Space Flashcards
(23 cards)
What is a parsec, and what is an arcsecond?
A unit of DISTANCE, where d = 1/p.
p is the parallax angle, measured in arcseconds, which are one degree divided by 60².
This works by comparing the position of a closer object in the sky to a much more distant one as the Earth spins around the sun. The closer object will seem to move due to the parallax effect.
What is the equation for redshift?
z = ∂λ/λ (recession is positive redshift)
≈ ∂f/f
≈ v/c
How can the Hubble constant be derived?
- The Hubble constant links recession velocity to distance from Earth as directly proportional, where v=Hd.
- Recessional velocity v of an object can be calculated using the redshift equation.
- Distance d of an object can be calculated for standard candles such as cepheid variable stars and type Ia supernovae.
- These known velocity and distance pairings can be graphed, and the gradient of the line of best fit gives us the Hubble constant.
What is the equation linking luminosity and flux?
L = 4πr²f
How can we calculate the distance of a type Ia supernova?
- A type Ia supernova occurs when a white dwarf gains mass (often form another star it was in a binary system with) until it reaches the critical mass of about 1.4 solar masses, after which it explodes as a supernova of known luminosity.
- Because we know its luminosity, and we can measure its flux (observed luminosity) from Earth, we can use the equation linking these two variable (L=4πr²f) to calculate its distance from Earth.
How can we calculate the distance of a Cepheid variable star?
- A Cepheid variable star is a type of star that regularly pulsates, changing in brightness, temperature and diameter.
- Its period of pulsation is directly proportional to its luminosity.
- Using this relationship, we can calculate its actual luminosity, and by measuring its flux, we can calculate its distance from Earth, using L=4πr²f.
What is a standard candle?
An object of known luminosity, such as a Cepheid variable star or a type Ia supernova.
What diagram links the luminosity of a star to its surface temperature? Describe it.
- A Hertzsprung-Russell diagram shows the luminosity and surface temperature of stars.
- The y axis measures increasing luminosity (although it is measured in ‘absolute magnitude’, which is logarithmic and uses lower numbers to indicate higher luminosities). It goes from about 20 to -10.
- The x axis measures decreasing temperature, also logarithmically. Go from 40000 to 2500 kelvin by dividing by two each time.
- Spectral class is also along the same axis of temperature: OBAFGKM.
- Main sequence stars lie in a roughly straight line, with slightly steeper gradients at either end.
- Supergiants lie along the top, giants lie just below them on the right, and white dwarfs lie in the bottom right.
What is evidence for dark energy?
The fact that galaxies nearer the edge of the universe don’t slow down expanding outwards, despite the centripetal force.
What is a neutron star?
When a star between 1.4 and 3 solar masses collapses and its gravity is such that it can combine electrons and protons to form neutrons.
What are the characteristics of a main sequence star?
- They mainly fuse hydrogen into helium.
- They maintain a relatively constant luminosity for most of their lifetime.
How is the distance to nearby stars calculated?
- The star is viewed from two positions at six month intervals, when the earth is at opposite ends of its orbit diameter around the sun.
- The change in position/angle is measured of the star against a fixed backdrop of distant galaxies.
- Trigonometry is used to calculate the distance from this change in angle. The diameter of the Earth’s orbit must be known.
How do you know that if all stars on a HR diagram are young?
- They will all be in their main sequence stage
- An older cluster would have some red giants
What is the Stefan-Boltzman law?
L = 4πr² * σT⁴
How can you approximate the age of the universe?
- Hubble plotted recessional velocity against distance and found a straight line relationship, and took the gradient as the Hubble constant (H0 = v/d).
- v=d/t, so t = d/v = 1/H0
- Actually, t ≈ 1/H0, because of these assumptions:
- Assumes the galaxy has always travelled at that speed.
- Ignores time between big bang and galaxy formation.
- Uncertainties in H0.
How do we know the expansion of the universe is accelerating?
- The gradient on the velocity-distance graph is shallower for galaxies further in the past, with more recent galaxies creating a steeper gradient. This suggests the rate of expansion is accelerating.
- Distant objects are seen in the past, as it takes a very long time for the light to reach Earth. If the universe expansion was slowing down, these distant objects would be travelling away faster than they currently are, so they would be closer than predicted, and have a higher recessional velocity than predicted. Observations of distance stars show that in fact the opposite is true; distant stars appear further away than Hubble’s law predicts, with a lower recessional velocity, suggesting that expansion is accelerating.
What are the different possible fates of the universe?
- Closed universe (Big Crunch) where gravity is greater than expansion.
- Open universe where expansion is greater than gravity
- Flat universe where expansion approaches gravity.
When calculating a recessional velocity from redshift, what must you remember to do?
Specify a direction, with a positive redshift indicating recession. Make sure you consider the sign of the change in wavelength when calculating.
Describe the different spectral classes.
O: blue, 25000-50000K
B: blue, 11000-25000K
A: blue/white, 7500 - 11000K
F: white, 6000 - 7500K
G: yellow/white, 5000 - 6000K (our sun)
K: orange, 3500 - 5000K
M: red, <3500K
Describe the lifecycle of a star.
- Nebula (gas cloud)
- Protostar
- Fragments of mass in nebula clump together under gravity.
- Irregular clumps rotate, and conservation of angular momentum spins them inwards to form a denser centre. - Main sequence
- Outward force of fusion and inward force of gravity are in equilibrium, so the star is stable.
- Hydrogen nuclei are fused into helium.
- The greater the mass of the star, the shorter its main sequence period, as it uses fuel more quickly. - Red giant (<3 solar masses)
- Hydrogen runs out.
- The core temperature increases and begins to fuse helium into heavier elements.
- The outer layers expand and cool. - Red supergiant (>3 solar masses)
- Same process as red giant, but larger scale. - White dwarf (<1.4 solar masses)
- Fuel runs out and fusion stops.
- The core contracts because the star isn’t balanced.
- The core becomes very dense, and is inactive.
- The star cools gradually over time, theoretically becoming a black dwarf when it loses most of its thermal energy. - Supernova (>1.4 solar masses)
- Fuel runs out, and the core collapses very quickly, becoming rigid.
- The outer layers of the star fall inward and rebound off the core, launching them back into space as a shockwave.
- As the shockwave interacts with surrounding material, elements heavier than iron are fused and flung into space. - Neutron star (<3 solar masses)
- When the core of the star collapses, gravity is so strong that it forces electrons and protons together to form neutrons. - Black hole (>3 solar masses)
- The gravity of the core is so strong that even neutrons are forced together.
- Gravity is so strong, that even light can’t escape.
What is evidence for dark matter?
Consider the centripetal force exerted on stars in the outer orbits of a galaxy. They should in general be traveling slower than stars nearer the centre, as speed is inversely proportional to orbital radius. However, it seems that all stars tend to travel at roughly the same speed. This suggests the existence of ‘dark matter’ that provides the extra mass needed to allow this higher orbital speed. This matter is unobservable.
Describe the vertical scale on a HR diagram.
The vertical luminosity scale, expressed in units of solar luminosity (3.9 x 10^26 W – the luminosity of our sun) extends over a large range, from 10^-4 to 10^4.
Why is trigonometric parallax suitable for measuring the distance of nearby galaxies?
The parallax angle of nearby galaxies is large enough to be measured accurately.
