closed’ universe.
A closed universe will happen if the density is high. Locally parallel light rays converge at a distant point = spherical geometry
What is absolute magnitude?
The brightness of a celestial object as it would be seen at a distance of 10 parsecs
Describe a flat universe.
When the density equals the critical density, creates a flat universe where parallel rays remain parallel
How does trigonometric parallax work?
You measure the angle of a relatively nearby star close to earth, and observe how it changes as the position of the Earth changes. Measure the angle 6 months apart (so 2AU apart), we can then use trigonometry to calculate the distance to the star
What are cephied variables?
Stars that brighten and dim periodically
How can Hubble’s Law be used to estimate the age of the universe?
time = distance / velocity
Ho = v/d
Age of universe is 1/Ho
Define luminosity.
the rate at which energy of all types is radiated by an object in all directions
What happens if the density of matter in the universe if greater than the critical density?
If greater then gravity will slow the expansion till it stops and collapses back on itslef - the ‘Big Crunch’
Describe an open universe
When the density is low, locally parallel raus will diverge to cause an open universe
